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And now . . . the perfect partner for our AM receiver: By John Clarke The Super-9 FM Radio 26 Silicon Chip Australia’s electronics magazine siliconchip.com.au This FM radio is easy to build and provides excellent performance. An entirely analog design, it has a sizeable internal speaker, with the ability to drive stereo headphones or external speakers. It can be battery or mains powered and is tuned with a hand-span dial. It looks great in its custom case, and building it is an excellent way to learn how FM radio works. O ur “Super-7” AM Radio (November & December 2017) has proven to be very popular. So we’ve developed this high-quality FM Radio with many of the same features. That includes ease of construction, good looks and great performance. It takes full advantage of the high audio quality that FM broadcasts are capable of reproducing. And it can receive in stereo, too. It’s powered from a 9V battery (making it truly portable) or 9V DC plugpack, and it automatically switches from battery to the plugpack when plugged in. Power consumption is moderate, so a small 9V battery should last for several hours of listening. All the components mount on one double-sided PCB (printed circuit board) which fits into a custom-designed acrylic case with a transparent back. That’s so the components are protected but you can still see its workings. It has a large hand-span tuning dial showing the current frequency plus many of the available FM radio stations around Australia. Once built and aligned, you will have a fully functioning radio. And that’s something the average person with basic soldering skills can do, as long as you follow our instructions. Besides the usual soldering and mechanical assembly using screws, nuts and spacers, you just need to wind a few coils. This FM Radio would make a great learning aid for people studying electronics. Most modern FM receivers use one or two integrated circuits (ICs), with a few external components. However, for this design, we have opted for a more discrete approach, so that the major circuit blocks are all clearly separated. Although we have used a few ICs, each only performs one or two major tasks. The circuit is therefore discrete in the sense that each functional block is separate, and that makes it easy to understand what it does and how it works. +10 Audio output 0 4 -20 3 -30 Signal level 2 -40 -50 1 Stereo threshold Signal level (V) 5 -10 Output level (dBV) Fig.1: these curves show how the unit’s performance varies with signal strength. The blue “Audio output” curve shows the test tone output level, with the cyan and red curves showing the corresponding noise levels. The distance between the Audio output and mono/stereo noise level is the signal-to-noise ratio for that input level. The corresponding voltage at TP SIGNAL is also shown in green, using the right-hand axis. Full limiting does not occur until the RF input reaches about 45µV, while stereo cuts out below 30µV. 0 -60 -70 Stereo noise level -80 Mono noise level -90 10 100 1000 10,000 Receiver SC  30,000 RF input level at 98MHz (µV) 20 1 9 siliconchip.com.au Australia’s electronics magazine November 2019  27 What is FM? FREQUENCY AND AMPLITUDE REMAIN CONSTANT CONTINUOUS Going back to the WAVE (NO MODULATION) times when radio was discovered, there have INTERRUPTED FREQUENCY REMAINS CONSTANT (A) ON-OFF been three basic methMODULATION ods of encoding a radio frequency wave, or “carrier”, with infor- FREQUENCY REMAINS CONSTANT, AMPLITUDE VARIES (B) AMPLITUDE mation. MODULATION The first of these (AM) is CW, or continuous AMPLITUDE REMAINS CONSTANT, FREQUENCY VARIES wave. If the RF carrier (C) FREQUENCY is fixed at one particular MODULATION (FM) frequency and the level, SC 20 1 9 or amplitude, is held constant, the only way that information can be conveyed is by switching the RF signal on and off. This is the technique used for Morse Code and other types of digital transmission, as shown in (A). Next to come was called amplitude modulation, or AM. Here a second signal is modulated, or mixed, with the radio carrier, which causes the RF signal level to vary in sympathy with the second signal. This makes it is possible to transmit speech, music or even video. This is shown in (B). A receiver that’s tuned to the carrier frequency can detect these changes in amplitude to reproduce the varying signal. But this type of encoding is quite prone to interference. Part of the reason for this is that the signal amplitude necessarily dips at times, and at these points, it can be more easily overwhelmed by interfering signals. Also, any distortion of the carrier waveform distorts the signal. The third method is called frequency modulation (FM). Instead of varying the carrier amplitude, information is conveyed by varying the carrier frequency, again in sympathy with the incoming speech, music or video signal. This is shown in (C). Note that the waveform amplitude is constant. At the receiver, the variations in carrier frequency are detected (or demodulated) to recover the original signal. Any variations in amplitude that may occur in the received signal are effectively ignored. Therefore, FM receivers are far less prone to interference than their AM counterparts. Broadcast band FM transmitters modulate the RF carrier by a maximum of 75kHz above and below the carrier frequency, which is typically around 100MHz. They also include pre-emphasis, whereby audio signals above 3.1831kHz (50µs time constant) are boosted. These signals are subsequently restored to normal in the receiver using a complementary de-emphasis circuit. The idea behind using pre-emphasis and de-emphasis is to reduce high-frequency noise, which may be injected by the modulating/demodulating circuitry or by interfering signals. By boosting high frequencies before transmission, then cutting them after reception, any high-frequency noise picked up along the way is also significantly attenuated. The radio is aligned with the aid of a simple 10.7MHz oscillator, which you can also easily build yourself. Along with the FM Radio construction details, we’ll have a project for one of these next month. Apart from that, the only other items required for alignment are a multimeter and a plastic trimming tool. These days, many components are only available in surface-mounting packages. Some of those can be quite tricky to hand-solder. We have done our best to use mainly through-hole components in this Radio, but in some cases, we had no choice. However, those few SMDs 28 Silicon Chip we’ve had to use can be soldered without too much difficulty, since they only have a few pins and the pins are not that closely spaced. Radio performance The performance of this FM Radio is shown in Fig.1 and described in the Features & Specifications panel. The minimum usable RF signal level is around 35µV, at which point the audio signal level is about 3dB down. With 100µV from the antenna, the mono signal-to-noise ratio is 70dB, which is quite good. The ultimate signal-to-noise ratio in mono is 85dB (ie, with a sufficiently Australia’s electronics magazine strong signal). Few commercial tuners would match that. The ultimate stereo signal-to-noise figure is 75dB, also very good. So while this is not the most sensitive FM Radio ever devised, it provides excellent performance on all local stations, with good reception for signals up to, say, about 70km away. In fact, this FM Radio sounds better than all but the best commercial receivers (and probably most FM receivers made in the last 10 years or so). Before you read the description below of how the FM Radio works, you may wish to first refresh your knowledge of FM Radio by reading the explanatory panel at left. Block diagram The Super-9 Stereo FM Radio is based on the superheterodyne principle. Fig.2 shows its general configuration. The antenna at upper left picks up signals in the FM band. These signals are fed to a bandpass filter, a parallel resonant circuit comprising one inductor (L1) and two capacitors. These heavily attenuate signals outside the 88-108MHz FM broadcast band. These signals then pass to a tuned RF amplifier stage. This stage has a parallel resonant circuit that is tuned by inductor L2 and varicap diode VC1. VC1 has a capacitance that changes with applied voltage. By adjusting the applied voltage, the RF amplifier can be tuned to any nominal frequency from 88 to 108MHz. Therefore, it only amplifies signals at the desired frequency and attenuates the rest. The tuning voltage comes from a tuning potentiometer (VR1), and the voltage is processed in the control voltage circuit to provide the required range for VC1 to tune over the broadcast band. Following the RF amplifier, the signal is fed to the mixer (Q2 & T1), where it is mixed with the local oscillator signal. This tracks the tuned RF amplifier frequency, which is achieved using a second varicap diode (VC2) in combination with inductor L3. The local oscillator tracks 10.7MHz below the tuned RF signal carrier. In other words, it is adjustable from 77.3MHz to 97.3MHz. So for example, if the FM Radio is tuned to 102.5MHz, the local oscillator will be at 91.8MHz (102.5MHz siliconchip.com.au Features & specifications Tuning range:................................. range:................................. 88-108MHz (FM broadcast band) 50dB quieting sensitivity: ........ 20µV Signal-to-noise ratio: .................. 85 dB mono, 75dB stereo with 150mV input (see Fig.1) Distortion (mono): ....................... 0.39% <at> 1kHz, 100% deviation; 0.31% <at> 1kHz, 75% deviation; 0.8% <at> 6kHz, 75% deviation Distortion (stereo): .................... 0.7% <at> 1kHz, 75% deviation; .7% <at> 6kHz, 75% deviation Frequency response: .................... 30Hz-20kHz, +0,-1dB with 150Ω 150Ω load (-3dB at 27Hz with 32Ω 32Ω load) Demodulator output: .................. 190mV RMS for 100% deviation at 1kHz De-emphasis time constant: .... 50µs Frequency capture range:.......... range:.......... ±200kHz Operating voltage range: ......... 9-12V DC Current consumption: ................ 75mA <at> 9V with low volume - 10.7MHz). The 10.7MHz frequency difference is a standard value for broadcast-band FM receivers. Tuning of this oscillator is also via VR1, with the control voltage for VC2 processed in the same control voltage block, to provide the required tuning range. The local oscillator frequency is fine-tuned (to ensure the correct 10.7MHz gap) via the automatic frequency control (AFC) signal from the demodulator block (described below). This produces a voltage that controls the capacitance of varicap VC3, which is connected to the local oscillator. AFC is voltage feedback to keep the local oscillator in-lock with the tuned signal, so the FM Radio does not drift off station. This also produces a snap-in effect, whereby the station suddenly locks in as the tuning approaches the station frequency. Note that the tuned amplifier is not affected by AFC. However, the RF stage bandwidth is sufficiently broad that it does not need to track precisely with the local oscillator. Superheterodyning of the two signals takes place in the mixer. By the way, the word “heterodyne” refers to a difference in frequency or beating effect, while the “super” prefix refers to the fact that the beat frequency is supersonic or ultrasonic (ie, beyond the range of human hearing). Four signals are produced as a result of mixing the tuned and local ANTENNA 88 108 MHz 108 MHz 88 IF AMPLIFIER BANDPASS FILTER (L1, 47pF & 39pF CAPACITORS) TUNED RF AMPLIFIER MIXER (Q1, L2 & VC1) (Q2, T1, 47pF CAPACITOR) 10.7MHz (IC2) 10.7MHz 10.7MHz BANDPASS FILTER 10.7MHz (XF1) A 77.3 – 97.3MHz K +5V VC1 LOCAL OSCILLATOR A K CONTROL VOLTAGE TUNING VR1 VC3 (IC1, VR2, VR3) (Q3, Q4, L3, L4 & VC2) A A K VC2 A 10.7MHz LEFT AMPLIFIER 10.7MHz AMPLIFIER, LIMITER & DEMODULATOR (IC3, L5,T2) REF AFC AUDIO STEREO DECODER (IC4) LEFT RIGHT (IC5) LOUDSPEAKER RIGHT AMPLIFIER (IC6) HEADPHONES OUTPUT CON2 Fig.2: the incoming RF signal passes through a bandpass filter STEREO/MONO SWITCHING and is then fed to a tuned RF amplifier stage. The tuned signal SC 20 1 9 is then mixed with the local oscillator signal to produce a 10.7MHz IF signal. This is then further amplified, filtered and fed to the demodulator. A stereo decoder and amplifiers for the left and right channels provide stereo for headphones and mono drive for the in-built loudspeaker. siliconchip.com.au Australia’s electronics magazine November 2019  29 This page and opposite: front and rear views of the complete FM receiver, before it is mounted in its Acrylic case. Everything mounts on this single PCB – but note that these shots are of an early prototype, hence a few “stray” components which are taken care of on the finished PCB. (Production PCBs will be black to highlight the dial markings). oscillator signals. These comprise the two original signals and the sum and difference frequencies. One of these is <at> 10.7MHz ±75kHz, due to the fixed difference between the RF carrier and local oscillator. The mixer output is fed to a bandpass filtering comprising transformer T1 and a 47pF capacitor. This filter is tuned for a centre frequency of 10.7MHz, so it rejects the other three signals and just keeps the 10.7MHz difference signal. This then passes to an amplifier stage, providing a gain of about 60 times (53dB). A much sharper-edged bandpass filter follows, which prevents signals passing through outside of a 280kHz band centred at 10.7MHz (ie, 10.7MHz ±140kHz). The big advantage of producing a fixed frequency signal to process is that we now only need to provide further gain at one frequency, rather than for the whole 20MHz broadcast band range, which would require complicated tracking filters. The amplifier, limiter and demodulator block includes a three-stage amplifier for this IF signal, to ensure that this signal is driven into limiting. Limiting Limiting is where the amplification factor is so high that the signal is clipped to the same level, even with a greatly varying input signal level. This is done to eliminate any amplitude variations in the tuned signal before it is fed into the demodulator. This is one of the factors that enables FM tuners to reject atmospheric and electronic noise that mainly affects RF signal amplitude. The amplifier, limiter and demodulator block also provides the AFC signal (mentioned above) and the audio signal output. This is obtained using Fig.3: the FM stereo encoding scheme, with the L+R signal extending out to 15kHz. The pilot signal at 19kHz is 10% of full modulation. The L-R signal is from 23kHz to 38kHz (a 15kHz bandwidth) and also from 38kHz to 53kHz with the 38kHz carrier suppressed (ie, not transmitted). 30 Silicon Chip Australia’s electronics magazine a quadrature detector comprising inductor L5 in series with a tuned circuit with variable inductor L6 and a parallel capacitor. This tuned circuit is adjusted to resonate at 10.7MHz. The inductor produces a fixed 90° phase shift while the tuned circuit provides an additional leading or lagging phase shift with frequency. A mix of these signals then produces a varying voltage that is the audio output. Stereo decoding Most FM radio stations broadcast in stereo for separate left (L) and right (R) channels. This is done by encoding the sum (L+R) and difference (L−R) signals in the FM transmission using a 38kHz subcarrier. This is shown in Fig.3. For mono reception, just the L+R signal is used. Since the left and right channels are the same for a mono signal, the L+R will be the same as 2L. For stereo reception, the left channel is derived as the sum of L+R and L-R (giving 2L) and the right channel is the difference of L+R and L-R (giving 2R). The left and right channels are decoded using a 19kHz pilot signal, which is exactly half the frequency of the 38kHz suppressed subcarrier. The phase of the pilot signal allows the left and right sum and difference signals to be decoded. siliconchip.com.au Fig.3 shows the FM stereo encoding with the L+R signal extending out to 15kHz. The pilot signal at 19kHz is just 10% of the full modulation. The L-R signal is from 23kHz to 38kHz (a 15kHz bandwidth) and also from 38kHz to 53kHz with the 38kHz carrier suppressed (not transmitted). The audio signal is processed in the stereo decoder (IC4) that separates the audio into left and right channels. This also includes the necessary 50µs deemphasis to compensate for the preemphasis in the transmitted signal. Amplifiers IC5 and IC6 provide the stereo signal output to drive headphones. Stereo decoding occurs only when headphones are connected — switch contacts within the headphone socket control whether there is stereo or mono output from IC4. Without the headphones connected, the sound is from the single loudspeaker in the Radio, so reception is in mono. Audio amplifier IC5 drives the loudspeaker. Circuit details Refer now to Fig.4 (overleaf) for the full circuit of the Super-9 Stereo FM Radio. Its main components are dualgate Mosfets Q1 and Q2, high-frequency transistor Q3, video amplifier IC2, amplifier/limiter/demodulator IC3, stereo demodulator IC4 and audio amplifiers IC5 and IC6. The function of each stage is shown siliconchip.com.au on the circuit, and each stage can be directly related to the block diagram (Fig.2). Starting at the antenna, the incoming RF signal is coupled to the junction of two capacitors (39pF & 47pF) which, together with parallel inductor L1, form the input bandpass filter. A 1kΩ resistor is included in parallel with L1 to reduce the filter Q, so that it covers the entire FM band without adjustment. This input filter helps to prevent signals with frequencies outside the FM band from entering the circuit and possibly overloading the following stages. Following the input filter, the RF signal is fed via ferrite bead FB1 to one gate (G1) of dual-gate Mosfet Q1. Q1 operates in a common-source configuration. Its quiescent current is set by the 330Ω source resistor, bypassed by a 10nF capacitor to ensure maximum AC gain. The gain is set to a high value by biasing G2 above its cut-off voltage, at around 4.5V, by the two 10kΩ bias resistors connected in series across the 9V supply. Q1’s drain load is a portion of coil L2, which ultimately connects to the 9V supply. The junction of L2 and the 47Ω decoupling resistor is bypassed by a 10nF capacitor. As a result, L2 is effectively grounded at this point, as far as RF signals are concerned. Scope1 shows the sinewave output of the local oscillator, as measured at TP1. This is low in distortion (low in harmonics) to improve image rejection. It also has low frequency jitter so that noise is not produced in the audio signal after FM demodulation. Australia’s electronics magazine November 2019  31 Fig.4: each stage in the circuit of the Super-9 Stereo FM Radio is labelled and can be related to the block diagram, Fig.2. Dual-gate Mosfet Q1 forms the heart of the tuned RF amplifier, while Q2 is the mixer and Q3 the local oscillator. IC2 and IC3 form the IF amplifier stages while L6 and associated resistor and capacitor form the quadrature detector for IC3, in conjunction with L5. Varicap diode VC3 provides Automatic Frequency Control for the local oscillator and is controlled from IC3’s AFC voltage output. The full L2 coil is tuned using the 220pF capacitor connected in series with varicap diode VC1. The 220pF capacitor reduces the tuning capacitance adjustment range to 88-108MHz. This capacitor also prevents DC voltage from reaching the anode (A1) from L2. The anode is then grounded via a 68kΩ resistor so that its DC bias is 0V. We’re using a dual varicap diode to minimise signal excursions from modulating the overall total capacitance of the varicap VC1. So if one of the varicap diodes has signal across it that reduces its capacitance, the opposite varicap diode connected in reverse will have a sig32 Silicon Chip nal that increases its capacitance. So these effects cancel out. Tuning is via adjustment of potentiometer VR1. This would normally have an adjustment range of 0-5V, over a travel of 300°. A mechanical stopper is used to restrict the travel range to 180°, so it has a usable voltage range of 1-4V. Op amp IC1b amplifies this voltage. When calibration trimpot VR2 is set for minimum resistance between pins 6 and 7 of IC1b, IC1b’s output range is 1-4V. With VR2 set for the maximum 10kΩ resistance between these pins, the amplification is 1.5 times (10kΩ Australia’s electronics magazine ÷ 20kΩ + 1), giving an output range of 1.5-6V. VR2 can be set to an intermediate position for a gain value between 1.0 and 1.5. VR2 is used to adjust the upper tuning frequency to 108MHz when VR1 is set for the maximum 4V at its wiper. The lower 88MHz tuning frequency (with VR1’s wiper at 1V) is adjusted by manipulating the inductance of coil L2, by slightly compressing or expanding it. The tuning voltage from the pin 7 output of IC1b is reduced by a factor of two using a voltage divider comprising two 4.7kΩ resistors. This voltage is then filtered by a 10nF capacitor siliconchip.com.au and applied to the common cathode of varicap diode VC1 via a 68kΩ resistor. The resistor is included to provide a high resistance to the capacitor, so that the resonance of the tuned circuit is not loaded. Local oscillator NPN transistor Q3 and its associated components make up the local oscillator. Its base is DC biased to about 4.5V by the two 10kΩ resistors connected across the 9V supply and by its 560Ω emitter resistor. The collector load is L4 to its series 47Ω resistor to 9V, with the junction of the two bypassed to ground by a 10nF capacitor. siliconchip.com.au Q3’s base is also connected to a tuned circuit comprising inductor L3 and varicap diode VC2. The other end of L3 is connected to ground via a 10nF ACcoupling capacitor, so that the DC biasing of Q3’s base is not affected by L3. Similarly, a 220pF capacitor between Q3’s base and the anode of VC2 isolates the base DC voltage from the varicap diode and reduces the overall capacitance variation for the tuned circuit from the varicap, as for the tuned RF amplifier. A 68kΩ resistor from the anode of VC2 to ground sets its DC bias to 0V. We are using a dual varicap here for the same reasons as described above. Australia’s electronics magazine The oscillation frequency is determined by L3’s inductance and VC2’s capacitance. Oscillation is caused by feedback between L4 and L3. These are mounted adjacent to each other to provide some magnetic coupling between them. This type of oscillator is known as an “Armstrong” or “Meissner” oscillator, after the original developers of the configuration. It’s also sometimes called a tickler oscillator due to the ‘tickler’ coil L4 exciting the tuned circuit incorporating L3. Transistor Q4 is a buffer connected in an emitter follower configuration. This provides a test point at the emitter (TP1) November 2019  33 for frequency measurement. Without Q4, an oscilloscope probe or frequency meter connected to the emitter of Q3 would alter the oscillation frequency. Scope1 shows the sinewave output of the local oscillator, as measured at TP1. Automatic Frequency Control is provided for the local oscillator using varicap VC3, which is coupled to the L3 tuned circuit via a 2.2pF capacitor. A single varicap diode is used since the signal level is very low across it, so the signal does not affect its capacitance very much. Its control voltage is derived from the tuning voltage produced by IC3, which will be described later. The local oscillator is also tuned using VR1. Op amp IC1a provides amplification of the voltage from VR1’s wiper, adjusted using VR3. The resulting tuning voltage is applied to the common cathode of varicap diode VC2 via a 68kΩ resistor, similarly as for VC1. VR3 is used to set the upper local oscillator frequency to 97.3MHz when VR1’s wiper is at 4V. The lower 77.3MHz setting (with VR1’s wiper at 1V) is made by compressing L3’s windings slightly for a lower frequency or expanding it for a higher frequency. Mixer stage The output from the local oscillator at Q4’s emitter is coupled via a 4.7pF capacitor to one gate (G2) of dual-gate Mosfet Q2. The 4.7pF and 330pF capacitors form a capacitive voltage divider, greatly reducing the local oscillator voltage applied to Q2, so as not to overload the mixer. Mosfet Q2 functions as the mixer stage. It mixes the local oscillator signal with the tuned RF signal fed via a 220pF capacitor and FB2, to its other gate input (G1). The bias for G2 is set to about 4.77V by two 10kΩ resistors and the 330Ω resistor from Q2’s source to ground, while G1 is biased to 0V by a 470kΩ resistor. FB2 prevents parasitic oscillation in Q2. Q2’s drain load is a tuned circuit, peaked at 10.7MHz using a 47pF capacitor and an adjustable ferrite-cored inductor which is the primary of IF transformer T1 (between pins 1 & 2). Since the pin 2 end of the primary is grounded for radio frequencies via a 10nF capacitor, the winding is effectively connected in parallel with the 47pF capacitor. As a result of this tuning, Q2 operates as a very efficient amplifier over a narrow band centred on 10.7MHz. Fre34 Silicon Chip quencies outside the wanted band (including the original RF signal, the local oscillator signal and the sum of these) are rejected. It is only the 10.7MHz difference signal that appears at the secondary of T1. Further gain The secondary winding of T1 (pins 3 & 4) couples the signal to the differential inputs (pins 1 & 8) of video amplifier IC2. Its inputs are DC-biased at half supply via a 10kΩ/10kΩ resistive divider across the 9V supply, with a 10nF filter capacitor to reject noise. The 10Ω resistance between pin 2 and 7 of IC2 sets its gain to around 400 times (52dB). Ceramic filter The output of amplifier IC2 is fed to ceramic filter XF1 via a 330Ω resistor. This resistor provides the 330Ω source impedance required for the filter to work as designed. The filter output feeds into another 330Ω load resistor, again required for impedance matching. XF1 provides further rejection of unwanted signals outside the 10.7MHz ±75kHz IF range. It is a bandpass filter with a 10.7MHz centre frequency and a 280kHz bandwidth. The filtered signal then goes to input pin 1 of IC3, the amplifier/limiter/detector. This is a part specially designed for FM radio decoding. It includes a three-stage IF amplifier and limiter, quadrature detector and an audio amplifier with a squelch feature. Squelch switches the output off if the signal level is so low that the output is just noise. IC3 also has a signal strength metering output at pin 13 and an automatic frequency control (AFC) output at pin 7. The voltage at pin 7 varies above or below the 5V reference voltage output at pin 10, depending on whether the signal frequency fed into pin 1 is above or below 10.7MHz. The 5V reference voltage is applied to the cathode of VC3 for the local oscillator via a 47kΩ isolation resistor. The AFC output is divided by two using a 47kΩ/47kΩ voltage divider, and this becomes the anode voltage for VC3. So when the tuning is spot on, VC3’s anode is at 2.5V. If it starts to drift off station, the AFC voltage will change, causing VC3’s capacitance to change, bringing the local oscillator back into tune. The quadrature components needed for demodulation comprise a fixed 22µH inductor (L5), variable inductor Australia’s electronics magazine (L6) and the associated 100pF capacitor and 3.9kΩ resistor. See the panel for an explanation on how IC3 and quadrature demodulation work. L6 is adjusted to resonate at 10.7MHz with the 100pF capacitor. The 3.9kΩ resistor lowers the Q of the tuned circuit to provide a linear voltage variation with frequency, over the frequency range of the FM signal. Stereo decoding The audio signal from the demodulator is fed to input pin 2 of the MC1310P stereo demodulator, IC4, via a 2.2µF coupling capacitor. IC4 decodes the left and right channel information included in the transmitted FM signal. It also provides the required 50µs de-emphasis (in both mono and stereo modes), rolling off the audio frequency response above 3.18kHz. The panel overleaf describes how the stereo signal is recovered. The de-emphasised audio outputs are from pin 4 for the left channel and pin 5 for the right channel. The 3.3kΩ resistor and 15nF capacitor at each output set the required 50µs time constant (3.3kΩ x 15nF = 49.5µs). The resulting left and right channel audio signals go to integrated amplifiers IC5 and IC6 respectively. These are used to drive the headphones in stereo mode, via 220µF electrolytic capacitors which remove the DC bias that’s present at the amplifier outputs. When the headphones are not connected, the IC5 drives the loudspeaker in a bridge-tied load (BTL) arrangement. So when pin 8 provides a positive signal swing, the pin 5 output provides a negative signal swing and vice versa. The result is that the loudspeaker is driven with more voltage and hence the amplifier provides more power (up to four times as much), compared to if only a single output from the amplifier were used. When driving the loudspeaker, we want IC4 to produce a mono signal so that the speaker reproduces a mix of both the left and right channels (assuming reception is in stereo). But when the headphones are connected, we want the speaker to be switched off and IC4 to provide stereo so that each headphone driver receives a different signal. Also, the headphones can only be driven in single-ended mode rather than BTL mode, because they share a common ground connection. This is because typical headphones connecsiliconchip.com.au tors such as TRS types only have three contacts: one for the left signal, one for the right signal, and a common ground. The LM4865 amplifier ICs we’re using have a clever solution to this. Pin 3 selects whether the output is singleended or BTL. The switching contact for the tip connection in the headphone socket goes to pin 3 of IC5 but is also tied to +5V via a 100kΩ resistor. With the headphones not plugged in, the 150Ω resistor pulls pin 3 below 50mV, and this sets IC5 in the BTL mode for driving the speaker. Pin 3 of IC5 is also applied to the gate of Mosfet Q5. Since this voltage will be low, Q5 is off and so the second Mosfet (Q6) has its gate pulled to 5V by a 100kΩ resistor. With Q6 switched on, it pulls pin 8 of IC4 to ground and this disables stereo decoding. IC5 therefore drives the speaker in mono. When headphones are plugged in, the switch contact in the headphone socket opens and pin 3 of IC5 is pulled to 5V via the 100kΩ resistor. This changes IC5 to single-ended operation, with output pin 8 floating. This prevents the speaker from being driven. Only pin 5 is driven, and this powers the left headphone channel. At the same time, the gate of Mosfet Q5 goes high, switching it on and pulling the gate of Q6 low. So Q6 switches off and allows the voltage at pin 8 of IC4 to rise, enabling stereo decoding. IC6 is always used as a single-ended amplifier, as its pin 3 is held high (5V) via a 100kΩ resistor. That’s because this IC is only used to drive the right headphone channel. How the CA3089 demodulator works original and phase-shifted signals are then fed into a mixer, followed by a low-pass filter. This arrangement effectively acts as a phase detector, producing a voltage proportional to the phase difference. The reason that this works as a demodulator is that the phase shift of the RLC network varies slightly with signal frequency; it will be a bit less than 90° at frequencies below 10.7MHz and a bit more than 90° at frequencies above 10.7MHz. Therefore, the output voltage of the phase detector tracks the frequency deviation of the incoming signal. The phase shift is not exactly linearly proportional to frequency variation; however, the frequency variation is a small percentage of the carrier (±75kHz compared to 10.7MHz, or about ±0.75%). The middle section of the frequency/phase curve is substantially linear, so this type of demodulator has very good performance. Distortion levels as low as 0.1% are possible with a well-designed and tuned reactive network. As shown in the spec panel, distortion is often a little lower for less than full deviation, because the demodulator is operating over a more linear part of the curve. For more details on its operation, see the CA3089 data sheet, which can be downloaded from: siliconchip.com.au/link/aav7 The block diagram of the CA3089 IC, extracted from its data sheet, is shown at bottom. The incoming signal passes through three separate balanced amplification stages, each with its own level detector. The level detector output currents are summed and fed to pin 13, allowing the signal level to be measured. Once the signal enters limiting, that current reaches a maximum value. The output of the last amplifier is fed to the quadrature detector, which converts the frequency deviation in the signal to a varying output voltage, recovering the audio signal. The way this demodulator works is shown below. SOURCE IMPEDANCE SIGNAL SOURCE MIXER 90° SHIFT <at> 10.7MHz LOW-PASS FILTER SC 20 1 9 The external RLC network (shown above as two capacitors, an inductor and a resistor) is designed to produce a 90° phase shift at the intermediate frequency; in this case, 10.7MHz. The CA3089 Block Diagram L QUADRATURE INPUT 22H V+ TO INTERNAL REGULATORS IF INPUT IF AMPLIFIER 1 1ST IF AMPL. 2ND IF AMPL. 11 IF OUT 8 C= 100pF 9 3RD IF AMPL. 10 REFERENCE BIAS QUADRATURE DETECTOR AFC AMPL. 7 AFC OUTPUT AUDIO AMPL. 6 AUDIO OUTPUT AUDIO MUTE (SQUELCH) CONTROL AMPL. 5 3 0.02 F 0.02F 2 DELAYED AGC FOR RF AMPL LEVEL DETECTOR LEVEL DETECTOR LEVEL DETECTOR 15 10K siliconchip.com.au FRAME SUBSTRATE 4 14 LEVEL DETECTOR TUNING METER CIRCUIT 150A METER MUTE (SQUELCH) DRIVE CIRCUIT 13 33K TUNING METER OUTPUT Australia’s electronics magazine 12 MUTING SENSITIVITY 470 120K 0.33F 500K TO STEREO THRESHOLD LOGIC CIRCUITS November 2019  35 Parts list –Super 9 FM Receiver 1 double-sided PCB coded 06109181, 313 x 142.5mm 2 shield PCBs coded 06109183, 13 x 35.5mm 1 antenna mount extender PCB coded 06109184, 7.6 x 27mm 1 pot travel stopper PCB coded 06109185, 23 x 26mm 1 set of laser-cut acrylic case and dial pieces [SILICON CHIP ONLINE SHOP Cat SC5166] 1 1.1m telescopic antenna [SILICON CHIP ONLINE SHOP Cat SC5163, Banggood Cat 1108129] 1 125mm (5-inch) 4Ω loudspeaker [Jaycar AS-3007] 1 Murata SFECF10M7FA00 10.7MHz ceramic filter (XF1) [Digi-key, Mouser, RS components] 1 DPDT push-on/push-off switch (S1) [Altronics S1510] 1 round knob for switch S1 [Altronics H6651] 1 20mm diameter knob for VR6 [Jaycar HK7786] 1 32mm diameter knob for VR1 [Jaycar HK7741] 1 2.1mm or 2.5mm inner diameter PCB-mount DC socket (CON1) [Altronics P0621/P0621A, Jaycar PS0519/PS0520] 1 6.35mm stereo switched jack socket (CON2) [Jaycar PS0190] 1 9V DC 250mA+ plugpack and/or 9V alkaline battery 1 9V PCB battery holder [Altronics S5048, Jaycar PH9235] 1 2-way polarised pin header, 2.54mm spacing (CON3) 1 2-way polarised plug to suit CON3 8 M3 x 15mm machine screws 8 M3 x 10mm machine screws 4 M3 x 15mm Nylon or polycarbonate machine screws 3 No.4 x 6mm self-tapping screws (for battery holder) 4 25mm long M3-tapped spacers 4 15mm long M3-tapped spacers 8 M3 flat washers 24 M3 hex nuts 18 PC stakes 1 300mm length of 0.8mm diameter enamelled copper wire (for L1-L4) 1 1m length of 0.125mm diameter enamelled copper wire (T1 & L6) 1 80mm length of 0.71mm diameter tinned copper wire 1 40mm length of light-duty figure-8 cable Coils & ferrites 2 Neosid M99-076-96 K3 transformer assemblies (T1,L6) (M76-403-95 Former K + M76-404-95 Can K + 76-409-95 Ferrite Cup Core S3/K3 + M76-410-95 Screw Core K3/F16) [SILICON CHIP ONLINE SHOP Cat SC5205; two required] 2 RFI suppression beads, Philips 4330 030 3218 2 (FB1,FB2) [Jaycar LF1250, Altronics L5250A] 1 22µH RF inductor (L5) Parts for IF alignment oscillator (to be described next month) 1 single-sided PCB, code 06109182, 52 x 30.5mm 1 Murata SFECF10M7FA00 10.7MHz ceramic filter (XF2) [Digi-key, Mouser, RS components] 1 74HC00N high-speed CMOS quad NAND gate, DIP-14 (IC7) 1 1N5819 40V 1A schottky diode (D1) 4 PC stakes Capacitors 1 100nF MKT polyester capacitor 2 10nF ceramic capacitor 1 330pF ceramic capacitor 1 8.2pF COG/NP0 ceramic capacitor Resistors (all 0.25W 1%) 1 1MW 1 330W 2 270W 1 1kW horizontal trimpot (code 102) (VR7) 36 Silicon Chip Semiconductors 1 LMC6482AIN dual CMOS op amp, DIP-8 (IC1) [Jaycar Cat ZL3482] 1 NE592D8R2G video amplifier, SOIC-8 (IC2) [Digi-key, Mouser, RS Components] 1 CA3089E FM IF amplifier and demodulator, DIP-16 (IC3) [SILICON CHIP ONLINE SHOP Cat SC5164] 1 MC1310P FM stereo decoder, DIP-14 (IC4) [SILICON CHIP ONLINE SHOP Cat SC4683] 2 LM4865MX/NOPB power amplifiers, SOIC-8 (IC5,IC6) [Digi-key, Mouser, RS Components] 2 BF992 dual gate N-Channel depletion mode Mosfets, SOT143B (Q1,Q2) [SILICON CHIP ONLINE SHOP Cat SC5165, Mouser 771-BF992-T/R, RS Components 626-2484] 2 30C02CH-TL-E NPN VHF transistors, SOT-23 (Q3,Q4) [Digi-key, Mouser, RS Components] 1 SUP53P06-20 P-channel Mosfet, TO-220 (Q7) [Jaycar ZT2464] 2 2N7000 N-channel Mosfets, TO-92 (Q5,Q6) [Jaycar ZT2400, Altronics Z1555] 2 BB207 dual varicap diodes, SOT-23 (VC1,VC2) [Digi-key, Mouser, RS Components] 1 BB156 (or 1SV304TPH3F) varicap diode, SOD-323 (VC3) [Digi-key, Mouser, RS Components] 1 7805 5V regulator (REG1) 1 15V 1W zener diode (ZD1) [eg, 1N4744] 2 3mm LEDs (LED1,LED2) Capacitors 2 220µF 16V PC electrolytic 2 100µF 16V PC electrolytic 2 10µF 16V PC electrolytic 1 2.2µF 16V PC electrolytic 7 1µF 16V PC electrolytic 1 470nF MKT polyester 2 220nF MKT polyester 3 100nF MKT polyester 1 47nF MKT polyester 2 22nF ceramic 2 15nF MKT polyester 14 10nF ceramic 1 470pF ceramic 1 330pF ceramic 3 220pF ceramic 1 100pF C0G/NP0 ceramic 2 47pF C0G/NP0 ceramic 1 39pF C0G/NP0 ceramic 1 4.7pF C0G/NP0 ceramic 1 2.2pF C0G/NP0 ceramic Capacitor Codes: 470n, 0.47 or 474 220n , 0.22 or 224 100n, 0.1, or 104 47n, 0.047 or 473 22n, 0.022 or 223 15n, 0.015 or 153 10n , 0.01 or 153 470p or 471 330p or 331 220p or 221 100p or 101 47p or 47 39p or 39 4.7p or 4p7 2.2p or 2p2 Resistors (all 0.25W, 1%) 1 1MW 1 470kW 1 120kW 4 100kW 4 68kW 3 47kW 1 33kW 2 20kW 1 16kW 13 10kW 1 5.1kW 4 4.7kW 1 3.9kW 4 3.3kW 3 1kW 1 560W 1 470W 4 330W 2 150W 3 47W 1 10W 2 Alpha 16mm 10kW linear taper potentiometers with 6.35mm D-shaft, 23.5mm long (VR1,VR6) [Jaycar RP7510] 1 10kW miniature horizontal trimpot (code 103) (VR2) 1 10kW multi-turn top adjust trimpot (code 103) (VR3) 1 500kW miniature horizontal trimpot (code 504) (VR4) 1 5kW miniature horizontal trimpot (code 502) (VR5) 1 100kW miniature horizontal trimpot (code 104) (VR7) Australia’s electronics magazine siliconchip.com.au How the MC1310P stereo decoder IC works Shown above is the internal block diagram of the MC1310, based on what is shown and described in the data sheet. The 76kHz oscillator at top middle has its frequency set via an external capacitor and resistor, which is usually connected in series with a trimpot to fine-tune its frequency. The 76kHz output is divided by two to get 38kHz, then again divided by two by a circuit that incorporates a phase shift, to obtain a 19kHz signal that’s 90° out of phase with the 38kHz signal. This is fed to the mixer at upper left, where it’s mixed with the incoming signal, then fed to a low-pass filter, then to a level detector to produce a DC voltage proportional to the difference product. The resulting voltage indicates the phase relationship between the 19kHz pilot tone and the oscillator, allowing the oscillator to be phase-locked with the pilot tone. A second divider produces a 19kHz signal that’s in-phase with the oscillator, which is fed to a second mixer. Its output then goes to a low-pass filter and then a trigger, which is activated when a 19kHz pilot tone is present, and the oscillator phase is locked to it. This then activates the external stereo indicator, along with the stereo switch, which admits the 38kHz signal to the stereo decoder. When that signal is present, the decoder recovers the L-R signal and then combines it with the L+R signal to recover the left and right channel audio, which is sent to the outputs. In the absence of the 38kHz signal, the decoder feeds the (L+R) mono signal to both outputs. IC4 has a stereo LED indicator (LED2) driven by pin 6, showing when IC4 is decoding in stereo. Stereo is available when a stereo jack plug is inserted into CON2, and there is sufficient signal level in the received radio signal for stereo decoding. All Australian FM stations broadcast in stereo. VR6 is the volume control, which controls the gain of both amplifiers, IC5 and IC6. Padding resistors set its wiper to product a voltage range of 0.8-3.4V. Balance control potentiometer VR7 alters the voltage applied between the pin 4 volume control inputs of IC5 and IC6, so that when it is rotated off-centre, one amplifier (left or right) delivers more signal. The maximum volume control signal of 3.4V prevents excessive volume from the headphones and also prevent the loudspeaker from being over-driven. reversed, but unlike a diode, it has a very small voltage drop when it is in conduction. If the supply polarity is correct, the gate of Q7 will be lower than its source, and so the Mosfet switches on. ZD1 protects the gate from over-voltage. LED1 lights up as a power indicator. Linear regulator REG1 derives a 5V supply for amplifiers IC5 and IC6 from the incoming 9V, as they require, and also supplies the tuning reference voltage for VR1. That completes the FM Radio circuit description. Next month, we will complete the project with full details on its construction and alignment and SC fitting it in its case. MIXER LEVEL DETECTOR INPUT BUFFER 76kHz OSCILLATOR 76kHz ÷2 38kHz ÷2 LOW-PASS FILTER 19kHz QUADRATURE STEREO INDICATOR MIXER 38kHz  STEREO SWITCH TRIGGER LOW-PASS FILTER DECODER SC 20 1 9 ÷2 19kHz IN-PHASE 38kHz MONO/STEREO OUTPUTS Power supply The FM Radio is powered either from a standard 9V battery or 9V DC plugpack. CON1 provides switching so that when the DC power plug is inserted, the 9V battery is disconnected. Switch S1 interrupts power from both sources, to allow the FM Radio to be switched on and off. Mosfet Q7 is included for reverse polarity protection. It will not conduct current if the supply polarity is siliconchip.com.au Qty Value  1 1MW  1 470kW  1 120kW  4 100kW  4 68kW  3 47kW  1 33kW  2 20kW  1 16kW  13 10kW  1 5.1kW  4 4.7kW  1 3.9kW  4 3.3kW  3 1kW  1 560W  1 470W  4 330W  2 150W  3 47W  1 10W Resistor Colour Codes 4-Band Code (1%) brown black green brown yellow violet yellow brown brown red yellow brown brown black yellow brown blue grey orange brown yellow violet orange brown orange orange orange brown red black orange brown brown blue orange brown brown black orange brown green brown red brown yellow violet red brown orange white red brown orange orange red brown brown black red brown green blue brown brown yellow violet brown brown orange orange brown brown brown green brown brown yellow violet black brown brown black black brown Australia’s electronics magazine 5-Band Code (1%) brown black black yellow brown yellow violet black orange brown brown red black orange brown brown black black orange brown blue grey black red brown yellow violet black red brown orange orange black red brown red black black red brown brown blue black red brown brown black black red brown green brown black brown brown yellow violet black brown brown orange white black brown brown orange orange black brown brown brown black black brown brown green blue black black brown yellow violet black black brown orange orange black black brown brown green black black brown yellow violet black gold brown brown black black gold brown November 2019  37