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~ ~ ~ 'P. , .II t _, ¼ , &) ri\o Cl{EO TEREO: WIDE-BAND AM STEREO 1- 2-Chip Stereo Radio You won't believe how good stereo AM radio can sound until you build this nifty little Walkman-style receiver. It's based on Motorola's brand new MC13024 IC which is virtually a complete AM stereo radio on a single chip. By STEVE PAYOR AM stereo W alkman-style radios are very thin on the ground in Australia or anywhere else for that matter so this little receiver is a big step forward. As far as most people are concerned, it sounds every bit 20 SILICON CHIP as good as FM stereo, unbelievable though that may seem. It has low distortion, wide bandwidth and wide stereo separation. It is also small and delightfully easy to tune. All you do is rotate the knob to light the LED indicator and then a moment or so later the sound flicks from mono into high and wide stereo. You listen via headphones, just as you do with any other Walkmanstyle receiver. And it is powered from just two penlite cells so batteries won't cost a mint. Best of all, you can build and align it yourself without any need for fancy tools and equipment. When you've got it going, you will have an AM stereo receiver of which you can be justifiably proud. It is hifi in a small package. Let's now discuss how the circuit works. Some of this description may 8 < m a u Cl i ::. , °l Above: virtually all the parts for the AM stereo receiver are mounted on a small PC hoard to give a compact assembly. The MC13024 IC is almost in the centre of the hoard. The completed receiver, shown on the facing page, is delightfully easy to tune and delivers hifi sound. be little foreign to you because it assumes some knowledge of how a superheterodyne radio works. But read on anyhow - you'll learn a lot! How it works The heart of this unique little receiver is a brand new chip [fresh out of the oven, in fact) from Motorola, the originators of the CQU AM compatible AM stereo system used by all AM broadcast stations in Australia. This chip is designated the MC13024 [ICl) and represents a considerable extension of the popular MC13020 AM stereo decoder IC. As well as a C-QUAM decoder, the MC13024 contains a sensitive mixer, a voltage controlled local oscillator [VCLO), an intermediate frequency [IF) amplifier, automatic gain control [AGC) and automatic frequency control [AFC) circuitry, a stereo pilot tone detector, a signal quality detector and a complete AM stereo decoder. By adding a modest number of passive components around the chip and a stereo head- phone driver amplifier, we get a complete AM stereo receiver. Another feature of the MC13024 is that it will operate from a supply voltage as low as 1.8V, with a current consumption of only 5mA. This makes it ideal for battery operation. Fortunately, it is not necessary to fully understand the intricacies of the C-QUAM [Compatible Quadrature Amplitude Modulation) process to use this IC. Superficially at least, the circuit diagram is no more complex than most ordinary integrated circuit AM radios. We will describe those features peculiar to stereo reception as we come to them. Signal & oscillator frequencies The incoming radio signals are picked up by the antenna coil (Ll) which is tuned to resonance at the signal frequency by variable capacitor C3. C3 forms one section of a ganged pair. The other section, C4, tunes local oscillator coil 12. The two sections of the tuning gang are designed so that the oscillator frequency will always be exactly 450kHz higher than the signal frequency. This difference of 450kHz is equal to the intermediate frequency (IF) of the receiver more about this later. Thus, as the signal frequency is varied from 531kHz to 1602kHz [the limits of the Australian broadcast band), the oscillator is tuned from 981kHz to 2052kHz. For this reason, the maximum capacitance of C4 is less than C3 (90pF vs 160pF). Precise tracking of the two frequencies is achieved by careful adjustment of 11, 12 and two small trimmer capacitors which are built into the top of the tuning gang. We have also added two small fixed capacitors across C3 and C4 (4.7pF and 8.2pF respectively) to bring the trimmers to the centre of their adjustment range. The precision of the tuning gang is most important, which is why we have specified the Tako HU-22124 type in preference to more common "no name" varieties. With this gang you can be sure that the frequency calibrations will be within a pointer's width across the entire dial. By the way, this SEPTEMBER1989 21 The only parts not mounted on the PC board are the batteries and the on/off switch. Note that close tolerance components must be used in some sections of the circuit, so be sure to obtain the exact parts specified in the parts list. gang also has two additional sections, marked Cl & C2, which are for FM tuning. These are not used in this circuit. The pre-wound antenna coil (11) is a standard "transistor radio" item, usually supplied wi.t h a short piece of flat ferrite rod. We have specified a much longer (100mm) ferrite rod for superior longdistance reception. The signal from the antenna coil is coupled to the low-impedance RF input (pin 10) of ICl via a small secondary winding. Note that the RF input of the MC13024, as well as the inputs to the pilot filter circuitry, are biased to + 1V DC from an on-chip voltage regulator, the output of which appears at pin 15. The ceramic filter bandwidth determines the upper audio frequency response limit (12kHz) and, unfortunately, the amount of broadband and adjacent channel noise as well. We have found ± 12kHz to be a suitable compromise for reception of local stations. To the hifi enthusiast, an audio bandwidth of 12kHz may seem a bit ordinary but it is not much different from the 15kHz bandwidth broadcast by FM stations. Hearing is believing as far as this aspect is concerned, and we think that you will be amazed at the sound quality this little receiver can achieve (given a good program source). Anyhow, back to the circuit description: ting, the IF output is connected directly to the top of 14, giving a slight amount of damping and widening the - 3dB bandwidth to around ± 9kHz. This is probably the best compromise between noise, sensitivity and audio bandwidth if you are interested in country as well as local radio reception. For city dwellers who are used to the sound of FM radios, the wide setting is the one to go for. This is selected by soldering both the medium and wide pads, which puts an additional lO0kn of damping across 14. This widens the bandwidth to :J;: 12kHz with just a slight loss of sensitivity. Mixer & IF stages IF amplifier & bandwidth selection The output of the IF amplifier is internally connected to an envelope detector, much the same as in a normal AM radio. The amplitude (or envelope) of a C-QUAM AM stereo signal is modulated with the sum of the left and right audio channels, so when the signal is demodulated in an ordinary radio, the signal you get is 1 + R (ie; the monaural sum of the stereo components). This is the key to the compatibility of the C-QUAM system with existing AM radios. When the MC13024 is in monaural mode, the output from the envelope detector (1 + R) is fed through to both audio outputs. The average envelope amplitude is a measure of the signal carrier strength and this voltage is used to adjust the gain of the mixer and IF The signal from the RF input is amplified and "mixed" with the local oscillator signal to produce a difference frequency of 450kHz, which appears at the mixer output (pin 11 ). Coil 13 is broadly resonant at this frequency and transforms the relatively high output impedance of the mixer down to the relatively low input impedance of the ceramic filter. The ceramic filter requires a resistive source impedance of approximately 2kO, so the output of 13 is loaded with a 2.7kO resistor to achieve this. The Murata SFP-450D ceramic filter contains four coupled resonant elements and provides an almost rectangular bandpass characteristic of 450kHz ± 12kHz at the - 6dB points. 22 SILICON CHIP The signal from the ceramic filter is amplified by the IF amplifier, between pins 16 and 19. At the output of the IF amplifier is another 450kHz tuned circuit (14) which can be wired for three different bandwidths: wide, medium or sharp. These a re selected by bridging some tiny solder pads on the underside of the PCB. In the sharp setting, the IF amplifier output is tapped about half-way up the coil, giving minimal damping of the tuned circuit and a very sharp resonance - approximately ± 3kHz at the - 3dB points. If you are interested mainly in interstate reception, where noise is a real problem, use this setting. For the medium selectivity set- Envelope detection & AGC amplifier to maintain a constant output level, regardless of signal strength. Any audio fluctuations are removed by the AGC filter which consists of a large internal resistance at pin 17 and an external 3.3µF capacitor. This sets the AGC time constant. In practice, the ACG voltage swings from slightly above the + 1V reference on weak signals to slightly below on very strong signals. Note, however, that the MC13024 uses a "delayed AGC" system for optimum signal to noise ratio. As the signal strength increases, the IF gain is reduced first, which reduces not only the signal but also the noise from the mixer. Further increases in signal strength then cause the MC13024 to reduce the mixer gain, to prevent overload. The result is that stations which vary by as much as 50dB in signal strength all sound about the same. This also means that it is not practicable to align the tuned circuits "by ear". We have therefore made the AGC voltage available at the edge of the circuit board, so that a multimeter can be used as a tuning indicator during the alignment procedure. In addition, an optional BC549C high-gain transistor (Q1) is used as a current amplifier to drive low impedance (50µA) moving coil multimeters, without disturbing normal operation. :z....,__...,,...1±: 0 O F-,•· + ....... 0 a: w > jjj (.) w a: w ...J ;;; al ~ I· a: 0 c. 0 w a: w ~ ~ ~;: :l!: i~~ ....--------;li <( NZ "' ,-. r--------1,~K ,a!: Phase detection & stereo We have already mentioned that the L + R (ie; mono) information is contained in the amplitude of the CQUAM signal. To complete our stereo signal we also need the L - R information, and this is contained in the phase of the C-QUAM signal. To extract the phase information, the MC13024 IC locks onto the average phase of the incoming signal using a phase locked loop ~§1""''-+-_..,,gMr-.,._-l~HI• -"';;o -·---'-"~!:; "' .,._-... ~-~M----1~~~ > "'+ "' "' c..a! " d > -o ...__ _-'+'-+'-'f'------og <C -----------4------o-w +"' > U.: (PLL). " Fig.1 (right): the heart of the circuit is the MC13024 IC which is virtually a AM stereo tuner on a single chip. Its left & right channel outputs drive a TA7376P stereo headphone amplifier IC. LED 1 provides tuning and stereo indication. SEPTEMBER1989 23 2-CHIP AM STEREO RADIO - CTD First of all, a stable 450kHz reference signal is required for the MC13024's internal phase detector. This is obtained from a 3.6MHz reference oscillator (tuned by 15), which is divided by eight internally to give the required 450kHz reference. The reason for running this oscillator at such a high frequency is to prevent its output signal from interfering with the broadcast signal. Nevertheless, for reception of very weak signals, the MC13024 shuts down the reference oscillator and internal dividers, to minimise " birdies" . In practice, the phase of the IF signal is compared with the phase of the 450kHz reference to derive an error voltage. This error is then fed back to adjust the local oscillator frequency, to bring the IF signal into phase lock with the reference. Two "pull in" speeds are used in the phase locked loop: a fast speed to give rapid lock as soon as a signal is tuned within the capture range, and one 50 times slower for precise fine tuning and low phase distortion. The PLL filter at pin 7 allows the loop to track slow deviations of the signal phase but not the L - R audio fluctuations. The 100 resistor in series with the 22µ,F capacitor damps the response of the loop so that it doesn't overshoot when locking onto a signal. When phase lock has been achieved, the MC13024 turns on the LED connected to pin 3 at half brightness. Full brightness comes a little later when, all being well, the MCl 3024 switches into stereo mode. Automatic frequency control You may well ask just how the local oscillator frequency is shifted when it is already set by the tuned circuit of 12 & C4. The answer lies in the fact that the resonance of the tuned circuit is not infinitely sharp. By advancing or r etarding the phase of the oscillator drive at pin 9, the MC13024 can pull the 24 SILICON CHIP oscillator frequency slightly to eitheT side of the resonant frequency. The 2 70 series damping resistor in the oscillator tuned circuit broadens the resonance and thus sets the locking range of the PLL. The locking range we have chosen is approximately 6kHz, and is fairly constant across the entire tuning range. Having a 6kHz locking range makes precise tuning a breeze, even without any form of vernier reduction drive on the tuning knob. All you have to do is turn the knob until the LED comes on and the MC13024 does the rest. Stereo decoding When the circuit is tuned to a stereo signal and the LED indicates phase lock, any phase fluctuations in the signal will be interpreted by the MC13024 as L - R stereo information. (Note: a full description of how this is done would take several pages. Readers who want all the details can refer to the data sheet for the MC13020 stereo decoder in the Motorola " Linear and Interface Devices " data book). Once the L - R signal is extracted, all that remains is to add and subtract it to the L + R mono signal to get our required stereo outputs. The MC13024 does this gradually, so there is a smooth "blend" from mono to stereo without any nasty pops. The mathematics of this process is shown below. In mono mode, we have: (L + R) + (0) = L + R (L + R) + (0) = L + R In stereo mode, the signals blend to: (L + R) + (L - R) = 21 (L + R) - (L - R) = 2R Note that the total output from both channels is the same in both cases; ie. 2(1 + R). Thus the volume doesn't change during the blend, only the stereo separation. The MC13024 will not switch into stereo mode on just any old signal, however. Valid C-QUAM stereo signals are identified by a low-level 25Hz pilot tone which is added to the L - R information. Don't worry about being able to hear the pilot tone - you can't. In fact the amplitude is so low (4 % modulation or - 28dB) that it takes a fair bit of circuitry to extract it. The output of the L - R demodulator is fed via a high impedance to pin 8 where it is filtered by a .068µ,F capacitor and a lOOkO resistor. This removes most of the audio but the pilot signal is still too small to be recognised at this stage. Bandpass filter To overcome this problem, the low-pass signal is buffered by a x2 amplifier between pins 5 & 4 and is then fed to a high-Q 25Hz bandpass active filter. This effectively extracts the pilot tone (if it exists) from the surrounding low frequency noise. This bandpass filter consists of an internal x-1000 op amp between pins 1 and 24, plus an external multiple feedback network of precision resistors and capacitors. These give the passband filter a gain of 6.5 and a Q of 8.6. Note that the combination of high gain and high Q makes the component values rather critical. For example, a 5 % shift in the capacitor values could cause a 20% drop in pilot tone amplitude. For this reason, 2% capacitors and 1 % resistors have been specified for the feedback network. The output of the 25Hz bandpass filter goes to the MC13024's pilot detector. If the 25Hz signal is present for more than seven consecutive cycles, the MC13024 switches to stereo mode. It takes 300ms to count seven cycles, so you will notice a slight time delay between when you stop tuning and the stereo light coming on. You will also find that the MC13024 switches straight back to mono as soon as you start to turn the tuning knob again. If it didn't, the disturbance to the phase of the signal would be interpreted as a stereo component and the sound would "flutter". The 10µ,F "lock" capacitor on pin 13 determines the sluggishness with which the decoder enters or leaves the stereo mode. Volume control The left and right audio output signals from the MC13024 are filtered by low-pass RC networks before reaching the volume control (VR 1) and again afterwards, to make sure that no IF signal components make it to the audio power amplifier. Even a few millivolts of RF here would find its way back into the front end of the tuner and cause all sorts of whistles and howls. Remember - the antenna is only centimetres away from the power amplifier circuitry. The .068µF coupling capacitors in series with the volume control roll off the low frequency response at 25Hz. There is no stereo balance control on this receiver nor is one really needed, as the channel matching is excellent. This is partly due to the "full-size" volume control pot specified. Maintaining balance at the very bottom end of the volume control is a little tricky though, as the slightest mismatch between the pot wipers will cause one channel to shut off before the other. We overcame this problem by inserting a 3300 resistor at the earthy end of each pot section. This preserves the balance at low volume levels. The minimum volume is just comfortably above the noise floor of the audio amplifier and should be low enough for most applications. If you prefer the minimum volume to be lower still, reduce the 3300 resistors to 2200. Audio amplifier IC2 (TA7376) is a complete stereo headphone amplifier in a 9-pin single in-line package (SIP). The only external components required are a number of bypass and coupling capacitors. The 22µF capacitors on pins 2 and 8 form part of the internal AC and DC negative feedback loops, while the 22µF capacitor on pin 7 is for an internal power supply ripple filter (common to both channels). The amplifier outputs are stabilised against high-frequency oscillation by two RC damping networks (2 .20 in series with O. lµF). As with all high-gain IC audio PARTS LIST 1 PCB , code SC06108891, 59 x 115mm 1 ABS plastic case with plastic I1d, 130 x 68 x 43mm 1 front panel label, 125 x 62mm 1 28mm dia. metal knob 1 22mm dia. metal knob 1 3.5mm PCB-mounting stereo headphone socket (see text) 1 2-cell side-by-side AA battery holder 1 battery snap connector 1 DPDT miniature slide switch (DSE Cat. S-2010, Jaycar Cat. SS-0821 or equivalent) 2 3/8-inch ID plastic P-clips (to fit around ferrite rod) 1 200mm length of light duty hookup wire (red) 1 200mm length of light duty black hookup wire 4 PC pins 1 1 OOkO log dual gang potentiometer Hardware 2 small right angle brackets (for mounting ferrite rod) 3 1/8-inch dia. x 1 /2-inch countersunk machine screws 4 1/8-inch dia. x 3/8-inch round head machine screws 2 M2 x 5mm pan head screws (to fit slide switch) 1 0 1 /8-inch full nuts 11 1 /8-inch flat washers 4 1/8-inch spring washers Parts for tuning dial 1 1 /4-inch dia. x 18.5mm extension shaft for tuning capacitor 1 1 O BA screw for tuning capacitor extension shaft 1 1/ 4-inch ID x 0 .5mm thick Teflon washer 1 29 x 47mm scrap of 1.6mmthick clear Perspex for cursor Parts for battery clamp 2 1 /2-inch long x 1/8-inch tapped spacers 4 1/8-inch dia. x 1/ 4-inch countersunk machine screws 1 82 x 33mm scrap of 1.6mm Perspex or aluminium sheet Semiconductors 1 Motorola MC13024P CQUAM AM stereo receiver (IC1) 1 Toshiba TA7376P stereo audio power amplifier (IC2) 1 BC549C NPN transistor (01) 1 3mm red LED (LED 1) Inductors 1 3/8-inch dia. x 1 00mm ferrite rod pre-wound AM broadcast band antenna coil (L 1 ); eg. from DSE Cat. L-0520 1 broadcast band oscillator coil (L2), Tako A7BRS-T1342AIX 1 450kHz mixer coil (L3), Tako A7NRES-T1341 AYN 1 450kHz IF coil (L4), Tako A7NRES-T1340AYN 1 3.6MHz reference oscillator coil (L5), Tako MF291 ACS-3688VL 1 Murata SFP-450D ceramic filter Capacitors 1 tuning gang, Tako HU-22124-MOOO-O 3 470µF 16VW PC electrolytics 4 22µF 16VW PC electrolytics 2 1OµF 16VW PC electrolytics 1 3.3µF 16VW LL electrolytic 3 0 .1 µF monolithic ceramics, 0.2-inch lead spacing 3 .068µF metallised polyester (greencaps) 2 .04 7 µF 2% polyester or selected 5% greencaps (see text) 6 .01 µF miniature ceramics, 0 .2-inch lead spacing 2 .01 µF metallised polyester 2 .0018µF metallised polyester 2 680pF miniature ceramics, 0 .2-inch lead spacing 1 8 .2pF NPO Philips miniature ceramic plate, 0.1-inch lead spacing 1 4. 7pF NPO Philips miniature ceramic plate , 0.1-inch lead spacing Resistors (0.25W, 5%) 2 1MO, 1 % 1 330k0 , 1 % 1 180k0, 1 % 2 1 OOkO 2 10k0 1 8.2k0 , 1% 1 2.7k0 2 3300 1 270 1 100 2 2 .20 SEPTEMBER 1989 25 This view clearly shows how the ferrite rod antenna is mounted on the PC board using aluminium brackets and plastic "P" clips. The four test points at bottom right are used during the alignment procedure. amplifiers, good high frequency bypassing of the power supply is essential. This is taken care of by a O. lµF ceramic capacitor right next · to the Vcc and GND pins of the TA7376. An additional 470µF electrolytic across the battery helps to extend the usable battery life by supplying the peak current demand on audio transients. On the subject of bypassing, you will notice quite a few .OlµF ceramic capacitors between Vee and ground around the RF circuitry, and also between the + 1V reference and ground. This effectively ties the ground and DC supply rail into an RF groundplane which serves to shield and stabilise the circuit. Although the TA73 76 is really intended for driving headphone loads (around 320 for the higher quality 26 SILICON CHIP types), it also does a pretty good job of driving a pair of 80 loudspeakers, provided they are not the insensitive "compact" variety. The maximum output is around 1V peak-to-peak, which translates into less than 20mW per channel into 80. But don't let this rather low figure put you off - try it and see. With normal headphones, there is sufficient available output to guarantee permanent hearing damage, so be prudent with the volume control. Batteries Both ICs will operate quite happily down to 1.8V but when you notice the stereo light dropping out on loud volume peaks, it is time to replace the batteries. You can expect several weeks of operation, 8 hours per day, from a pair of alkaline penlight cells. In fact, the current drain is low enough to give economical performance from the lesser battery grades. This brings us to an interesting point. The MC13024 and TA7376 both have a current drain of 5mA each and, at normal listening levels, the drain of the TA7376 only increases by a few milliamps or so. However, the tuning indicator LED draws about 5mA when it first turns on and about 20mA when indicating the stereo mode. We thus have a situation where the stereo indicator consumes twice as much battery power as the rest of the radio combined! Well that more or less completes the circuit description. Next month we'll continue with the construction procedure and give the alignment details. ~