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This neat little FM receiver is controlled by your PC and tunes the 144-148MHz amateur band. Alternatively, just by changing the software, you can use it to tune the 132-144MHz band for weather satellite frequencies or, with just a few hardware changes, the 118-132MHz band. PC-controlled VHF FM receiver Design by MARK ROBERTS, VK2GND I F YOU’RE LOOKING for a basic FM receiver capable of moni­toring the 144-148MHz amateur band, this unit should do the job quite nicely. It’s all built on a PC board measuring just 90 x 74mm and plugs into your PC’s parallel port via a DB25 male-to-female printer cable. An on-screen display lets you control the receiver and does away with expensive hardware such as meters, digital displays and tuning knobs. You don’t even need to house the device in a case if you don’t want to, although a low-cost plastic case to protect the circuit would probably be the way to go. Fig.1 shows the on-screen display that’s used to “drive” the VHF Receiver. There’s really not much to it! The top half is dominated by the large digital frequency display and a tuning meter, while between these are three memory preset buttons and a large vertical fine-tuning “knob”. The bottom half of the display carries a Power button, a coarse tun- 26  Silicon Chip ing “knob” and squelch and volume slider controls. You tune the unit in 5kHz, 10kHz or 100kHz steps, either by dragging the tuning “knob” with the mouse or by clicking the Up and Down arrows on either side. Clicking anywhere on the circumference of the knob will also tune the receiver to that spot. The 5kHz, 10kHz or 100kHz tuning steps are selected by clicking the large buttons immediately to the right of the tuning knob. Clicking the top button toggles between the 10kHz and 100kHz tuning steps, while clicking the (misnamed) Help button selects 5kHz tuning steps. One nice feature of the unit is its ability to automatically scan the frequency band. Just click the Scan button, and the receiver automatically scans up the band, incrementing at the selected frequency steps. This scanning automatically stops when the received signal strength exceeds the squelch control setting. The functions of the Squelch and Volume controls are self-explanatory. As you’ve no doubt guessed, they are also adjusted using by dragging them with the mouse. As you drag the squelch control, the level is indicated by a dark-brown “bar” on the meter, so that you can instantly see the squelch setting in relation to the signal level. Finally, there are three memory buttons for you to store your favourite channels. All you have to do is tune to the re­quired frequency, click the Read button and click the desired memory preset button (Ch-1, Ch-2 or Ch-3) and voila! ... the frequency is programmed in. Block diagram Fig.2 shows the block diagram of the VHF 144-148MHz FM Re­ceiver. It’s built around a Motorola MC13135 radio IC, which is virtually a complete narrowband FM radio on a single chip. It drives an LM386 audio amplifier stage via a 4051 8-stage analog multiplexer, the latter providing the volume control function. Quite a lot of circuitry is packed into the MC13135, includ­ ing two local oscillators, a varicap tuning diode, two low-noise mixer stages, a high-gain limiter, a demodulator and a received signal strength indicator (RSSI) – see Fig.5. However, the first local oscillator isn’t used in this design as its maximum fre­quency is only about 100MHz. Instead, our circuit uses an external VCO (voltage con­ trolled oscillator) which is controlled by a PLL (phase locked loop). In operation, the PLL compares a divided-down VCO signal with a reference signal and, based on the phase error, produces a tuning voltage for the varicap diode inside the MC13135. The varicap diode sets the VCO frequency, which is pulled into lock with the divided reference. The analog-to-digital (A-D) converter stage is there simply to monitor the received signal strength and the external power supply rail. It converts these analog voltages to digital values so that they can be processed by the software and displayed by the onscreen instrument panel. The signal strength meter indi­ cates the RSSI in analog fashion, while the supply voltage is indicated in the bottom righthand corner of the meter. Dual conversion Before moving on to the circuit description, let’s take a closer look at the MC13135 receiver IC. This is a “dual conver­ sion” receiver and basically Fig.1: this is the on-screen virtual instrument panel that’s generated by the VHF FM Receiver software. You tune the unit (in 5kHz, 10kHz or 100kHz steps) by dragging the tuning knobs or by clicking the Up and Down arrows. functions just like a conventional superhet but with one important difference. A conventional superhet receiver has only one local oscilla­tor and this is mixed (or heterodyned) with the incoming FM signal to produce an intermediate frequency (IF). This IF signal is then amplified and filtered before being demodulated to recov­er the desired audio signal. This is referred to as a single conversion and the IF is typically 10.7MHz for FM receivers and 455kHz for most AM receiv­ers. By contrast, a dual conversion receiver has two local oscil­lators (LO), two mixers and two intermediate frequencies. The first LO is mixed with the incoming signal to produce an IF of 10.7MHz. This is then ampli- fied and mixed with the second LO operating at 10.245MHz to produce a second IF of 455kHz (ie, 10.7MHz 10.245MHz = 455kHz). Dual conversion receivers are commonly used for narrowband FM reception, where the deviation is typically only ±5kHz (as compared to ±75kHz for commercial FM radio stations). Circuit details OK, now let’s take a look at the main circuit diagram (Fig.3). The incoming RF signal is picked up by the antenna and fed to first mixer input (pin 22) of IC3 via C4 and an LC filter net­work. This filter is tuned using trimmer capacitor VC1, while C6 (22pF) sets the bandwidth. Transistor T1 forms the external local oscillator. This voltage con- Fig.2: the VHF FM receiver is controlled via the parallel port of a PC. A dual-conversion FM receiver chip (IC3) forms the heart of the design and this is tuned using a PLL and external voltage controlled oscillator (VCO). JUNE 2000  27 28  Silicon Chip 2000 SC DB1 R7 2.7k R14 1M 18 4 2 A1 12 16 GND VAG 5 VHF FM RECEIVER 7 6 5 4 3 +5V 16 C B A +5V 13 R23 1.5k C18 56pF C17 120pF Y1 14 R22 2k C20 .01F 12 4 VCC1 R20 5.6k Y5 5 R19 6.8k DECOUP2 DECOUP1 1stMIXOUT 1stMIXIN2 1stMIXIN1 Y6 2 E Z VEE Y7 4 8 7 6 3 AMPOUT AMPIN- AUDIO AMPIN+ RSSI QUADIN 2ndMIXIN GND R18 3.3k 8 GND 19 VCC2 IC3 MC13135 2ndMIXOUT LIMITIN 2ndLOB 2ndLOE VARICAPa VARICAPc 1stLOB 1 Y4 IC4 4051B Y3 11 10 7 9 6 5 23 24 1 R21 2.7k C19 .01F 4 2 R8 2.7k 15 Y2 3 1 XTAL2 10.245MHz F1 455kHz CERAMIC FILTER C33 0.1F Y0 VDD R24 1.2k 9 8 R4 2.7k R16 12k +5V 1 9 13 10 1 C26 .01F 11 11 A0 R6 39k TP1  10 12 R12 56k C15 0.47F R11 2.7k R13 100k T2 MPSH10 R17 560 E C C16 22pF 14 K IC1 MC145041 R5 330k GND VDD +5V 13 B T1 MPSH10 A10 A9 A8 A7 A6 A5 A4 A3 A2 SCLK DIN DOUT CS 20 VDD +5V 14 R25 2.7k OSCOUT OSCIN VREF 2 PDOUT 4 FIN E C10 120pF R9 560 IC2 MC145170-2 CLK ENB DIN C8 8.2pF B C +5V 17 LED1  17 3 A 16 9 15 2 REF LM385Z-2.5 _ + 10 C12 3-12pF 7 6 1 5 6 XTAL1 8MHz E2 470F 16VW 7 C11 68pF E4 10F 16VW +5V C7 22pF R10 C9 100k 56pF 8 5V DC INPUT _ + D1 IN4001 L3 C14 330pF +5V G 2 3 1 S D _ + ADJ CATHODE LM385Z F3 455kHz QUAD COIL L1 C6 22pF 2N7000 E B C MPSH10 D G S +5V C4 10pF R15 12k C29 .001F 2 3 4 1 IC5 LM386 6 +5V 7 SPEAKER 8 E3 470F 16VW 5 C2, C13, C23, C25, C28, C34, C35, C116 ALL 0.1F +5V F2 10.7MHz CERAMIC FILTER V1 5-60pF C21 120pF T3 2N7000 R1 16k C22 0.1F R3 1k C27 .01F C3 .01F +5V C24 0.1F 16 15 17 14 12 13 18 20 21 22 ANTENNA Fig.3 (facing page): the full circuit diagram of the VHF FM Receiver. The dual conversion receiver (IC3) is tuned by the external VCO (IC2) and the PLL (IC1). The demodulated audio output appears at pin 17 and drives audio amplifier IC5 via IC4 which functions as the volume control. trolled oscillator (VCO) is tuned by the vari­cap diode between pins 24 & 23 of IC3, along with C14 and inductor L3. The output appears at T1’s emitter and is fed to the LO input (pin 1) of IC3 via C16 where it is mixed with the received RF signal. In operation, the VCO runs at between 154.7MHz and 158.7MHz (ie, 10.7MHz higher than the received frequency), depending on the capacitance of the internal varicap diode. After mixing with the antenna signal, the first IF at 10.7MHz is filtered by ceram­ic filter F2 and then fed to pin 18 (mixer 2) of IC3 where it is mixed with the second local oscillator. The second local oscillator operates at 10.245MHz, as set by crystal XTAL2 and its associated capacitors. As a result, the second IF is at 455kHz and this is filtered using ceramic filter F1 which has a bandwidth of 12.5kHz. This second filter limits any out-of-band noise and increases the selectivity. Following F1, the signal is fed to an internal limiting circuit and then to a quadrature demodulator. F3 is the external quadrature coil and is tuned during the adjustment procedure to 455kHz using a ferrite slug. The recovered audio signal appears at pin 17 of IC3 and is fed via R3 & C22 to the top of a resistive divider network (R18-R24). The eight steps of this resistive divider are fed to the Y0-Y7 inputs of the 4051 analog multiplexer (IC4) which we’re using as the volume control. This IC is controlled by a 3-wire interface from the PC’s parallel port (LPT1). The control signals are applied to the binary control inputs at pins 11, 10 & 9 (designated A, B & C) of IC4 and select which of the eight input channels is switched through to the output at pin 3. Basically, IC4 functions as a single-pole 8-position switch. It selects one of the possible eight signal levels and applies it to pin 3 of the following LM386 audio amplifier stage (IC5). Parts List 1 PC board, 90 x 74mm 1 PC-mount DB25 male connector 1 455kHz ceramic filter, 12.5kHz bandwidth (F1; see text) 1 10.7MHz ceramic filter 1 8MHz crystal (Xtal1) 1 10.245MHz crystal (Xtal2) 1 5-65pF trimmer capacitor (V1) 1 3-13pF trimmer capacitor (C12) Semiconductors 1 MC145041 8-bit A-D converter (IC1) 1 MC145170-2 PLL synthesiser (IC2) 1 MC13135 dual conversion FM receiver (IC3) 1 4051B 8-channel analog multiplexer (IC4) 1 LM386 audio amplifier (IC5) 2 MPSH10 VHF NPN transistors (T1,T2) 1 2N7000 N-channel MOSFET (T3) 1 1N4001 diode (D1) 1 LM385/2.5 2.5V reference (REF) 1 miniature LED (LED1) Inductors L1 5T of 0.7mm ECW on 3mm former L3 5T of 0.7mm ECW on 3mm former F3 455kHz quadrature coil (F3) IC5 operates with an AC gain of 20 by virtue of its internal feedback components. The amplified output appears at pin 5 and is coupled to the loudspeaker via a 470µF capacitor. Tuning The tuning for the receiver is controlled by IC2 which is a Motorola MC145170 PLL frequency synthesiser. Its internal refer­ence oscillator operates at 8MHz due to crystal XTAL1, although this can be “tweaked” slightly using trimmer capacitor C12. This reference frequency is divided down by an internal 15-stage counter to either 100kHz, 10kHz or 5kHz, depending on the re­quired tuning steps. Emitter follower T2 buffers the VCO output and feeds the signal to the FIN input of IC2 via C10. We won’t go into all the inner workings of the Capacitors 2 470µF 16VW electrolytics (E2, E3) 1 10µF 16VW electrolytic (E4) 1 0.47µF ceramic (C15) 11 0.1µF ceramic (C2, C13, C2225, C28, C33-35, C116 5 .01µF ceramic (C3, C19-20, C26-27) 1 .001µF ceramic (C29) 1 330pF (C14) 1 120pF (C10, C17, C21) 1 68pF (C11) 1 56pF (C9, C18) 1 22pF ceramic (C6-7, C16) 1 10pF ceramic (C4) 1 8.2pF ceramic (C8) Resistors (0.25W, 1%) 1 1MΩ (R14) 1 330kΩ (R5) 2 100kΩ (R10, R13) 1 56kΩ (R12) 1 39kΩ (R6) 1 16kΩ (R1) 2 12kΩ (R15, R16) 1 6.8kΩ (R19) 1 5.6kΩ (R20) 1 3.3kΩ (R18) 1 2.7kΩ (R4, R7, R8, R11, R21, R25) 1 2kΩ (R22) 1 1.5kΩ (R23) 1 1.2kΩ (R24) 1 1kΩ (R3) 1 560Ω (R9, R17) MC145170 here; suffice to say that the VCO frequency is divided down using a 16-stage counter. The phase of the divided VCO signal is then compared to the divided reference signal to generate an error voltage at the pin 13 phase detector output (PDOUT). This voltage is filtered using a low-pass filter made up by R11, R12, C15 and C26. This then becomes the tuning voltage and is applied to the varicap diode inside IC3 via R13. What happens in practice is that the PLL tunes the VCO so that its divided frequency exactly matches the divided reference frequency – either 100kHz, 10kHz or 5kHz. Controlling the PLL The PLL is itself controlled by a 3-wire interface from the parallel port to pins 5 (DIN), 6 (ENB) and 7 (CLK). JUNE 2000  29 C9 L3 C14 TP1 Fig.4: follow this parts layout diagram to build the 144148MHz and 132-144MHz versions of the VHF FM Receiver. The component side of the PC board is shown in grey, while the underside pattern is in blue. The corre­ sponding control outputs on the parallel port are pins 8, 7 & 6. DIN is the serial data input and the number of bits clocked in determines which registers are accessed to set the division ratios for the internal 15-stage and 16-stage counters. Pin 7 is the clock (CLK) input to the MC145170, while pin 6 (ENB) is the enable input. When pin 6 of IC2 is taken low, the data on pin 8 of the parallel port is clocked into the DIN input to set the division ratios for the This photograph shows the completed PC board assembly and will assist you to identify the parts. The parts that have to be changed for the 118-132MHz version (C9, C14 and L3) are indicated with red arrows. counters. OK, now that might all sound terribly complicated in theory but in reality, it’s quite simple. To set the tuning steps, the data on the parallel port (as generated by the software in response to user inputs) sets the appropriate division ratio for the 15-stage counter. Let’s say that we want 100kHz steps. In that case, we have to divide the 8MHz reference frequency by 80. If we want 10kHz or 5kHz steps, then we have to divide by 800 or 1600 respectively. Now let’s say that we want to tune the receiver to 146MHz and that we have selected 100kHz steps. To receive this frequen­cy, the VCO must be tuned to 156.7MHz (ie, 10.7MHz higher) and so the 16-stage counter must be set so that it divides 156.7MHz down to 100kHz exactly; ie the counter must be set to divide by 1567. If we now select a frequency of 146.1MHz, the software sets the Table 1: Resistor Colour Codes  No.   1   1   2   1   1   1   2   1   1   1   1   1   1   1   1   1 30  Silicon Chip Value 1MΩ 330kΩ 100kΩ 56kΩ 39kΩ 16kΩ 12kΩ 6.8kΩ 5.6kΩ 3.3kΩ 2.7kΩ 2kΩ 1.5kΩ 1.2kΩ 1kΩ 560Ω 4-Band Code (1%) brown black green brown orange orange yellow brown brown black yellow brown green blue orange brown orange white orange brown brown blue orange brown brown red orange brown blue grey red brown green blue red brown orange orange red brown red violet red brown red black red brown brown green red brown brown red red brown brown black red brown green blue brown brown 5-Band Code (1%) brown black black yellow brown orange orange black orange brown brown black black orange brown green blue black red brown orange white black red brown brown blue black red brown brown red black red brown blue grey black brown brown green blue black brown brown orange orange black brown brown red violet black brown brown red black black brown brown brown green black brown brown brown red black brown brown brown black black brown brown green blue black black brown 16-stage counter to divide by 1568 and the tuning voltage generated on pin 13 of the PLL “pulls” the VCO into lock so that it now runs at 156.8MHz. A-D converter IC1 is an 8-bit A-D converter with 11 analog input channels (A0-A10). It is used here to monitor the received signal strength and the external supply voltage. As shown, the RSSI output from IC3 appears at pin 12 and is fed to pin 14 which is the non-inverting input of an internal op amp. The buffered output appears at pin 16 and is fed via R4 to the A0 (pin 1) input. Similarly, the supply voltage is sampled using R5 and R6 and the divided voltage applied to the A1 input. An LM385Z-2.5 (REF) sets the reference voltage to 2.5V on pin 14 of the A-D converter (IC1). The latter is controlled via a 3-wire interface from the PC to pins 15, 17 & 18, while the data is clocked out of pin 16 and applied to pin 10 of the parallel port. Transistor T3 is used to mute the receiver. This transistor is con­trolled via pin 5 of the parallel port and turns on to mute the audio from IC3 as required. Finally, pin 9 of the parallel port controls the power indicator LED (LED 1) via R7. This is turned on and off via the power switch on the front panel. Table 2: Capacitor Codes            Value IEC Code EIA Code 0.47µF 470n 474 0.1µF 100n 104 .01µF   10n 103 .001µF    1n 102 330pF 330p 330 120pF 120p 120 68pF   68p   68 56pF   56p   56 22pF   22p   22 10pF   10p   10 versions. Start the assembly by installing all the resistors, the capacitors and the ICs. Table 1 shows the resistor colour codes, while Table 2 shows the codes for the MKT polyester and ceramic capacitors. It’s also a good idea to check each resistor on a digital multimeter, just to make sure you have identified it correctly. Keep all component leads as short as possible, to avoid stray capacitance and inductance effects. Next, install the three transistors (T1-T3), followed by the ICs which should be are directly soldered to the PC board. Take care to ensure that these parts are all orientated correctly and don’t get them mixed up. In particular, note that T3 is a 2N7000 MOSFET, while T1 & T2 are MPSH10 bipolar types. Now for the two inductors (L1 and L3). These are both made by winding five turns of 0.7mm enamelled copper wire (ECW) onto a 3mm former (eg, a 3mm drill bit). After winding each coil, slide it off the drill bit, scrape away the enamel from its leads and push it all the way down onto the PC board before soldering. The turns should be evenly spaced so that each coil is about 9mm long. Now complete the assembly by installing the two ceramic filters (F1 & F2), the two crystals, the quadrature coil (F3), the trimmer capacitors (V1 & C12), the DB25 connector and the power supply terminal block. You can also install pin headers for the loudspeaker and antenna connections. Note that if you want to receive weather satellite pictures on 136MHz, ceramic filter F1 (455kHz) should have a bandwidth of 50kHz (these filters are available from Jaycar and Dick Smith Electronics). 118-132MHz version In addition to changing the software, three component changes are Power Power for the circuit is derived from an external 5V supply. This supply must be well regulated; eg, by using a 5V 3-terminal regulator. Note that a 5V plugpack isn’t good enough, since its regulation will be quite poor. Diode D1 is there to provide shortterm reverse polarity protection. A 100mA fuse should be included in the supply line if the supply isn’t short-circuit proof. Typically, you could use a 9V AC or DC plugpack or 9V battery to the regulator. Fig.5 shows a suitable circuit, with an optional LED power indicator. Construction Building the VHF FM Receiver sure is a lot easier than understanding how it works. All the parts, except for the loud­ speaker, are mounted on the PC board and the alignment is easy. Fig.4 shows the assembly details for the 144-148MHz and 132-144MHz Fig.5: the MC13135 radio IC is virtually a complete narrowband FM radio on a single chip. It’s a dual conversion receiver with two local oscil­lators (LO), two mixers and two intermediate frequencies (IFs). It also includes a varicap tuning diode, a high-gain limiter, a demodulator and a received signal strength indicator (RSSI). JUNE 2000  31 Fig.5: this simple regulator circuit will let you power the receiver from a 9V AC plugpack. Alternatively, you could use a 9V DC plugpack or a battery pack to directly feed the 7805 regulator and eliminate the four rectifier diodes. required if you want to tune from 118-132MHz: change C9 to 22pF; change C14 to 1500pF; and use six turns for coil L3. These parts are all in the VCO and the component changes are necessary so that it now tunes over its new range from 128.7MHz to 142.7MHz. Software The software runs under Windows 95/98 but not under Windows 3.1x. The main software version covers the range from 144-148MHz and is provided with the kit (see panel). Range updates for 132-144MHz and 118-132MHz bands are also available and can be downloaded free of charge from the SILICON CHIP website at www.siliconchip.com.au or from Softmark’s website at www.ar.com. au/~softmark Note that there are three range updates to choose from: 144vhf.zip for the 144-148MHz band; 132vhf.zip for the 132-144MHz band; and 118vhf.zip for the 118-132MHz band. You install the main program by running setup.exe. This will install the various files into a folder named C:\Program Files\FM-Receiver (you can change this if you want to) and install the necessary entries in your Start menu. To install the range updates, first unzip the file, then run the “.exe” file. Note that the updates only work if you have the main program installed on your computer. Test & alignment Connect the receiver to your PC and to a loudspeaker, apply power from an external 5V DC source (eg, batteries), Note To keep costs low, the interface to the parallel port has been kept very simple, with no surge protection fitted for the external circuitry. For this reason, we suggest that you use a short cable (say less than 1-metre long) to connect the VHF FM Receiver to the parallel port. In addition, you should always apply power to the VHF FM Receiver first, before booting the computer and loading the software. The reverse order applies when switching off – ie, turn off the computer first before removing power from the receiver. Where To Buy The Parts A full kit of parts for this design is available from Softmark, PO Box 1609, Horns­by, NSW 2077. Phone/fax (02) 9482 1565; email softmark<at>ar.com.au Full kit (hardware and software; specify CD-ROM or floppy disks)....................... $85 Payment by cheque or money order only. Please add $6 for postage. Range updates can be downloaded free of charge from the Soft­mark website at www. ar.com.au/~softmark or you can download from the SILICON CHIP website at www. siliconchip.com.au Note 1: the above prices do not include GST which comes into force on 1st July, 2000. Note 2: copyright of the software and PC board associated with this project is owned by Softmark. 32  Silicon Chip then boot the computer and run the software. The software will ask you which parallel port you wish to use (either LPT1 or LPT2), after which you turn the on-screen display on by clicking the power button. Assuming that everything is working OK, the first step in the alignment procedure is to adjust coil L3 so that the VCO tunes the required range. To do this, adjust the tuning so that the on-screen display reads 146.000MHz and stretch (or squeeze) L3 so that the voltage at test point TP1 is 2V (see photo for location of test point). Now tune the receiver across its entire range. The voltage at TP1 should vary from about 0.2V at 144MHz to about 4.0V at 148MHz. It should never be at 0V or at 5V. Similarly, for the other two frequency ranges, simply tune to the centre of the band and adjust L3 for 2V at TP1. The next step involves adjusting L1 and trimmer capacitor V1. This involves tuning to a station that you can receive and adjusting these two components for maximum signal strength, as indicated on the meter. Initially, you should try setting V1 to mid-position; if you find that V1 is at the end of its travel for maximum signal level, try adjusting L1. Alternatively, you can use a VHF signal generator if no on-air stations are available. Don’t connect the generator directly to the receiver though. Instead, attach a 200mm antenna to the generator’s output and attach a similar length of wire to the antenna input of the receiver. Adjust V1 and L1 as described above. Next, the quadrature coil (F3) should be adjusted for best audio quality. You will probably find that the ferrite slug will be just proud of the top of the can but note that this adjustment isn’t particularly critical. Frequency calibration Trimmer capacitor C12 provides the frequency cali­bration. To do this, tune to a station or repeater of known frequency and adjust C12 so that the indicated frequency is correct. Alternatively, if you have an accurate frequency meter, you can adjust C12 so that the reference frequency is exactly 8MHz. Note that you will have to use a sniffer probe to pick up the oscillator signal, as a direct connection will provide enough loading to shift the frequency one way. SC