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AMATEUR RADIO By GARRY CRATT, VK2YBX Build a simple VHF FM • monitor receiver, Pt.2 Last month we described the operation of our simple VHF FM monitor receiver. This month we cover construction and alignment of the unit. The basic configuration of the MC3362 was covered in last month's article, so we will discuss the additional circuitry here. We found that the receiver produced reasonable sensitivity without the suggested external BFR91 broadband amplifier shown last month, so this has been deleted as an unnecessary complication. As it stands, the receiver gives quite reasonable sensitivity (in the order of several microvolts). An LM386 audio amplifier was used to produce adequate volume levels for normal listening conditions. The squelch is configured so that the DC output from pin 11 of the MC3362 is used to bias the audio amplifier off when no signal is being received. This also minimises the current drawn by the receiver when no signal is being received, an important point to con- sider if the receiver is to be run from batteries in a portable, application. Construction The first prototype receiver was built on a single sided circuit board but we subsequently designed a double-sided PCB (using a groundplane on the component side) for additional stability. The board measures 108 x 55mm and is coded SC 06103891. Normal RF contruction techniques apply. Basically the most important thing is to keep component leads as short as possible. This The parts are all mounted on a small double-side printed circuit board. Note that where the ground plane comes right up to the edge of a hole, the component lead must be soldered on both sides of the board. 68 SILICON CHIP ANTENNA J C2 100pF lOOpF 23 C3 120pF VC1 2-20pF TUNING VR3 50k C9 ClO .01~.1 ~ .,.. C23 + 10 25VW+ C21 CfJ + 01 - 116VW+ C20I 0.1 R3 47k 22 Fll C24 27pF 455kHz FILTER L3 21 19 ICl MC3362 FL2 10.7MHz FILTER 18 17 SQUELCH VRl 200k 10 A0JUST~NJv-......--WfH,----"'t C11I 0.1 C12 .047+ R9 100 C7 0.1 11 455kHz QUADRATURE COIL 13 12 24 CB .01:t 16 .,.. SQUELCH R5 2.2k C16 + 100 25VWJ R6 22k C19 220 16VW +) VOLUME VR2 10k LOG ':' L1 : 4T, 25B&S ECW ON 4mm FORMER L3 : 21, 258&S ECW ON 6mm FORMER C18 .047 1 BO SPEAKER RlO 10[! - NARROW BAND FM RECEIVER Fig.1: the circuit is based on the Motorola MC3362 (IC1) which is virtually a complete narrowband FM receiver on a single chip. L3 and C24 set the frequency band while the output (at pin 13) drives an LM386 audio amplifier (IC2). means that all capacitors, resistors and transistors should be pushed down close to the board before soldering the leads. Fig.2 shows the parts layout on the PCB. Begin construction by installing all the resistors, working down the parts list from Rl to Rll. Bend the component leads against the copper side of the circuit board so that the components don't fall out of the PCB when it is turned upside down for soldering. Solder the resistor leads, then trim the excess lead lengths with a pair of side cutters. After soldering the resistors, proceed to the capacitors, working through the parts list from Cl to C23. It is a good idea to cross the components off the parts list as they are installed, so that nothing is missed during construction. Make sure that you install all the electrolytic capacitors with correct polarity. If you don't do this correctly, circuit damage can result. For example, on one of the prototype receivers the 220µ,F capacitor was installed incorrectly. When powered up, the reverse voltage across the capacitor caused it to go short circuit. This caused pin 5 of IC2 to deliver about 6 volts DC (about half the available supply voltage) to the speaker and consequently the IC blew its top. Learn by our mistake - put the capacitors in the right way around. At this stage the trimpots for squelch (VRl) and volume (VR2) can be installed, as can the two IF filters (10.7MHz and 455kHz) and the 10.245MHz crystal. Be careful not to use excessive heat when soldering the crystal, as the body of the crystal will be close to the tip of the soldering iron when soldering the leads. The next step is to install the semiconductors. Ensure that the ICs are oriented correctly before you start soldering the pins. We do not recommend the use of IC sockets in the receiver as they can be unreliable. They also add stray capacitance and inductance to every IC pin and this can prejudice operation at very high frequencies (VHF). So be careful to install the ICs correctly the first time - desoldering all those IC pins can be a pain and can render the ICs unusable. Note that where component leads are connected to earth, they should be soldered on both sides of the PCB. This applies to the GND terminal of the 3-terminal regulator, pin 16 of ICl, pin 2 of IC2 and to quite a few of the resistors and capacitors. The only components requiring any degree of preparation prior to installation on the PCB are the input coil Ll and the tank coil 13. 11 is made by winding four turns of 25 B&S tinned copper wire (TCW) or enamelled copper wire (ECW) on a 3.5mm former. After winding, stretch the coil as shown MARCH 1989 69 VR3 PARTS LIST X1 - 10.245MHz crystal FL 1 - 455kHz ceramic filter (Murata SFB-455) FL2 - 10. 7MHz ceramic filter or crystal Semiconductors IC1 - MC3362 narrowband FM receiver IC2 - LM386 audio amplifier IC3 - 7805 +5V 3-terminal regulator Inductors L 1 - 4 turns 25B&S tinned copper wire x 3.5mm dia. L2 - 455kHz coil (from DSE L-2060 coil pack) L3 - 2 turns 25B&S tinned copper wire x 6mm dia. Capacitors C1 ,C2 - 1 00pF ceramic C3 - 120pF ceramic C4 - 56pF ceramic C5,C6,C7 ,C11,C20,C21 0 .1 µF monolithic C8,C9 - .01 µF ceramic C10 - 0 .1µF ceramic or metallised polyester C12,C 14 ,C18 - .047µF ceramic C13 - .01 µF ceramic or metallised polyester C15 - 2.2µF 16VW electro C16 - 1 00µF 25VW electro C17 - 1 0µF tantalum C22,C23 - 1 0µF 16VW electro C19 - 220µF 16VW electro C24 - 27pF (depends on frequency band) VC1 - 2-20pF trimmer Resistors R1 - 1kn R2 - 68kn R3 - 47kn R4 - 18kn R5 - 2 .2kn R6 - 22kn R7 - 1 0kn R8 - 56kn R9,R1 o - 1 on R11 - 3 .3kn Potentiometers VR1 - 200kn miniature trimpot, horizontal mount (squelch) VR2 - 1 0k0 miniature trimpot, horizontal mount (volume) VR3 - 50k0 multiturn potentiometer (tuning) Miscellaneous 1 x PCB (code SC06103891, 108 x 53mm) , 1 x 8-ohm loudspeaker , 1 x 9V DC plugpack supply, 1 x case to suit. 70 SILICON CHIP J~ ~L1 ~ C2 VC1~ec::>41 Fig.2: wire up the PCB as shown in this parts layout diagram. Keep all component leads as short as possible and don't forget to solder to both sides of the PCB where appropriate (note: ground plane not shown). in the photograph so that there is a 3mm gap between windings. The l00pF input capacitor (Cl) is connected 1-1/2 turns from the C2 end of Ll. If using enamelled copper wire it will be necessary to scrape the enamel from both ends of the coil and pre-tin the leads prior to insertion into the circuit board. 13 is made by winding two turns of wire on a 6mm former. Once again, 25 B&S wire should be used although the gauge is not critical as we will be stretching the coil mechanically to set up the frequen- cy coverage required. The tuning control is a multi-turn 5okn potentiometer which is specified for ease of tuning. Because the tank circuit operates directly at VHF, the varicap tuning voltage is quite sensitive to adjust. Note that any drift of the tank frequency with temperature will cause a change in operating fr equency. This can only be compensated for by changing the tuning voltage, via the tuning control. In fact , to tune over the entire 2-metre amateur band requires a change of only 2V (from 2V to 4V) Where to buy the parts The 10.245 MHz crystal can be obtained from HY-Q Crystals, 1 Rosella St, Frankston, 3199 . Phone (03) 783 9611 . It is also used in many Dick Smith Electronics kits and may be available through their stores. The SFB-455 455kHz ceramic filter is available from Dauner Electronics, 51 Georges Crescent, Georges Hall , NSW 2198. Phone (02) 724 6982. It is listed in the current Murata catalog (Murata components are handled by IRH Components, phone 02 648 5455). Other types can be used if the pin configuration is modified . The 10.7MHz filter can be either a crystal type (available from HY-Q Crystals or DSE) or a ceramic type which could even be salvaged from a surplus cordless telephone or other communications equipment. They are also available from Murata distributors. The MC3362 is available from VSI Electronics in Sydney phone (02) 439 8622. The 50k0 1 0-turn potentiometer is available from Geoff Wood Electronics (phone 02 427 1676) or David Reid Electronics (phone 02 267 1385). The 455kHz quadrature coil came from a DSE " IF coil pack" , Cat. L 0260. Circuit Notebook continued from page 33 L3 is made by winding two turns of tinned copper wire on a 6mm former. The band of operation is set by squeezing or expanding L3. The input coil L1 is made by winding four turns of tinned copper wire on a 3.5mm former. Note the location of the 100pF input capacitor (Cl). on pin 23 of IC1. And to change the tuning by 600kHz (the difference between input and output of a repeater) requires very little change in tuning volts. For this reason a standard single-turn pot is not practical - you must use a multi-turn control. The tuning control itself should be located away from the circuit board, so that the capacitance of the user's hand when tuning the receiver does not affect the tank frequency. Because the tuning control carries only DC, ribbon cable can be used to connect it to the circuit board. An alternative would be to crystal lock the receiver by using an external overtone oscillator fed to pins 21 & 22. Motorola mention in their data sheet that a level of around 100mV is required for this style of operation. Power up Because the unit has its own 5V regulator, the receiver can be run from virtually any DC source over 7 volts or so. Normally this would be either a 9V or 12V battery, or a 9-12V power supply. If you have a power supply with current limiting, it is wise to set the limit as low as possible to ensure that there are no unexpected fireworks when power is first applied. A correctly assembled receiver will draw less than 20mA from a 12V source. + 3.6V and hence by Ohm's law the current through this resistor and Q2 is about 1.6 mA. The output voltage at the collector of Q2 will then be a little above the emitter voltage. Now if we increase the input voltage above 5V, Ql will begin to conduct. This lowers the voltage at the base of Q2, thus decreasing the current flowing through this transistor. However, the current through Ql increases so that the total current flowing through the emitter resistor is almost constant. Increasing the input voltage further will cause Ql to switch on harder and this in turn will switch Q2 off as the voltage at the collector of Ql rapidly diminishes. Further increase in the input voltage will cause the emitter voltage to rise, cutting off Q2 even further. The output voltage will now be around the supply voltage. If we now decrease the input voltage, we find that Ql does not begin to turn off until the input voltage is around 4.5V. When Ql begins to switch off, it's collector voltage rises and this in turn switches Q2 on. The loop gain of the circuit can be altered by varying the emitter resistance. Decreasing this to around 6800 from 2.7k0 will make the loop gain equal to unity. This eliminates the hysteresis (and defeats the purpose of this circuit), so the circuit behaves like a normal buffer. Increasing this resistance will have an opposite effect. The addition of a O. lµF capacitor between the collector of Ql and the base of QZ will allow enhanced operation at higher frequencies. If the input of the circuit is to be direct coupled to a stage which also operates from a 9V supply, the two 22kn input bias resistors may be omitted. Malcolm Young, SILICON CHIP. MARCH 1989 71 Assuming that you have checked the component placement and found no errors, power may be applied. Check initially that the current drawn is not excessive and that the receiver generates white noise or "hash". You should also check that the squelch control (VRl) operates correctly; turning it anticlockwise should mute the receiver. Check the volume control trimpot (VR2) too; rotating it clockwise should cause an increase in audio level. If a signal generator is available, it should be set to the desired frequency of operation and at a reasonable output level [say 100µ V) to make identification of the signal reasonably easy. If no signal generator is available, connect a suitable VHF antenna to the receiver, so that the local VHF repeater or beacon can be utilised. Once a suitable signal is available the receiver can be tuned by squeezing or expanding the tank coil to shorten or lengthen it. This varies the inductance and hence the tank resonant frequency. If you have access to a spectrum analyser, it is simple to check the actual tank frequency, which will be 10.7MHz above the desired input frequency. This could also be checked by using a sniffer probe hooked up to a frequency meter. It may also be helpful at this stage to replace C24 with a 2-20pF trimmer to allow greater flexibility in setting the correct tuning range. Once a signal has been located, peak 12 [the 455kHz quadrature coil) for maximum audio output. In fact, this can be done without any input signal - you simply adjust 12 for maximum receiver noise. The only other adjustment is to peak the VCl input trimmer for maximum quieting using a suitable off-air signal. Fig.3: here are the actual size patterns for the printed circuit board. The ground plane (bottom) ensures receiver stability. Alternatively, a simple discone antenna will provide good results on a far wider range of frequencies. The frequency coverage of the receiver is not limited to the 2-metre amateur band and it can be successfully used on the VHF marine band, paging frequencies and VHF commercial frequencies. Footnote The purpose of presenting this project is to stimulate active experimentation on the amateur bands. Obviously, a receiver of this simplicity will not perform as well as a $1000 2-metre transceiver. However , it does show that reasonable performance is available at very modest cost. No doubt amateurs will develop improvements to this circuit that will make it easier for others to build. We welcome comments on these projects and will endeavour to incorporate suggestions in future articles. Now who will be first to produce the 6-metre version? ~ Antennas You will find that the receiver performs quite well with just a single piece of hookup wire connected to the input capacitor. The tap on the input coil ensures that a reasonably close match to 50 ohms is available if using a 2-metre antenna having coaxial feed. 72 SILICON CHIP RCS Radio Pty Ltd is the only company which manufactures and sells every PCB & front panel published in SILICON CHIP, ETI and EA. 651 Forest Road, Bexley, NSW 2207 Phone (02) 587 3491 for instant prices 4-HOUR TURNAROUND SERVICE