|An FM Receiver for the 144MHz (2m) Amateur Band
Originally Published in Radio and Electronics World in May 1982
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|This was originally designed to receive the telemetry from the UOSAT (Univesity of Surrey LEO) Satellites. From the days when synthesisers were not always used, it is a basic 144MHz, crystal controlled FM receiver with AFC. The design is usable for other VHF bands by changing the filters and multiplier components.
This receiver is a six channel crystal controlled design. An AFC facility is available on two channels, since the doppler frequency shift on the 2 meter beacon is about + /-3 kHz which will cause a significant degradation to the signal if a fixed frequency receiver is used. The AFC range is just sufficient to cope with the +/- 15 kHz doppler shift found on the 70cm beacon. A suitable low noise 70cm converter was described in reference (5).
The low noise figure and high gain of the 3SK88 RF and mixer stages are exploited to give the receiver a sensitivity exceeding O.15uV (-l24dBm) for 12dB SINAD. The heart of the receiver is an MC3359/ULN3859 IF IC, which is a tidied-up and slightly improved version of the MC3357. The use of this IC simplifies the design of the IF considerably, and helps achieve the overall excellent price/performance of the unit. A nine times multiplication is used in the local oscillator chain to give the required range from the VXO - which also means that a readily available crystal may be used.
Components List and Placement Information
The RF and mixer stages, Q1 and Q2 are similar to the well tried 2m converter design described in Ref (I), except that the local oscillator is injected to the the source of the mixer. This method produces a more stable mixer with improved intermodulation characteristics.
The drain of Q2 is tuned to 10.7 MHz by TI, which also matches the mixer to the crystal filter. The output of the filter is the same impedance as that found at the input to ICI (pin 18). The high sensitivity and impedance at this point (about 5uV for 12dB SINAD and 3k respectively) may be demonstrated by the ready reception of short wave signals if you place your finger near pin 18. The double balanced mixer converts the 10.7 MHz signal to the second IF at 455 kHz.A Colpitts oscillator is provided in Id, which in this case is crystal controlled by X7. The output at pin 3 has a 1k8 impedance which matches directly to a ceramic filter. Most of the gain of the IC is at 455 kHz in the limiting amplifier, and the input to this stage is also matched to the filter impedance. The amplitude - limited FM signal is demodulated using a quadrature detector, and the recovered audio is filtered by R12, C23, C24 and C26 to give the desired response. Unfiltered audio is fed to a bandpass filter formed by an inverting op-amp at pins 12/13 and RI I, R8, CIS. C16. Any noise above the normal audio range, present in the absence of signal, is selected, amplified and then detected by the combination of DI and C14. The squelch sensitivity is adjusted by varying the bias fed via R9 to the squelch detector input. A level of 0V7 at pin 14 will activate the squelch detector causing pin 15 to be open circuit and pin 16 to be grounded via pin 17. Shorting the audio at the top of RV4 mutes the receiver. The ubiquitous TDA2002 is used as the audio power amplifier
AFC & Local Oscillator
About +2dBm is required at 133.3 to 135.3 MHz as local oscillator. Q3 forms a Colpitts oscillator at about 14 MHz. The frequencies of the crystals are trimmed either manually (by C34-37) or by the AFC voltage via D2 and D3. T3 and 14 select the third harmonic of the crystal frequency. Q4 triples again and its output is tuned by T5 at the LO frequency. T6 forms the rest of the filter and matches the output to Q2. A high multiplication factor is needed to allow sufficient range to be extracted from the VXO. Despite the nine times multiplication, the output at 16 is very clean because of the use of double tuned filters 13/14 and T5/6. The AFC voltage derived from pin II of ICI is scaled by 1C3 and added to an offset which is set by adjusting RV2 97 this allows the receive centre frequency to be adjusted. Manual tuning is available if the input of 1C3 (pin 2) is connected to a fixed bias source, which is set by RV3.
Assembly of the PCB is straightforward, but it helps if the following few notes are adhered to. All components that require an earth connection should be soldered to the top of the board. The earth leads on IC1 and 1C3's sockets are bent outwards before soldering. If an SLF-D type filter is used, its earth connectors are made likewise. Fl,T1 /3/4 connections and the earth leads of C42/44/48/50 should be soldered top and bottom. Qi source and the earth track under F2 should be connected through the PCB to the earth plane. C19 must be fitted before IC1's socket.
Remove IC1 & 3 from their sockets. Connect the power and check the supply voltages. Check the audio amplifier operation by applying a signal to pin I of 1C2 (finger or signal generator). Fit a crystal in channel 3, set T3 and T4 cores level with the can top, and monitor the voltage on Q4 emitter, TPI, tuning T3 and T4 for maximum reading. Monitor TP2 and tune T5/T6. Retune T3 and T4 for maximum. Switch the supply off, insert ICi and switch on again. Set the mute fully anticlockwise. Using either a signal generator or strong local signal, tune T2, TI, and Cl for best signal - to - noise ratio. If a signal generator is available, inject signal at 10.7 MHz to pin 18 of ICI and set the voltage on pin 11 to about 2.8V by tuning T2. Without a signal generator, the descriminator may be set up by tuning T2 as before, but with no signal input to the receiver. Inserting 1C3 activates the AFC to channels I and 2. RV2 provides coarse tuning for these channels. With SWI set to manual - and RV2 to centre of its travel - adjust RV3 to give about 5V at pin 6 of 1C3. This sets the manual tuning range. Although Fl is supplied fully aligned, it may require some minor adjustment to take account of circuit variations. When the receiver has been tuned as above, it is a good idea to slightly retune Ll, Fl, TI and T6 for best signal - to - noise.
1. Timothy Edwards, A state of the art 2m Receiving Converter, R&EW;October 1981.
2. Peter Whatley, Motorola mc, FM Receiver Mixes High Gain with Low Power, Electronics June 2nd 1981.
3. MC3359 Data Sheet, Motorola Inc.
4. ULN3859 Data Sheet, Sprague
5. Leighton G.R., 70cm to 2m and TV converter, R&EW January 1982.
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© 1982 G. Leighton © 2006 radioshop.co.uk