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AMATEUR RADIO By GARRY CRATT, VK2YBX Build this simple VHF FM monitor receiver Need a spare VHF receiver for monitoring the local repeater? How about a dedicated unit for foxhunting, or a radio direction finding receiver? If the answer to any of these question is yes, then this may be the project for you. A few hours of construction time is all it takes to build this little unit. The heart of the receiver is a new integrated circuit from Motorola, the MC3362. Unlike many previous "single chip receivers" (such as the CA3089), this new integrated circuit provides all receiver functions from the antenna input to the audio preamp output. It is a low power dual conversion design with low power drain, excellent sensitivity, and good image rejection in narrow band voice and data link applications. Dual conversion A dual 'Conversion receiver is an extension of the basic superheterodyne principle. A normal RF lnpul to 200 MHz Vee . . - - - . . - From PLL Phase f--:i_ 5 0 pF a._______ __. 10 245 MHz Detector 001 -:22t----~ 0 41 µH 21t---+--~ Ceramic Filter - c - - - - - l 5 455 kHz rt---=Ceram,c FIiter tO 7 MHz Lp ep Fig.1: this diagram from the Motorola applications data shows how the MC3362 is used in a typical PLL frequency synthesised receiver. 50 SILICON CHIP superhet has one local oscillator which is "heterodyned" or mixed with the incoming signal to produce an intermediate frequency which is then amplified and demodulated. This is referred to as single conversion and the intermediate frequency is typically at 10.7MHz for FM receivers or 455kHz (or 450kHz) for most AM receivers. By contrast, a dual conversion receiver has two local oscillators. The first one beats with the incoming signal to produce an intermediate frequency (IF) of 10.7MHz. This is amplified in the first IF stage and then mixed with the second local oscillator which operates at 10.245MHz. This produces the second intermediate frequency of 455kHz (ie, 10.7MHz 10.245MHz = 455kHz). Dual conversion receivers with this arrangement are commonly used for narrow band FM reception. By "narrowband" we are referring to the fact that normal broadcast FM has a deviation of ± 75kHz and a total channel bandwidth of 150kHz. This is wideband FM. For the amateur bands and commercial use, narrowband FM normally refers to a much smaller frequency deviation, such as ± 5kHz. This can vary though, for amateur operations, depending on which band is in use. This will be the subject of a future article. Using the chip Fig.1 shows a typical application of the MC3362 in a circuit from Motorola's application literature. It shows the incoming signal fed from ANTENNA RF AMPLlflER 8FR91 2ND IF 455kHz 1ST IF 10.7MHz 1ST LOCAL OSCILLATOR LIMITER L 2ND Dr LOCAL 10.245MHz OSCILLATOR -t QUAD DETECTOR LM386 AUDIO AMPLIFIER SPEAKER MC3362 Fig.2: block diagram of the receiver to be described in next month's issue. A simple RF amplifier stage precedes the MC3362's input while the output drives a single-IC audio amplifier stage to provide loudspeaker volume. the antenna via a matching network and .0lµF capacitor to pin 1 of the MC3362. This is the input to the first mixer. The first local oscillator can be run using a free running LC tank, or as a VCO using PLL (phase lock loop) synthesis. With variable tuning, a range of 4MHz is available using the internal varicap at pin 23. Alternatively, for single channel use, the local oscillator can be driven from an external crystal oscillator. A buffered output of the first local oscillator is available at pin 20. The Motorola data sheet says that the local oscillator can be run as high as 170MHz which means that, in this configuration, the highest receiver frequency we could cover would be around 160 MHz. As the chip itself was designed as a cordless telephone receiver for the US market (49MHz), the design lends itself immediately to either a 6-metre or a 2-met.r e receiver for amateur use. A 10. 7MHz crystal or ceramic filter connected between pins 17, 18 and 19 sets the bandwidth for the first IF stage where the signal is amplified before being fed to the second local oscillator. The second local oscillator is a common base Colpitts type which is typically run at 10.245MHz under crystal control. A buffered output is available at pin 2. Pins 2 and 3 are interchangeable. The mixers are double balanced to reduce spurious responses. The first and second mixers have conversion gains of 18dB and 22dB respectively and the mixer gain does not vary with changes in the supply voltage. For both conversions, the mixer impedances and pin layout are designed to allow the user to employ low cost, readily available ceramic filters. Sensitivity is quite high. The input level for 20dB (S + N)/N is 0. 7µ V at 50MHz. The 455kHz IF is filtered and fed to the limiter input, at pin 7. The limiter has a 10µ V sensitivity for - 3dB limiting, flat to 1MHz. The output of the limiter is internally connected to the quadrature detector which includes an internal quadrature capacitor. A parallel LC tank is connected externally from pin 12 to Vee. In addition, a 68k0 shunt resistor is included which determines the peak separation of the quadrature detector. A smaller value will increase the spacing and linearity but decrease recovered audio output and sensitivity. For data communications, the recovered audio output from pin 13 can be connected to an internal comparator between pins 14 and 15. FSK (frequency shift keyed) data rates of 2000 to 35,000 baud are detectable. The hysteresis of the internal comparator can be varied by connecting a high value resistor from pin 14 to pin 15. Values below 120k0 are not recommended as the input signal cannot overcome the hysteresis. A meter drive circuit at pin 10 detects input signal level by monitoring the limiting of the limiter stages. It can be used to trip a carrier detect circuit, pin 11, at a particular input level. Our receiver Veiru-· olp Cnntrnl -. 22 151 LO Tank I I 21 1st LO hnk I 20 1sl LO Output 19 1st Mo,et Ou1put 18 2nd M1aer Input L,m,te, Decoupling l 1m 1ter Decoupling l 7 2nd Mixer lnpul 16 VEE Fig.3: the MC3362 is virtually a complete narrowband FM receiver on a single chip. Our receiver circuit uses a number of variations c:in the theme suggested by Motorola. The block diagram for the receiver is shown in Fig.2. It shows the main functions of the MC3362 which is preceded with a single BFR91 RF amplifier stage. Our design uses a 10.7MHz crystal filter, although a ceramic unit could be used at the expense of adjacent channel rejection. Following the MC3362 we used an LM386 as an audio amplifier to provide sufficient drive to a small speaker. Next month we will present the full circuit of this receiver, configured for the 2-metre amateur band. A printed circuit pattern and constructional information will be included. ~ FEBRUARY1989 51