10.7 MHz FM Demodulator Circuit


This is a simple add-on circuit to provide FM audio demodulation capabilities to a system with a 10.7 MHz IF.  This circuit was designed primarily for use in the 1 GHz RF spectrum analyzer project which was documented in GBPPR 'Zine Issue #80, but it can be easily adapted for other uses.  The circuit as shown and documented here has no squelch circuit, which simplifies construction greatly, but it's not really that useful for scanning radio applications.  It just sometimes handy to be able to listen to a signal which is displayed on the spectrum analyzer.

The FM demodulator will be based around the common Motorola MC3361 narrowband FM IF chip.  This IC is very common in older 49 MHz baby monitors and analog cordless phones, so you'll want to check your local thrift store for those.  The most critical component is the 10.245 MHz crystal used for the second local oscillator within the MC3361 itself.  Custom crystals are very expensive but, fortunately, 10.245 MHz crystals are very common in older Regency and Bearcat scanning receivers.  Keep an eye out for those old "crystaled" scanners or the ones with the vacuum fluorescent displays.

The MC3361 will also require a 455 kHz IF resolution filter (Murata or equivalent) and a 455 kHz quadrature coil, but these can be salvageable from the same radio circuit you found the MC3361 in.

The circuit shown here has an optional Mini-Circuits PSC-2-1 RF splitter on the 10.7 MHz IF input.  This is to send one signal to the FM demodulator and the other signal to a front-panel connector for future use.

There is also a Motorola MWA120 hybrid RF amplifier driving the RF input to the MC3361.  This RF amplifier is optional, but useful for increasing the sensitivity of the MC3361.  Just about any MMIC amplifier will work.  With only one MWA120 amplifier, the MC3361 still gave good audio quality with a RF input of only around -80 dBm.

It's also possible to do AM demodulation by tapping the 455 kHz IF signal after the 455 kHz resolution filter (pin 5 of the MC3361) with a simple transistor buffer and then sending that signal to a 1N34 diode detector.  Take the raw output after the diode detector and send that to the audio amplifier.

Pictures & Construction Notes

Overview of the FM demodulator circuit.

10.7 MHz IF input is on the upper-left.  It passes through a Mini-Circuits PSC-2-1 splitter.  One RF output from the PSC-2-1 is sent to an external BNC connector, and the other RF output is sent to a Motorola MWA120 RF amplifier.

The RF input (pin 16) to the MC3361 has an impedance of around 3,000 ohms.  Instead of using a inductor/capacitor impedance matching network on this input, we'll just put a 51 ohm resistor parallel with it to ground.  Pin 1 and 2 of the MC3361 are for the 10.245 MHz crystal and the little black box to the right is the 455 kHz IF resolution filter.  This filter is equivalent to a Murata CFW455D and has 6 dB bandwidth of around 10 kHz.

There is a quadrature coil on pin 8 of the MC3361.  This coil provides the 90 degree phase shift on the 455 kHz IF signal which the MC3361 requires for proper FM demodulation.  You'll need to salvage this coil from the same circuit you got the MC3361 from, but document the coil's pin out very carefully.  Only two pins are used and the rest are usually grounded.  A 39 kohm resistor across the two pins of the quadrature coil "de-Qs" the tank circuit, which eases any off-frequency tuning condition.

You may need to slightly tune the quadrature coil if the audio output appears to be distorted at all.  Just tweak the trimmer inductor (black thing on top of the quad coil) with a non-metallic object until the audio sounds decent.

The recovered audio output (pin 9) of the MC3361 is sent to a LM386 audio amplifier to drive a small speaker.  There is no attempt at a squelch circuit or proper FM de-emphasis and filtering, so the audio quality may be a little "raw."

Alternate view.

The MC3361 will require a proper PC board ground plane and RF bypassing on the Vcc line.

The LM386 audio amplifier was built on a little vertical riser board.

The circuit is powered from a switched +12 VDC line with the MC3361 run off a separate 78L05 voltage regulator.

Installing the FM demodulation circuit in the sweep & sync case for the GBPPR 1 GHz RF Spectrum Analyzer project.

The coax cable on the left carries the 10.7 MHz IF input from the 10.7 MHz Sample output on the IF amplifier and log detector board.

The blue shielded wire is the audio output from the LM386 and goes to a panel-mounted RCA jack for the speaker.

The gray shielded wire goes to a 10 kohm volume potentiometer mounted on the front-panel.  That potentiometer also includes a power switch which controls this entire circuit.

The white coax cable along the bottom-right is the second 10.7 MHz IF output from the Mini-Circuits PSC-2-1 splitter.  It goes to a panel-mounted BNC jack.

A Motorola MC3361 and a MC3359 narrowband FM IF chip in the receiver section of some old 49 MHz baby monitors.

Older 49 MHz baby monitors and analog cordless phones are a good source for the Motorola MC3361, the 455 kHz quadrature coil, and a useable 455 kHz IF filter.

The quadrature coil is the silver box with the black screw top to the lower-right of the MC3361 and MC3359 in the above picture.

10.245 MHz 2nd local oscillator crystal in an old Regency radio scanner.

You'll need to salvage the 10.245 MHz crystal from older Bearcat or Regency scanners or even some older two-way radios.  Be sure to make note of the loading capacitor values used on the crystal, as these will be critical for proper oscillation.

Look around at ham fests for these older scanners or radios.  You can usually pick them up pretty cheap and it's not worthwhile to order a new 10.245 MHz crystal.