The NE602 and its variants like the NE 612, SA602, and SA612 have been around longer than I have. They make excellent mixers and are used in thousands of amateur radio designs as IF mixer, DSB / SSB modulators, product detectors, and even as plain oscillators.
One thing that seems to be fairly unknown about this Gilbert cell mixer is that the Gilbert cell was initially not designed and patented as a mixer. The Gilbert cell was originally patented as a standard exclusive OR gate (XOR). The use as four-quadrant multiplier was discovered and implemented in practical designs much later.
In fact, the NE602 and its derivatives act just like an XOR if they are slightly overdriven. Thanks to the amplifiers that are built into the NE602, only very little drive level is needed to accomplish the overdrive.
Using an XOR gate in a FM demodulation circuit is nothing new. A XOR gate configured to act as a phase detector and a phase shift network is all that is needed to get an FM demodulator type that is referred to as a quadrature demodulator.
In a nutshell, what happens is that the phase shift network creates a defined 90 degree phase shift between the two mixer inputs at the desired IF frequency. If two signals with 90 degree phase shift but same frequency are applied to the inputs of an XOR gate, the output will result in a 50% duty cycle rectangular signal with twice the input frequency. 50% duty cycle means the output is ‘on’ (represents logic 1) half the time and ‘off’ (represents logic 0) the other half of the time. If any signal with 50% duty cycle is passed through a low pass filter, the output will be the arithmetic middle (average) of the minimum (logic 0 state) and maximum (logic 1 state) voltage of the input signal. A TTL (0 = 0 V, 1 = 5 V) signal with 100 KHz clock rate and 50% duty cycle would turn into a constant 2.5 Volt DC signal after integration through a low pass filter. The DC voltage decreases as the phase difference increases and increases as the phase difference decreases. I will write another in-depth article on how quadrature demodulators work because I want to limit this article to the simple NE602 FM demodulator. UPDATE: How it works: Quadrature detector
Since the exact phase shift is frequency dependent and the DC voltage on the output of the XOR gate (our NE602) is dependent on the phase shift, this behavior makes a perfect FM demodulator.
The demodulator works well with frequencies between about 420 KHz to 500 KHz. The required input level is between 0.5 Volts peak-to-peak and 2 Volts peak-to-peak. T1 in conjunction with R2 is an emitter follower amplifier boosting the input signal before handing it off to the input of the NE602 through C2. Instead of a BC547 any other NPN type small signal transistor, such as the 2N3904, will work fine in this circuit.
A tiny amount of the IF signal is branched off through C4 into the LC tank circuit consisting of L1 and C5. C6 brings the now phase shifted signal into the second input of the NE602. The low pass filter consisting of R3 and C8 is filtering out the recovered modulation signal.
The sensitivity of this simple demodulator is about 13 mV/kHz. That’s more than enough even for narrowband FM signals.