FM demodulation using a mixer IC

Created: Jun. 2020

Introduction

Some twenty years ago (1999), I needed an FM demodulator, but such component wasn't available at my workplace. However, my (then) employer did manufacture a popular mixer component: IAM-81008. So, I attempted to re-purpose the said mixer into a quadrature demodulator. Although I have only experimented with one specific mixer model, there is no reason why the concept would not work with other mixer models.

Material & method

The underlying principle is this: if the input signal is split and fed into both RF & IF ports, the IF port will produce a DC output proportional to the frequency change. An IAM-81 application note did briefly describe a phase detector using a similar arrangement, but without any circuit detail.

A three-way resistive divider is used to split the input signal (fig. 1). In the LO path, a 12p capacitor provides ~90 degree phase shift. A slug-tuned parallel LC tank provides fine control of the phase shift and its secondary lowers the impedance to match the LO port's 50 ohm. The tank component is a common 10.7 MHz intermediate frequency transformer (IFT) - Sumida F35 type with a 9+9 turn primary and 4 turn secondary. The turn ratio is likely not optimum for this circuit, but it was what I had in the junk box. However, this component is not critical and other tunable 10.7 MHz IFTs should work too. The RC network at the output, in conjunction with the IF port 50 ohm impedance, provides de-emphasis (although the resultant 1 uS value is far short of the standard 50 uS).

Fig. 1 circuit to re-purpose mixer into a 10.7 MHz quadrature discriminator

Fig. 2: the components were retrofitted to a PCB originally intended for the IAM-81 mixer application

Results

FM demodulators work by converting the input frequency deviation into a varying DC voltage, thereby, producing the S-curve. The prototype produced a DC output voltage that is inversely proportional to the input frequency (fig. 3). These S-curves' slopes increase with input power level . The slope rates are 0.15 V/MHz, 0.31 V/MHz and 0.57V / MHz for -5 dBm, 0 dBm, and 5 dBm, respectively. The steeper slope produces a larger demodulated signal. The changing slope is probably inconsequential in actual applications as the input power is more or less held constant by the preceding limiter stage. The IFT is tuned so that the S-curve's top and bottom halves are approximately equal at 10.7 MHz. A minor niggle is the midpoint of the S-curve shifts slightly (tens of mV) with input power.

Fig. 3: Measured output voltage vs. input frequency (S-curve) as a function of input power

As expected, the demodulator's Signal-to-Noise (S/N) ratio improves with input power (fig. 4). The test signal has 22.5 kHz deviation (m=0.3) and a 1 kHz audio. Probably even better S/N could be achieved if the de-emphasis had been correctly dimensioned to 50 uS.

Fig. 4 Measured Signal-to-noise ratio vs. input power

Conclusion

The IAM-81 mixer IC can be re-purposed into a FM quadrature detector. Although demonstrated at 10.7 MHz, it should work at other frequencies by changing the relevant phase-shift components. The shown circuit can probably work with other mixer components too.