Synchronous Demodulator

The circuit

To decode a message in amplitude modulated carriers, a waveform has to be multiplied with itself using a product demodulator. In many ways, a simple nonlinear device like a semiconductor diode can be used for this purpose. Its exponential characteristic can be mathematically expressed by a Taylor series. The message then is decoded by filtering out the quadratic term.

In applications like spectrum analysers and lock-in amplifiers, problems with higher order Taylor terms may occur. In this case, the diode should be replaced with a device having a parabolic characteristic, a junction field effect transistor for example. However, in zero-IF systems, dc terms have to be eliminated too. This is the area of synchronous demodulators.

The input file

synchronous demodulator

.control
tran 50ns 10us
plot v(1)+3 v(2)/5-1 v(3)-3 xlabel t[s] ylabel 'input reference output[V]'
  + title 'TRAN analysis'
.endc

v1 1 0 dc 0 sin 0 1 550kHz
v2 2 0 dc 0 pulse 0 10V 0 0 0 1uS 2uS

r1 1 4 1meg
r2 4 3 1meg
r3 1 5 1meg

e1 3 0 5 4 1meg

j1 5 2 0 pkanal

.model pkanal pjf
 
.end

The results