Voltage Modulation Circuit

Thread Starter


Joined Sep 28, 2021
Hi all :),
I tried to simulate a modulation circuit which uses a comparator, following this click here .

The output I get at the 1 kΩ resistor is weird; it should be the same shape as the send signal above.

May I know what values of capacitor and inductor are used, and whether the resistor value at the output also affects the shape of the send signal?

Is my LTspice output correct (Oscillating wave output seems triangular) and should the carrier signal, audio signal and send signal be AC or DC? The video diagram shows all of them are DC.

I attached my LT Spice file below, can anyone make some adjustments so that the send signal would be more sinusoidal?

Thank you for reading and have a nice day:)



Joined Aug 21, 2008
The three transistors around the middle of the circuit form a Gilbert Cell multiplier which multiplies the output of an RF oscillator (left 1N4401) by an audio signal fed into the lower 2N4401. The inductors and capacitors in the upper-right of the circuit are for converting the collector current at the output of the cell to drive a short antenna. This was originally designed by Charles Wenzel.

The 2N4401 is usually from the same process to make the 2N3904, but has a typical Ft of 200 MHz instead of 300 Mhz -that does not matter at 1 MHz.