I am working on an ASK modulator and de-modulator. The modulator works fine (thanks to great folks at SE). However when I try to pass the modulator output to the input terminal of the receiver, the output becomes 0 mostly due to the interference of components of receiver.

Below is the image showing simulation output when modulator is not connected to the receiver:

Green is the ASK output which looks good, red is the digital signal and blue is the carrier.

enter image description here

However when I connect the green output to the receiver (where antenna would be there in real scenario,) this output is generated:

Yellow is supposed to be demodulated output.

enter image description here

We can see that green is now 0. Please help me figure out a way so that I can test in simulation whether my demodulator is working or not.


As requested, pfb the circuit. It's built using multisim on a windows machine.

enter image description here

  • \$\begingroup\$ Show your circuit. The whole thing. Modulator, carrier wave generation, connection between (simulated) transmitter and (simulated) receiver. The whole thing. \$\endgroup\$
    – JRE
    Apr 4, 2019 at 17:56
  • \$\begingroup\$ @JRE Green wire from terminal D of osc is coming as input. Antenna would be there instead. \$\endgroup\$ Apr 4, 2019 at 18:01

1 Answer 1


Take a look at C2.

When just your scope is on it, all is good. The scope is practically no load at all, and the high pass made of C2 and R4 has a cutoff of around 16MHz (below the 27 MHz being discussed in your other question.)

When you put your demodulator on there, you change that high pass filter. I expect the impedance of your demodulator input is low enough to raise the cutoff to the point that nothing gets through.

Easily checked:

Move the scope connection to the other side of C2. If you have your expected signal at the junction of C2 and R2, then you've found the problem.

In that case, use a larger C2 or put a largish resistor between C2 and L1.

  • \$\begingroup\$ actually before C2, the output contains DC bias. Need to remove that before feeding to the receiver \$\endgroup\$ Apr 4, 2019 at 18:26
  • \$\begingroup\$ Yes. Of course. I said to use a larger capacitor in place of C2, and maybe put a resistor between C2 and L1. No mention of removing C2. \$\endgroup\$
    – JRE
    Apr 4, 2019 at 18:28
  • \$\begingroup\$ Thanks sir. This answers the mentioned problem. However there is no change to the de modulated output which i expected to resemble the red wave. Can you help me in this matter? \$\endgroup\$ Apr 4, 2019 at 18:31
  • \$\begingroup\$ Does the demodulated output still look like your yellow line? \$\endgroup\$
    – JRE
    Apr 4, 2019 at 18:36
  • \$\begingroup\$ Unfortunately, yes \$\endgroup\$ Apr 4, 2019 at 18:37

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.