# Why doesn't this FM receiver work in LTspice?

After years, I'm back again to the electronic world. To start off, I just tried to simulate this circuit in LTspice:

Here is what I've done in LTspice:

But it doesn't work. Here is a screenshot for the signal of the ANT:

And the output of the speaker:

Why doesn't it work? Any idea?

Edit1: Here is the screenshot for 100 MHz frequency:

Edit2: Here is the screenshot for 100 MHz frequency (corrected):

• What carrier frequency? What modulation frequency? What depth of modulation? Or, put more simply, what did you expect to see? Commented Jan 8, 2023 at 16:13
• (100kHz carrier, no modulation? What do you expect, what makes you state It doesn't work?) Commented Jan 8, 2023 at 16:15
• @Andyaka Hey Andy, How you doing man? I think 100MHz should be Ok. FM!
– Roh
Commented Jan 8, 2023 at 16:16
• @greybeard 100kHz is just to test the circuit.
– Roh
Commented Jan 8, 2023 at 16:17
• You need a circa 100 MHz carrier; your screen shot shows 100 kHz on the antenna. That is miles off. Commented Jan 8, 2023 at 16:19

I'm answering this question because it's easier than leaving any more comments: -

Why doesn't this FM receiver work in LTSpice?

You haven't done anything that suggests you can make that statement because, it looks to me like it should work on paper. For instance, you originally used a carrier frequency of 10 kHz when, clearly stated in the linked article in your question; the carrier frequency is circa 100 MHz.

But, you can't just feed a pure carrier into the receiver antenna input and expect to see some audio signal on the demodulated output. Your carrier needs to be modulated like this: -

• trace_A should be the unmodulated carrier at 100 MHz
• trace_B is the modulating signal (typically 1 kHz)
• trace_D is the modulated carrier

Trace_C is irrelevant to frequency modulation so ignore it.

A signal like trace_D is what needs feeding into your receiver. If the carrier is 100 MHz then maybe the modulation signal moves this up to 100.1 MHz and down to 99.9 MHz by the act of modulation.

You don't appear to have a signal that modulates the carrier. You need one or, the output will flat-line as you are currently seeing.