fm-am transmitter/receiver

[nwar1994]

I must be putting something in wrong.

 

[nwar1994]

demodulator works well

 

[RadioRon]

When I try it on the breadboard again, I'll show you my configuration. I must be putting something in wrong. I understand the breadboard is bad, but there should be ATLEAST some sound from the speaker correct?

Well, not necessarily. If the receiver is not oscillating you would get no sound at all. If the receiver is oscillating, then you should get no (or very little) sound until you tune the oscillation frequency to be very close to your transmitter frequency. I recommend that you confirm that the receiver is oscillating first. Not by checking it with a scope because the mere contact of the probe changes things a fair bit and might induce oscillation where there wasn't any. The better way to check if it is oscillating is to attach a bare wire to a spectrum analzyer to act as an antenna and then bring that wire next to your receiver. Then tune your variable capacitor and you should be able to see a pip on the spectrum analyzer moving left and right on the spectrum display.
I confirmed that mine was working simply by tuning across the FM broadcast band and hearing different channels as I varied my capacitor. This requires an antenna, but I just used an alligator clip lead and that worked fine as an antenna.

However, at the very least you should hear clicks or pops or static from the speaker if you touch a bare wire to the LM386 input.

 

[nwar1994]

The inductor is 4 turn 22 gauge. I think my air-core inductor may not be too exact, but I thought the var-cap should take care of that.

 

[RadioRon]

Verify that your oscillator is running and measure what frequency it is running at.

 

[audioguru]

Your large vari-caps look like they have 10 times too much capacitance. The perfboard has parts too far apart. The solderless breadboard has stray capacitance between the rows of contacts. Your long wires on every part have inductance. Then it will not work at 100MHz, maybe at 10MHz it might work.

 

[nwar1994]

My transmitter is working fine. Measuring the frequency of the LC tank on the receiver is tough. It's giving me sporadic values in the range of 98.7 - 180 MHz, then a random 50GHz value

 

[RadioRon]

Do you have a spectrum analyzer available?

 

[Nigel Goodwin]

My transmitter is working fine. Measuring the frequency of the LC tank on the receiver is tough. It's giving me sporadic values in the range of 98.7 - 180 MHz, then a random 50GHz value

I'm with AG, it's badly built with no regard for the high frequencies involved, and a breadboard is TOTALLY unsuitable for use at these frequencies.

I wouldn't expect it to work.

I don't know how you are expecting to measure the frequency?, but that's fraught with problems as well.

 

[JimB]

I am with AG and Nigel.

My transmitter is working fine.

Are you sure?

It's giving me sporadic values in the range of 98.7 - 180 MHz,

Correctly working transmitters do not give "sporadic values" across a 2:1 frequency range.

then a random 50GHz value

This thing has become surreal, 50 gigahertz, you are pulling our wires!

Have a look at this ancient thread:

https://www.electro-tech-online.com/threads/breadboards-how-bad-are-they-at-rf.109193/

JimB

 

[RadioRon]

My transmitter is working fine. Measuring the frequency of the LC tank on the receiver is tough. It's giving me sporadic values in the range of 98.7 - 180 MHz, then a random 50GHz value

I'm not clear on how you are measuring the frequency. What is "it" that is giving you sporadic values?

The other members are correct about your construction. The solderless breadboard is not appropriate for these high frequencies. I recommend building as shown in the post #98 photo.

 

[rjenkinsgb]

The other members are correct about your construction. The solderless breadboard is not appropriate for these high frequencies. I recommend building as shown in the post #98 photo.

Agreed. You can also use "dead bug" construction, that's fine for VHF - UHF if done carefully.
The PCB copper surface is ground, all other connections are in mid air, with the leads as short as practical.
eg.
**broken link removed**

 

[audioguru]

I recommend that you confirm that the receiver is oscillating first.

I laugh when it is said that this "Mickey Mouse" regen radio must oscillate when we were talking about poor construction causes an amplifier to oscillate.

However, at the very least you should hear clicks or pops or static from the speaker

Because this "Mickey Mouse" regen radio has an AM detector instead of an FM detector.

 

[nwar1994]

The receiver LC tank is the circuit that is causing issues, not the transmitter.

 

[unclejed613]

It's giving me sporadic values in the range of 98.7 - 180 MHz, then a random 50GHz value

exactly how are you measuring this? s 50Ghz frequency counter would be an impressive (and expen$ive) piece of equipment. the sporadic values between 98 and 180Mhz could be caused by false triggering, you really need a reasonably clean waveform for a frequency counter to trigger reliably.