Simple RF transmitter Audio Amp interference

[eduardoarnoldh]

I am trying to implement **broken link removed** design of a simple RF transmitter.

I have successfully mounted the oscillator and was able to make it work with low frequency drift on the FM range. I have only changed the transistor to an 2N3904, and adjusted the bias resistors, since that ZTX is not available anymore AFAIK.

I also mounted the audio pre-amp and when tested without energizing the oscillator, by using the probe on the buffer output, it worked fine.

The problem resides in energizing the oscillator, checking the buffer output, even removing the connection from the buffer to the transistor, there is a lot of oscillator signal interference, the audio signal is completely lost. I have tried increasing the cap C5 to try and reduce high-frequency interference (as it is the case), but it did not produce significant change, still the audio signal unrecognizable among the interference.

As far as I understand this is the reason why I can't hear anything but noise when I tune my FM radio on the frequency of oscillation. To ensure that the basic design was working I tried using a signal source to produce a sinusoidal signal and couple it through cap C6. I could hear, although with some noise, the sinusoidal wave on the receiver. So I guess the problem is the interference on the amplifier that shades any audio signal I have on the mic.

It makes me wonder how the circuit from that schematic would work with this kind of interference. Does anyone have a suggestion on how to proceed in this case?

Thanks in advance,
Eduardo.

 

[jiripolivka]

I am trying to implement **broken link removed** design of a simple RF transmitter.

I have successfully mounted the oscillator and was able to make it work with low frequency drift on the FM range. I have only changed the transistor to an 2N3904, and adjusted the bias resistors, since that ZTX is not available anymore AFAIK.

I also mounted the audio pre-amp and when tested without energizing the oscillator, by using the probe on the buffer output, it worked fine.

The problem resides in energizing the oscillator, checking the buffer output, even removing the connection from the buffer to the transistor, there is a lot of oscillator signal interference, the audio signal is completely lost. I have tried increasing the cap C5 to try and reduce high-frequency interference (as it is the case), but it did not produce significant change, still the audio signal unrecognizable among the interference.

As far as I understand this is the reason why I can't hear anything but noise when I tune my FM radio on the frequency of oscillation. To ensure that the basic design was working I tried using a signal source to produce a sinusoidal signal and couple it through cap C6. I could hear, although with some noise, the sinusoidal wave on the receiver. So I guess the problem is the interference on the amplifier that shades any audio signal I have on the mic.

It makes me wonder how the circuit from that schematic would work with this kind of interference. Does anyone have a suggestion on how to proceed in this case?

Thanks in advance,
Eduardo.

I do not see your schematic. The basic problem lays in that most transistors used in audio amplifiers can and do respond to VHF and UHF. What you should do is to adjust the audio amplifier response to only amplify audio and not RF, and block the RF access to audio input.
The audio amplifier must be designed as a low-pass filter blocking or rejecting any signal frequency above 20 kHz. This is easy, you can add a 500 pF capacitor between base and collector of the audio amplifier transistor.
The RF access to audio input can be blocked in two ways: RF output from your transmitter may only go to the antenna by a coxial cable and connector, and the trasmitter should be located in a metal case with DC and audio capacitive feedthroughs. Audio amplifier section should be located separately from the transmitter, you can also use a metal enclosure to block interference.

If you can look at professional products and how they are designed.

 

[eduardoarnoldh]

I do not see your schematic. The basic problem lays in that most transistors used in audio amplifiers can and do respond to VHF and UHF. What you should do is to adjust the audio amplifier response to only amplify audio and not RF, and block the RF access to audio input.
The audio amplifier must be designed as a low-pass filter blocking or rejecting any signal frequency above 20 kHz. This is easy, you can add a 500 pF capacitor between base and collector of the audio amplifier transistor.
The RF access to audio input can be blocked in two ways: RF output from your transmitter may only go to the antenna by a coxial cable and connector, and the trasmitter should be located in a metal case with DC and audio capacitive feedthroughs. Audio amplifier section should be located separately from the transmitter, you can also use a metal enclosure to block interference.

If you can look at professional products and how they are designed.

Thanks for the reply, the link for the schematic is here:
**broken link removed**

I had turned it into a Hyperlink within the original post.

I am not using a transistor to do the audio amplifier, but an ampop, LM358. I don't think it should amplify in the RF frequency range since it has a unity frequency of about 1 MHz. The problem must be the wires then, but still, I am using capacitor C5 to filter high-frequency signals, but it does not seem to be enough.

This transmitter is supposed to be really simple and I have seen many implementations of similar designs in PCBs without metal insulation for RF interference. I guess there has got to be something I am missing.

 

[jiripolivka]

Thanks for the reply, the link for the schematic is here:
**broken link removed**

I had turned it into a Hyperlink within the original post.

I am not using a transistor to do the audio amplifier, but an ampop, LM358. I don't think it should amplify in the RF frequency range since it has a unity frequency of about 1 MHz. The problem must be the wires then, but still, I am using capacitor C5 to filter high-frequency signals, but it does not seem to be enough.

This transmitter is supposed to be really simple and I have seen many implementations of similar designs in PCBs without metal insulation for RF interference. I guess there has got to be something I am missing.

Thanks for the schematic! I think I would add a 500 pF capacitor across R4 to reduce audio bandwidth, and a 0.1 uF ceramic cap across C4 for a better DC blocking.

I would still enclose the RF oscillator in a metal enclosure, and use a coax connector and cable tp feed the antenna, so no RF can enter the audio section. Also take care to use a good coax cable from the microphone to audio input. Opamps do not amplify RF but their inputs can detect RF and shift their operation point.

 

[E-design]

You might be completely over deviating (modulating) the oscillator so that it sounds like just crackle on an FM receiver. Play around with injecting a sine wave externally and varying the input amplitude. It takes a very little audio input level to deviate the carrier by 75kHz.

 

[chuckey]

The gain on the audio side is 1, so you are applying very little audio to the oscillator, just the output from the mic. Try putting 10K in series with C2. If your FM receiver "hisses" when it is not tuned to a station, then when it is tuned to your transmitter it should go quiet. If it does not hiss, then it has inter station muting and all you will get is a plop as you tune through the carrier.
The circuit generates more AM then FM.
Frank