Identifying Oscillator Topology

[rossco_50]

Hi,

There have been are a number of retailers selling cheap (£5) FM wireless headphones in the UK. There is a transmission unit into which you plug your source, and a scanning fm radio in the headset (transmission seems to be around 86 Mhz). This is the sort of thing **broken link removed**

I was surprised (others might not be) to open the transmitter and see that the circuit had no crystal; the performance and reliability of the wireless is quite good. At the heart of it seems to be something resembling a colpitts oscillator resembling many of the 'bug' circuits you can find on the web. Although I don't fully understand the buffer / attenuator / bandwidth filter at the antenna, I am quite confident that is what it is - I imagine the arrangement is ensuring short range fm regulations are met in terms of power output.

However, the audio input is split so that it is fed directly into the oscillator and is also feeding another 4 transistor arrangement which appears to eventually feed into the tank circuit.

I have attached a rough outline (I know there will be mistakes, but hopefully close enough for the expert eye) of what I could determine from the circuit. Could anyone explain or provide a further reading reference that would help understand the arrangement of 4 transistors outwith the main oscillator? Is it for stability to meet regulations or overcome the frequency drift of simple 2 transistor transmitters?

Can't find anything online or in the texts I have that would explain this multiple audio paths into the oscillator arrangement.

Thanks,

Ross

View attachment transmitter3.pdf

 

[E-design]

Q1 and Q2 are audio amplification stages. Q3 is some sort of peak detector that connects to the modulation LED driver indicator Q4. Q5 is a common Colpitts oscillator arrangement fed into an RF buffer amp Q6 with a low-pass filter connected between the output and antenna to remove harmonics. Audio is combined into a mono signal and fed directly into the Colpitts for modulation. Output signal will contain significant components of both AM and FM modulation.

 

[rossco_50]

Thanks, that makes sense now. Looks like Q4 is also switching the battery supply to the oscillator on when an audio signal is detected. Would it be correct to say that the audio amplification of Q1 and Q2 is only to feed the audio signal detector to ensure it is triggered even at low signal levels? I don't see that any audio from this section is being fed into the oscillator.

I just didn't think a 2 transistor transmitter would be useable in a commercial audio product and therefore assumed they were using some kind of special stabilising circuit - seems not.

 

[biff44]

crystal oscillators are very stable, and you would not be able to FM modulate them hardly at all with your audio input, maybe a few KHz at most if you try hard.