Small VHF crystal-controlled audio transmitter

[KEØBRS]

This is my version of a "small" crystal-controlled voice transmitter. It uses 5-transistor array chip, otherwise, it could easily be made with individual components. It transmits at around 166.900MHz using a 50MHz crystal. The input audio jack (for the microphone) is simply attached with the blue-white twisted pair and uses the "matched pair" of the array as a preamplifier. The signal is then fed into the oscillator section--a crystal and varactor diode. Then the RF signal is amplified by a set of multiplier tank circuit and a final buffer.
Power is supplied by a 9 volt battery. The green line is a simple wire antenna. A short piece of RG174 could also be used to a SMA jack is you want to get fancy. The output is around 100mW.
The board was etching using toner-transfer/iron-on process.

It was a fun experiment.

Pete
KEØBRS

 

[betwixt]

Very nice but how come it has a 50MHz crystal but transmits on an unrelated frequency. 166.9MHz isn't harmonically related or are you using one transistor as a mixer with a different oscillator?

Brian.

 

[D.A.(Tony)Stewart]

Could it be a clever circuit using harmonic injection locking the 3rd harmonic of the VCO with the 10th harmonic of 50MHz pulse 1ns wide? nah

 

[KEØBRS]

I am absolutely not sure why the strongest output it at this frequency. It also has a good signal at 150MHz--which makes perfect sense. But 166.9xx gave me an even clearer audio signal.
I'm just a simple radio hobby mechanic, so I couldn't figure this one out if I tried. But I suspect one reason is the 50MHz crystal I plugged in might already work on an "overtone" and I amplified some spurious harmonic. Yeeeaaahh...

 

[betwixt]

I think SunnySkyguy found the right solution to the puzzle. 50MHz 3rd overtone has a fundamental at 16.667MHz, what you are hearing is the 10th harmonic at 166.67MHz +/- some pulling due to crystal loading effects. If he is right, you should be able to hear signals at 16.667MHz as well, if they are there at all it proves the theory.

Brian.
(aka. GW6BWX)

 

[KEØBRS]

The transmitter also puts out a strong signal on my scanner at 150.215MHz. My cheap (GOOIT, Chinese brand) frequency counter shows a strong signal at 100.145MHz. To be fair, this crystal was off eBay sent from HongKong and probably was never "to spec". But this might explain why the 3rd overtone is also slightly higher than calculated. You guys are great puzzle-solvers.