2018-12-26 UHF Oscillator

I built an oscillator using the same circuit used for the FM wireless microphone. The difference is that the capacitors and inductors are much smaller in value and the 1 transistor is a 2SC3355 which can oscillate at several gigahertz. I did this to see how high a frequency I could attain with just discrete components, which are known as ‘lumped constants’. The oscillator has an RF detector which turns on a transistor that drives a LED. If the oscillator quits, the LED will not light. I plan to change the transistor to a socket so I can switch to various transistors to see how high I can get it to work with common transistors.

First a schematic, then a photo.

So far my measurements showed the L1 coil had 20 nanohenrys. The variable capacitor has from 5 down to a fraction of pF. But the transistor and circuit have stray capacitance, so I don’t know what the actual total capacitance is. But I set the variable capacitor at about 2.5 pF and it’s oscillating strongly so I’m guesstimating it’s over 600 MHz and might be closer to 700 MHz.

Update Dec 27 –

I found my Gooit frequency counter. It shows the frequency of 376 MHz. I didn’t add enough stray capacitance to my calculations so my frequency guesstimate was 600 MHz using a stray capacitance of 3 pF. The actual stray capacitance is 6 pF for a total of about 8.5 pF. So I will have to reduce the coil turns and lower the inductance from 21 to 11 nanohenrys. That should increase the frequency a lot.

I removed one turn from the coil so it has about 2.5 turns. I got 433 MHz, but I moved the variable capacitor slightly and it went up to 447 MHz. The 433 MHz band is where the remote controls operate, so I don’t think it would cause much interference. If my garage door opens I’ll know why! 😮😮😂😂

The resonance calculator shows that 16 nanohenrys and 8 picofarads gives 444 MHz, so the actual total values are in that range. I’m not sure how much of that is the stray capacitance and inductance. The big concern is since these are not controlled by the tuned circuit, how much those uncontrollable stray values change with temperature and voltage.

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