Rebuilding/downgrading a High Q Farnell sine wave generator to a simple circuit 100KC-12MC (VLOG 2)
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Rebuilding/downgrading a high Q Farnell sine wave generator to a simpe circuit 100KC - 10MC (VLOG 2). A schematic of a FET VFO is showed (at least for 6 MC-18 MC, more info in this description). Aim of the project: making a sine wave oscillator with a FET going from 100 KC-12 MC.
First VLOG is here • Rebuilding/downgrading a high Q Farnell si...
Update: BEST schematic of this oscillator is here (22 feb. 2023) • VLOG 6: Sine Wave oscillator 130 KC-8 MC a...
Pure waveforms from 130 KC tot surely 8 MC and even 12 MC.
After damaging the High Q Farnell professional sine wave oscillator during electronic experiments in the past year I decided to scrape out all the components of that Farnell generator and use the bare frame and glass-fiber board to make an HF oscillator for 100 KC-12 MC.
Perhaps with (variable, in terms of depth) an AM modulation circuit on 1000 Hertz, via a sine wave or a square wave.
I don’t know if I can get to 100 KC, I will try. Minimum frequency that surely can be reached with this simple setup is 300 KC.
Of course the dial tuning (in this new “hobby” setup) will not/cannot correspond any longer to the original frequencies on the Farnell dial. After scraping the copper clad side off and removing the components there was room to make a new build up.
Of course is downgrading…. On the other hand: the maximum frequency to be generated with my hobby circuit will be much higher, say to 12 MC (instead of the original max. 1 MC).
This hobby circuit will, of course, never be as stabile as the professional circuit. But that does not have to be a problem when you have some understanding about HF & electronics and know how to use this test oscillator.
This video/Second VLOG shows the first experiments in this remaking/downgrading the Farnell circuit. It is now no longer an R-C generator but an L-C generator.
In this video I show a FET-oscillator that was made to work between 6 MC and 16 MC. It was originally published (page 105) in my Dutch (=Dutch text) book “Retro Radio”, issued by Elektor International Media in the Netherlands in 2008. ISBN 978-90-5381-234-1. NUR 950.
The showed schematic in this video (with the 100 pF capacitor from Source to Ground, that is paralleling the coil of 260 windings, ape-hair, insulated copper wire, and with the 220 pF capacitor from the Source of the BF 256 A FET to the Gate) does NOT work on all frequencies. Because it was originally made for 6 MC-16 MC.
Thus especially the lower frequencies, say below 1 MC will give a problem.
Anyway: try & experiment. Some very high Q coils can work below 6 MC with this setup. And changing the value of the 200 pF capacitor to a higher value, say 470 pF or 1000 pF, will/can work too to get the circuit working below 6 MC.
The waveform purity & the oscillation (yes or no) depends also on the quality (Q factor) of the coil.
Tested today (6 Feb. 2023): with a 56 uH (Micro Henry) coil (factory made, ferrite core) bridged with the original Farnell tuning capacitor of 1000 pF (2 x 500 pF) it oscillates between 989 KC and 1800 KC (=1.8 MC).
When turning the tuning capacitor to a certain position (moving to 1000 pF) the oscillation stops suddenly. It means that the oscillation conditions are breached in this (specific) case when the tuning capacitor goes to a too high value, say 700-800 pF.
There is a FET oscillator circuit on my YT Channel with a much broader frequency band. It can work between 300 KC (even on lower frequencies) and 12 MC, also when the Q (quality factor) of the coil is not very high.
That (better) oscillator circuit (300 KC-12 MC) is on You Tube (named: the “test oscillator”):
Link: • FET test oscillator sine wave with very go...
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I was very surprised that many small signal N-FET’s did not want to work in this circuit. Only the BF245 (obsolete, no longer made..?) and the BF 256 A wanted to work.
Upload 6 February 2023. Perhaps more video’s on YT will follow, about further developments.