Re: Valve frequency multipliers

legg <legg@nospam.magma.ca> wrote:

On Sun, 9 Feb 2025 10:23:28 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 

legg <legg@nospam.magma.ca> wrote:
>
>
[...]

 
I'd be more selective of the crystal frequency. Two triplers might get
you there with a lot less grief.

>
You were right: with a crystal frequency of 16.656 Mc/s and two triplers
the results are much better.
>
New block diagram:
<http://www.poppyrecords.co.uk/other/Transceiver/BlockDiag6c.gif>
>
New oscillator circuit:
<http://www.poppyrecords.co.uk/other/Transceiver/XtalOsc4c.gif>
>
The stages are: EF91 Reactance, EF91 Oscillator, ECC91 Tripler+ tripler.
>
The new crystal necessitated changes to the component values in the
'pulling' circuits, but the basic concept remain unchanged.   Over large
frequency swings the reactance valve characteristic appears curved but
with 0dBm audio input at 400 c/s  there is no audible distortion and the
frequency swing is around 15 Kc/s pk/pk, which is three times larger
than the permitted deviation in the amateur bands.
>
I tried putting a parallel-tuned circuit in the anode circuit of the
oscillator, to resonate at the third harmonic, but there was very little
signal, so I decided to use one triode of the ECC91 as the first
tripler.  I then resonated the anode tuned circuit of the oscillator at
crystal frequency but discovered that this made it unreliable at
start-up.  Eventually I found that just an aperiodic [untuned] anode
choke gave plenty of drive to the grid of the first tripler and allowed
the oscillator to start reliably.
>
The parallel-tuned circuit of the first tripler and series-tuned circuit
between the second tripler and the output cable both have comfortably
gentle tuning, which is an advantage because they should hold their
settings without adjustment for a long time.  The output voltage is only
around 300mV rms into 75 ohms, but this can be stepped up when it gets
to the mixer grids by a further resonant circuit.

 
You seem to have got all your ducks in a row.

I'm beginning to have my doubts now: a search receiver suggests that
something funny is going on at about 200 Mc/s, but I can't work out
which stage is generating it.

>

 

Tripling within the oscillator seems to have been pretty popular
back in the day. I'm not sure what kind of stress they were putting
on the crystal to do this, but they wanted >5V from a substituted
VCO to get similar downstream responses.

I think you have to make the oscillator work very hard to get much third
harmonic and most crystals in amateur gear were around 7 Mc/s or lower.
At16 Mc/s things begin to get much more difficult.  My main concern was
getting the crystal to respond reliably to the pulling and frequency
modulation, even if it meant losing the tripler effect.


--
~ Liz Tuddenham ~
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www.poppyrecords.co.uk