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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: Valve frequency multipliers
Date: Mon, 10 Feb 2025 19:26:15 +0000
Organization: Poppy Records
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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 ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk