Re: Valve frequency multipliers

Tauno Voipio <tauno.voipio@notused.fi.invalid> wrote:

On 9.2.2025 12.23, 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.
 
 

 
Are you sure that the crystal lets you pulll it by 11 kHz and add angle
modulation to it?

It pulls on receive and is modulated on transmit, the two never happen
together.  According to a frequency meter I am getting the correct shift
on receive and the correct centre frequency on transmit.  The
disadvantage of pulling a crystal is that it becomes less stable the
further you pull it.  This is less important when receiving, which is
where the largest 'pull' occurs.

As a modulation test, I connected the output of the multipliers to a
length of wire to act as a transmitting aerial and fed 400 c/s at 0dBm
into the audio input.  I then tuned an Eddystone 770R to 150 Mc/s and
connected it to a panoramic display unit, Eddystone EP14, which can be
adjusted so that the sidebands cover a marked width of the display
screen.  My accurate signal generator doesn't go up as far as 150 Mc/s.
so I set it to 75 Mc/s and used the second harmonic.   I adjusted the
F.M. modulation until its sidebands covered the same width as the VXO
signal, then read-off the deviation from the signal generator's scale.
Because of the effect of frequency doubling, the actual deviation was
twice the scale reading.

Please design and verify the 144 MHz transmit and receive filters
associated with the mixers for enough attenuation on the 150 MHz band.

I am nowhere near ready to make the power section of the transmitting
chain, but when I do, I shall be very careful to check it for spurious
emissions before using it with an aerial.

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk