Re: Grounded grid VHF front-end

On Sun, 10 Nov 2024 10:09:22 -0800, john larkin <JL@gct.com> wrote:

On Sun, 10 Nov 2024 16:03:25 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
>

john larkin <JL@gct.com> wrote:
>

On Sun, 10 Nov 2024 08:00:17 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 

john larkin <JL@gct.com> wrote:
>

On Sat, 9 Nov 2024 20:26:18 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 

Cursitor Doom <cd@notformail.com> wrote:
>

On Sat, 9 Nov 2024 19:27:13 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 

john larkin <JL@gct.com> wrote:
>

On Sat, 9 Nov 2024 16:35:45 +0000, liz@poppyrecords.invalid.invalid
(Liz Tuddenham) wrote:
 

My current receiving aerial system is very inefficient at 2 metres
(144 Mc/s) and I have thought about making a sleeve dipole for that
band.  My VHF receiver is an Eddystone 770R, which covers the band
but only in a small portion of the whole scale.  While I am
improvomg the aerial system, I could also make a crystal-controlled
down-converter, that would allow me to use an HF communications
receiver or the lower ranges of the 770R, so that the  band 2 Mc/s
wide would cover a much greater scale length.
>
It's been a few years since I designed anything with valves, so I
thought I might have a go at making a down-converter using valves -
but not necessarily the expensive 'cult' ones which everyone seems
to regard as having magical powers.  The EF91 is plentiful and
cheap as New Old Stock, so that seems like a good valve to start
playing about with.
>
The EF91 was used as an RF amplifier in the input stages of
television sets working at about 45 Mc/s, so it can't have too bad
a noise figure (although Mullard don't quote one in their data
sheet).  If I triode-strapped it and ran it in grounded grid mode,
that would reduce the noise and increase the maximum frequency it
could usefully amplify. From the data sheet, with 200v on anode and
grid 2 and an anode current of 6mA, the gm is about 6mA/V, which
gives an input impedance at the cathode of  160 ohms.  A 75-ohm
feeder could be matched to this with a Pi tank or by tapping the L
or the C of an input tumed circuit.
>
The voltage gain may not be as high in this configuration as in
grounded cathode mode, but it allows the valve to be triode
strapped for low noise without instability problems or the
dependence on neutralising that a cascode stage would have
(especially the need for correct neutralising to obtain the best
noise figure).  If I also use an EF91 as a mixer, I might need one
more stage of RF gain to get the signal up to a level where the
mixer noise is negligible - but this isn't such a bad thing because
it would allow extra tuned circuits to give better image rejection
and allow a lower output frquency if I wanted one.
>
Anyone with experience of doing something like this with valves?

 
 
How about a tube/valve XO and a diode mixer to start?
 
A good HF receiver may have a low enough noise figure that
atmospheric noise still dominates.

>
Good thinking but there are several snags with that system:
>
If the down-converter is at the aerial end of the feeder, the HF
receiver is almost certain to suffer from strong HF signals picked up
on the downlead.  If the down-converter is adjacent to the HF
receiver, there will be significant losses at VHF in the downlead, as
the aerial needs to be mounted as high as possible.
>
If there is no amplifier ahead of the mixing diode, the local
oscillator signal could be radiated by the aerial - especially if it
happens to lie at a frequency where the dipole has another resonance
or the dipole and downlead form a resonant system.
>
I was thinking in terms of the converter being right next to the
aerial (the sleeve dipole has a 'cold' bottom end and could be joined
directly onto the converter box).  The HT and LT could be supplied
either by a separate multi-core cable or by superimposing 40v A.C.  at
50c/s on the co-ax and feeding it into the 200-220-240v tappings.of a
mains transformer primary.  The full primary winding would act as an
auto-transformer to give 250v H.T. and the secondary could give 6.3v
or 12.6v to run the heaters.

 
This is really ham territory so I don't think JL - with all due respect
- will be able to assist you very much in this endeavour. However,
there should be tons of info on this in one of the old ARRL handbooks.
If you have any from the early 60s lying around it should be well worth
a look through.

>
I have read most of that sort of literature in the past and still have
copies of most of it but don't remember this particular approach being
used before - that was why I though it might make a good fun project.
There are some grounded-grid circuits but they use triodes intended for
the purpose.  There are cascode circuits with double (and sometimes two
single) triodes  but, again, the triodes are intended for that purpose.
The idea of using a bog-standard descendent of the ubiquitous EF50 for
frequencies it wasn't supposed to cover - and making it do that
adequately - appealed to me.
>
The only place I have come across anything like this is in Geoff
Woodburn's design for the Eddystone Panoramic Display Unit, where a
triode-strapped E180F is used as a grounded-grid untuned wideband
front-end amplifier.  I did copy that successfully with a ZTX450 as the
wideband front end of a noise-measuring set that I designed; it gave very
satisfactory results.

 
Seems to me that the lowest noise voltage gain - no noise in fact -
comes from a high-Q LC resonator. And that best drives a small
capacitance like a grid.
 
Driving a cathode can be wideband, but a cathode looks like a low
value resistor, a Q killer.

>
Series-tuned input circuit.

 
Or a parallel tank with taps. The Q killers are the radiation
resistance of the antenna and, a little bit, the ohmic component of
the grid impedance from electrons being ornery.

>
The point I was making about grounded-grid operation is that the input
impedance of the valve is very nearly the characteristic impedance of
the co-ax (voltage ratio 3:2 for a triode-strapped EF91 drawing 6mA from
a 200V HT line).   A Pi network or a 3:2 winding on a ferrite core could
be used to match them

>
A tuned circuit into the grid has voltage gain, but the grounded-grid
with ohmic matched impedance throws away at least half the available
signal voltage. Impedance matching isn't good when it throws away
signal.
>
It's the voltage difference between the grid and cathode that gets
amplified against the tube's inherent noise.
>
Of course you can never get a better s/n than what the antenna
provides, and that will be pretty bad, so working hard to get a very
low noise fig in a HF receiver is entertaining but not terribly
useful.
>
At some wavelengths, in the microwave, looking at things way overhead
with a very directional antenna, low noise figures are worth the
hassle. The effective temperature of the universe is low.
>
A very directional antenna is a big win on s/n. It ignores a lot of
junk. I don't think it improves the inherent thermal background if
it's receiving terrestrial transmitters. That would violate COE.
>
I wonder if one can tell the difference in thermal noise by aiming an
antenna north or south from the USA or Europe. It's certainly less
aiming up.
>
I guess a good antenna feeding a matched resistive load will heat up
the load; steal power from the BBC. Or aim up and cool it.

Under 5Mhz is where it gets particularly bad. 80m is often unusable.