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From: john larkin <JL@gct.com>
Newsgroups: sci.electronics.design
Subject: Re: Grounded grid VHF front-end
Date: Sun, 10 Nov 2024 10:09:22 -0800
Organization: A noiseless patient Spider
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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.