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From: liz@poppyrecords.invalid.invalid (Liz Tuddenham)
Newsgroups: sci.electronics.design
Subject: Re: Grounded grid VHF front-end
Date: Sun, 10 Nov 2024 08:00:17 +0000
Organization: Poppy Records
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Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

> Cursitor Doom <cd@notformail.com> wrote:
> > On Sat, 09 Nov 2024 12:21:41 -0800, john larkin <JL@gct.com> wrote:
> > 
> >> On Sat, 09 Nov 2024 20:02:05 +0000, 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 was never interested in rag chewing, but signals is still signals.
> > 
> > Indeed, but this is niche and there are so many fine points and
> > trade-offs and gotchas that need to be factored in that only a
> > dedicated VHF RF designer could assist here. For sure the best people
> > here could come up with a workable design, but in practice it would
> > stink for the above reasons. There's not a single person on this group
> > today who can really add any value here. Ham group, Liz; ham group.
> > 
> 
> 2 metres is pretty much DC nowadays anyhow. 
> 
> HF receivers don’t have to have good noise performance because the
> atmosphere is so noisy, and AFAICT they usually don’t.  Intermod is more of
> an issue. 
> 
> The atmosphere is quieter above 100 MHz, though, so you care more about the
> Rx noise figure. 
> 
> A mixer front end is going to have a noise figure of 6 dB or so, on account
> of the conversion loss, and that adds to the NF of the HF back end. 
> 
> Some gain ahead of the mixer, and some more following the band select
> filter should help a lot. Don’t overdo it, of course. 

Thanks, that was the path my thoughts were going down.  A grounded grid
stage on a 200v HT supply should have reasonable linearity according to
the data sheet.  I hope to get a voltage gain of x10 from it from it if
the anode circuit can be designed to be around 2 kilohms at resonance.
That should lift the signal above the noise of the mixer stage with a
bit to spare but still give good immunity to overloading.

With one tuned circuit at the front and a slightly-overcoupled bandpass
circuit to the mixer, the bandwidth over the band from 144 to 146 Mc/s
should be fairly flat.  Alternatively a bandpass troughline at the front
and a single tuned anode load would give the same bandwidth and better
out-of-band rejection ahead of the EF91.


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