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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 07:47:32 -0800
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On Sun, 10 Nov 2024 14:59:29 +0000, piglet <erichpwagner@hotmail.com>
wrote:

>On 09/11/2024 22:52, Phil Hobbs 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.
>> 
>> Cheers
>> 
>> Phil Hobbs
>> 
>> 
>
>Remarkably commutating switch mixer like diode rings etc can be made 
>almost lossless if the source and load impedances are manipulated right. 
>British ham Peter Martinez the inventor of varicode psk31 has shown that 
>if the mixer RF port is fed from a parallel tuned circuit and IF port 
>loaded with a series tuned circuit in the right ratio then the 6dB loss 
>always assumed to be unavoidable goes.
>
>piglet

If a mixer is just made of switches (diodes, phemts, relays) there is
no loss mechanism.

You do get sum and difference outputs, but any single mixer does that.