[Collins] Collins 62S-1 Transverter Downconversion Gain??

[Dr. Gerald N. Johnson]

On 9/21/2010 5:21 PM, Carl wrote:
>
> ----- Original Message ----- From: "Dr. Gerald N. Johnson"
> <geraldj at weather.net>
> To: <collins at mailman.qth.net>
> Sent: Tuesday, September 21, 2010 1:11 PM
> Subject: Re: [Collins] Collins 62S-1 Transverter Downconversion Gain??
>
>
>>
>>
>> On 9/21/2010 9:32 AM, Carl wrote:
>>> In 1965 I modified a 75A4 in the shop at National to a 6GM6 RF and 7360
>>> mixer but havent found the test data yet. The improvement however was
>>> substantial on 10-15M and so was the signal handling. My primary
>>> operating
>>> was DX and contests on CW and SSB. Before that I had a Drake 2B which
>>> would
>>> fold up on 40M almost nightly.
>>
>> Mixers have always been the achilles tendon of receivers. Pentagrid
>> mixers as used in the A4 were about the poorest possible, lots of
>> inherent noise meant they needed plenty of RF stage gain for weak
>> signals and poor strong signal handling was made worse by that high RF
>> stage gain. The 7360 was so good a low noise and high dynamic range
>> mixer that it sometimes was used connected directly to the antenna. In a
>> class all by itself.
>
>
> Its a great tube and getting pricey but I put away a couple of NIB
> sleeves when they were almost free. The Squires Sanders SS-1R used that
> as the front end but it had problems with reactive antennas as found
> with ham installations.

Likely a result of maximizing gain with the resistive 50 ohm antenna and 
loosing gain when the antenna wasn't resonant and the front end didn't 
have any reserve gain or an antenna trimmer. Probably a low gain and low 
noise RF stage would have solved that at the cost of a little of the top 
end of the dynamic range. The beauty of the mixing part was that it was 
a true switcher by beam deflection, not something nonlinear that was 
stomped by strong signals. There have been some really good mixers 
invented in the last few years using logic switch chips, like used in 
the Softrock receivers.

> Some have tried regular TV versions but Ive not seen any conclusive data.

The TV type was used for the balanced modulator in the KWS-1 exciter 
brassboard I used to have. It seemed to do that job well.
>
>
> The Pullen mixer is another interesting circuit, Ive used it in several
> BA receivers with outstanding results. I might try it in the 75A3 when I
> get to recapping and overhauling it.
>
Cathode follower input stage, grounded grid mixer with the RF on the 
cathode driven by that input stage low output impedance with control 
grid injection. I suspect the mixer section would work even better if 
the grid impedance at RF was truly low, like as if it was also driven by 
a cathode follower or a low impedance link that effectively grounded the 
grid at the RF and IF frequencies.
>
>
>>> In 1980 I used a 5722 NF device to tweak the A4 and wound up with a
>>> 8.6dB
>>> system NF on 10M.
>>
>> The 5722 is nearly a primary standard of noise generation when the plate
>> current is emission limited, but very dependent on the quality of the
>> resistive termination at least below 400 MHz where capacitance and
>> resonances within the tube affect the noise level.
>>>
>>> In 2002 I bought a surplused HP 4970A NF meter from work and attacked
>>> the A4
>>> again resulting in a 6.8dB NF on 10M. Swapping the front end tubes
>>> with NOS
>>> didnt improve it by more than .2dB so I left the oldies in place. Id
>>> always
>>> wondered about the 5722 accuracy.
>>
>> How did you use the NF meter? most HP depend on a 30 or 140 MHz IF
>> frequency, not an audio output meter. And they look at noise levels
>> coming out of the stage under test to compute the gain. With gain in
>> hand they compute the RF stage NF ignoring the effects of its finite
>> gain and the generally higher NF of the following stages. So a stage
>> could easily show 6.8 dB NF while the system had 8.6 dB NF.
>
>
> The same way I used it at work when operating above its range, an
> external mixer and LO. Tap off the last IF, mix and filter, cal out the
> mixer/filter, and let it rip.

There you get another complication, the IF and RF bandwidth at HF is 
often narrower than that of the NF meter and the calibration for that is 
almost impossible because the NF meter figures input noise power for its 
4 MHz IF bandwidth.

Since a low noise 14 MHz RF stage I took to CSVHF last year measured 
poorer in gain and NF that I expected, I looked into coming up with a 
calibration correction. There isn't one. I found an article on the topic 
by the AIL guru of automatic NF meters and he came up with some 
experiments that worked for a few cases stuffing in some fudge factors.

Part of the rub is that the relative noise contribution by the stage 
being tested depends on whether the selectivity is before or after the 
stage. In a receiver, there's selectivity on both sides that makes the 
active device's noise power contribution different than that from the 
noise head to the active device. I concluded that the only way to 
measure a narrow RF or IF is the old fashioned way, adjusting the 5722 
current by adjusting its filament voltage for a 3 dB rise in IF or audio 
output, tossing the NF meter. Or you can use a Y factor, but you have to 
measure at the narrow IF which the NF meter won't do.
>
> Most of my work the last decade before retiring was from 5.6 to 75 GHz.
>
>
73, Jerry, K0CQ, Technical Adviser to the Collins RAdio Association.