[R-390] Measuring tangential sensitivity

Sun Mar 18 12:48:50 EST 2007

Mark,

In the 75S-3, the IF gain sets the resistance in the cathode of the 1st IF
amp.  It has two stages of IF amplification.  I guess as you say the best
thing is to try it and see what happens.  I have all the gear; HP 606B SG,
TEK 465 scope, and a BK 3011 FG.

The next time I have my R-390 on the bench, I will try it on that one too.
I'm getting too old to take the R-390 out of the rack and move it to the
bench unless it needs service.

David

> -----Original Message-----
> From: Mark Huss [mailto:mhuss1 at bellatlantic.net]
> Sent: Saturday, March 17, 2007 8:38 PM
> To: dhallam at rapidsys.com
> Cc: r-390 at mailman.qth.net
> Subject: Re: [R-390] Measuring tangential sensitivity
>
>
> What this type measurement is originally developed for was for Radio
> Astronomy (if what I was told as a tyke is true.  Might be since we
> traditionally had connections to Millstone/Haystack Radio Astronomy
> Observatory up the road in Westford) for measuring improvements in
> system noise.  Someone in ASA had the bright idea to use it with the
> R-390'a, and later the R-390A's.  Unlike most HF receivers, you can
> adjust the system noise in the receiver, only because the stage
> noise/gain for the R-390 and A is set in the IF Deck rather than the
> normal first RF stage.  This was either a deliberate design choice by
> Collins, or a byproduct of making the R-390 and R-390A suitable for
> Diversity reception by allowing the system gain to be adjusted to match
> the other receiver.  Collins did this by allowing the IF Stage Gain to
> be adjusted by changing the bias on the first and second IF amplifiers
> in the R-390, and the third IF amplifier in the R-390A.  All things
> being equal, the stage gain/noise for each stage of an R-390 and R-390A
> result in the 455 kHz IF setting the noise floor of the R-390 and R-390A
> when set according to the manual unless you have a really noisy RF/Mixer
> tube(s).  When we did it in the shops, over half showed improvement, I
> guess.  Some a little, some a lot.  I can't remember specific figures.
> The R-390A I have shows about an eight to ten dB improvement, enough
> that leakage around the attenuator messes up the normal S+N/N measurement.
>
> Stage Gain/Noise is simply the gain of the amplifier stage compared to
> the noise the amplification stage produces.  This is the noise
> temperature of the stage divided by the sum of the gain of all previous
> stages.  See http://www.veron.nl/amrad/art/sysnoise.pdf if you need more
> info.  Usually the first RF stage of a receiver has a lot of gain, so it
> swamps the noise temperature of the following stages.  The R-390 really
> does not have much gain at all through the RF and Mixer stages, I guess
> (never measured them), or else the IF stages are very noisy.
>
> Does the Collins 75S-3 have the same stage gain/noise parameters as the
> R-390?  I don't know.  I do know the Hammarlund SP-600 has a similar
> setup, but adjusting the IF Gain does not help the noise floor.  The RF
> Amp in the SP-600 is notoriously noisy, so much so that in the early
> 60's the Air Force bought a bunch of plug-in modules to replace the
> first RF tube with a cascode triode circuit.
>
> Besides, it is simple enough to try out.  Make sure you have the proper
> equipment and procedure to set the 75S-3 IF Gain pot.  Do a measurement
> of the S+N/N as it is.  Then try the technique.  If you get results,
> check that the S+N/N has indeed improved.  If it doesn't work, you can
> always return everything to normal.
>
> Usually, worrying about dropping the receiver noise figure in an HF
> receiver doesn't pay below about 20 MHz.  And for most high-level
> receiver, below 50 MHz.  Because, lets face it, atmospheric noise is
> going to massively overpower the receiver noise unless you are working
> with very narrow bandwidths.  Latest figures I have access to show a
> atmospheric noise level on average of -131 dBm at 400 hz bandwidth in a
> rural location (greater than ten kilometers from a population center).
> Suburban locations are around -110 dBm, and urban is a whopping -90
> dBm!  And these are daytime figures.  When the skip comes in, these
> noise figures increase dramatically.  Since your R-390A runs about -137
> dBm noise floor, even a rural location will be sufficient unless you are
> working with small, low-gain antennas.
>
> Of course, the previous paragraph is pretty irrelevant as far as most of
> us are concerned.  We are in it to make our R-390's perform well above
> the originals.  Sort of like who in their right mind would put an 1100
> cu. in. Ford GAA Tank Engine in a 1970 Mustang?   Sure, you can boost it
> up to about 1400 HP with stock parts, but you got to cut a hole in the
> hood big enough for the cylinder heads, valve covers, dual distributors,
> and an air cleaner that is higher than the roof!  Not exactly something
> you want your son to take his Drivers Test in :-)   (OK, maybe you would)
>
> David C. Hallam wrote:
> > Mark,
> >
> > Do I assume this method will also work on the Collins ham band receivers
> > that have an IF gain adjustment such as the 75S-3 series?
> >
> > David
> > KC2JD/4
> >
> >> -----Original Message-----
> >> From: r-390-bounces at mailman.qth.net
> >> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Mark Huss
> >> Sent: Tuesday, March 13, 2007 8:56 AM
> >> To: Bill Hawkins
> >> Cc: r-390 at mailman.qth.net
> >> Subject: Re: [R-390] Measuring tangential sensitivity
> >>
> >>
> >> Bill, you are right. It's most common use is to test the
> Signal to noise
> >> ratio of video and wideband amplifiers and detectors. It is
> also used in
> >> Cable TV Amplifiers. And in Radio Telescopes. It is not often
> used in HF
> >> receivers because there is no way to really adjust the system noise
> >> floor in them. The R-390 and R-390A, with their IF Gain
> adjustment which
> >> not only adjusts the total receiver gain for matching
> purposes, but also
> >> has a large effect on the receiver noise floor, are exceptions to
> >> this rule.
> >> I first ran across this technique at Capitol Cable in Austin
> back around
> >> 1972. I was reintroduced to it again when being trained to repair
> >> R-390's in ASA School. And later when doing VHF/UHF and Satcom systems.
> >>
> >> Before you reject it out of hand, give it a shot. The only
> thing you are
> >> adjusting is the IF Gain pot. It is easy enough to adjust it
> back if you
> >> find it does not help your receivers noise floor.
> >>
> >> Bill Hawkins wrote:
> >>
> >>> Roy,
> >>>
> >>> Seems like I was off the mark. As is usual, one should do some
> >>> research before hitting the keyboard, unless one's intent is to
> >>> liven things up a bit.
> >>>
> >>> >From a blurb for Agilent wide range laboratory detectors:
> >>>
> >>> "Tangential sensitivity is the lowest input signal power level
> >>> for which the detector will have an 8 dB signal-to-noise ratio
> >>> at the output of a test video amplifier. Test amplifier gain
> >>> is not relevant because it applies to both signal and noise.
> >>> Agilent detectors are designed for optimal flatness and SWR."
> >>>
> >>> Since R-390s do not contain video detectors, I let the matter drop.
> >>>
> >>> Regards,
> >>> Bill Hawkins
> >>>
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> >
> >
> >
> >
> >
>
>
> --
> ----------------------------------------------------------------------
> Some people are like a Slinky .. not really good for anything,
> but you still can’t help but smile when you shove them down the stairs.
> ----------------------------------------------------------------------
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