[R-390] Measuring tangential sensitivity

Mark Huss mhuss1 at bellatlantic.net
Sat Mar 17 19:38:18 EST 2007 

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
>>>
>>> _____________________________________________________________
>>> R-390 mailing list
>>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>>> Help: http://mailman.qth.net/faq.htm
>>> Post: mailto:R-390 at mailman.qth.net
>>> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>>>
>>>
>>>       
>> --
>> ----------------------------------------------------------------------
>> 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.
>> ----------------------------------------------------------------------
>> _____________________________________________________________
>> R-390 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/r-390
>> Help: http://mailman.qth.net/faq.htm
>> Post: mailto:R-390 at mailman.qth.net
>> Unsubscribe: http://mailman.qth.net/mailman/options/r-390
>>
>>
>>
>>
>>
>>     
>
>
>
>
>   


-- 
----------------------------------------------------------------------
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.
----------------------------------------------------------------------

More information about the R-390 mailing list