[R-390] AN/URM-25s

[2002tii]

Bob wrote:

>So such extreme measures really aren't all that necessary to perform a
>high degree of adjustment and tweaking of equipment.

It depends on what one means by a "high degree of tweaking."  If that 
includes measuring the sensitivity of a very sensitive receiver, they 
most assuredly are.  But there is lots of "high degree tweaking" that 
has nothing to do with receiver sensitivity.

Note also that radar transmissions use very predictable, relatively 
low-duty-cycle pulses, so it is standard practice to use blanking 
techniques to eliminate their effect on measurements.

If you want to measure HF receiver sensitivity down in the vicinity 
of -140 dBm and get results that really reflect the ability of the 
equipment -- measurements good enough to put on a spec sheet and 
guarantee, as if you were the manufacturer -- you need to take 
extraordinary measures to ensure the integrity of your setup.  If 
someone says they used a URM-25D to measure the sensitivity of an 
R390A, without doing any of that, we know with certainty that the 
number they got is wildly optimistic.  That does not mean it is 
useless -- one may be able to adjust a receiver for maximum 
sensitivity even if one does not know the actual number (although 
adjusting for maximum sensitivity is not what one would normally want 
to do), and one can compare results with one's previous tests or with 
others who used similar test setups (although leakage is very 
unpredictable, so this is of limited usefulness).  Just like a 
butcher selling meat by the pound -- it is less important that his 
scale is accurate to milligrams than that it is repeatable, so 
everyone pays the same, time after time.

But if one wants to say that one's receiver has an MDS, or 
sensitivity in uV or dBm, or Sinad of xxx, one DOES need to get all 
of the measurement errors down significantly below the quantity being 
measured.  And when that quantity is -140 dBm, that is a very 
exacting standard to meet and does require extraordinary measures.

Best regards,

Don