[R-390] R-390 sensitivity measurements

[Lester.veenstra K1YCM]

One unexplored problem with all the signal generator based sensitivity 
measurements is that they are dependent on a flat bandpass response for 
the results to be meaningful. For example. take a 4 kHz filter with 2 dB 
od pass band ripple. In terms of uV, a two dB difference in measured 
performance, what would appear to be a significant difference, as 
possible on the same receiver, as a function of where in the pass band 
the signal generator is placed.

The solution, although not the most practical in most shops, is to go to 
a noise generator based system, which will be independent of both IF 
bandwidth in which the measurement is made and independent of any pass 
band ripple.

     Les Veenstra
     K1YCM/3
      NNN0HWW

Bob Camp wrote:

> Hi
>
> As with many things, what matters the most in sensitivity measurements 
> is that you define how you got your numbers.
>
> There are a number of ways to change the setup that will impact the 
> result that you get. That's neither good or bad in it's self. What is 
> a problem is a undefined method. If I use method A and you use method 
> B we really can't be sure that the results are comparable to each 
> other. If we each have fully characterized and defined (no small task) 
> our methods then we may be able to guess at a comparison.
>
> A lot of restoring these radios comes down to "I got 0.001 uv on all 
> bands, time to tell the world".  I think that has lead to a number of 
> conversations about measurement technique ....
>
> A far more interesting question - does *any* single sensitivity number 
> actually tell what's going on in the real world? I would claim that 
> the answer is no and that is the root of the problem. At that point 
> (no one number)  we more or less divide into those who give up on 
> numbers and those who measure a whole bunch of numbers. That makes for 
> interesting conversations ....
>
>     Take Care!
>     
>         Bob Camp
>         KB8TQ
>
>
>
>
> On Jan 16, 2005, at 10:46 PM, DJED1 at aol.com wrote:
>
>> Here's another bit of trivia which I hope will entertain.   In 
>> looking at
>> Dallas' other postings, he discusses a method of measuring receiver 
>> sensitivity.
>>  From my view, I'm not sure there is an absolute correct way, but 
>> just for
>> fun I looked up the MIL spec for the R-390 (MIL-R-13947B) to see what 
>> the test
>> requirement was.   It's pretty close, but not exactly what Dallas 
>> discussed.
>> The spec has two tests: AM and CW.   The AM is performed by turning the
>> generator modulation on and off, with 30% at 400 cycles modulation.   
>> The signal
>> level is set for 10 mw audio out with modulation, and 1 mw with no 
>> modulation.
>> The only difference from his test is that it is done at 8 Kc 
>> bandwidth, and a
>> 125 ohm matching resistor is in series between the generator and the 
>> receiver.
>>   You still need to do the correction for the generator reading to 
>> get the
>> voltage at the receiver.
>> For the CW case, the setup is the same, except the generator is 
>> unmodulated,
>> where signal is carrier on and no signal is carrier off (still in the 
>> 8 Kc
>> bandwidth).    The AM spec is 3.3 uv except in the 16-32 bnd, where 
>> it is 4.4 uv.
>>   The CW spec is 1 uv.
>> This, I believe, was the final spec for the R-390 and was used for 
>> acceptance
>> testing.   I never did much like the concept of turning the 
>> modulation on and
>> off, but this gives that approach significant legitimacy.
>> Ed
>>
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