RE: Low receiver sensitivity

Posted by Ron D'Eau Claire-2 on May 12, 2005; 7:24am
URL: http://elecraft.85.s1.nabble.com/Low-receiver-sensitivity-tp378175p378178.html

Bart, PA3GYU wrote:

>Look up "Receiver Sensitivity Testing" (Page 4 of the XG1 manual) and
>"Signal-to-Noise and MDS Calculations (page 5) for instructions on how
>to use the XG1 to measure receiver sensitivity.
I did, and found this procedure and example:

>A. Divide S+N by N; call the resulting ratio R.
>B. Take the base-10 logarithm of R ("log" key on most calculators). C.
>Multiply the result by 20 to obtain the S+N/N ratio at 1 microvolt, in
>dB. D. If the S+N/N is greater than 10 dB, then the MDS is
>approximately equal to the result from (C) subtracted from -107 dBm.
>
>Example: DMM readings of 1.0 Vrms (XG1 on), and 0.030 Vrms (XG1 off).
>
>A. R = 1.0/.03 = 33
>B. log(30) = 1.52
>C. 20 x 1.48 = about 30 dB (this meets the requirement for step D) D.
>MDS = -107 dBm - 30 dB = -137 dBm

Two remarks:
1. I think the second step in the example should read: "B. log(10) = 1.52".
2. I may be missing something here, but if the outcome of B) equals 1.52,
why is "1.48" used in C)? (Not that it changes much, only 0.8 dB, but hey:
it might confuse some people!)

----------------------------------

Sorry for the delay responding Bart! I've been away all day and I see that
none of our other sharp-eyed contributors have answered your question!

Yes, you are right. B. should read log(33) = 1.52 (the base 10 is
"understood" unless a different base is shown and, if so, the base is shown
as a subscript following log, not in parenthesis). The procedure is to take
the result of A. and find the logarithm of that.

I'm really confused about where the 1.48 came from!! That should be 1.52 so
20 x 1.52 = 30 dB.

That instruction sheet was not one if my projects, so I never looked at that
example closely. I'll pass this info along to Wayne.

Ron AC7AC



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