[R-390] R-390A dB meter as as "S" meter

[2002tii]

David wrote:

>How do you calibrate your S meter.  I was trying to that on a 75S-3 and
>gave up other than to just set the IF gain to give a S9 reading when the
>meter on my HP-606B read 50 uV.
>
>Since the 606B meter reading are accurate only when working into a 50
>ohm load, what load is the input circuit of the receiver?  Is it the
>same for all bands?  If you use a 50 ohm terminator for the signal
>generator, do you put it at the signal generator end of the coax
>connector or at the receiver end.  What is the impedance at the end of
>the piece of coax that connects the receiver to the signal generator?

No, not all receivers have 50 ohm inputs, although there is a modern 
trend toward 50 ohms.  One would typically get this information from 
the manufacturer's spec sheet, but it can be measured with an RF 
impedance bridge if you don't have the manual or if you have reason 
to believe the manufacturer's figure is not accurate.

Since the "S" scale is a power scale, one would ideally use a 
transformer to match the generator to the actual rx input 
impedance.  -73 dBm (50.2 uV) at the meter would then be S9 for all 
radios, regardless of the voltage at the transformer secondary.  In 
actual practice, it is more common to build a resistive pad to 
transform the impedance.  In this case, one must figure the insertion 
loss into the determination.  In either case, one would use 50 ohm 
cable from the generator, and put the transformer or pad right at the 
radio antenna input terminals.

The balanced input of the R-390A is specified as "125 ohm terminating 
impedance."  The impedance of the unbalanced input is not 
specified.  The specified procedure for measuring sensitivity 
(NAVSHIPS 0967-063-2010 pp. 4-2 and 4-3) involves connecting an 
AN/URM-25( ) generator to the unbalanced input and adjusting the 
generator output level for 10 dB quieting of a 30% modulated signal 
at 12 specified receive frequencies.  The generator level should be 
less than 3 uV for 10 dB quieting.  (Per Table 1-1, the sensitivity 
spec is 5 uV for 10 dB quieting on AM signals; which input is not specified.)

According to the NAVSHIPS manual ((table 5-5, step 17), the R-390A 
carrier level meter should read at least "40" with 10 uV from an 
AN/URM-25( ) connected to the unbalanced input, and "60" +/- 2 with 
100 uV, measured at 15 MHz.

Best regards,

Don