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

[2002tii]

Ed wrote:

>If you look at the original Military tech manual, it has a curve of 
>meter reading vs signal level.  What the meter is supposed to read 
>is dB relative to 1 microvolt, just as the meter on the SP-600 
>does.  This means that 0 dB is 1 microvolt, and 40 dB is 100 microvolts.

Interesting.  Which manual are you referring to?  As I mentioned in 
another post, the manual I generally use (NAVSHIPS 0967-063-2010) 
says the 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.

One of the older manuals I have (TM-11-856A, published by the Army in 
1956) specifies carrier level readings only relative to whatever 
signal reads "20" (page 171).  The only adjustment is for meter zero 
(p. 164).  I didn't check all of the older manuals I have.

Unlike the NAVSHIPS manual, however, TM-11-856A specifies measuring 
AM sensitivity through the balanced antenna input using Electrical 
Dummy Load DA-121/U (see pp. 166, 173), and through the unbalanced 
input using Electrical Dummy Load DA-124/U and adapter UG-636A/U (p. 
173).  Schematic diagrams for the dummy loads can be found here:

http://www.tpub.com/content/electronicmeters/TB-SIG-319/TB-SIG-3190002.htm

DA-121 is a 50-to-125 ohm L-pad with 5 dB insertion loss (in addition 
to the 6 dB loss that the 50 ohm load causes from the 50 ohm source 
impedance of the generator, which is generally taken into account in 
the generator calibration), while DA-124 is a 50 ohm termination with 
a 56 pF capacitor coupling the generator to the radio 
input.  UG-636A/U is just a coax adapter.

Best regards,

Don