[R-390] R-390A actual S/N measurement values

[Larry H]

I've know for a very long time that the balanced input impedance of the R-390A is around 125 ohms and varies depending on frequency.  Thus an impedance adapter is required when doing sensitivity measurements so that the meter on the signal generator will read the correct value.  The adapter specified in the TM11-5820-358-35 Army tech manual is the DA-121/u that is part of the MK-288/URM maintenance kit.  It is a 68 ohm resistor across the signal line to ground and a 100 ohm resistor is then in series with the signal line to the RX.  This 'L' pad approach helps keep the load on the signal generator close to the required 50 ohms for reading correctness for different frequencies.  Although not quite perfect, it does reduce the impedance mismatch good enough for close readings.

However, anytime a resistor is placed in series in the signal line, loss will occur.  In this case it is about 55% (about 9.1 db).  So, I've not seen anything about adjusting the sensitivity numbers to account for this loss.  When using this adapter, if your reading is 2 uv, then the real sensitivity is less than 1 uv.  Is this not important?  Comments please.

As a test, I made a simpler adapter, an 82 ohm resistor across the signal to ground.  125 ohms and this also makes the impedance of 50 ohms, but with no loss.  It seems to work quite well as the readings I get on it are the same as the DA-121 'L' pad after mathematical adjustment.

comments please.

Regards, Larry Haney