[R-390] R-390 alignment questions

[John KA1XC]

Hi Miles,
technically you are correct, and this is one of the several ways of
measuring sensitivity that have been cussed and discussed here and on the
different R390A web pages around.

-Some folks want to terminate the sig gen for 50 ohms
-others want to terminate the sig gen AND match the RX antenna input for 120
ohms so they make a minimum loss matching pad, do the measurement, and then
add in the loss factor for the pad.
-others connect the 50 ohm sig gen directly to the RX.

I used to worry a lot about what is the proper "correct"  method, but in
reality a 390x balanced input is anything but 120 ohms; I believe it
actually rises to a few hundred ohms on some bands.

So now I just use the simplest method with a direct connection to the sig
gen and be done with it, and BTW also use the sig gen Modulation on/off
method when measuring the AM 10 db SN ratio. Both these are described in
Para.166 of the TM-856A 1956 Army manual (my fave) and give me R-390A AM
sensitivity values in the 0.4 to 0.5 uV range after a refurb and alignment.
My feeling it this technique gives the most "honest" sensitivity measurement
(or at least in the ballpark) and also aligns the receiver to a 50 ohm
impedance which is a standard ham shack value.

I suppose it would be cool if everybody could get on the same page with
these sensitivity measurements but I doubt that'll ever happen :^)

John



----- Original Message ----- 
From: <K2CBY at aol.com>
To: <r-390 at mailman.qth.net>
Sent: Wednesday, October 13, 2004 8:50 AM
Subject: [R-390] R-390 alignment questions


> You may be deceiving yourself with respect to the sensitivity measurement.
>
> You make no mention of terminating the 8640B signal generator.
>
> To get an accurate reading, the output attenuator of the 8640B (and any
> other signal generator) has to be terminated with a specific load
resistance -- 
> in this case, 50 ohms. If it is terminated with a high impedance  -- i.e.,
the
> unbalanced input of the R-390A -- less current is going to be  pulled
through
> the series element of the attenuator, and the "multiply by"  marked on the
> skirt of the output attenuator dial isn't going to be correct.  What
appears to
> be 0.2 or 0.3 uV is actually going to be higher.
>
> The BALANCED input is nominally rated at 125 ohms (though this is probably
> far from constant either from one band to another or across a signle
band). A
> 50-ohm signal generator connected to the balanced input should therefore
> terminated by an 82 ohm resistor (with short leads) connected across the
antenna
> input terminals.
>
> The impedance of the UNBALANCED input is not indicated in any of the
> documentation I have, but it is supposed to be "high." I would therefore
terminate
> the signal generator with a 51 ohm resistor so that it sees a proper
load.
>
> Only when the signal generator is properly terminated do the output
signal
> readings shown by the panel meter and the switch  attenuator correspond
with
> reality.
>
> Miles, K2CBY