[R-390] R-390 alignment questions

[N4BUQ at aol.com]

Are you implying we all need sensitivity training?

Barry(III) - N4BUQ

>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
>
>
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