[R-390] R-390 alignment questions

[Cecil Acuff]

The big corp. buzz now days is "Diversity Training".  I think we got that 
one down!

Cecil....
----- Original Message ----- 
From: <N4BUQ at aol.com>
To: ""R-390 reflector"" <r-390 at mailman.qth.net>
Sent: Wednesday, October 13, 2004 2:46 PM
Subject: Re: [R-390] R-390 alignment questions


> 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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