[R-390] R-390 alignment questions

[Bob Camp]

Hi

In just about every other measurement I know of what you say is exactly 
true.

For a variety of reasons receiver sensitivity is not measured that way. In 
the case of a receiver you do not know the impedance the receiver presents 
to the antenna. It may be high and it may be low, who knows. It is rarely 50 
ohms.

One solution would be to measure the actual impedance that the radio 
presents to the antenna. With a reasonable amount of gear you could do that. 
Once you know the radio's impedance you could come up with a combination of 
coils, capacitors, and resistors that converted that impedance to 50 ohms. 
In doing that you would have to make some decisions about the impedance 
transform you applied in the process. Different individuals could make 
different assumptions and their data would be significantly different as a 
result.

Since that's more than anybody wants to go through they came up with a 
different approach to this particular measurement:

1) Make sure the *source* impedance of the generator is correct. The old 
Measurements generators came with an attenuator to do this. Most modern 
generators have a number of fancy attenuators in them that do this quite 
nicely.

2) Calibrate the generator into a 50 ohm load or into what ever impedance 
you are running.

3) Hook the generator straight into the receiver. Use the "calibrated" 
numbers on the generator to report the data.

Obviously this means that a radio *could* have twice the calibrated voltage 
at it's input. In fact this is exactly what most radios do. You could look 
at this as cheating but that's how it's done.

    Enjoy!

        Bob Camp
        KB8TQ




----- 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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