[R-390] Signal Generator Impedence Questions

[Barry]

The GR1001A doesn't have a microvolt meter.  In reality, I was guessing at
150uV by setting the output level to 100uV and setting the adjustable
attenuation control to 150%.  Kludgy, but probably not too far from accurate
(if it is in calibration which I'm sure it isn't).  I was just trying to get
something close to the procedure that's in the manual.

I was disappointed that with 150uV(?) at the input, I was not able to
get -7V on the diode load.  I'm guessing I have a gain problem (bad tube,
drifted resistors, etc.) in the IF deck, but haven't checked all that yet
(but plan to).  I was thinking that the 10-ohm impedance mismatch might be
affecting my ability to set the gain by the manual properly.

Thanks for the input guys!

Barry - N4BUQ

----- Original Message ----- 
From: "Bill Hawkins" <bill at iaxs.net>
To: <r-390 at mailman.qth.net>
Sent: Thursday, June 02, 2005 1:06 PM
Subject: RE: [R-390] Signal Generator Impedence Questions


> It's true that if you measure the input in microvolts then
> the impedance doesn't matter. But the low impedance puts a
> load across the antenna coil that isn't there if you use a
> long wire antenna. The purpose of the network was to simulate
> the antenna and not load down the antenna coil.
>
> As it is, most people aren't set up to do microvolt tests
> because their shielding isn't good enough. Might just as
> well connect the generator direct to the antenna and do
> the alignment without worrying about impedances. You'll
> use the antenna trimmer with a real antenna.
>
> You can't really test the receiver sensitivity that way,
> though. You can check your shielding by seeing if the jump
> in dB from 1 microvolt to 10 is the same as 10 to 100.
>
> If your generator has a microvoltmeter, it may be looking at
> the input to the attenuator, which has to be properly
> terminated to have the right voltage at the output.
>
> Bill Hawkins
>
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