[R-390] R-390A IF deck input impedance, sig gen reading

[Bob kb8tq]

Hi

If the output changes very little with a change in load resistance, then the output impedance 
is very low. If it was very high and you put a 50 ohm load on it …. you get close to nothing at 
all out. 

The URM-25 (and many early generators) use a “waveguide beyond cutoff” sort of attenuator. 
That takes the attenuator out of the equation. They mismatch the load coil in this approach. 
The net result is an output impedance that may be in the vicinity of 10 ohms. 

Bob

> On Mar 15, 2019, at 8:54 PM, Larry H <larry41gm at gmail.com> wrote:
> 
> I was curious about the IF deck input impedance, so I checked the loading
> it has on my URM-25.  It had very little affect on the input voltage, so
> the impedance must be high.  IE: I set my sig gen for 150 uv and terminated
> it with a 50 ohm load and it measured 152 uv volts RMS with my HP 400FL AC
> voltmeter.  I removed the 50 ohms and connected it directly to the IF deck
> input and centered the URM-25 frequency to the IF deck.  I then measured
> the voltage and it was 304 uv.
> 
> For making the IF gain adjust -7 vdc on diode load, the tm-11-5820-358-35
> book says to terminate the sig gen with a 50 ohm load and then connect that
> to the IF deck input.  Now we know why.  They want a real 150 uv to go into
> the IF deck.
> 
> Regards, Larry
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