[R-390] Signal Generator Impedence Questions

[Flowertime01 at wmconnect.com]

Barry,

As it was done way back when (68-75) and taught at the ASA school house at
Ft Devens Mass. Skip all those adapters. None were actually used. We all
read that same TM. Nice theory, good education, likely the real exact science
way to do things. Just not what was practiced in the Field world wide for 
years.

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The IF alignment procedures for the R390A state to connect the URM-25* to
the input connector using an impedance matching adapter  (either Test
MX-1487/URM-25D or CU-206/URM-25F depending on which generator you have).
Assuming the URM-25* is 50-ohm output, what does this converter do?  

>From Para 73 page 114. of the R390/A TM 11-5820-358-35 Dec 8 1961 copy.
Fellows, does someone have the AN/URM 25 TM and tell us exactly 
what is in these two Adapters? Thanks Roger KC6TRU
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I find it odd that the adapter is mentioned for the IF GAIN ADJUSTMENT 
procedure,
but not for the IF alignment procedures even though the generator is
connected to the IF module the same way for both procedures.

Just one of those errors in the TM. Do you know what it cost to get an
errata page distributed for a TM? This was our excuse for never using any
adapter. Just look at the preferred paragraph and get it done.

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I assume the input impedance of the IF module is something other than
50-ohms and the adapter is being used to match the two impedance, but I
don't know what the input impedance of the IF module is.

No one at the school house or student of the school house knows either.

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I have a GR1001A signal generator which, for most attenuation settings, has
an output impedance of 10 ohms.  The reason I'm asking about the above
adapters is I'd like to construct a proper matching network for my 10-ohm
generator to work the same way as the 50-ohm generators do for the IF
alignment procedures.

Old 33's knew the input of the IF deck was not 50 Ohm's. 
Its likely not 10 Ohms either.

Field practice was to skip all the adapter stuff. Just cable it up.
We did use 150uv into the IF deck. BNC to adapter on the back panel and
mini BNC jumper cable moved from the 50 ohm output to The IF deck input.

Chuck Ripple had a good procedure to readjust the IF gain for over all
receiver best signal to noise ratio. This procedure is not dependent on
signal generator impedance matching and gives the real world best performance.

Just cable up your generator for alignment and tube noise testing.
Every thing is relative. An adjustment either provides more output or less 
output.
A tube change either provides a better signal to noise or a poorer signal to 
noise when the generator is adjust for the same signal plus noise level. This 
checks the 
noise of a tube compared to another tube in the same socket. Changing tubes
into the same socket with the signal generator held constant, checks tube 
gain.
More tube gain may or may not be more tube noise. This is a definite place
where YMMV.
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Another question:  The instructions state to set the output at 150uV for the
IF GAIN ADJUSTMENT procedure.  If I'm using a different adapter for 10-ohms
versus 50-ohms, will I need to change the output setting to something other
than 150uV to account for the different adapter impedances?

For alignment and test, just cable it up and use 150uV. 
Run what you need to get a 1/2 watt of audio out.

You are running a 10 ohm source into a higher impedance.
The higher impedance will not load the source.
The receiver is essentially a voltage circuit as opposed to a current circuit.

You hang a 1 watt 600 ohm resistor on the local audio output and start 
measuring
the output while reading the DB scale of the AC meter. You quickly find that 
when turning the audio modulation on and off, you either are getting a 30 DB 
change between modulated signal and un modulated signal. 

As you play with the IF gain to change the diode load DC voltage (-7 volts) 
and the
signal generator output level to get the 1/2 watt output. you are either 
getting the 30 DB change. Changing the exact gain and drive will not change the 
signal to noise ratio. Making adjustments will make changes. Changing tubes will 
make changes.
Work on it until you get the  30 DB change. 

Then just dial 150uV and set the IF gain for -7 volts.
Go on to the RF deck alignment.
When you get that done then set the IF deck gain as
Chuck Ripple details it.

Barry, If this is all not as clear as mud or darker, ask some
more questions.

Roger KC6TRU