[R-390] Signal Generator Impedence Questions

[Barry]

Roger,

It's pretty clear and makes sense.  I have Chuck's method for setting the IF
gain and will probably use it, but was wanting to try the method in the
manual.

One thing I did notice that was rather humorous.  In the section for
aligning Z503, the manual states that if Z503 is defective, replace it.  I'm
not sure why they felt the need to include that blurb.  Kind of obvious,
isn't it?

Thanks!

Barry - N4BUQ

----- Original Message ----- 
From: <Flowertime01 at wmconnect.com>
To: <r-390 at mailman.qth.net>
Sent: Thursday, June 02, 2005 12:33 PM
Subject: Re: [R-390] Signal Generator Impedence Questions


> 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.
>
> ----------------------------------------------
> 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
> -------------------------------------------------------------------------
>
> 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.
>
> --------------------------------------------------------------------------
----
> ------
> 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.
>
> --------------------------------------------------------------------------
----
> ---------
> 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.
> ---------------------------------------------------
> 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
>
>
>
>
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