[R-390] R-390A Use of CX-2919 and CX-1363 adapters

dog agfa at hughes.net
Thu Mar 14 19:02:36 EDT 2019


I don't have any of those pieces of equipment, so I have to use what I 
have, a Moto 2005HD with 50Ω output and is only accurate into a 50Ω load 
and a Moto RF milivoltmeter (Boonton knock off) and it has the hi-Z 
(2pf) input as well as a 50Ω adapter and none of my regular voltmeters 
have any input Z to speak of (10MegΩ). So I was always wondering what to 
set for the level from the sig gen. The book talks about a 125Ω as a 
match to the balanced input and also the unbalanced with one side 
grounded. So I always put a 50Ω load at the end of the coax I use feed 
to the balanced input (but in parallel with 125Ω?). But with all that 
said, the numbers that I find are all close within spec even if I don't 
use the 50Ω load or even use a 125Ω load. I'm not sure it matters all 
that much. I've heard several opinions however. Can someone really 
enlighten me on the values? But I still haven't gone through both my IF 
modules to see what they both read at each grid/plate with a defined 
(measurable) input to the IF, and then what is the input Z of the IF?

Dave N3DT

On 3/13/2019 23:39, Larry H wrote:
> I was checking the stage gain of a couple stages in the IF deck I am
> working on and noticed that the TM-856A specifies to use the CX-1363
> adapter, but the TM-11-5820-358-35 specifies to use the CX-2919 adapter and
> that it comes with the AN/URM-25D.  Well, the book I have for the 25D
> specifies that the CX-1363 adapter comes with it and not the CX-2919
> adapter.  However, the book for the 25F specifies that the CX-2919 adapter
> comes with it.  I know this seems trivial, but since I don't have a CX-2919
> adapter and wasn't sure what it was and it isn't listed in my 25D book, I
> had to go look it up.  Turns out that the CX-2919 adapter is simply a .047
> mfd cap in series with the signal to block DC from damaging the URM-25's.
> And, the CX-1363 adapter is a .1 mfd cap in series with the signal for the
> same reason.  It looks to me that either adapter would work fine for this
> purpose.  I searched through our online doc about this, but couldn't find
> it.
>
> I'm just curious - do many of you URM-25 users have a CX-2919 adapter?
>
> It also looks to me like a 50 ohm load should be used in front of these
> adapters in order to have the URM-25's meter read as accurately as possible
> (they need a 50 ohm load).  This is because the books are specifying a
> particular level of signal and these adapters provide no loading at all.
> But where a specific signal level is not specified or need to be known, the
> loading is not necessary.
>
> Regards, Larry
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