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

[Larry H]

Dave, good question.  Let's look at a sig gen first.  Most good ones have a
50 ohm output and most need to be impedance matched to a 50 ohm total load
in order for the meter to correctly indicate the signal level.  So let's
look at what I mean by a  50 ohm total load.  Just for the sake of
discussion, if you hooked up your sig gen directly to a receiver that had a
100 ohm input, that would be a mismatch and the meter reading on your sig
gen would be off be a known amount.  If you were taking sensitivity
measurements, then you would want it to be accurate and you would  need to
make the load look like 50 ohms.  In this case, it's easy.  Add a 100 ohm
res in parallel with the rx to make a 50 ohm load.  IE: connect one end of
the res to the coax center and the other end to the shield.

Now for the R-390A: It has an input impedance on the balanced input of
between 50 and 450 ohms, but the powers at be say about 125 ohms.  So the
powers at be designed an impedance adapter DA-121 that does that.  The big
problem with it is it reduces the signal level at the receiver by 55% or
9.1 db.  This means that you need to do a math calculation for each
measurement using it.

I tried an alternative method that does not reduce the signal level - an 82
ohm res in parallel (shunt) with the coax center conductor.This is not
perfect, but then neither is the DA-121, but it's fairly close.

Regards, Larry

On Thu, Mar 14, 2019 at 4:02 PM dog <agfa at hughes.net> wrote:

> 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
> > ______________________________________________________________
> > R-390 mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/r-390
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:R-390 at mailman.qth.net
> >
> > This list hosted by: http://www.qsl.net
> > Please help support this email list: http://www.qsl.net/donate.html
> >
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html