[R-390] URM-25 RF output measurement

Tue Jul 6 19:40:18 EDT 2004

Bob and Roy, I agree, this is tough. Here are a few ideas.
Calibrating the generator all the way down to the "knat level" that an R390A
can handle may well be impossible in the shack. The leaking around problem
(RF leaking around the attenuator) makes you think that the receiver is more
sensitive than it actually is. (BELIEVING that the R390A really is that
sensitive seems easier....)

However, finding, rolling and calibrating some decent attenuator pads may
not be so difficult, at least at HF frequencies. If you can establish a
reference that you can trust like 0 dBm or 1mW RMS into 50 Ohms or 0.224V
into 50 Ohms or God forbid 0.775 V into 600 Ohms (something that you can
easily measure with an AC meter), this makes a good starting point.  You can
also see how the generators attenuator works at a high level using the
meter. This proves that the generators attenuator works fine and establishes
that you can vary the generator by some trustworthy amount like 20 dB.

Sometimes you see a pile of attenuators at hamfests. Usually these are
microwave grade and are well shielded - grab them! They may be the best
pieces of radio gear you will ever own.

Measure (verify that they are not smoked) each attenuators loss with your
generator and meter. Finally you put the attenuators in series until you get
down to the signal level your need. If you can find a pair of those HP
microwave attenuator banks, you havce hit gold! The attenuators will have to
be a completely shielded type. Since the generator is always being used at
high level, this will give you say 20 dB of range at the lower level set by
the "calibrated" attenuators. If you put 100 dB of attenuators in line, you
would end up with a range of -100 to -120 dBm, for example. Don't forget
cable loss. This should be low at HF unless the cable or connectors are not
correctly terminated. Double shielded cable like RG-223 is a must for
accurate measurements.

I have found that the problem (even with a lab calibrated generator) is that
the signal easily sneaks around- over and through the generator chassis,
cables, connector interfaces and thus around the attenuator or attenuators.
I was having problems getting decent readings at work below -120 dBm at
least at VHF. Having a 100 dB screen room at work, I used a feedthru
connector with 20 dB attenuators on each side of the wall and got the extra
isolation I needed. Fortunately HF should be easier to deal with.

Mike WB2UID

----- Original Message -----
From: "Bob Camp" <ham at cq.nu>
To: "Roy Morgan" <roy.morgan at nist.gov>
Cc: <r-390 at mailman.qth.net>
Sent: Tuesday, July 06, 2004 5:29 PM
Subject: Re: [R-390] URM-25 RF output measurement

> Hi
>
> So now I'm both in hot water for Lutefisk threads and in trouble for
> being to technical. I just can't win this week .....
>
> Every time I have gotten the "can I trust my signal generator" bug the
> attenuator has been the part that I trusted the least. Admittedly the
> waveguide beyond cutoff style used in the URM-25 and it's cousins is
> less prone to trouble than the switch based gizmos used in more modern
> equipment. The attenuator check is pretty simple and generally it gets
> me back to worrying about other things.
>
> The CMOS signal generator is fairly easy to build, but to your point if
> you don't get it quite right then it's more trouble than it's worth.
>
> There is one other way to do the whole thing. It's nice because it
> takes out all the errors at the low signal end of things. That's
> normally where you really care about it being accurate anyway. The
> stuff I came up with last time was all aimed at the high level end of
> the range.
>
> There are a number of ways to generate a fixed level of RF noise at
> R-390 type frequencies. One easy way is to pick up a used GR tube based
> noise source. You might also go looking for a tube for it as well.
>
> Since you have a R390 handy you have a radio with known IF bandwidths.
> Once you know the bandwidth *and* the noise level into the radio you
> can calibrate the radio in terms of micro volts. Poof! you have an
> accurate RF micro volt meter. Just the thing to check out your URM-25
> with. Since you have the full attenuator in on the URM all the
> tolerance build up from the accuracy of your substitution attenuator in
> the other procedure goes away.
>
> Note that no animals were harmed in any of the above proceedings ....
>
> Enjoy!
>
> Bob Camp
> KB8TQ
>
>
> On Jul 6, 2004, at 4:10 PM, Roy Morgan wrote:
>
> > At 11:13 AM 7/6/2004 -0400, Paul H. Anderson wrote:
> >>  ... How can I tell if my URM-25 is actually
> >> putting out 150 microvolts?
> >
> > Actually, you can't. You can be more or less confindent after some
> > testing.  Bob Camp's advice on building a new rf generator or level
> > setting device sounds like a find but challenging project.
> >
> >>  Do I need a calibrated RF meter with a known
> >> input impedance and make sure that is matched to the output of the
> >> URM-25?
> >
> > I see two ways to do this, both depending on the assumption that your
> > URM-25 attenuator is working as it should.
> >
> > 1) Follow the calibration procedure in the URM-25 manual. Basically,
> > you set the oscillator output to two volts and then depend on the
> > attenuator to do the rest.  the URM-25 front panel meter measures that
> > two-volt level during normal operation.
> >
> > 2) Get or borrow a millivoltmeter of some sort.  A good oscilloscope
> > may do in a pinch.  Measure the URM-25 output level at some reasonable
> > level like 100 millivolts (being sure to terminate the thing
> > correctly) and then assume the attenuator is working on other (lower)
> > settings.
> >
> > If you then think clearly about what the signal generator is doing,
> > what the termination or external attenutaor "pad" is doing, and what
> > the load of the receiver is on that setup, you will come to some
> > conclusions about what the actual rf voltage is at the receiver
> > terminals.  (note that the input impedance of the balanced antenna
> > input can vary from 50 to 700 ohms.)
> >
> > Roy
> >
> >
> > - Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
> > 7130 Panorama Drive, Derwood MD 20855
> > Home: 301-330-8828 Work: Voice: 301-975-3254,  Fax: 301-948-6213
> > roy.morgan at nist.gov --
> >
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