[R-390] Measuring Sensitivity

Sat Mar 10 02:48:42 EST 2007

If the signal generator is outputing 1 microvolt across the 1/2 ohm 
resistor, the receiver
will see 1 microvolt at it's input with any reasonable receiver input 
impedance. The exact
value of 1/2 ohm in parallel with 50 ohms is very, very close to the 
value of 1/2 ohm
in parallel with 700 ohms.

I believe that 50 ohm output signal generators are calibrated so that 
the output reads
correctly when the generator is loaded with 50 ohms.  So the receiver 
would see 1 uV
at frequencies where it's input was 50 ohm, but almost 2 uV at freqs 
where it's input impedance
is 700 ohms.

John
KK6IL

At 11:38 AM 3/9/2007, you wrote:

>On Fri, 09 Mar 2007 11:38:06 -0500, Roy Morgan <roy.morgan at nist.gov> wrote :
>
> > At 09:27 AM 3/9/2007, n4buq at knology.net wrote:
> > >... I'm interested in measuring the sensitivity of my R390A and I have
> > >a signal generator (GR1001A) that has a 10-ohm output.  I have an adapter
> > >that houses a 40-ohm resistor that matches the output to a 50-ohm cable.
> >
> > Barry,
> >
> > The GR 1001A is one of my favorite generators, so I have some information
> > and ideas for you:
> >
> > The manual for this thing is on BAMA.  Reading it and puzzling out what
>is
> > going on is a good idea. (Link is below.)
> >
> > The 1001A has an odd set of output impedances.  "The output impedance at
> > the ATTENuator jack is 10 ohms for all except the 100 MV  MULTIPLIER
> > setting, where the output impedance is 50 ohms."
> >
> > >When I feed the balanced input with this (40 ohm weries unit), there is
>an
> > >obvious mismatch between the 50-ohm cable and the input.
> >
> > Yes, but it may not be severe.  The input impedance of the R-390A may
>vary
> > from about 50 ohms to about 700 ohms depending on frequency, tuning of
>the
> > input circuit, and component variations.  I quote from MIL-R-13947b, the
> > Military Specification for the R-390/URR ("non-A").
> >
> > "3.13.3 Antenna input impedance.- The rated input impedance for the
>balanced
> > input circuit shall be 125 ohms. In the range from 500 kc to 16 mc, the
> > measured input impedance shall not be less than 50 ohms nor greater than
> > 375 ohms; for the range from 16 mc to 32 mc the measured input impedance
> > shall be not less than 100 ohms nor greater than 700 ohms."
> >
> > If you accept the fact that the input impedance actually does vary over
> > such a range, then you can do a number of things:
> > 1) Make sure you can reasonably expect the actual RF voltage at the
>antenna
> > terminals to be what you think it is.  (This is relatively simple to do.)
> > 2) Simply follow the manual procedure for sensitivity measurement,
>keeping
> > in mind that the maintenance manual procedure is meant to deliver radios
>to
> > service that are meeting some sort of minimum performance measure, not
>make
> > an accurate measurement of actual sensitivity.
> > 3) Measure the input impedance of the radio and do a bunch of figuring.
> > 4) Just wing it with informed engineering as Collins seems to have with
>the
> > S-line.
> >
> > >...how do I determine how much signal is present at the radio for a
>given
> > >setting on the generator?
> >
> > The 1001A generator will deliver to the output end of a (relatively
>short)
> > cable the voltage level indicated by the panel meter and cotrols if:
> > - on the 100 microvolt setting there is only the cable
> > - on other settings, the 40  ohm series resistor unit is in place.
> >
> > NOTE: If you terminate the cable with 50 ohms, in either case above, the
> > actual voltage at the termination will be ONE HALF what the panel
>controls
> > indicate.
> >
> > Realizing this, if you simply use the 40 ohm series unit, and feed the
> > R-390 balanced input (one side grounded) with the end of the un
>terminated
> > cable, AND you make the assumption that the input impedance if the radio
>is
> > 125 ohms or more, you will have somewhere between the voltage indicated
>on
> > the generator and half that.  This is equivalent to method 4 above.
> >
> > Note: it is quite clear from the reports of a Collins List member, who
>has
> > spoken to at least one of the design engineers who was at Collins during
> > the S-line production period, that they simply ran a simple cable from
>the
> > generator output to the radio, and took the panel indications as the
>value
> > of the input voltage.  The generators they used were meant to deliver
> > indicated voltage when the output was terminated in 50 ohms.  This works
> > out pretty well, IF the input impedance/resistance of the radio is
> > approximately 50 ohms.  I don't yet have any idea of what the actual
>input
> > impedance of S-line radios is.
> >
> >
> > >  I realize I need to make up a matching network at the
> > >termination of the 50-ohm cable where it connects to the receiver, but
>not
> > >sure how all these matching networks play into the signal levels between
> > >the generator and the radio.
> >
> >
> > Here is what I suggest, if you want to "Make sure you can reasonably
>expect
> > the actual RF voltage at the antenna terminals to be what you think it
>is."
> >
> > Make up a voltage divider with a 50 ohm resistor and a one-half ohm
> > resistor in series from the input to ground.  The output is taken off the
> > one-half ohm resistor.  This will do a number of things:
> > - It will present a 50.5 ohm load to the end of the cable from the
>generator.
> > - It will divide the voltage at it's input by 100.
> > - It will let the cable operate at 50 ohms, so there will be no or little
> > standing waves to bother anything.
> > - It will reduce the effect of any possible leakage into the cable or
> > connections up to the antenna connector, either from the generator or
>from
> > broadcast band signals or whatever.
> >
> > I suggest you make this thing up inside a twinax connector with a BNC
> > connector fitted into the back end.  This will provide a convenient
> > divider/load for use with all your R-390 radios.  The half ohm resistor
> > (two one-ohm units in parallel perhaps) go from twinax pin to pin.
>Ground
> > one of them (there is a correct one to be grounded). Feed the other from
> > the center pin of the BNC connector via the 50 ohm resistor.  The BNC
> > connector can be mounted into the cable clamping nut of the twinax
> > connector either by filing the threads and force fitting or soldering, or
> > by threading the clamping nut.  (I think it's a 3/8 by 32 thread.)  Allow
>a
> > flexible wire from BNC center connector to the 50 ohm resistor to allow
>for
> > threading the clamping nut into the connector body.
> >
> > Now, if you have a GR 1001A, use the 100 micro volt setting of the
> > attenuator with no series resistor.  If you use other settings of the
> > attenuator, use the 40  ohm series unit. You have a generator system that
> > delivers to the antenna terminals one percent of the voltage indicated on
> > the panel.  If you have a generator that operates correctly with the
>cable
> > terminated in 50 ohms,  just use the cable from generator to voltage
> > divider unit.
> >
> > The voltage at the antenna terminals will be one hundredth of the panel
> > indication.  The input impedance of the radio will not matter enough to
> > bother.  (The "output" impedance of the signal source is about one half
> > ohm, and the input impedance of the radio is from 50 to 700 ohms.)
> >
> > >I assume that once this calculation is made,
> >
> > I did just above.  It's convenient to divice the generator output
> > indication by 100.
> >
> > >then when I feed the radio with a modulated AM signal with the BFO off
>and
> > >get -7V on the diode load, then I should be able to know the sensitivity
> > >level, correct?
> >
> > Yes, correct. You will likely get DIFFERENT results if you use the method
> > in the manuals, with a URM-25, for instance.  It's likely the numbers
>from
> > the manual method will be from two thirds down to one half of the numbers
> > from using a low-output-impedance attenuator as I describe above.
> >
> > The reason:  An unterminated generator meant to indicate correctly if the
> > cable IS terminated will deliver TWICE the indicated voltage at the end
>of
> > the cable.  The receiver most likely has in input impedance above 50
>ohms,
> > possibly three to 14 times as high, and so will appear more or less like
>an
> > open circuit to the cable.
> >
> > A note on leakage:  If you have an EICO, HEATH or other similar
>generator,
> > the question is not whether or not it leaks, the question is how
> > much.  Almost certainly it is leaking and the leaked signal probably ADDS
> > to the signal from the end of the cable at the receiver input
> > terminals.  So the sensitivity readings you get will be misleading.  To
>see
> > what it takes to get a moderately low leakage generator, examine the HP
>606
> > generators: every lead into and out of the oscillator box is filtered
> > thoroughly, the box is cast aluminum with rf seals on every opening, the
> > attenuator is similarly constructed, and so on.  There have been reports
> > that the common HP 8640 generator leaks somewhat unless it's seals and
> > fastenings are cleaned and tight. To see what's going on with a
>generator,
> > put a nice 50 ohm load directly on the output connector and sniff around
> > with your properly running receiver set to full gain.
> >
> > >The reason I ask is that I'm having to crank the generator up to around
> > >10uV to do the "switch the modulation on and off to get 10dB S+N/N
>readings
> > >and that seems high.
> >
> > You may well have leakage from the generator.  The 1001A leakage spec is
> > "Fields at one mc are less that one microvolt per meter at two feet from
> > the generator."  Do test it and then take it mostly apart to clean,
>tighten
> > and reseal all and every place there might be leakage.  One report from
>our
> > now-absent Nolan Lee told how he had to replace the coax inside the URM-
>25,
> > because it was leaky or lossy, and also the BNC-to-BNC cables that came
> > with the thing were so lossy as to completely foul up any measurements.
> >
> > >  The radio "hears" quite well, but I'm wondering just
> > >how sensitive it really is.
> >
> > It sounds like really you don't know yet.
> >
> > >The problem I'm having with the "modulation on, modulation off" method,
>is
> > >that the cable and generator make a very, very good antenna, especially
>on
> > >the BC band, making these measurements  quite difficult.
> >
> > Terminate the INPUT end of the cable with 50 ohms. Then try to figure out
> > where the signals are getting into the radio.  The generator set to a
>high
> > level with a few-turn loop on the end of another output cable can help
>you
> > find the route of signals into the radio.  Bottom and top plates may
> > help.  De-Oxit on RF and IF connectors may help.  (Compare a Mini-BNC
>used
> > in the R-390A with a General Radio 874 locking connector to get an idea
> > what the differences are!)
> >
> > When I replace line cords on receivers, I locate a shielded computer cord
> > (these are marked "shielded" and are a bit thicker than normal) and use
> > that.  Any R-390 which is running without its line filter may be leaking
> > via the cord.
> >
> > Speaking of loops, GR made the 1001-P3 Voltage Divider,meant to inject
>the
> > correct CURRENT in series with a household radio loop antenna.  This
>thing
> > is, I think, a 50:1 divider, and if you find one it could easily be
> > modified to do the 100:1 division.  Just add a one-ohm resistor across
>the
> > little wirewound card resistor you find in there.
> >
> > GR made a standard loop antenna (type 1000-P10) for use with the
> > GR-1001A.  It had the needed matching inside it, and was meant to create
>a
> > standard field for testing of loop-equipped radios.
> >
> > Also useful at times is the 1001-P4 Dummy Antenna.  This thing was
>defined,
> > in the 30's I think, to simulate a wire antenna over the BCB-HF frequency
> > range.  The URM-25 manual and many other sources contain the details of
>the
> > thing (a little inductor, a resistor and two caps.)
> >
> > >Any advice here would be most appreciated.
> >
> > See the R-390A FAQ page for LOTS of manuals and such:
> > http://209.35.120.129/faq-refs.htm
> > Including the R-390/URR engineering report, the R-390 Cost Reduction
> > report, and also the Mil Spec, which is at:
> >
> > Military Specification, MIL-R-13947B - Receiver, Radio (Radio Receiver
> > R-390( )/URR) 1960-Oct-26
> > http://209.35.120.129/mil-r-13947b.pdf
> >
> >
> > There is a document: sensitivity_alignment.pdf that is also on the Pearls
> > of Wisdom page
> > http://209.35.120.129/Pearls/index.htm
> > that is most helpful. It is also most long, but very good reading.
> > http://209.35.120.129/Pearls/sensitivity_alignment(s).pdf
> >
> > Hewlett Packard, now Agilent, published many tutorials, tech notes and
>such
> > on their generators.
> >
> > The GR 1001A manual on BAMA is at:
> > ftp://bama.sbc.edu/downloads/gr/1110a/
> > (No home should be without a 1001A, and the manual.)
> >
> > (Comments, corrections, and discussion welcome.)
> >
> > Roy
> > Who needs an output attenuator dual pot for his 1001A
>
>Roy,
>
>Lots of great info here (understand the updates to the 100 millivolt vs 100
>microvolt settings).  I bought some components and had intended to build
>one of the 100:1 dividers and never got around to it.  Looks like I need to
>do that as it will make the measurements a bit easire (I think).
>
>What I'm still a bit confused about is the fact that the receiver is
>connected across the 1/2-ohm resistor.  Isn't the radio seeing a pretty bad
>impedence mismatch here?  I realize this makes the generator immune to
>impedence changes brought on by the receiver's input, but doesn't this
>mismatch have a detrimental affect on the radio's ability to "see" the
>voltage across the 1/2-ohm resistor?
>
>Sorry if this sounds goofy, but I'm having a bit of a hard time
>understanding all the impedence values and their effects in this setup.
>
>Thanks again,
>
>Barry - N4BUQ
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