[R-390] Measuring Sensitivity

Fri Mar 9 11:38:06 EST 2007

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 Morgan, K1LKY since 1959 - Keep 'em Glowing
13033 Downey Mill Road, Lovettsville, VA 20180
Phone 540-822-5911   Cell 301-928-7794
Work: Voice: 301-975-3254,  Fax: 301-975-6097
roy.morgan at nist.gov --  