[R-390] GR Adapter and Signal Generator findings

Roy Morgan roy.morgan at nist.gov
Fri Jun 3 11:18:22 EDT 2005


At 10:06 AM 6/3/2005, Barry wrote:
>All this talk about the GR1001A reminds me that I need a GR 874 to BNC
>Female adapter.  Does anyone know where I can get one of these?

Barry,

Watch Ebay and go to Hamfests.  (Sorry, none of mine are for sale.)
Email the seller of item 7519425441 to ask if they have what you need. 
Several small lots of adapters have sold in the last couple of weeks. Item 
7519914142 includes one 874 to bnc female adapter and three other items.

>... With the output level set to 1uV, I could
>hear the modulated signal....could still hear the signal as low as 
>0.2uV.  I ...
>didn't have the series 40-ohm or the parallel 50-ohm resistors in place yet
>so I'm not sure how this works into the overall equations,

Here is how it affects it:

The "source impedance" of the GR 1001A generator is (engineered to be) 10 
ohms.  With no series output resistor, and no 50 ohm load, the open circuit 
output voltage will be about twice what the meter-attenuator-controls indicate.

If you just hook the 1001A output to the receiver input, you have in effect 
a voltage divider of 10 ohms and whatever the input impedance of the 
receiver is. Let's assume it's 120 ohms (it varies with band and 
frequency). Then the actual voltage at the input terminal will be 120/130 
of twice the indicated 0.2 uV, or roughly 0.4 uV.

The accessory I mentioned earlier is the GR Type 1001-P3 Voltage 
Divider.  I seem to remember that it is a 10:1 divider with an input 
impedance of 50 ohms and an output impedance of 0.5 ohms.  It this thing is 
used at the input to your receiver (along with the 40ohm series unit) you 
divide the indicated generator output by 10, and can rest assured that a 
receiver input impedance of 120 ohms or the like will not change that 
voltage value. Further, leakage is likely to cause less trouble.

A further note on the 1001A:  Another poster is right, the thing has a 
voltmeter (a "simple" diode peak rectifier) calibrated to indicate at the 
red line on the output meter when 2.0 volts RMS is present at the input to 
the attenuator.  The attenuator, multiplier, and output variable control 
are arranged to indicate the correct voltage at the cable termination if:
  - the 40 ohm series resistor is in place
  - the cable has a characteristic impedance of 50 ohms
  - the cable is terminated in 50 ohms.

(There are some circuit refinements that are switched in by the operator 
below 400 kc or some such to retain accuracy of the system.)

If I remember correctly, the "high level" output connector is at the point 
in the circuit where the 2.0 volts is measured.  The manual does say to 
cover that connector when making low level measurements.

>I also realize there is (can be) signal leakage from the generator, but if I
>turn the output down below 0.2uV, the signal is inaudible so I'm not sure
>I'm hearing leakage or not.

What *may* be going on is that the control you are moving is a dual wire 
wound potentiometer whose "bottom ene" is grounded. You may be simply 
getting to the ground point and cutting off the signal to the attenuator 
system. Leakage can exit the generator via many paths: bypass routes around 
the attenuator, power line, meter holes, other connectors such as the high 
level output, shaft openings and so on.  I would expect the 1001A to show 
moderately low leakage, but by no means superb by modern standards.  I've 
never tested one.

You can make a test though: Terminate the cable with a good 50 ohm load 
(not a resistor twisted to the raw end of a coax cable).  Set the generator 
output to some moderate or low level, and place the cable on the table near 
the receiver, and use a sniffer loop (a few turns of wire at the end of a 
coax cable) connected to the antenna input to see if you can find any 
signal at or around the generator or cable.

Inversely, set the receiver up fed by the generator on some frequency, and 
use another generator with a sniffer loop on it's output to see if you can 
"inject" signals into the system from outside.

>   I have the 2V output on the GR1001A plugged so
>I don't think there's much (if any) juice getting out from that point.

Those connectors were provided with a snap-on cap that simply covered the 
opening.  Later GR 874 connectors were made in "locking" style that reduced 
leakage quite a bit.. if your generator does not have the locking style 
connector, and you are using the snap-over cap, you are likely getting some 
leakage from that place.  Use of a GR-874 "open" termination with a locking 
connector would likely reduce leakage quite a bit.

>What really surprised me was with the signal generator as my "antenna",
>signals on the 9mc band came blasting in.

This is the best indication that you may have been getting leakage. If 
radio signals can get INto the generator, it's signals can get OUT.

>   I've heard some signals with the
>generator connected before, but not that loud.  Does this indicate the
>generator could be really leaking more than I suspect?

It could be.  You can experiment quite well with those signals coming 
in.  Wiggle things and unhook and short circuit points in the system till 
you find where those signals are getting into the receiver.  Of course, 
loose coax braids in your R-390A are a possibility.  You have not told us 
whether or not you are using a proper GR 874 connector at the output of the 
generator. If you are using banana plug-ended leads (as GR says you can) 
then that is a likely source of the signals.

Roy


- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
Home: 301-330-8828 Cell 301-928-7794
Work: Voice: 301-975-3254,  Fax: 301-948-6213
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