[R-390] AN/URM-25s

[2002tii]

Curt wrote:

>Former life as a Tektronix field engineer, I had the opportunity to
>support several well equipped Rx development labs in the area.  The
>efforts they took to get isolation were pretty remarkable.  Screen
>rooms, floating floors, anechoic coatings, etc.  Hard to imagine anyone
>doing similar work without extraordinary measures.

It can be done, but it is a challenge.  I generally find that most 
hams' measurements of sensitivity at HF are optimistic by 10 dB or 
more due to signal leakage.  For all of their good qualities, R390As 
are particularly egregious offenders -- they leak RF like a sieve, so 
ANY leakage from your sig gen will make them appear to have very high 
sensitivity.

Note that there are two issues -- signal from the generator leaking 
out of the cabinet and into the DUT, and external QRM and QRN leaking 
into the DUT (possibly including RF that leaks into the sig gen and 
then appears at its output jack, or is induced into the cable 
connecting the sig gen to the DUT).

It helps greatly to use a well-shielded external coaxial attenuator 
(e.g., HP355A + 355B, or JFW adjustable attenuators, or Mini-Circuits 
in-line attenuators), placed right at the input of the DUT, rather 
than using the internal attenuator in the sig gen.  (By "right at," I 
mean connected using just a coax adapter, no cable at all -- prop the 
attenuator up on a stack of books or something so it isn't just 
hanging there.)  That way, the level in the connecting cable is much 
higher and any QRM/QRN introduced before the input jack of the DUT is 
attenuated by many dB.  Ideally, you would move all of the 
attenuation to the DUT end of the cable and run the sig gen at 0 dBm 
or more.  If nothing else, get three or four Mini-Circuits HAT-30 
fixed attenuators (http://www.mini-circuits.com/pdfs/HAT-30+.pdf) 
($10 each) and put them right at the input of the DUT to do 30, 60, 
90, or 120 dB of your attenuation there, and do the rest (always less 
than 30 dB) at the sig gen.

Preventing leakage into the DUT other than through the antenna jack 
can be harder (and, of course, to the extent that you reduce it, your 
results will not represent how an off-the-shelf unit works).  A GOOD 
power line filter is mandatory, on both the DUT and the sig gen.  Use 
copper mesh gaskets at all cabinet joints (or these days, conductive 
polymer shield gaskets).  One of the greatest sources of this ingress 
(or egress, in the case of signal generators) is the chassis openings 
for controls.  One thing that helps is to put mesh grounding collars 
on the shafts of rotary switches, capacitors, and potentiometers to 
ground the shafts to the cabinet (look inside the front panel of an 
HP8640B for an example).  These days, you will probably have to make 
your own -- coarse stainless-steel wool works OK (do NOT use regular 
steel wool).

If the input jack of the DUT is floating (not galvanically connected 
to the chassis right there at the jack), make sure the body of the 
connector is RF-grounded using a 0.01-0.1 uF capacitor (best practice 
is to use the cap in parallel with a 10-100 ohm resistor).  This 
applies to any other floating connectors, as well, not just the input 
jack.  You also may want to wind 10 turns or so of the cable 
connecting the sig gen to the DUT around a ferrite toroid core (Type 
43, say 2.4" OD) to keep any QRM/QRN riding on the shield from 
getting to the DUT input.  Put this choke immediately before the 
external attenuator (or the DUT input, if you are not using an 
external attenuator -- but understand that if you are putting -130 
dBm and lower signal levels into the sig gen end of the cable, you 
are unlikely to get good results no matter what else you do).

If you have a chance, taks a Harris 590 apart and observe what they 
did -- the 590 is the best-RF-sealed receiver I have seen 
(interestingly, the 590As I have seen are not as good by 15-20 dB at 
some frequencies).

Best regards,

Don


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