IMDR tests of my ICOM 7700
I finally got
around to doing one of several tests for IMDR on my Icom
7700. ( this is the first, I will repeat the test in a
few weeks)
The goal is to get
numbers for the MDS ( minimum discernable signal), the
BDR (blocking dynamic range) and the IMDR ( intermodulation distortion response). Tom Roach W8JI has a pretty good, if somewhat
opinionated ( I enjoy opinionated opinions !) on his
website. See http://www.w8ji.com/receiver_tests.htm
.... Well worth studying.
The figures of
interest for serious
amateurs are the BDR and IMDR for 2 kHz spacing. These are often
absent from the manufacturers specs. They are of much interest to Dxers and contesters who operate in crowded band
conditions. ( The IC7700 manual does have a
number specified for 100 kHz spacing, but let's face it... that's like comparing
strong signals from another planet !) What this means is that we
are evaluating how powerful signals inside the receivers passband will
compromise the weak signal that we are targeting. The pass band in this case refers to
what’s inside the receiver system passband.. not just one stage of IF. The IC7700 has specified 3kHz roofing filters as it’s narrowest
set. I never change this as I use CW most of the time, and 3 kHz is still fine
for SSB use ( heaven forbid).. hey,
I DO have a mic, I just can’t find it most of the
time.
Test equipment:
As mentioned, I
now have two HP8640B’s in good condition. Before use, I checked output levels on a Tektronix digital
scope across a known 50 ohm load to compare the power calibrations. Both were
good. Next I check both internal frequency counters against each other, the
scope ( a Tektronix TDS 360) and another HP frequency
counter. Everybody in agreement to 10 Hz.
Attenuators include the MFJ unit
with 1 dB steps ( a great value for an attenuator, in my mind), and the built
in attenuators of the 8640B generators. A hybrid combiner with a 6 dB insertion
loss was used to combine the generator outputs.
I have found my best detector is usually a pair of MK 1 ears coupled to the radio with Bose noise
canceling headphones.
The layout looks
better than the actual setup. I didn’t actually use all the attenuators shown
in the photo. The sketch above is accurate. Detecting consisted of carefully
listening for an increase in hiss at the listening point, or the first actual
discernable signal tone in the ears.
And, here are the
results:
160M, no pre-amp 1:
Generator A 1840 kHz, and B
1842 kHz.
MDS: -131 dBm
BDR: 89 dB
IMDR: 85 dB
20M, no pre-amp 1:
Generator A 14.010 mHz, and B 14.012 mHz
MDS: -121 dBm
BDR: 82 dB
IMDR: 80 dB
20M, with pre-amp 1:
Generator A 14.010 mHz, and B 14.012 mHz
MDS: -135 dBm
BDR: 82 dB
IMDR: 81 dB
I reckon these
numbers are OK to about +- 1 dB. I intend on repeating the test in a few weeks
to insure I didn’t do anything too stupid the first time around. If the results
are different and they well might be, I will provide analysis.
Here is a shot of the
rig’s display during the IMDR test… the little blip to the right of the big
ones is my ‘signal of interest” which is 2 kHz away from the left hand signals.
To the right of this point there isn’t much because we are outside of the rigs
3 kHz pass band.
The scope display
is at +- 5 kHz spacing and no attenuation. There is a tiny little signal at the
marker which I could just detect with the “MK 1 ears”.
So, there is one round
of an actual measurement, for good or bad. Actually, I think these are pretty good
numbers. I hope it is repeatable. Note
that the 85 dB signals close by are the same as 30 dB over S9… monstrous
signals.
BTW, the feet
under the rig are custom jobs I made to tilt the front panel up for good
contrast. There is another pair underneath for the back feet and the back of
the transceiver just clears the desk top.
VE1ZAC