Many sets of receiver measurements do not address the close-in measurement
critical to DX and contest operation. The new DSP rigs look quite good on
SSB because they have crystal filters before the DSP to protect it from adjacent
signals. However on CW these wide filters provide the DSP no protection and
hence the performance suffers.
I made two sets of measurements:
A. ONE INTERFERING SIGNAL
I set the desired signal to 3 microvolts (about S5) and increased the
interfering signal at various spacing to degrade the desired to 12db SINAD
(25 percent distortion)
B. TWO INTERFERING SIGNALS.
This was set-up the same way as "A" except a another generator was
introduced at the same level as the first interfering generator but at twice
the frequency spacing, thus creating IM on the desired signal. For instance
a +60db reading would indicate that two 3 millivolt signals (about S9+35db)
would degrade the desired 3 microvolt signal to 12db SINAD.
Measured was a FT-1000, an IC-746, a Kachina 505, a late vintage TS-870, and
an IC-781. Results were as follows:
FT-1000 IC-746 KACHINA TS870 IC781
SPACING 0.5 KHZ:
A: 57db 50db 30db 45db 55db
B: 58db 45db 30db 45db 55db
1.0 KHZ:
A: 67db 60db 40db 55db 65db
B: 67db 50db 40db 50db 65db
5.0 KHZ:
A: 80db 77db 70db 72db 85db
B: 70db 50db 65db 75db 65db
10 KHZ:
A: 90db 80db 75db 85db 95db
B: 75db 50db 75db 88db 70db
Notes: Switching to 250 Hz. filters in the FT-1000, IC-746,TS-870 and
IC-781 improved the 0.5 khz readings to be about the same as the 1.0khz
readings. Switching to more narrow filtering on the Kachina made the
interference worse. It appears that some of the improvement in inter-
modulation performance ("B") in the Kachina and the TS-870 isthe result
of front-end AGC. You can sometimes hear the audio level drop in the
presence of interference while the S/N ratio stays the same. Doing the
equivalent to the other radios by manually switching in attenuation also
increases their intermodulation protection significantly.
John K9DX
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