Topband: RECEIVER MEASUREMENTS

[John Battin]

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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