TopBand: TS-870S IMD improvement Mod
Robert A. Kile
Wed, 3 Dec 1997 07:28:00 -0800
>The so-called "IMD" problem on CW in the early TS-870S's is due to the
>wider IF selectivity stages before the IF DSP. i.e., if the DSP is set
>to say, 100 Hz, strong signals within +/- 1.5 kHz or so of the receive
>frequency will cause AGC "pumping". Apparently, from what Lars, SM3BDZ
>wrote on 1 Dec, Kenwood has alleviated this problem by decreasing the IF
>gain in the offending IF stage before the DSP. To make up for the gain
>loss, they've increased the gain in a later non-offending IF stage.
>This problem should not apply to the TS-570S because it uses AF DSP.
The AGC loop in the TS870S is controlled by the DSP filter/detector and is
variable in time constant. Saturation of mixers and IF amplifiers will
generate IMD that will blow right through a perfectly good filter. The
dynamic range of the DSP in the TS870 is more than adequate to handle the
duty if the signals getting to it are linear.
You are correct the TS570D uses audio DSP. The AGC loop is controlled by the
detector/BM. Thus mixers and filters ahead of DSP must do their job just as
well. In the TS570 the IF AGC time constant is slightly too long, and the
2nd mixer is saturating allowing blowby of unwanted images. I point out that
the TS570S/S has adjustable IF gain post filter just like the TS870. By
changing time r/c time constant and gain distribution the TS570D could
probably compete with the best of them.
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