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From: "Robert A. Kile" <midnite2@concentric.net>
To: <topband@contesting.com>, "Earl W Cunningham" <k6se@juno.com>
Subject: Re: TopBand: TS-870S IMD improvement Mod
To: <topband@contesting.com>
Date: Wed, 3 Dec 1997 07:28:00 -0800
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Sorry if I seem dense today, Maybe I had too much listening for 5A2A
on 1829.5 last nite!!
I seem to remember a west coaster put a filter or two in the 455 IF of
an early TS-870S to knock down the bandwidth before the electronic
gates controlled by the front panel determined the passband.
Is the updated version of the TS-870S good enough to take out the
S9+60 and S9+80 db adjacent signals on either CW or SSB with the lower
early IF gain settings?
Thanks for your replies.
George, K8GG
______________________________ Reply Separator _________________________________
Subject: Re: TopBand: TS-870S IMD improvement Mod
Author: "Robert A. Kile" <midnite2@concentric.net> at BC_INTERNET
To: <topband@contesting.com>
Date: 12/3/97 7:28 AM
>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.
>
Earl,
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.
73
Bob KG7D
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