TopBand: TS-870S IMD improvement Mod

[km1h @ juno.com]

On Wed, 3 Dec 1997 07:28:00 -0800 "Robert A. Kile"
<midnite2@concentric.net> writes:
>>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



Bob, Earl, etc

It was my understanding that the 870 uses minimal filtering in the 8.8MHz
and 455KHz IF's...just simple 2 or 4 pole types and depends on the 11KHz
DSP IF to do all the work. 
Another point that was mentioned is that the pre DSP IF filters are
either 3 KHz or 6KHz so even if you select sharp DSP you are a captive of
the earlier stage filtering. 

This is just what I was told back in 1995 by Kenwood...I have never
followed thru by breaking down and buying a Svc Manual. 

Looking at the block diagram the 870 uses the same 8.8 and 455 IF schemes
as from the days of the TS-830. 930,940, 850, etc.  So why not just dig
in and add the appropriate filters which are readily available? 
Granted, the SMD PC boards are a challange but there is certainly a load
of room inside the case for a complete set of KW or IRCI filters and some
micro size relays...or PIN switching. 

Am I all wet or is there really hope for the 870???

73  Carl   KM1H


>
>
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