[Kenwood] TS-870S Test Drive

[Vladimir A. Gordienko]

> (2) The TS-870 has got to have the poorest noise immunity
>       of any rig I have used. Same goes for the TS-570. Reason
>       is no roofing filter ahead of the IF's. In the TS-870, all
>       the noise gets funneled into the IF's where the DSP filtering
>       takes place. The noise contaminates the DSP filtering
>       which renders the 200 and 400 Hz BW settings virtually
>       unusable if you live in a QTH which is fairly noisy. My 18
>       year old Ten Tec Corsair with just a narrow 2.3 KHz SSB
>       XTAL filter will pull signals out of the noise
>       with good copy that are barely audible on the TS-870.
>       The speced sensitivity of the Corsair in only .25 uV for
>        a 10 db S/N ratio with the Preamp on. From memory now,
>        think the 870 is speced at .15 uV for a 10 db (S + N)/N
>        ratio. Never did agree with the change from S/N to
>        (S + N)/N - the latter produces a fudge factor like the
>        audio boys who spec a stereo channel at 20W and do
>        not bother to tell you it is peak power and not rms power
>        at a certain % of distortion.

Product Review Column from QST Magazine February 1996
Kenwood TS-870S MF/HF Transceiver:
....
"Like the rest of the current crop of DSP-equipped amateur MF/HF transceivers,
the TS-870S performs digital signal processing at a low IF, about 11.3 kHz in
this case. On receive, the signal is first mixed to 73.05 MHz and passed through
the 15-kHz-wide "roofing" filter, then shifted down to 8.83 MHz to pass through
a
3 or 6-kHz filter (or through no filter in FM). The signal is then shifted to
455 kHz,
where it passes through a 3, 6 or 15-kHz filter. Finally, the signal is mixed
down to
11.3 kHz and applied to the DSP's analog-to-digital converter.
The various oscillators that set the mixing frequencies are all digitally
controlled,
and that turns out to be important. The TS-870S uses a time-honored technique to
achieve adjustable bandwidth and IF shift by varying these oscillator
frequencies
so that one side of the receiver passband is set by the 8.83-MHz filter and the
other
side by the 455-kHz filter. The resulting band-limited signal then is
demodulated by
the DSP. It is also bandpass filtered by the DSP, and the DSP generates the AGC
voltage. Since both the oscillators used for mixing and the DSP unit are
digitally
controllable, the TS-870S can adjust the analog passband and the digital
passband
in tandem.
Consider what this means. If the TS-870S relied only on DSP to narrow the
receiver's passband, signals outside the DSP filter but inside the analog
passband
would pound away at the IF circuitry, possibly causing gain compression
(blocking)
and/or IMD generation. On the other hand, if the TS-870S relied only on the
analog
filters to set the passband, strong-signal performance wouldn't be as bad, but
you
wouldn't have the advantage of the "brick-wall" shape of a DSP filter's
frequency
response. But the TS-870S uses both kinds of filtering. That means the receiver'
s
ultimate passband is set by the DSP filters, but signals outside the DSP filter
passband are attenuated by the analog filters.
The results of this tradeoff are particularly noticeable in CW. To achieve a
typical
CW passband of 400 or 600 Hz, the analog filters are shifted so that their
passbands
overlap only by the desired amount...."

I think it is one of most convenient CW receivers.
(Try NR with SPAC function ON (i.e. Menu No13 - OFF) - the best I ever
heard for weak and noisy signals!)

73 Vladimir ("Bob")  UT1IA
http://www.qsl.net/ut1ia/