Re: FT-990 wide-band audio hiss:
> As far as the noise is concerned, Hardy wrote:
> "Added a 5.8kHz Collins mechanical filter on the output
> of T2023 to remove broadband hiss. This is what you are
> hearing with the SC filter off. This happens when the
> front end gain is low (as it should be for decent
> dynamic range) and the IF gain is high. The trouble
> comes because the IF bandwidth widens out with the use
> of the small 10mm tuned circuit transformers that have
> low Q.
Hi everybody!
Mr. Hardy has it exactly right about the causes of
"wide-band-post-main-selectivity hiss".
For good IP3 performance, the main selectivity of the radio should be
as far forward in the IF chain as possible. On the other hand, for
low wide-band audio hiss from the receiver, the main selectivity should
be as far rearward in the IF chain as possible.
Professional radios use high slope-factor selectivity at BOTH ends of
the IF chain, just after the first mixer and just before the product
detector. But, alas, amateurs have a reputation for an unwillingness
to pay for a really good radio -- high slope-factor selectivity costs
lots of money [crystal/mechanical filters]. So, amateur radios get
good selectivity only at one end of the chain.
In the Drake R-4 series (I used to work as engineer there at the time),
the main selectivity was just before the product detector at 50 KHz.
Great sound! The IP3 wasn't too bad because tube circuits can handle
BIG signals if biased for them. Not so with solid-state.
The FT-990 WITH the SC filter turned on, is a pretty good device when
compared to the R-4s of yesteryear. I had one for years and liked it,
but only with the SC filter turned on.
Then I bought a JRC JST-245. Amazing, no wide-band audio hiss! In fact,
it sounded just as good as my FT-1000D, -- maybe better. As a dedicated
"receiver snob", I appreciated its low in-the-passband IM distortion.
Wow, I was impressed. But JRC is not the end of the story.
When the FT-1000MP arrived (the FT-990's evolutionary decendant), I was
tempted by the built-in DSP functions, so I bought one. After having it
for a few days, I sold my FT-990 AND my FT-1000D. The 'MP has it all
as far as I'm concerned. Good IP3, good sounding, low-IM, low-wide-band
hiss audio, two-receivers, built-in DSP noise-reduction, and the rest.
The inclusion of DSP into the last IF of the 'MP receiver allows the
radio to have crystal/mechanical filters up front near the first mixer,
little gain before the second filter set, and DSP clean-up of the
high-gain back-end IF stages. Technology marches on!
My station now consists of the FT-1000MP for HF and the JRC JST-245
for 6 meters native and 10 meter IFs for 144 on up. I can cheerfully
recommend either radio for anyone looking for a great RECEIVER.
73
Mike Valentine - WA8MSF (for another few weeks)
Cincinnati, Ohio
ex-R.L.Drake, but still a receiver snob
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