[Yaesu] Re: Pin Diodes FT-990 and FT-990 HISS

Wed, 13 Feb 2002 15:45:28 -0600

Stuart,
I ran test on my 990, RF gain minimum, AF gain maximum, 15 meters SSB, 2.4
filter, and almost no noise with digital filter off, lots of noise with
digital filter on. What does that mean?
Darryl, KK5IB
----- Original Message -----
From: "Stuart Rumley" <[email protected]>
To: <[email protected]>
Sent: Monday, February 11, 2002 10:31 PM
Subject: Re: [Yaesu] Re: Pin Diodes FT-990 and FT-990 HISS


> This hiss is a common problem with the Yaesu FT-990 and FT-1000.  For this
> radio it is the limiting factor on sensitivity excluding secondary factors
> such as local oscillator phase noise and intermodulation distortion from
> adjacent high level signals.
>
> You can experience the problem first hand yourself by doing the following
> experiment:
>
> 1) Disconnect the antenna from the receiver input.  Just to be consistent,
> turn on the on the RF attenuator.
>
> 2) Turn the RF gain off by turning the RF gain control fully
> counterclockwise.
>
> 3) Tune the receiver to any band, 15 meters is fine, and set the
selectivity
> and mode for USB (2.4kHz BW).
>
> 4) Now the receiver has no RF, or IF gain, only the product detector and
AF
> stages are running.  IF you turn up the audio gain you will hear the hiss
> coming from the product detector and audio stages.   This is not
desirable.
> There should be very little noise of any kind coming from a receiver with
RF
> gain at minimum.  Try this on another radio for comparison.
>
> Here's some more interesting things you can try depending on what filters
> you have installed in your radio that will further illuminate this
problem.
>
> 5) With the antenna still disconnected and the RF attenuator in, turn up
the
> RF gain to maximum (typical normal SSB operation).
>
> 6) Now turn up the AF gain so the noise is a comfortable level.  If you
have
> an audio voltmeter calibrated in dB connected it to the speaker output.
>
> 7) Now change selectivity while remaining in SSB mode.  For example if you
> can go from a 2.4kHz SSB filter to 500Hz CW filter you should here the
noise
> drop.  In fact it should drop according to 10*LOG (2400Hz/500Hz)= 6.8dB.
>
> On the FT-990 the noise doesn't drop indicating that majority of the
> receiver noise is not generated in the front end but downstream behind the
> high selectivity filters.  This is exactly were you don't want this to
> occur.  This means the sensitivity of the radio is not determined by the
> noise performance of the front end but by excessive noise in the IF.  You
> don't get the full benefit of using narrower filters for CW or PSK.
>
> Hope the helps
>
> Stuart,
> ki6qp
>
>
>
>
>
>
>
> ----- Original Message -----
> From: "Ronald KA4INM Youvan" <[email protected]>
> To: <[email protected]>
> Sent: Monday, February 11, 2002 2:10 PM
> Subject: Re: [Yaesu] Re: Pin Diodes FT-990
>
>
> | > This IF hiss in the FT-990s has been something I've been interested in
> | > working on for some time.
> |
> |
> | > Kind of provocative but maybe we can make this otherwise good radio
> close
> |
> |  > perfect.
> |
> |    What HISS are we all talking about?  I haven't noticed no HISS.
> |
> |        73 (= Best Regards) de: Ron [email protected]
> |     100% LINUX, 100% of the time.   SENT Time and Date are UTC
> |         Visit my HAM Web SITE at: http://www.qsl.net/ka4inm
> |
> | _______________________________________________
> | Yaesu mailing list
> | [email protected]
> | http://mailman.qth.net/mailman/listinfo/yaesu
> |
>
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