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

[Stuart Rumley]

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" <ka4inm@tampabay.rr.com>
To: <yaesu@mailman.qth.net>
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 ka4inm@tampabay.rr.com
|     100% LINUX, 100% of the time.   SENT Time and Date are UTC
|         Visit my HAM Web SITE at: http://www.qsl.net/ka4inm
|
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