[Yaesu] FT-1000MP: main rx AF hiss

[George Cutsogeorge]

Hi Steve and the list,

Some time ago I posted a message about the "audio hiss" which answers some
of the questions that have recently come up.  The previous msg is repeated
below.  Since that time I've built a prototype and run some dynamic range
tests on the modified receiver.  The net effect due to the increased
sensitivity is actually a small increase in dynamic range.  Yaesu was kind
enough to sell me some connectors and cables, so I am laying out a PC
board.

George, W2VJN

----------
11/24/97


I have been looking into the problem and have what I believe to be a good
solution.  The noise coming out of the receiver is mostly IF noise.  This
can be verified by removing a pair of filters, one in each IF, and
measuring the audio output noise and setting a reference on the meter. 
Then, return the filters and take another measure.  Any additional noise
will be coming from the front end of the radio.  The front end noise must
over ride the IF (and audio) generated noise or the receiver noise floor
will be degraded.  The result of my test showed from 0 to +2 dB of
additional noise with various filter combinations.  This is not enough to
over ride the IF noise and shows that the gain before the second IF is too
low.

The RF gain reduces the noise because it operates on the agc buss which
reduces the IF gain as well as the RF gain.  Another way to observe the
problem is to switch in front end attenuation.  You will notice that the
noise heard out of the radio does not reduce very much.  If the noise were
coming from the antenna, attenuation would reduce it.  This is not a
problem on 10 meters when the tuned front end is employed as the dual gate
mosfet has more gain than the push pull RF stage used on the lower bands. 
You will notice more front end noise on 10 meters for this reason.

In my unit I have installed a small, high dynamic range amplifier with a
few dB of gain at the 70 MHz input to the IF board.  It doesn't take much
gain to over ride the IF noise and excessive gain would reduce the strong
signal capabilities of the radio.  The measured noise floor of my radio has
improved 3 to 4 dB and it really sounds good to me now.  Antenna noise is
very apparent and I have been able to reduce the IF gain using menu item
9-1 by about 3 dB.  This further reduces the IF hiss problem by an
equivalent amount.

After the WW contest I'll measure the dynamic range of the modified radio
and maybe adjust the gain a bit.

The amplifier is very simple.  A major problem is that I have not found a
supplier for the pc mounted coax connectors used in the radio.  If I can
come up with connectors, I intend to make a mod kit available through
International Radio.  

George
w2vjn@rosenet.net


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