[amsat-bb] Re: front-end overload was: echo
Andrew T. Flowers, K0SM
Sat Aug 26 09:59:29 PDT 2006
>the radio set in the Narrow FM mode for greater sensitivity.
Two things come to mind, and I want to make sure anyone reading this
thread doesn't get confused:
The first is to make sure that the NBFM bandwidth is wide enough to pass
9600 baud transmission. Doesn't one need about 20 KHz? I'm sure
someone out there knows. Don't forget that you have to account for
being off frequency due to doppler, particularly if you are using 5-kHz
Secondly, the mode setting on the radio is something that affects a
filter in the IF chain, not the front-end of the radio. I think the
issue with the FT-8800--and generally any other transceiver designed to
double as a wider VHF receiver--is that the the front end amplifier is
overloaded by a strong signal at *any frequency*. The classic receiver
design has a (preferably low-noise) amplifier as the first thing in the
receive chain, which boosts the incoming signal enough to overcome the
following mixer losses. That ampflifier is being bombarded by
everything from DC-GHZ, and will amplify whatever the transistor is
capable of. (There is usually some sort of filter ahead of it in
commercial equipment, but in the case of wide-band VHF receiver it's
likely to be pretty wide).
Now as we all know, amplifiers have a limit as to how much output they
can supply, and after an incoming signal gets too loud it will become
distorted. This is familiar to many of us in urban areas when we
suddenly hear a pager on the frequency of our favorite repeater. That
is often an effect of that very strong signal being clipped and
introducing new frequency components. Another effect is that a strong
incoming signal to that front end--not matter what frequency it is
on--effectivly swamps the amplifier so that the weaker signals one
desires to listen to are not amplified faithfully. This often manifests
itself as "deafness", but it is really because the front end is spending
what juice it can supply on that big signal 50 MHz away from where you
are trying to listen. No changes to IF filters are going to make that
signal go away. When this happens one usually has to use some sort of a
filter ahead of the amplifier, either to notch out the offender, or pass
only the band of interest. This will allow the front-end amplifier to
run in its linear region, and thus amplify all the incoming signals
without (significant) distortion.
The lab specifications for front-end overload can generally be figured
from the thrid-order intercept point (IP3), which essentially tell you
how much distortion one gets for an input signal of a given amplitude.
The "sensitivity figure" doesn't tell you anything about how the radio
will perform in a high-RF environment.
I don't mean to lecture, but rather clear up some possible confusion.
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