[amsat-bb] Re: front-end overload was: echo

[Andrew T. Flowers, K0SM]

McGrane wrote:

>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 
steps.

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.

-Andy K0SM/2