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Re: front-end overload was: echo




Hi andy- thanks for the education but I'm using narrow fm for the phone
downlink , not packet. I did try a diplexer with a .25 db loss between the
antenna and the mast mounted preamp but noticed significantly lower
recieve at or near fringe.
I'm in a rural area. Big power stations are not a problem. Thanks for
info, pat

On Sat, 26 Aug 2006, Andrew T. Flowers, K0SM wrote:

> 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
> 
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