[amsat-bb] Re: front-end overload was: echo
Sun Aug 27 17:19:44 PDT 2006
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
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
> 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
> Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author.
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