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Re: front-end overload was: echo



----- Original Message -----
From: "Andrew T. Flowers, K0SM" <aflowers@frontiernet.net>
To: <amsat-bb@amsat.org>
Sent: Saturday, August 26, 2006 6:59 PM
Subject: [amsat-bb] Re: front-end overload was: echo

> 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

Hi Andy, K0SM/2

I have just read your thread it is a good good article, congratulations !

I would like to add few comments.

I agree with you that the third-order intercept point (IP3) describes the
amplitude of third-order distorsion products when more than one signal
is present at a receiver input.

But when a receiver is tuned to a weak signal than a strong adiacent signal
may cause an apparent decrease in receiver gain and this is better described
by the 3 dB compression point wich is essential to tell you how much the
gain of your receiver will be reduced in presence of an in-band or out of
band strong and undesired signal.

I aegree with you that the only cure is to increase the selectivity at the
front end of the receiver to attenuate the strong and undesired signals
wich are adiacent to the receiver passband because gain compression or
desensitization occurs when the input voltage from the undesired signal is
large enough to exeed the bias on an RF amplifier or mixer and drives the
base or the grid to conduct.

I aegree with you that high receiver sensitivity is the antithesis of good
cross-modulation and compression performance expecially in a high-RF
environment.

For those who are interested to go in more details in to the above matter
I recommend to read the article:

"Receiver noise figure sensitivity and dinamic range,what the numbars
means" by James R. Fisk, W1DTY Ham Radio Magazine October 1975

Upon request I can send a compressed file of the above article to
everybody.

Best 73" de

i8CVS Domenico





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