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Date: Wed, 25 Apr 2001 09:43:49
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: Pre-amps.
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Hello Laurie,

In most circumstances SNR will be determined by the 'band noise' (QRM -
QRN), so the noise figure of a pre-amp won't matter too much (anything
below 10dB should be fine). But, unless you have a very small antenna in
combination with a very 'deaf' receiver a pre-amp won't be of any use, it
will just crick-up the S-meter reading but will not improve readability of
a signal.

I did some tests woith pre-amps and found out that most simple designs have
a too poor IM behaviour and make things worse instead of better. If I would
design one I would primarily aim at a high IM rejection and don't worry too
much about noise figure or gain.
If you want to use one of the monolithic amps from Mini-Circuits I would go
for something better than a MAR-6, that one has the worst IM behaviour of all.
To compare :

type	gain (dB)	noise fig. (dB)		3rd order IP (dBm)
MAR-6	20		3			+15
MAR-8	33		3			+27
MAR-4	8		4			+26
MAR-3	13		6			+23
ERA-1	12		7			+26
ERA-2	16		6			+27
ERA-3	22		5			+23

Personnaly I wouldn't recommend the ERA-8 as it tends to be unstable in non
resistive loads, but I would go for a ERA-2.
Price of all these amps is very similar (MC suggested resale price at 30
QTY is between 1 and 2 USD for all of them) and mainly depends on what your
local supplier stocks (and how greedy he is .... ..).

An old wisdom, but still valid : the best RX amplifier is a good antenna
(0dB noise figure and almost endless IP3).

73, Rik  ON7YD

At 18:41 24/04/01 +0100, you wrote:
>Hi All,
>I have been thinking about weak signals on 136 using QRS and DFCW. These
techniques enable signals to be read which are buried 30db down in the
noise(see Extreme narrow bandwidth techniques ON7YD ) Does this mean that
signals 30db down on atmospheric noise are seen,if so what about the noise
from the RX pre-amp ? In my case this is about 20/30 db less than the Ant.
noise,so would I be looking at the pre-amp noise or is this also reduced by
the narrow bandwidth ?
>I should like to improve the noise performance of my pre-amp.
>filter combination. and would appreciate any information on high
performance circuits or amp. units. Is the MAR6 likely to be worth while.  
>By the way if my thoughts are correct this may be why high Q loops are
reported to be superior to lower Q types, higher signal output overriding
amp. noise. Any ideas ?      73s  Laurie.  
>
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