From: "Paul Keinanen" <keinanen@sci.fi>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Pre-amps.
Date: Wed, 25 Apr 2001 21:06:02 +0300
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References: <001501c0cce5$fde69ca0$4527893e@g3aqc> <3.0.1.16.20010425094349.309747e8@mail.cc.kuleuven.ac.be>
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On Wed, 25 Apr 2001 09:43:49, Rik Strobbe
<rik.strobbe@fys.kuleuven.ac.be> wrote:

>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.

Mini-Circuits does not seem to publish noise figures for low
frequencies (below 100 MHz). 

However, in general, it is not a good idea to use microwave
transistors for very low frequencies, since the corner frequency for
the 1/f noise can be quite high. 

In normal transistors, the  corner frequency for 1/f noise is in the
lower audio frequencies, but watch out for 1/f noise in VLF and LF
frequencies when using microwave transistors.

Paul OH3LWR