On Wed, 25 Apr 2001 09:43:49, Rik Strobbe
<[email protected]> 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
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