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G0FTD > TECH 24.10.07 18:48l 59 Lines 2038 Bytes #999 (0) @ WW
BID : 000025296PZT
Read: GUEST OE7FMI
Subj: RF gain vs IF gain.
Path: DB0FHN<DB0MRW<DB0PV<DB0WGS<OE2XUM<OE5XBL<OE6XPE<DB0RES<ON0AR<GB7YKS<
GB7SYP<GB7PZT
Sent: 071024/1634Z @:GB7PZT.#24.GBR.EU [Kidderminster] #:2500 XSERV410h
Date: Wed, 24 Oct 2007 17:34:04 +0000
From: g0ftd@gb7pzt.#24.gbr.eu (Andy)
To: tech@ww
Subject: RF gain vs IF gain.
I'm starting to wonder if modern rigs have a generic design flaw in them.
And that's the lack of IF gain control or a poor AGC chain.
In a typic amateur rig you'll find that the pre-amp before the 1st mixer
is used in a simple off or on state. It's not part of the AGC or RF gain
control mechanism.
This creates big problems.
In the presence of strong signal the amount of preamp gain ensures that
the 1st mixer has too much gain, forcing the 1st rx mixer to overload
and creat plenty of intermodulation products. It's real pain in the
backside, as I find when out in my /P QTH.
I've been playing for just over 1 year with this problem.
Firstly I've played with tuned LC notch filters.
This has been mostly effective,the idea is that if a 2nd order IP is
evident then so long as you kill one of the frequencies involve the
intermod products go away.
The other is to use the tuned variable LC circuit as an attenuator
for near operating frequency attenuation.
By tuning the LC circuit on the *slope* of an offending frequency you'll
then have am infinitely controllable attenuator, so that you can
provide only the 2 or 3 db reduction needed to stop the pre-amp gain
overloading the 1st front end mixer.
The other solution is to reduce front end gain (1st pre-amp) and increase
the IF gain. This seems to be quite an effective way of maintaining
overall rx sensitivity by compensating for the reduced 1st pre-amp
gain.
This latter solution has been tested on my IC706 MK2, whilst the former(s)
have been tested on an old 706 MK1.
The problem is that if you turn the pre-amp off, then you may eradicate
IP's, but lose weaker signals due to lack of gain.
What we need is a controllable IF gain AND front end pre-amp
gain control which no rig seems to offer.
This way the user can change the rx gain *distribution* and control
intermodulation products more effectively.
Grrrrr...
- Andy -
G0FTD @ GB7PZT
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