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<DIV>In principle, this is correct. There is no useful low noise ,high IP3 and
variable gain discrete ciruit know to do this. I would suggest to use a low
noise high IP3 GaAs amp, if necessary in pus-pull and put an GaAs binary
coded step attenuator ahead of this stage with 40 dB range and 0.2 dB minimum
loss in 1 dB steps , connected to the main AGC, being activated after 50dB AGC
of the IF AGC. This does NOT eliminate an high IP3 first mixer , second preamp
and second mixere. A similar system was implemented in the R&S ESH3
(10Khz to 30 MHz )and ESVP (20 MHz to 1300MHz ) test and measurement
receivers, using a relay based attenuator. The ESH3 did not have a preamp, the
ESVP did have one.</DIV>
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<DIV>The R&S XK2100L I am using has an optional preamplifier, which so far i
have only needed above 21 MHz....</DIV>
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<DIV>73 de Ulrich, N1UL</DIV>
<DIV> </DIV>
<DIV>In a message dated 8/13/2006 9:02:12 P.M. Eastern Daylight Time,
k3bu@optonline.net writes:</DIV>
<BLOCKQUOTE
style="PADDING-LEFT: 5px; MARGIN-LEFT: 5px; BORDER-LEFT: blue 2px solid"><FONT
style="BACKGROUND-COLOR: transparent" face=Arial color=#000000 size=2>*****
High Performance Software Defined Radio Discussion List *****<BR><BR>Amen to
that!<BR><BR>Another important requirement for superior RF preamp is
continuous gain control and AGC control with possible programmable levels for
various antenna selections.<BR><BR>Continuous gain control is very important
for top notch RX performance, the gain of the preamp should be set just at the
treshold of band noise, so you know that there is enough gain to hear the weak
one, but not more than needed.</FONT></BLOCKQUOTE></DIV>
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