At 06:17 AM 12/11/2005, Christoph - HB9AJP wrote:
What is the tx ssb image rejection you obtain with sdr-1000?
According to my ham friends I have only 20-25dB of rejection.
Is that with a tone or two tone modulation? Or is it on speech?
I'm curious, because it's entirely possible that you might get really good
image rejection at some audio frequencies and terrible at others (not that
-25dBc is terrible, but it doesn't meet the usual requirement of
-40dBc). If the two audio paths aren't identical (in terms of
phase/amplitude vs frequency), then calibrating at one frequency will give
you great performance at that frequency, but not so hot at others.
The phase balance has to be pretty good to get -40dBc.
With PERFECT phase balance, the amplitude balance has to be within about a
quarter dB. With perfect amplitude balance, and a phase balance of 2
degrees, you'll only get 35dB of rejection. To get 30dB you need about 3.7
degrees balance.
This is very tough to do over a wide bandwidth (i.e. 0-40 kHz)
http://www.microwaves101.com/encyclopedia/mixersimagereject.cfm
I tried to
optimize the rejection with Transmit Rejection in the DSP setup menu while
measuring with a second receiver, a spectrum analyzer and a xy-KO (circle at
line out). I am using sdr-100O, Delta44, v1.4.5 Beta7. Sorry if the theme has
been treated before.
Chris, HB9AJP
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