Hi Jim
It is with speech: my ham friends say they hear me with S9+20 on 80m/LSB and
when listening to USB they hear my signal at abt S8. When I try to adjust the
image rejection with transmit rejct in the DSP setup, I get abt 40db of
rejection of the single tone.
73, Chris HB9AJP
Jim Lux schrieb:
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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James Lux, P.E.
Spacecraft Radio Frequency Subsystems Group
Flight Communications Systems Section
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