|Subject:||Re: [Discuss-gnuradio] CW between bursts|
|Date:||Mon, 12 Dec 2011 05:52:51 +0000|
Thanks for your response. Please see below.
From: discuss-gnuradio-bounces+address@hidden [discuss-gnuradio-bounces+address@hidden on behalf of Marcus D. Leech address@hidden
Sent: Monday, December 12, 2011 12:05 AM
Subject: Re: [Discuss-gnuradio] CW between bursts
On 11/12/11 11:26 PM, Nowlan, Sean wrote:
>Mixers generally have between 30 and 40dB of LO suppression, so what you see is roughly what you'd
> expect. Calibrating the I/Q phase and magnitude will help move the suppression towards the lower
> end, I think.
I'll give this a shot, but I'm not sure calibration is supported on RFX900 yet. (It doesn't appear here yet: http://files.ettus.com/uhd_docs/manual/html/calibration.html)
>There's a "tension" here between wanting to maintain phase-coherence in the mixer between bursts,
> and wanting to suppress the LO between bursts. Not sure that it's configurable, but perhaps
> it should be?
>In a heterodyne system, the LO is very often outside the passband of the TX filters, but in
> direct->conversion or small-offset up-conversion, you may not have that luxury. I looked at the git-log
> of the most recent UHD, and it looks like Jason added shutting-down the TX mixer between bursts, so
> I don't know which strategy "won" (keep mixer up between bursts to give phase coherency across
> bursts, or shutdown the mixer between bursts to suppress the LO between bursts).
Is this phase coherence helped by using a stable reference clock? Is phase incoherence caused by random drift in a PLL or some other component?
>You might try the most-recent UHD and appropriate FPGA, etc, to see if you are now getting
> LO suppression between bursts.
I'll try this too. I think I'm using a version that is a month or 2 old.
>In a more traditional radio, the combination of the LO being out-of-band with respect to the final
> TX filters, and keying of the TX amplifiers makes this a non-problem. But turning on and off
> the various components in the analog chain has its own problems--they take a finite time to turn
> on, and stabilize, so you end up with *other* issues as a result.
So could I use LO offset and push it outside the transmit filter bands? (Whatever those are; I'll have to look through the specs; but I believe I'm limited by the range of the offset). Would this LO bleedthrough be fully suppressed at that point, and what negative effects might I expect? I'm not a very experienced RF guy, so a lot of the front-end stuff is new to me.
-- Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org
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