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From: | Marcus D. Leech |
Subject: | Re: [Discuss-gnuradio] CW between bursts |
Date: | Mon, 12 Dec 2011 00:05:53 -0500 |
User-agent: | Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.15) Gecko/20101027 Fedora/3.0.10-1.fc12 Thunderbird/3.0.10 |
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. 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). You might try the most-recent UHD and appropriate FPGA, etc, to see if you are now getting LO suppression between bursts. 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. -- Principal Investigator Shirleys Bay Radio Astronomy Consortium http://www.sbrac.org |
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