Thanks for your response. Please see below. Sean ________________________________ From: discuss-gnuradio-bounces+sean.nowlan=gtri.gatech....@gnu.org [discuss-gnuradio-bounces+sean.nowlan=gtri.gatech....@gnu.org] on behalf of Marcus D. Leech [mle...@ripnet.com] Sent: Monday, December 12, 2011 12:05 AM To: discuss-gnuradio@gnu.org Subject: Re: [Discuss-gnuradio] CW between bursts
On 11/12/11 11:26 PM, Nowlan, Sean wrote: I'm not sure whether this is a GNU radio or USRP issue, so I'm posting to both lists. I played around with GNU radio's stream tagging feature and managed to implement something that does timed bursts (partially by taking a lead from tag_source_demo.h, etc. in gr-uhd/examples). I'm observing a continuous wave at the carrier frequency in between bursts, and also during start-up when the gr_uhd_usrp_sink object is instantiated before any samples get transmitted. The CW is about 30 to 40 dB down from my signal, but I'd like the radio to be silent in between bursts. I've looked into changing the LO offset, but this just seems to move the CW to another part of the spectrum, but not knowing any better, this may be the expected behavior. Would UHD calibration or playing around with the DC offset help at all? I'm at the extent of my knowledge with this one. Thanks, Sean >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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