Re: [Discuss-gnuradio] CW between bursts

[Marcus D. Leech]

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.

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