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Re: [Discuss-gnuradio] Cross band relais / bridge - DMR?

From: Vijay Pillai
Subject: Re: [Discuss-gnuradio] Cross band relais / bridge - DMR?
Date: Fri, 29 Apr 2011 18:36:07 -0700 (PDT)

Hi Nick,

High end fractional N PLL's generally have a settling time of 100us or less if it is shifting to another frequency in the same band (say within 100MHz) assuming a high enough loop bandwidth. If it is starting up for the first time, then a few ms of settling time is ok.

I am just wondering what other factors/latencies that cause a settling time of several ms all the time?

Best regards,

--- On Fri, 4/29/11, Nick Foster <address@hidden> wrote:

From: Nick Foster <address@hidden>
Subject: Re: [Discuss-gnuradio] Cross band relais / bridge - DMR?
To: "Feng Andrew Ge" <address@hidden>
Cc: address@hidden
Date: Friday, April 29, 2011, 3:51 PM

On Fri, 2011-04-29 at 15:42 -0400, Feng Andrew Ge wrote:
> Nick,
> Could you confirm that switching TX frequency takes "a few ms"?  How did
> you get this number?  Is it the same for switching TX frequency after
> the code has been running for a while, that is, reconfiguring the TX
> frequency dynamically?

We don't have a guaranteed spec for PLL settling time on the WBX, and we
encourage users to experimentally determine the settling time for their
particular setup and frequency. The figure I gave is based on what I've
seen in the lab, to the best of my recollection. =) It should be pretty
straightforward to determine the settling time for your setup and
frequency step by taking a data capture of a known source.

Any time you retune a PLL VCO, it will take some time to settle. If you
need to avoid that settling time, you can do CORDIC-only tuning with the
advanced tuning parameters to shift around within the bandwidth of your
ADC. This will provide a basically instantaneous tuning speed within a
range of 50MHz for USRP2/N2XX, or 32MHz for USRP1/E100.


> Andrew
> On 04/29/2011 12:00 PM, address@hidden wrote:
> > Every time you tune the TX you'll have to wait a few ms for it to become
> > stable. For voice transmission purposes, a few ms is effectively
> > instantaneous anyway. And if you aren't retuning, then it will come up
> > instantly, because the LO will still be running in between sample
> > bursts.
> >
> > --n
> >

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