Re: [Discuss-gnuradio] USRP tuning time

[Marcus Müller]

Hi Mat,

the answer is a bit more complicated, I'm afraid:

so first of all, we need to distinguish latency from time it takes to tune.
Latency happens if you want the device to tune as soon as possible, and will be mainly dominated by the latency of your general purpose OS running on your general purpose computer. It wildly varies, but good estimates on a properly configured PC might be around 5ms; your actual time might be different.

Now, you can hide that latency by using timed commands if you know in advance when you want your tuning process to start. Then all there's left is the "invalid" signal during the time the device actually tuned.

USRPs have a two-stage tuning: You get the configurable synthesizer that generates the LO for mixing down (and up), and you have the digital frequency shifter [1].
The digital tuning happens almost instantly. Basically, the moment the right register is set, the CORDIC uses a different phase increment [2]; that can happen at every clock cycle of the master clock (ie. 200MHz by default on X300)[2].

Then, there's the analog part. That's where things get tricky, because a) we now have to take the time into account it takes to tell the LO synthesizer what we want to do, and b) the time it takes for the synthesizer to get stable, and c) there might be DC offset filters that might distort your signal.

a) communication time

This depends on the synthesizers used. The UBX uses MAX2871 synthesizers. These are SPI chips; from the top of my head I don't know at which rates the bus is clocked, but let's assume it's "sufficiently fast" (>>1Mb/s). So let's subsume that with 20µs max.
Now a speciality for the UBX: if the LO synthesizer you want to tune is turned off, it must be started first. Typically, you'd only have that at initialization, but since the UBX spans so much spectrum, different clock lines have been introduced, and hence, a separate synthesizer for frequencies below and above 500MHz each. So when crossing that boundary, assume at least 20ms additional startup time.

b) analog/real-world hardware stuff

Things get tricky here, and I can't actually give you a definite answer. Tuning speed depends on too many factors, some of which aren't even controllable. Looking through datasheets and at the loop filter components, I'd say a locking time of up to 100µs might be realistic. However, that's a bit up to you as user; the less accuracy you need, the shorter you can wait for the LO to successfully lock. After all, "locked" is just the notion that PLL oscillations have dropped below an arbitrary level.

c) DC offset filtering

RX only:
To eliminate the DC offset that the ADC might measure, there's a DC offset filter. It's an IIR whose sole accumulator gets reset at every tune. Depending on how far the real DC offset is from 0, the IIR's step response might or might not be visible for up to 40ms, if I remember correctly. You can disable the DC offset removal, however, with the multi_usrp::set_rx_dc_offset(false); however, DC offset removal is then up to you; this might or might not be a problem for your application.

Best regards,
Marcus

[1] http://files.ettus.com/manual/page_general.html#general_tuning_process
[2a] https://github.com/EttusResearch/fpga/blob/master/usrp3/lib/dsp/duc_chain.v#L41
[2b] https://github.com/EttusResearch/fpga/blob/master/usrp3/lib/dsp/duc_chain.v#L82
[2c] https://github.com/EttusResearch/fpga/blob/master/usrp3/lib/dsp/duc_chain.v#L193
[2d] https://github.com/EttusResearch/fpga/blob/master/usrp3/lib/dsp/cordic_z24.v

On 09.08.2015 19:05, Mat Mat wrote:

Dear all,

After quite some searching, I wasn't able to find information about how
long it takes to retune a USRP (say, change the centre frequency). Do
you guys know anything about that? Specifically, I'm using a USRP X300
with a UBX160 daughterboard. Does anyone know how long it takes in this
specific setup to retune the daughterboard to a different centre
frequency?

Kind regards and many thanks,
Mat