[USRP-users] Resolution of center frequency settings

Marcus Müller marcus.mueller at ettus.com
Fri Dec 26 15:30:04 EST 2014


Hello Mahaveer,

thanks for following up on this!

There has been some work done on Radar with USRPs [1] and also a Google
summer of Code project this year, demonstrating the usage of USRPs, GNU
Radio and Radar algorithms [2], if this is of interest to you.

Greetings,
Marcus

[1] http://digbib.ubka.uni-karlsruhe.de/volltexte/1000038892
[2] https://github.com/kit-cel/gr-radar
On 12/25/2014 04:02 AM, mahaveer gupta wrote:
> Thanks very much for the detailed response. This is really helpful. We
> were trying to develop a RADAR based application using USRPS whose
> effectiveness depends on the frequency resolution
>
> Regards,
> M
>
> On Wed, Dec 17, 2014 at 4:21 AM, Marcus Müller
> <usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>> wrote:
>
>     Hello Mahaveer,
>
>     tuning USRPs is a two-stage process:
>     1. the daughterboard has gets tuned.
>     2. the DSP logic performs digital frequency shifting.
>
>     1. Daughterboard tuning
>     -----------------------
>
>     The daughterboards have local oscillators (LO) which they use to mix
>     down the RF signal to baseband (or vice versa).
>     These LOs are generated from the motherboard reference clock,
>     usually by
>     fractional interpolation; thus, the LO can only tune to a discrete set
>     of frequencies.
>     How many of these frequencies exists, and how they are distributed,
>     depends on the daughterboard.
>
>     2. DSP frequency shifting
>     -------------------------
>
>     RX side:
>     The FPGA has logic that generates a complex sine (it's actually a
>     12-stage CORDIC) and multiplies the ADC samples with it, before
>     anti-aliasing filtering and decimating the ADC samples (coming in at
>     100MS/s) to the user-requested sampling rate.
>     This allows the USRP to transparently tune to "any" real frequency in
>     the daughterboard's frequency range.
>
>     "any" is in parentheses, because this is where accuracy comes in: the
>     sine calculation of course happens with fixed point numbers, and so do
>     the multiplication and the rest of the DSP. Thus, everything is
>     quantisized (at 16bit, most of the time); this, by considering
>     quantization noise, will give a maximum SNR you can get. With that
>     maximum SNR you could calculate the maximum accuracy a given frequency
>     estimator could achieve. And that will be your lower boundary for
>     frequency resolution.
>
>     TX:
>     exactly the same, other way around.
>
>     Overall frequency accuracy considerations
>     -----------------------------------------
>
>     1Hz *is* very small. At 2.45GHz, that would be 0.4 parts per billion
>     accuracy. The accuracy of the reference clock is (I don't have exact
>     numbers in my head, just putting down something intuitively good)
>     about
>     50 times worse. With other words: the variation you'll see because of
>     your reference clock should be expected to be 50 times as big as the
>     steps you want to do.
>
>     Even when driving the N210 at rather low user sampling rates, for
>     example 1MS/s, and considering the frequency shift in baseband, a
>     frequency accuracy of 1ppm is something that most receiving
>     systems try
>     to autonomously correct by design.
>
>     This question comes up once or twice every year, and it's always
>     interesting to hear the motivation behind it, so would you mind
>     explaining why you want to tune so finely?
>
>     Best regards,
>     Marcus Müller
>
>     On 12/17/2014 10:52 AM, mahaveer gupta via USRP-users wrote:
>     > Hello,
>     >
>     > Could you please tell me the resolution at which I can vary the
>     center
>     > frequency settings in USRP N210.
>     >
>     > For example, I can set the center frequency to be at 2.45 GHz. If I
>     > want to increase the center frequency by 1 Hz, I could use
>     2.450000001
>     > Hz. Not sure, how accurate the small increment would be and if it
>     > would make any difference at all.
>     >
>     > I know 1 Hz is very small, but would like to know the step size at
>     > which frequency increments would have effects
>     >
>     > --
>     > Thanks,
>     > M
>     >
>     >
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>
>
>
> -- 
> Thanks,
> M

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