[USRP-users] Resolution of center frequency settings
mgupta1616 at gmail.com
Wed Dec 24 22:02:27 EST 2014
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
On Wed, Dec 17, 2014 at 4:21 AM, Marcus Müller <usrp-users at lists.ettus.com>
> 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.
> 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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