[USRP-users] Resolution of center frequency settings

mahaveer gupta mgupta1616 at gmail.com
Wed Dec 31 05:36:42 EST 2014


Thanks Marcus!

On Fri, Dec 26, 2014 at 2:30 PM, Marcus Müller <marcus.mueller at ettus.com>
wrote:

>  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
> > 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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>> > USRP-users at lists.ettus.com
>> > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
>>
>>
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>>
>
>
>
>  --
>  Thanks,
> M
>
>
>


-- 
Thanks,
M
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