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Re: [Discuss-gnuradio] Bandpass Sampling

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] Bandpass Sampling
Date: Mon, 08 Dec 2014 20:38:16 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0

Hi Rich,

just a hint first: this is an interesting question, but it's very USRP-centric; there's the usrp-users mailing list especially for USRP-related things, which I find quite interesting, most of the time [1] :)

The N210 is a modular USRP, meaning that the mixing is usually done on a daughterboard, which will usually actually take your input signal and bring it down to baseband. Therefore, the ADC always sees -50MHz to +50MHz of signal (it's a complex sampling ADC, hence the negative frequencies), and all daughterboards for the N2x0 are limited to a 40MHz baseband bandwidth, so there is no aliasing (^1).

The N2x0 architecture _always_ uses the 100MHz sampling rate on RX, and the user is free to choose any integer fraction of that rate (^2). Using digital filtering and decimation, the 100MHz Nyquist bandwidth is reduced to the desired sampling rate.

Thus, in usual operation, your daughterboard will be tuned, to generate a 300MHz (complex) tone, with which it mixes your signal down to -160kHz and 160kHz, which you will see with any sampling rate >320kHz. You are free to specify which frequency the daughterboard tunes to, so you can also tune to let's say 295MHz and see your signal at 5MHz +- 160kHz, although you'll need to use a much higher sampling rate, for example 100MHz/18~=5.56MHz, or your signal will be filtered out.

I hope that answered your question :)


^1 there's the exception of the basicRX daughterboard, which neither mixes nor has the 40MHz low pass characteristics; if you use that, you will have to make sure that your signal fits into the ADC's bandwidth, otherwise you *will* see images.
^2 as long as the resulting rate fits through gigabit ethernet, ie. 25MS/s for 16bit, 50MS/s for 8bit max.
[1] http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

On 12/08/2014 08:23 PM, Richard Bell wrote:
I have a binary FSK signal centered at 300 MHz with 160 kHz offset peaks
from the center freak. Therefore the lowest signal frequency of interest is
300-0.16 = 299.84 MHz and the highest is 300.16 MHz.

I know the maximum sampling rate of the USRPN210 ADC is 100 MSPS. If the
USRP uses that sample rate of 100 MSPS, it will alias my signal onto itself
in an unwanted manor.

My question is, how does the USRPN210 decide what sample rate to set the
ADC to? Is it a function of both the center frequency and the sample rate
setting of the UHD source block?


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