|Subject:||Re: [Discuss-gnuradio] Question about sampling rate and center freq|
|Date:||Sat, 30 Apr 2016 21:23:23 -0700|
Not at all!Sorry, I was taking pictures of hardware and hence just took one forthe GRC too. As I showed in previous picture, I am only using RX-B. Iam not sure how to connections will be for dual mode, but currently Iam just using one. I get complex values in the GRC and I can plot
complex FFT of them
Interesting! Why is there a time axis? I must admit those graphs look like they took some effort to create; really, a clean drawing on paper would have been as good for me, but thanks.
No, I didn't mean that.And, as I mentioned in IRC when I asked for them, it is important to define wheter you're doing I&Q or real sampling.
I am dong I&Q sampling as shown in . By real sampling, I supposeyou meant that I am taking magnitude before feeding to the Spectrogram.
I mean: Is the signal you *want* to observe a single voltage signal varying over time, or is it an I & Q signal pair?
I'm not convinced you actually are in both cases! That's why I'm asking :)It's no use discussing any of your question until we know whether we're looking at a complex equivalent baseband or at a real signal.
I am doing complex baseband as in the picture.
OK, we need to talk about sampling:My question was relatedto the sampling. I thought doing sampling (complex baseband as shownin GRC)
Sampling takes place where your analog signal becomes a series of digital numbers, in the ADC of the N210. That is before you digitally tune to e.g. 125 kHz. The LF and Basic daughterboards don't have tuners!
That really really depends! It depends on whether you want to observe a single real signal or an I&Q signal pair. So: what do you want to observe?in 250kHz at center frequency 125kHz is equavelent to doing500kHz, with center frequency 0Hz using LFRX.
So here's the problem I'm having with your original question:
But: What *are* the first 250 kHz?I want to sample first 250KHz on the channel.
If you're just sampling a real signal with a single ADC, then obviously, you get a real-valued stream of samples, and to cover the bandwidth of 0 Hz – 250 kHz you will need to sample at 500 kHz.
If we're talking about complex baseband, I'd define (and that's purely something that I like, not something that is uniformly defined somewhere) the "first 250 kHz" to be the spectrum from -125 kHz – +125 kHz, because those 250 kHz correspond to the 250 kHz bandpass signal "closest" to the center frequency of your down-mixer. To get those 250 kHz, your ADC will need to run at 250 kHz, but it will need to produce one I and one Q sample simultaneously.
You can configure the N210 carrying a Basic or LF board into either mode – real sampling of up to two channels, and complex sampling of an I&Q pair that you feed into the two SMA connectors on the daughterboard.
We really need to understand what you *want* to be doing!
So, I really don't know what literature you've got access to – if you have access to the scriptum for a communication theory introduction, that might be a good start.
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