[Discuss-gnuradio] Sampling rate mismatch and rational resampler vs Repeat

[Jeon]

I am building a certain system whose clock rates can be 200k, 400k, 3.75M, 7.5M, 15M, 30M, 60M and 120 MHz.(It's not an RF communication system, but a wired communication system using a square wave on-off keying.)

First of all, I've tested a USRP with only available rates for USRP (i.e. 200k, 400k, 3.75M, 7.5M and 15 MHz). But, when I ran a flow graph, clock rates of 3.75M, 7.5M and 15 MHz are tuned to 3.703704M, 7.692308M and 14.285714 MHz.

I see that it's because of dsp rate should be divisible by sampling rate. So, the GNU Radio, or USRP tunes sampling rate as close as one given by a user.

What I concern is, will such differences between given sampling rate and tuned sampling rate make a big difference on a waveform, throughput and performance? I don't think it would make a big difference since ratio of differences are less than 5 percent.

But it would be better to know about it before I run a real experiment.

For the second, I want to fix the sampling rate of USRP as the common multiple of clock rates, 15 MHz. And I am thinkinkg about using a rational resampler block or a repeat block to make a low clock rate signal fit 15 MHz clock.

For example,

1. 200 kHz signal -> rational resampler (interp: 15M, decim: 200k) -> USRP (15 MHz)

2. 200 kHz signal -> rational resampler (interp: 75, decim: 1) -> USRP (15 MHz)
3. 200 kHz signal -> repeat (interp:75) -> USRP (15 MHz)

When I run those three, first two using rational resampler blocks gave me ugly waveforms. I think it's because band limiting filter is applied to the original signal or something like that. (Forgot to take a picutre of FFT, sorry.)

In this situation, I think I don't have to use rational resampler and it's okay to use a repeat block because a clock rate of USRP (15 MHz) is a multiple of every given clock rates (200k, 400 kHz, ...) and because it's not a sinusoid, but a simple on-off keying.

So, how do you think of it? Are there something that I am missing in a real implementation?

Regards,

Jeon.