UHD Warning on ODD Interpolation

GNU Radio
1

Hello list,

I have a graph consisting of a modulator mixed with a signal source, and
a USRP sink.

I have it configured with a sampling rate of 720KHz.

When I run it, UHD fires the following warning (and transmission seems
fine).

 The requested interpolation is odd; the user should expect CIC

rolloff.
Select an even interpolation to ensure that a halfband filter is
enabled.
interpolation = dsp_rate/samp_rate -> 89 = (64.000000
MHz)/(0.719101 MHz)

What is this about? Thanks.

2

Hey Marcus, thanks for answering.

(1) 720Khz is not an exact integer fraction of 64MHz (the sampling
rate
of the device)–so it rounded

(2) It’s warning you that you’ll get a “humped” frequency response
with
more rolloff than you’d like, due to the rounded-down sample rate
requiring an interpolation rate that is not divisible by two.

If I understood correctly, this it means that the transmitted signal
gets filtered more on the edges than expected?

Note that by setting a sampling rate of 720KHz, the transmission and
receiving work fine.

That is, can I fix the sampling rate warning and preserve the
transmitted signal by modifying the sampling rate (e.g. to 480KHz) and
by adding a band-pass filter before the USRP sink block?

Thanks for your insights. I appreciate.

3

On 08/01/2013 08:16 AM, embyte wrote:

The requested interpolation is odd; the user should expect CIC

[email protected]
Discuss-gnuradio Info Page

Two problems:

(1) 720Khz is not an exact integer fraction of 64MHz (the sampling rate
of the device)–so it rounded

(2) It’s warning you that you’ll get a “humped” frequency response with
more rolloff than you’d like, due to the rounded-down sample rate
requiring an interpolation rate that is not divisible by two.

–
Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

4

Dear Marcus,

How wide is your signal?

My signal is 25 KHz wide, centered at 25 KHz. (I mix the baseband signal
with a sin function at 25 KHz)

Then find the nearest sample rate > your_bandwidth that satisfies the
desired outcome that the computed
interpolation rate (64.0e6 / desired_rate) is an even number.
If the signal has artefacts that “spill” outside the required sample
rate, then, yet, filter it first.

While experimenting, I discovered that with 870 KHz sampling rate works,
i.e. without the half-band filter problem. However, I get the baseband
signal mirrored at 50 KHz lower than the center frequency configured in
the USRP. It seems like related to the sampling theorem (I mean
aliasing, 2*Fmax=Fs)

I can’t figure out how the USRP works internally, and why moves the
baseband signal to the transmitting frequency (Yes, I’m not an RF/DSP
expert, just intermediate :slight_smile:
Looking on-line,I found out that the function block diagram is something
like: CIC (Intepolation) -> half-band filter -> DAC -> TX. Can you
confirm it?

I appreciate.

5

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