# Arbitrary symbol rate transmission by USRP2

Arbitrary symbol rate transmission by USRP2.

I would like to transmit dqpsk 1,536,000sps with using benchmark_tx.py
and
benchmark_rx.py thru USRP2.
Since USRP2 does have 100MHz as clock source.
With the clock, seems decimation and interpolation only allow me at
1,538,461sps(100MHz/65) is the nearest.

I thought I need to make the clock which familiar to the desired symbol
rate(1,536,000sps).
Which I modify FPGA’s DCM clock manager to prescale 100MHz by 3125
(100,000,000/3125=32KHz) and
multiply it by 384(12.288Msps = 8x 1,536,000) and feed it to the
I feel decimation and interpolation number is too huge for DCM.

Would someone please advice me out where I should tackling to at first?
Should I still go for modify DCM clock manager?

Yoshiya K…

On 01/04/2010 07:29 AM, åŠ è—¤ç¾©ä¹Ÿ wrote:

Which I modify FPGA’s DCM clock manager to prescale 100MHz by 3125
(100,000,000/3125=32KHz) and
multiply it by 384(12.288Msps = 8x 1,536,000) and feed it to the DAC/ADC.
I feel decimation and interpolation number is too huge for DCM.

Would someone please advice me out where I should tackling to at first?
Should I still go for modify DCM clock manager?

Do not modify the DCM. That will not do what you want. What you will
need to do is make the software output samples at a rate which the USRP2
can handle. There are 2 ways to do that:

1 - Create a modulator which will output a non-integer number of samples
per symbol.

2 - Use the existing modulator, and add a rational resampler to your
transmit chain to get to a useful rate. The ratio between 1.536M and
100 M is 3125/48 or (5^5) / (2^4 * 3). So you would need to interpolate
by a total of 5^5 and decimate by 2^4*3. You can get 5^2 in the USRP2
interpolator by setting the main interpolator to 25. This means you
would be supplying 4 MS/s over the ethernet to the USRP2.

You would then generate you sample stream at 5 samples per symbol,
giving you 7.68 MS/s. Now the ratio between 7.68 MS/s and 4 MS/s is
only 25/48. So you would use the GNU Radio rational resampler block to
do this sample rate conversion, which is not hard.

Matt

Matt. Thank you very much for very prompt and highly accurate reply.

To have maximum number of options for considering with my limited
resources.

-Change 100MHz crystal to 76.8MHz or 122.88MHz(lined up on Crystek
CVHD950)
can not be a option?
Will this idea give significant impact to USRP2?

maximum frequency for each cores.

Why I thought this may be a option was because Scott tried “re-clocking”
motherboard on below.

scott scott
Mon, 26 Oct 2009 19:49:56 -0700

In my understand 100MHz(CVHD950) is simply used for the drive DAC/ADC
and is
able to replace.
Unfortunately, I could not reach to the document address specified in
above
mail.

Best Regards.

2010å¹´1æœˆ5æ—¥6:14 Matt E. [email protected]:

I thought I need to make the clock which familiar to the desired symbol

total of 5^5 and decimate by 2^4*3. You can get 5^2 in the USRP2
interpolator by setting the main interpolator to 25. This means you would
be supplying 4 MS/s over the ethernet to the USRP2.

You would then generate you sample stream at 5 samples per symbol, giving
you 7.68 MS/s. Now the ratio between 7.68 MS/s and 4 MS/s is only 25/48.
So you would use the GNU Radio rational resampler block to do this sample
rate conversion, which is not hard.

Matt

Yoshiya K. [email protected]

On 01/04/2010 07:17 PM, e\$B2CF#5ALie(B wrote:

Matt. Thank you very much for very prompt and highly accurate reply.

To have maximum number of options for considering with my limited resources.

-Change 100MHz crystal to 76.8MHz or 122.88MHz(lined up on Crystek
CVHD950) can not be a option?
Will this idea give significant impact to USRP2?

You should be able to use the 76.8 MHz option without any trouble. You
would not be able to meet timing in the FPGA with the 122.88 MHz one.
You will also need to change the firmware in the FPGA which sets the PLL
tuning if you are using an external reference.

It will be MUCH easier to use the rational resampler I mentioned in my
previous email. I would strongly advise that you try that first, as it
shouldn’t take you more than an hour or two.

Matt