Re: [Discuss-gnuradio] Ettus N210 GMSK 9600

[Tom Golden]

The modem's output is ~30dBm and I have ~30dB of attenuation.  I figured that the FFT is showing a lot of power, so the gain wasn't necessary.

Thanks for the idea.  Implementing a LPF in front of the resampler doesn't seem to make a difference though.

On Wed, Mar 23, 2016 at 1:29 PM, Marcus D. Leech <address@hidden> wrote:

On 03/23/2016 03:14 PM, Tom Golden wrote:

Here's my flow-graph along with a snapshot of the constellation and FFT.

Thanks!!

-Tom

What output level is the RF modem producing?  Are you using attenuators between it and the RF input of the USRP?

I note that you have the RF gain on your capture set to 0.

Also, I don't recall whether the polyphase resampler requires that the input already be Nyquist-limited before it gets resampled.
  I know some of the other resamplers do that, otherwise, you get aliases in your output passband.

Since you're looking at a 9600BPS signal, you should probably have sampled this at 250ksps--the lowest rate possible with the N210--
  that makes filtering later less computationally intensive.

On Wed, Mar 23, 2016 at 1:01 PM, Marcus D. Leech <address@hidden> wrote:

On 03/23/2016 02:48 PM, Tom Golden wrote:

Hi,

I'm a novice gnu radio user.  I'm using gnuradio with an Ettus N210 cabled to a modem transmitting GMSK 9600bps.  This is just for a test to verify the modem transmit bits.

I'm having issues with resampling.  The N210 clock can't be set to a multiple of 9600, so I'm attempting to resample. I've tried various mechanisms but the output after resampling to 96000 is too noisy to successfully decode bits.  I've tried the GMSK demod block as well as the combination of Quadrature Demod->Clock Recovery MM->Binary Slicer - and neither works.

I've also played with the Polyphase clock sync but I don't see any noticeable difference. Can anyone recommend a solution?

Thanks!!
-Tom

For a first step, it would be useful for you to share your flow-graph with the list.




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