Multiple signals - messy spectrum

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bpadalino
Posts: 303
Joined: Mon Mar 04, 2013 4:53 pm

Re: Multiple signals - messy spectrum

Post by bpadalino » Thu Dec 10, 2015 7:52 am

Interesting it wasn't the PLL issue - I've seen that cause this type of issue. The RX and TX PLL's can internally pull on each other since they are both on all the time.

If you just use a constant to transmit and output a tone using just DC, does that exhibit the same spurs? If so, if you short out FB11, does it appear that the spectrum changes? You should be able to see results just by using tweezers while it's running and short the sides.

With TX, I've seen the PLL fighting issue cause lots of problems. Otherwise I've just seen tones with the phase noise skirt out to around +/-50kHz or +/-100kHz, depending on where you think the skirt ends.

I know there are large harmonic sample rate spurs that can show up as well. I am not sure what's best with the sample rates. When doing receive only applications, I've tried setting the TX samplerate to 160kHz just so it would be extremely slow and it would be many more harmonics before it would get into the RF.

Zortz
Posts: 15
Joined: Mon Oct 26, 2015 1:50 am

Re: Multiple signals - messy spectrum

Post by Zortz » Thu Dec 10, 2015 8:32 am

bpadalino wrote:Interesting it wasn't the PLL issue - I've seen that cause this type of issue. The RX and TX PLL's can internally pull on each other since they are both on all the time.

If you just use a constant to transmit and output a tone using just DC, does that exhibit the same spurs? If so, if you short out FB11, does it appear that the spectrum changes? You should be able to see results just by using tweezers while it's running and short the sides.

With TX, I've seen the PLL fighting issue cause lots of problems. Otherwise I've just seen tones with the phase noise skirt out to around +/-50kHz or +/-100kHz, depending on where you think the skirt ends.

I know there are large harmonic sample rate spurs that can show up as well. I am not sure what's best with the sample rates. When doing receive only applications, I've tried setting the TX samplerate to 160kHz just so it would be extremely slow and it would be many more harmonics before it would get into the RF.
Here is constant tone spectrum:
constant.png
Constant tone
Shorting FB11 had no effect at all. Lobes are not stable, jumping about 3db.
This spectrum is similar to the figures presented in http://www.analog.com/media/en/technica ... AN_927.pdf . Maybe some kind of power supply noise?

Waiting for further instructions, master :D

Zortz
Posts: 15
Joined: Mon Oct 26, 2015 1:50 am

Re: Multiple signals - messy spectrum

Post by Zortz » Thu Dec 10, 2015 9:31 am

Okay now i get different results.

Spectrum changes when i tune the TX frequency to X and then back to 1G. I used 300M or 3.5G for X atm.
Here are some results, all measured at 1G:
Attachments
2.png
1.png

bpadalino
Posts: 303
Joined: Mon Mar 04, 2013 4:53 pm

Re: Multiple signals - messy spectrum

Post by bpadalino » Thu Dec 10, 2015 11:41 am

The big spikes that come up when you output DC only is due to the PLL's fighting. Change your RX PLL to something like 433MHz or something not close to the 1GHz you are transmitting. Or change your transmit to some other area. You should see it go away.

The spikes to the left and right of the tone you're transmitting we've been able to recreate and are looking into it.

Can you let us know what versions of the library, FX3 firmware and FPGA image you're using?

Zortz
Posts: 15
Joined: Mon Oct 26, 2015 1:50 am

Re: Multiple signals - messy spectrum

Post by Zortz » Fri Dec 11, 2015 1:55 am

It seems that i am not using the latest firmware:

Code: Select all

  bladeRF-cli version:        1.2.1-git-19c7f7c
  libbladeRF version:         1.4.3-git-19c7f7c

  Firmware version:           1.8.0
  FPGA version:               0.3.3
I used the apt-get method for installing just a few weeks ago, it seems that those repositories are not updated.

After i updated to latest firmware:

Code: Select all

  bladeRF-cli version:        1.2.1-git-19c7f7c
  libbladeRF version:         1.4.3-git-19c7f7c

  Firmware version:           1.8.1
  FPGA version:               0.4.1
The lobes around DC output was due to RX PLL. I once again changed the RX to lower frequency and they went away, but the 78.1kHZ spurs for tone signal remained. As i understand you were able to reproduce it?

Dual tone spurious emission retests:
I have retested the TX and RX module for dual tone(20M separation) spurs, due to I/Q imbalance:
1gTxDualSpur.png
TX 1G, dual tone with 20M between
As can be seen i am already getting close to my -55dBc goal. There are some other spurs present ...maybe related to this 78.1kHz spur?

Also retested dual tone for RX(TX set to 300M) and sadly results are the same - wont even make it to -40dBc :(
1gRxDualSpur.png
RX 1G, dual tone with 20M between
We are currently evaluating this SDR development path and while it seems promising we are afraid that the quality is just not good enough. So if you have any other ideas/solutions to these problems please let me know ;)
Thanks for all the support.

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