[USRP-users] Master Clock Supression

Ian Buckley ianb at ionconcepts.com
Fri Jul 17 16:20:55 EDT 2015


Just correcting MarcusM here…the MCR (200MHz clock) *is* fed to the daughter boards…that is the clock reference that drives all LO's.

And if you suspect that its harmonics are getting into your signal chain the quickest litmus test you can do is invoke UHD with arguments for a non-standard master_clock_rate (--args="master_clock_rate=120e6"), it will be quite clear then if you are seeing harmonics. 200/184.32/120 MHz are current supported rates.

-Ian

On Jul 17, 2015, at 12:04 PM, devin kelly via USRP-users <usrp-users at lists.ettus.com> wrote:

> Right now I've got two X310s each with two UBXs.  I have each TX/RX antenna on subdev A connected to each other (with 40 dB of attenuation), same for each B subdev.  I have all the RX2 antennas terminated.  
> 
> I see the tones on regardless of the subdev/antenna combination.  I also tried connecting a spectrum analyzer to the TX/RX antenna ports and saw no tones.  I also put an antenna on my spectrum analyzer and saw no tones over the air with the antenna right next to each X310.
> 
> Thanks Again,
> Devin
> 
> On Fri, Jul 17, 2015 at 11:19 AM, Marcus Müller <usrp-users at lists.ettus.com> wrote:
> Shielding/separation-wise, the X310 should be pretty much superior in every aspect -- not only is the ADC, DAC, mixer, amplifier and LO synthesizers in one package on the B2x0 (the AD9361); the X310 is modular in the sense that the MCR doesn't even appear to the daugtherboards, necessarily; it's an FPGA-only clock, generated by the clock distribution IC on the mainboard, and the UBX, especially to suppress strong interferers, has a metal can (which is unique among our RF hardware, so far). Under that metal can, there's a lot of partitions, which align with the different jobs and frequency ranges on the board, compare the schematic [1].
> 
> So since you're seeing 200MHz-harmonics, these might either spuriously get into the antenna ports, or for some reason, at multiples of 200MHz LO leakage is worse (I don't expect that). Have you tried using offset tuning [2] to reduce these spurs?
> 
> Best regards,
> Marcus
> 
> 
> [1] http://files.ettus.com/schematics/ubx/ubx.pdf
> [2] http://files.ettus.com/manual/page_general.html#general_tuning_process
> 
> 
> On 17.07.2015 17:03, devin kelly via USRP-users wrote:
>> Hello,
>> 
>> I have an X310 with a UBX daughtercard.  I've noticed that there are tones every 200 MHz (I checked 2.4GHx, 2.6 GHz, 2.8 GHz, 3 GHz, 3.2 GHz).  Do I have any options for suppressing the tones?  Is there any shielding or RF absorber I can add somewhere?  
>> 
>> The B210s seem to have each section of radio gated off (to make it easier to add shielding?).  But  the X310s don't as far as I can tell.
>> 
>> Thanks for any help,
>> Devin
>> 
>> 
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