[USRP-users] TX Image rejection B210

Ian Buckley ianb at ionconcepts.com
Mon Apr 13 15:16:44 EDT 2015


So this this one has me curious. That image is way too big. I can reproduce it exactly using the same command line.
*However* when I use one of my standard GRC flow graphs to generate a CW with arbitrary LO/Offset etc the image is far better suppressed.
I see -40dBc vs -25dBc. I also see 10dB more power in the carrier with the same RF gain…that's with a digital CW from GR at 0.95 p-to-p amplitude.

I'm going to suggest that there is something in this UHD example thats less than ideal. I note that the code as supplied by Ettus has no "--lo_off" argument, so perhaps Sebastian has edited this example? Regardless I see the issue in the factory code and I'm going to file a bug.

To the observation that you are seeing improved improved spectral mask conformance with an OFDM signal…I suspect this is mostly because the recent update you did from maint caused you to pick up the new FPGA images that were published last week. These have specific filter changes to improve performance with exactly this kind of wideband signal.

The discussion of what is the ideal master_clock_rate vs what is merely an OK one is a very broad topic and differs between USRP's. If you want to give some specifics of your signal then I can give some suggestions. 


On Apr 13, 2015, at 6:49 AM, Marcus D. Leech via USRP-users <usrp-users at lists.ettus.com> wrote:

> On 04/13/2015 09:33 AM, Sebastian Held wrote:
>> I updated my UHD today to the newest maint.
>> 
>> The problem was in fact the master clock rate. I had to change it from
>> the default, because of the sample rate. After that I forgot about it.
>> Increasing it to 40 MHz, my OFDM signals now pass the spetrum mask! (I
>> did not check with the sinusoids again, will do this tomorrow).
>> 
>> One problem remains: how to select the best master clock for a specific
>> sample rate?
>> 
>> Sebastian
>> 
>> Am 13.04.2015 um 15:24 schrieb Marcus D. Leech via USRP-users:
>>> What version of UHD are you using?
>>> 
>>> Have you tried specifying a master clock rate that is a multiple of
>>> your (10Msps in your example) sample rate?
> Actually, in the latest UHD, if you don't specify a master clock rate, UHD will pick an appropriate one.
> 
> 
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