[USRP-users] LFTX output amplitude (Michael Rahaim)

Ian Buckley ianb at ionconcepts.com
Fri Dec 12 02:42:52 EST 2014


OK, time for a DSP recap I think. Whilst it makes life easier when we draw a system diagram and get to add a single block labelled CORDIC, I think we run the risk of forgetting what function it's actually performing.

Referring to a nice old fashioned pencil diagram: http://www.ionconcepts.com/files/CORDIC.jpg

In the context of a DDC/DUC we are using a CORDIC algorithm processor to replace a logical DDS and complex mixer, which in practical terms amounts to a PHASE accumulator that endlessly counts 0->2*Pi then wraps at a rate determined by the PHASE_INCrementor.
That PHASE value is the input to lookup tables for SIN and COS functions thus synthesizing a complex sinusoid digitally.
To effect a frequency shift of a complex signal we mix that DDS output with the input signal, which in terms of digital operations means we are doing a complex multiply of the SIN/COS values with the complex components of the input signal.

So when PHASE = 0 and PHASE_INC = 0, IOUT = IIN and QOUT= QIN. Now whilst streaming if we retune the CORDIC and introduce a frequency translation, PHASE_INC != 0 and PHASE increments by PHASE_INC every clock cycle. We then later retune the CORDIC so that no frequency translation is introduced and now PHASE_INC = 0 again. However PHASE is left at an unknown and likely non-zero value which means our complex input signal is still rotated around the unit-circle. to produce the output signal:
IOUT = IIN*COS(PHASE) + QIN*SIN(PHASE)   and QOUT = QIN*COS(PHASE) - SIN(PHASE)

Now, referring back to Michaels original post he has a flow graph that introduces a real only signal into the I channel and leaves the Q channel at 0…I think the problem becomes apparent. Now if the input signal is instead complex valued then the issue goes away.

When a new streamer is set up, (TX or RX) PHASE will be initialized to 0, and PHASE_INC will have been step-up by UHD to satisfy the current tuning request. The only way to return PHASE to zero predictably is to restart the streamer.

-Ian


On Dec 11, 2014, at 7:53 AM, Michael Rahaim via USRP-users <usrp-users at lists.ettus.com> wrote:

> Thanks Lou. I think I get what you're saying, but the end result seems to end up as a channel gain as opposed to a DC offset. For example, if I set my signal so that it initializes to a sine wave with peak to peak amplitude of 1V, after turning the carrier on and off it might end up having a pk-pk amplitude of 0.5V or 0.2V or 0.8V. 
> 
> Is there a way to reset via software such that the accumulator gets set back to 0? 
> 
> Thanks again,
> 
> -Mike
> 
> On Thu, Dec 11, 2014 at 9:08 AM, Louis Brown <rfengr00 at me.com> wrote:
> Ok what you are describing is the phase accumulator halting to a non zero state when you tune the oscillator back to 0 Hz.  Try generating a very low frequency sine wave (a few Hz) and you see what I mean when you tune it away from 0 Hz and then return it.  When you start you flow with a 0 Hz source, the accumulator is initialized to zero, then it starts accumulating when you tune it to some frequency.  Return it to 0 Hz and it stops at a non-zero phase, which is equivalent to a DC offset.  I'm sure this is the way it is supposed to work, and is what I have observed before.
> Lou
> 
> 
> On Dec 11, 2014, at 07:21, Michael Rahaim <mrahaim at bu.edu> wrote:
> 
>> Hi Lou,
>> 
>> I have the output of the LFTX connected to a scope. I'm not using the LFRX in the test loop.
>> 
>> Best,
>> 
>> -Mike
>> 
>> On Wed, Dec 10, 2014 at 7:44 PM, Louis Brown via USRP-users <usrp-users at lists.ettus.com> wrote:
>> How are you measuring the LFTX output?  With additional test equipment or looping it back into the LFRX?
>> 
>> Lou
>> 
>> 
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> 
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