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Observer
Observer
435 Views
Registered: ‎01-17-2019

ZCU111 RF Aliasing Issue

Greetings,

We have been working with ZCU111 RF and have been stuck with aliasing problem on the ADC side for quite some time now. Non coherent (energy based synchronisation) detection works fine, but we are unable to recreate the incoming ADC signal back to the original form.
 
We have tried to recreate rectangular pulses of 100MHz BW and OFDM signal of 100 MHz BW. It seems like the signals are still not in baseband as we can see high frequency components in the obtained signal. We tried digital filters, the results of which are not acceptable. Do we need to use Anti Aliasing Filters externally at the input of ADC in order to solve this issue? Is it part of the kit because we have not obtained it in the package. 
 
Please find the attached screenshot taken from Xilinx guide. Along with it, screenshot of the ZCU111 RF output showing aliasing, is attached.
 
Also, can you please share the following ZCU111 RF configuration
 
1. Sampling Rate for ADC and DAC
2. Interpolation and Decimation Factors
3. Number of words in one clock cycle
4. AXI Stream clock
5. QMC if applicable
6. Threshold values??
7. Dithering and Sinc Filtering enabled or disabled
 
for ADC as well as DAC for the below NR configuration.
 
Mixer for DAC and ADC: IQ <=> Real
IQ Data Rate = 122.88 MSPS
Bandwidth = 100MHz
NCO (Center Frequency?) = 1GHz
 
Let us know the way forward in the implementation that we are trying.
2019-12-16.jpg
Screenshot from 2019-12-16 12-43-24.png
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3 Replies
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Moderator
Moderator
349 Views
Registered: ‎04-18-2011

Re: ZCU111 RF Aliasing Issue

Hi @arkesari 

That looks like a presentation and not from a user guide. 

In this case it can make sense to employ some filtering but the requirements are not so stringent since with careful frequency planning you can downconvert in the signal in the digital domain. 

What is the sampling frequency of the respective DAC and ADC in this case?

For the DAC what mixer setting do you use? Are you in the 2nd nyquist zone, if so are you operating in Mixed mode?

have you looked at the DAC output on it's own on an spectrum analyzer, is it where it is supposed to be in the spectrum. 

In the ADC case, where is signal located here, where does it end up in the 1st Nyquist zone? What mixer settings do you have? What decimation is employed?

Keith 

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Observer
Observer
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Registered: ‎01-17-2019

Re: ZCU111 RF Aliasing Issue

 
The input to the ADC is coming from an R&S signal generator.
 

What is the sampling frequency of the respective DAC and ADC in this case?


Required Sampling Rate = 122.88 MSPS
 

For the DAC what mixer setting do you use? Are you in the 2nd nyquist zone, if so are you operating in Mixed mode?


Carrier frequency is 983.04 MHz. Mixer type is Fine. Mixer mode is Real => IQ. NCO phase is 0.
 

have you looked at the DAC output on it's own on an spectrum analyzer, is it where it is supposed to be in the spectrum. 

Yes. Proper 100 MHz bandwidth can be seen.
 

In the ADC case, where is signal located here, where does it end up in the 1st Nyquist zone? What mixer settings do you have? What decimation is employed?


Since there are constraints in place for decimation factor, sampling rate and number of samples per clock cycle, we either need to decimate it in digital domain or we need to reduce the AXI clock to half its required rate in order to get the correct number of samples for the configuration mentioned below. We are getting double the number of samples which should come in a given time frame. We decimated it by a factor of 2 (apart from the decimation in ADC) to pass it through the reference MATLAB chain but it is unable to decode the samples. We have tried both ways and nothing is working for us.
 
Analog RF => IQ Samples
Required Data Rate = 122.88 MSPS
Bandwidth = 100 MHz
Carrier frequency = 983.04 MHz
 
We have tested this same experiment using GUI too but without any success.
 
Can you please suggest the IP configuration in detail that we should use for our application? 
 
Alok.
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Teacher
Teacher
310 Views
Registered: ‎07-09-2009

Re: ZCU111 RF Aliasing Issue

With sampling systems, one needs to always know where the signal and nyquest point is,

 

Out of band signals alias back to the base band, and as such can not be filtered digitaly, to the digits they look just the same as the real signal you want,

have look at page 9 in particular

https://www.analog.com/media/en/training-seminars/tutorials/MT-002.pdf

For an ADC, 

 Fist thing one needs to work out, is what frequency range is your signal over ?

      then you can decide what sampling frequency you want, and by definition , what nyquest zone you are interestd in,

signal / noise in any other nyquest zone than the one your interested in, will reflect back to the base band, so needs to be filtered out before it hits the ADC 

 

For a DAC, the same, work out your signal bandwidth, and then decide the DAC frequency to put that signal into the band you want, 

    the output of the DAC is a sinc waeform, so you get out your band and all the harmonics. 

            Your applicatoin needs to either fiter out the DAC outputs you dont want, or be in sensetive to them,

  ( i.e. an audio DAC ouput is oftern not filtered much, as the speakers make a fine low pass filter )

  

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