ADC sampling with custom LMX2594 frequency

[dcwhitehead]

Hi all,

I am trying to configure the ADC on my ZCU111 board for a 1018MHz sampling frequency with 8x decimation. I think I've got everything set up right, but when I sample a known frequency it appears to be shifted by ~1.7MHz from where it should be.

Signal Generation:

I am generating a 10.335MHz sine wave on DAC Tile 0,Channel 0 (P) of one ZCU111 board using the RF Data Converter User Interface tool at 6389.76MHz with a NCO setting of -400.000MHz. This produces a signal at 410.335MHz as verified by a spectrum analyzer.

Signal Capture:

On a second ZCU111 I have configured the LMX2594 to output a 1018MHz reference clock based on the output of the TICS Pro tool from NI as shown in the image below. I added the registers generated by this program to the ClockingLmx structure xfdc_clk.c file and call LMK04208ClockConfig & LMX2594ClockConfig from my application.

I have the ADC configured for 1018.000MHz sampling frequency and 8x decimation on ADC Tile 2 Channel 1 (P). The NCO is configured for -400MHz. Currently, I am running without a balun on both the ADC and DAC and am just using the P end of the balanced signal (and expecting a ~6dB loss because of it).

When I capture the sine wave on the receiving RFSoC using chipscope and take the FFT of it in Matlab (exported .csv file) I see a sine wave with a frequency of 12.058MHz, or a difference of 1.723MHz from the expected value.

What am I doing wrong in my process?

[klumsde]

Hi @dcwhitehead

I don't understand why you are trying to use the DAC and ADC single ended like this. That is not what was intended. 

Can you try

Use the evaluation design and gui to loopback the signal from the DAC to an ADC using one of the low frequency balún paths to make sure that this works for a single board then try expand to the second board using the reference design image?

[dcwhitehead]

@klumsde, I'm operating this way as a quick and dirty test setup because the baluns I have for those ports don't support these low of frequencies but I wanted to have something low enough frequency to where I could see it on the oscilloscope available to me. The fact that I'm only using half of the balanced signal should have no significant impact on the results of the test other than power level.

As for you suggestion, I setup both boards with the RF Data Converter Evaluation User Interface (RFDCEUI) tool (still on DAC T0,C0, P & ADC T2,C1, P) with an ADC sampling frequency of 3194.9MHz (which is the default) and a decimation of 8x. Using this setup, I receive the signal perfectly fine.

Now, the main difference between the two tests is when I use the RFDCEUI tool, the LMX2594 ADC clock is set to 247.760MHz with an internal PLL bumping it up to 3194.9MHz where as in my design the LMX2594 ADC clock is set to 1018MHz and the internal PLL is bypassed.

I would love to load my .tcs file generated from TICS Pro into the clock settings of the RFDCEUI but everytime I adjust the clock settings in the tool (whether pre-defined Xilinx .tcs files or my own) I lose communication with the board.

This seems like a clocking problem but everything appears to be correct.

[dcwhitehead]

@klumsde, I was finally able to get the RF Data Converter Evaluation User Interface tool to accept clock changes (I thought I could just configure a single RFPLL and was leaving the REF CLK and other RFPLLs blank). I can reproduce the same problem on a single board with the onboard LF baluns.

New setup:

DAC Tile 1 Channel 3 => ADC Tile 0 Channel 1

Where DAC nco is set to -400MHz, and Fs is set to 6389.76MHz. Still a 10MHz sine wave input with 16K samples (actual 10.335MHz)

The ADC nco is also set to -400MHz, Fs is 1018.00000MHz (internal PLL bypassed), and 8x decimation.

I'm still seeing the same 12.054MHz I was seeing in the previous setups.

I've attached the .tcs file I'm using to generate the 1018.000MHz clock. The extension has been changed to .txt since the forum won't let me upload a .tcs file.

[klumsde]

Hi @dcwhitehead 

I was about to reply and say that it seems to expect all fields to be populated in the advanced mode. 

OK Let me  give this a try here. 

Keith 

[dcwhitehead]

@klumsde

Have you been able to reproduce what I'm seeing? I can go through my setup in more detail if required.

Thanks,

Dan

[klumsde]

Hi Dan, 

Unfortunately I haven't had a chance to try this. I've asked a colleague to take a look and try it on our side 

Keith

[dcwhitehead]

@klumsde,

As a quick update, I also see frequency discrepencies on the DAC to a spectrum when I change the LMX2594 that drives the DAC. Below is a table of a few different NCO frequencies, the expected output, and the measured output. All units are MHz.

NCO	Measured	Expected	Diff
410	418.1	419.84206	1.74206
405	413.125	414.84206	1.71706
400	408.15	409.84206	1.69206
399	407.15	408.84206	1.69206
385	393.2	394.84206	1.64206
315	323.5	324.84206	1.34206
285	293.625	294.84206	1.21706
215	223.9	224.84206	0.94206
75	84.5	84.84206	0.34206

[klumsde]

ok so are we saying that this only happens with your custom LMK configuration?

A good test would be to try with a known good configuration for the LMK for both the ADC and DAC. 

Also we could set the sample clock with your custom LMK settings and use the coarse mixer, that should show the fs/4 is correct or not say. 

I really suspect that the issue is with this LMK setting. I will carve out some time and look at it again in my set up. 

Keith 

[dcwhitehead]

@klumsde,

Sorry for the late reply. The issue is definitely related to my LMX2594 settings.

I set up the DAC to bypass the Mixer and just output a 200MHz sine (or rather the frequency the GUI chooses to round to) and with the default configuration and a couple of the other Xilinx created .tcs files I get exactly the expected frequency on the spectrum analyzer.

However, with my .tcs file, which was previously attached, I am off from the expected by ~1MHz with what is otherwise the exact same setup. As far as I can tell my file is setup correctly and given a 122.88MHz input oscillator it should produce a 1018.0000000028MHz output clock.

Thanks for the help.

-Dan