topic Re: RF Synchronization Suite in RF Measurement Devices

RF Synchronization Suite

Betty Boop — Fri, 06 Jan 2012 16:14:06 GMT

Hello! 😄 I’m currently trying the sample codes included in the RF Synchronization Suite. However, I’m having a hard time understanding the effect of the “phase offset” on the IQ data which I generate (or acquire).

For the generation part, I’m testing the sample code named “PXIe-5673 Phase Coherent Arb Waveform Generation with NI-TClk Example”. If for example, I entered 0° as my phase offset for channel 1 and then entered 10° as my phase offset for channel 2, does that mean that my data for channel 1 is AMPLITUDE1∠0° and my data for channel 2 is AMPLITUDE2∠10° in polar form? If so, do I enter these values before running the code, or while running the code (and after which I click the “Update Phase Offsets” button)?

Thank you very much for your time! God Bless! 😄

Re: RF Synchronization Suite

Andy_Hinde — Fri, 06 Jan 2012 17:43:02 GMT

Hi Betty,
The Phase Offset control manipulates the phase of the onboard signal processor (OSP) of the arb component of the PXIe-5673.It doesn't change the phase of the actual IQ data/waveform downloaded to the arb, but does allow you to change the phase of the RF output (on the fly) as it is generated.

However, these changes should be viewed in a relative sense between channels. We can't guarantee what the phase of a single channel is since phase is a relative metric and the absolute phase of a single channel is just telling you a relationship between the RF output and the frequency reference being used for that channel. However, if you are comparing phase between multiple channels as in your phase-coherent use case, you can adjust the phase difference between two or more channels.

If you generate two signals out of two PXIe-5673s, you can adjust their phase relationship by changing the phase offset of one of the channels. Normally you would leave the first channel unchanged and simply change the Phase Offset of the second channel, but you could also do this vice versa.

The answer to your question though is that you're not setting an absolute phase relation ship of an individual channel starting from a 0 degree point. If both channels have Phase Offset set to 0, you shouldn't assume both channels are phase-aligned at the same phase, and that this phase is 0 degrees. Both channels will come up outputting RF signal with an unknown phase relationship to the reference, and an unknown phase delta between them. You can then use the Phase Offset controls tweak the outputs from this starting point.

Please let me know if this answers your question, and if you have any further ones. I'll also invite you to look at some updated phase coherency code posted on our RF Multi-Channel page on NI Comunity (feel free to join this page for updates!). This is what we consider our latest and greatest phase coherency code at the moment, and we need to go update the RF Synchronization Suite software to remove any mismatch. One thing you will notice is that this newer example set doesn't expose the Phase Offset control the way the examples you are looking at do. You can add this by using the same NI-RFSG attribute Phase Offset if you like.

RF Multi-Channel parent page:

https://decibel.ni.com/content/groups/rf-multichannel?view=overview

Phase Coherency Examples child page:

https://decibel.ni.com/content/docs/DOC-19433

Regards,

Andy Hinde

RF Systems Engineer

National Instruments

Re: RF Synchronization Suite

Betty Boop — Sat, 07 Jan 2012 18:10:28 GMT

Thank you for your response! 😄 Actually, I’m currently working on a code for direction finding application. As we know, the direction can be calculated through the phase difference (unique propagation delay to each receiver). I’m sort of confused whether the ‘phase offset’ in the sample code affects this phase difference (that is, adjusting the phase offsets of both channels implies adjusting the phases of the two generated signals that are involved in the computation of the phase difference), or is the ‘phase offset’ plainly for the alignment of the channels? Thank you very much! God Bless! 😄

Re: RF Synchronization Suite

Andy_Hinde — Mon, 09 Jan 2012 23:27:35 GMT

Hi Betty,

There are phase offset controls in both the generation and acquisition code, which manipulate the OSPs of the vector signal generaotrs (VSGs) and vector signal analyzers (VSAs).

I'm not sure I understand the distinction you are communicating in your prior post about phase offset, can you elaborate?

Regards,

Andy Hinde

RF Systems Engineer

National Instruments

Re: RF Synchronization Suite

Betty Boop — Fri, 20 Jan 2012 09:15:49 GMT

Thank you for your response! 😄 I'm tasked to generate and acquire 2 signals which resemble real RF signals, provided that the I/Q data of each signal can be manually adjusted. However, I'm having problems implementing this using the RF Synchronization Suite. I tried downloading NI RF Phase-Coherent Multi-Channel that you recommended, but some VIs are missing (as seen in the attached pictures). Where can I get them?:D Thank you! 😄

Re: RF Synchronization Suite

Andy_Hinde — Fri, 20 Jan 2012 15:12:07 GMT

Hi Betty,

You need to download and install the NI-Sync driver. These reference examples support multichassis expansion, which requires the use of NI PXIe-6674T or NI PXI-6672 modules, both of which are programmed via NI-Sync. You won't be using this feature, but the driver needs to be installed to open the examples nontheless.

NI-Sync 3.3:

http://joule.ni.com/nidu/cds/view/p/id/2496/lang/en

Regards,

Andy Hinde

RF Systems Engineer

National Instruments

Re: RF Synchronization Suite

Betty Boop — Sat, 21 Jan 2012 08:41:48 GMT

I have just finished installing NI-Sync and there were no missing VIs unlike before. Thank you! 😄 I am going through the codes and I'm just wondering where I could manually adjust the I/Q data that I generate because I could not find it? 😞

Re: RF Synchronization Suite

Andy_Hinde — Mon, 23 Jan 2012 13:54:23 GMT

Hi Betty,

In Case (25) Configure Generators, within the sub-case Arb, you will see where some default arb waveforms are generated in software and downloaded to signal generator memory. This is the code that gets run when Arb is selcted from the front panel selector. You can generate your own IQ data here, or read it from a file for example as well.

Regards,

Andy Hinde

RF Systems Engineer

National Instruments

Re: RF Synchronization Suite

Betty Boop — Tue, 24 Jan 2012 09:43:01 GMT

Thank you! 😄 May I just ask where can I get the missing VIs in the attached pictures?:D

Re: RF Synchronization Suite

Betty Boop — Tue, 24 Jan 2012 10:10:59 GMT

Hi, Sir! Are these the right toolkits to download for the missing VIs?

1) NI Spectral Measurements Toolkit 2.6.1 - Windows Vista/XP x86/Server 2003 R2 (32-bit)/Server 2008 R2 (64-bit)/7

2) LabVIEW Modulation Toolkit 4.3.1

Thank you, Sir! 😄

Re: RF Synchronization Suite

Joel Khan — Tue, 24 Jan 2012 16:21:22 GMT

Betty,

"SMT" in the missing VIs stands for Spectral Measurments Toolkit. The NI Spectral Measurments Toolkit should cover the missing VIs.

Re: RF Synchronization Suite

Andy_Hinde — Tue, 24 Jan 2012 16:30:13 GMT

Hi Betty,
The Acquisition example uses functions from the Spectral Measurements Toolkit (SMT), which comes with NI's vector signal analyzer products.

Modulation Toolkit is another software dependency for the acquisition example, and the Mopdulation Toolkit comes with both our vector signal analyzer and vector signal generator products.

Here is a complete list of dependencies for these examples as listed on the software download page:

https://decibel.ni.com/content/docs/DOC-19433

Dependencies

Acquisition

NI LabVIEW 2010 or later
NI-RFSA
NI-RFSG
NI-Sync
NI Modulation Toolkit
NI Spectral Measurements Toolkit

Generation

NI LabVIEW 2010 or later
NI-RFSG
NI-Sync

Regards,

Andy Hinde

RF Systems Engineer

National Instruments

Re: RF Synchronization Suite

Betty Boop — Wed, 25 Jan 2012 15:23:45 GMT

Thank you! 🙂

Re: RF Synchronization Suite

Betty Boop — Wed, 25 Jan 2012 15:30:11 GMT

Hello! Thank you for your response! 😄 I have successfully generated and acquired signals, particulary arbitrary signals. However, their I and Q waveforms are constantly 'moving'/changing amplitude. How can I generate I and Q waveforms with constant amplitude?:) I would also like to verify what you said in one of your replies in this thread that the phase offset in the code does not affect the I and Q data/waveform? I'm still confused 😞 Sorry for repeating my question 😞 Thank you so much for your time! 😄

Re: RF Synchronization Suite

GovBob — Wed, 25 Jan 2012 20:54:10 GMT

Maybe the Baseband Developers Kit will help you out. It's a great resource.

Re: RF Synchronization Suite

Betty Boop — Fri, 27 Jan 2012 23:12:13 GMT

Hello, Sir! 🙂 This is about the 'NI RF Phase-Coherent Multi-Channel' Generation VI. I would just like to ask if the phase offset button is the only way to compensate for the delay of the other I/Q vector modulator since the LO is connected in a daisy chain manner. I mean, the first I/Q vector modulator would be generating/operating ‘earlier’ than the second I/Q vector modulator so that there is an inherent phase delay/difference between the two. Thank you!

Re: RF Synchronization Suite

Andy_Hinde — Sat, 28 Jan 2012 00:07:37 GMT

Hello Betty,

What you are seeing is expected behavior, and requires a little explanation.

To start, assume you have a single RF generator and a single RF analyzer that are both locked to the same reference clock source. Locking Tx and Rx to the same ref clock source removes any frequency offset between the generator and analyzer. This is a requirement because frequency is a measure of how fast phase is changing - frequency is the first derivative with respect to time of phase. A frequency offset is the same as a constantly changing phase, so the first thing to do is make sure you are frequency locked.

Next, imagine the analyzer operating in a continuous acquisition mode, where it is continuously acquiring 'gap-free'data. In this situation, you are acquiring a sine wave with the analyzer that has no frequency offset relative to the generator, and therefore, the acquired data will show no phase change as a function of time. The I vs time and Q vs time plots would be unchanging horizontal lines, and the I vs Q plot would show a point with a constant magnitude and phase angle.

One important point to stop and think about though, is that this phase angle is not going to be 0 degrees (unless by chance it happens to end up at 0). Phase is a relative metric, and in a single channel Tx/Rx case, ask yourself what you are measuring when you measure the phase of a sine wave. The phase of the sine wave is constantly changing, so measuring the phase of a sine wave depends on when you start your acquisition. There is no such thing as absolute phase, phase is a relative metric.

Next, imagine the same setup except instead of continuous IQ data, you are acquiring multiple single-shot acquisitions over and over. Each acquisition will display the phase of the sine wave depending on when you start the acquisition. Every time you acquire a new set of data, the measured sine wave will be at a different phase and this is what will be displayed. This is why you are seeing movement in your I and Q waveforms vs Time. Each new acquisition results in each channel starting its acquisition at a new point in time of the sine wave. This is why you always still see horizontal lines on the I and Q vs Time plots, but over time these horizontal lines are changing the Y axis point at which they have 0 slope. If there were some kind of frequency offset, you would see some slope or sinusoidal change in the I and Q vs Time plots, and some curvature (like an arc) in the I vs Q plot.

You can try this out for yourself. If you set your IQ Rate to 10 kSPS, and the samples to acquire to 1,000, you will acquire 100 ms records. While running with these settings, you will see horizontal lines in I and Q vs Time (that move around from acquisition to acquisition) and a point in the I vs Q plot that has a constant magnitude but changing phase angle. Next, change the sample to acquire to 10,000, increasing the record length by 10X to 1 second. Notice there is no change in the slope of the I and Q vs time traces, and no increase in any arc length of the I vs Q point. Increase it again to 100,000 for a 10 second acquisition and see the same thing. If there were a frequency offset, you would see essentially a circle plotted in the I vs Q plot, and sinusoidal traces for both I and Q in the I and Q vs Time plots. By the way, any arc you see in the I vs Q plot in this case is showing you phase noise...

So, you will only measure the same phase value in a single channel situation if you synchronize the start of your acquisitions with the same phase of the incoming sine wave (resulting in non-moving I and Q vs time plots, and a point in the I vs Q trace that doesn't move).

Since we're talking phase-coherency here with multi-channel use cases, what is important is the phase relationship between each channel. This should stay stable in a phase coherent system. If you plot phase delta vs time, or a combined mag/phase delta on a polar I vs Q plot, these will stay stable since even though each channel is acquiring the incoming sine waves at a different phase, the changes are all the same across channels and the phase deltas stay constant.

Regards,

Andy Hinde

RF Systems Engineer
National Instruments

Re: RF Synchronization Suite

Betty Boop — Wed, 01 Feb 2012 03:33:39 GMT

Hello! I would just like to ask if I am missing something in my settings because the generator and analyzer are still out-of-phase.
The I vs time and Q vs time plots still show frequency offset (changing horizontal lines).

The master NI PXIe-5601/PXIe-5611 has a baseband module and LO module associated with it, with the remaining slave having a baseband module associated with it and LO source set to external.
For the master's reference clock source, I set it to 'PXI_CLK' in the front panel for both the generator and analyzer.

Thank you for your time! 😄

Re: RF Synchronization Suite

Andy_Hinde — Fri, 03 Feb 2012 22:09:12 GMT

Hi Betty,

The goal of my previous post was to explain why what you are seeing is expected behavior. The only way to get the types of behavior you are expecting to see is to either look at the I and Q vs time plots for phase offset data (instead of the individual channel data), or to switch from repeated single shot acquisitions to a continuous, gap-free acquisition.

Perhaps some simple examples may help illustrate this.

I have attached a simple NI-RFSG shipping example that generates a CW signal and lets the user set the reference clock source, The default settings are a 1 GHz sine wave @ 0 dBm with the PXI backplane set as the reference clock source. This is RFSG Reference Clock.VI.

I have also attached RFSA Getting Started IQ.vi. This example acquires repeated single-shot acquisitions and plots the data on a I vs Q plot. The default settings are for 1 GHz center frequency, a 0 dBm reference level, and the PXI backplane as the reference clock source. The assumption is that the signal generator and analyzer are both locking to the same PXI backplane clock (meaning they are in the same chassis).

You are expecting a constant phase angle for the IQ data, and the RFSA Getting Started IQ.vi shows that this angle will move. Again as explained in the previous post, each new NI-RFSA acquisition will occur at some different starting point of the incoming CW sine wave. Since there is no such thing as absolute phase, the phase reference used is changing and shows up as a rotating point on the I vs Q plot.

I have also posted RFSA Acquire Continuous IQ.vi to illustrate this further. If I am performing a gap-free, continuous acquisition, there is only one acquisition instead of multiple, separate acqusiitions each with a different starting reference phase. You can see that with the continuous gap-free acquisition, the phase angle is constant.

Again, there is no difference in how the hardware is operating except for the simple difference of one case being multiple, separate acquisitions with slight breaks in time between the end of one acquisition and the start of the next acquisition, and the other case being a single acquisition with multiple fetches and no gaps in the data, meaning the starting reference phase is always the same and you don't see any change in phase angle.

In both cases, if you were to plot phase offsets between additional channels and a master, they will always have constant phase angles assuming the generator and analyzers are locked to the same ref clock.

Regards,

Andy Hinde

RF Systems Engineer

National Instruments

Re: RF Synchronization Suite

Betty Boop — Tue, 07 Feb 2012 15:15:58 GMT

Hello, Sir! Thank you very much for your last reply (and all others :D). I would like to ask if the NI RF Phase-Coherent Multi-Channel acquisition code (main VI) is configured for repeated single-shot acquisitions. This is because when I choose CW in the generation front panel, I get changing horizontal lines for the I and Q vs t waveforms. On the other hand, I get 2 points (since I deal with 2 channels) which both move in circles for the I vs Q plot.

May I also ask if you have any suggestions on how I can fix my I and Q vs t waveforms when I choose Arb in the generation front panel. The resulting I and Q vs t waveforms are changing sinusoids which I thought to indicate frequency offset between the generator and analyzer. However, I already had my generator and analyzer both locked to the same reference clock source (they are in the same chassis and both set to PXI_CLK reference clock source in the front panel of generation and analyzer VIs).

Attached is a summary of the results I obtained.

Thank you very much for all the help! 😄