How to build a PXI Vector Signal Analyzer using different module combinations

[emc2006]

I understand NI Vector Signal Analyzers are composed by 2 or 3 separate PXI modules: 1 digitizer, 1 RF downconverter and 1 RF signal generator (model 5663).

1.       May I use the digitizer and signal generator as general purpose oscilloscope and signal generator separately?

2.       May I build my own VSA by choosing different combinations of digitizers and signal generators? Or replace the signal generator by an arbitrary signal generator?

3.       I intend to purchase a digitizer/oscilloscope and an arbitrary waveform generator to perform frequency response analysis on transformers. Later I plan to purchase a downconverter and build a Vector Signal Analyzer. Is that possible?

[Speedy_Badger]

Hello,

The PXIe-5663 is composed of the PXIe-5622 IF digitizer, the PXIe-5601 RF downconverter, and the PXI-5652 CW signal generator. Their specific roles are as follows:

5622 - ADC for conversion of analog IF signal into a baseband digital signal.

5601 - Translation of an analog RF frequency signal to corresponding analog IF frequency signal via a single stage mixer architecture.

5652 - CW LO signal to drive the 5601 downconverter, allowing for RF signal translation.

The IF frequencies of the 5663 are specific to the following table:

Input Frequency	Translated IF Frequency	NI 5601 Instantaneous Bandwidth	LO Frequency†	RF Image Frequency*
10 MHz to <120 MHz	187.5 MHz	10 MHz	LO = 187.5 MHz + fIN‡	fIN + 375 MHz
120 MHz to <330 MHz	53 MHz	20 MHz	LO = 53 MHz + fIN	fIN + 106 MHz
330 MHz to <3.0 GHz	187.5 MHz	50 MHz	LO = 187.5 MHz + fIN	fIN + 375 MHz
3.0 GHz to 6.6 GHz	187.5 MHz	50 MHz	LO = fIN – 187.5 MHz	fIN – 375 MHz

With the above information in mind you need a digitizer capable of acquiring signals at the above bandwidths and IF frequencies, as well as the necessary SNR and phase noise specifications to maintain enough dynamic range such as ADC bit resolution and clock accuracy. We also make use OSP functionality of the 5622 to apply IF equalization and bandpass filtering to increase the quality and accuracy of our acquisitions - all of which will need to be manually provided in hardware of software depending on the capabilities of any substituted digitizer.

The Signal generator will need to be capable of providing the appropriate LO frequencies and power levels to downconvert signals within the range of 10MHz to 6.6 GHz. The integrity of the signal generator will also be important for the same reasoning of phase noise and frequency accuracy etc.

That being said, the PXIe-5622 is a fully functioning single channel digitizer with 150 MS/s sample rate. It has OSP functionality allowing for digital downconversion, decimation, and onboard fractional resampling. It is fully supported in the NI-Scope driver API as a standalone device. See the specifications document for more information.

The PXI-5652 can be a standalone signal generator using the NI-RFSG driver API. It is capable of CW generation from 500 kHz to 6.6 GHz, along with some basic digital modulation schemes (2-FSK and OOK). It  is not an arbitrary waveform generator however. See the specication document for more details.  

The PXIe-5601 downconverter is controllable in the NI- RFSA driver API in three modes of operation: as a PXIe-5663, as a PXIe-5601 standalone with NI LO source (5652), and as a standalone PXIe-5601 with an external third party LO source. The performance you will have on the latter two modes is not guaranteed or specified in any way by Naitonal Instruments aside from the PXIe-5601 specifications.

Hope that helps,

[emc2006]

Hello Speedy Badger,

Thank tou for your prompt response!

Could you post the IF frequencies table for the 5660 and 5661 PXI vector signal analyzers too? Why the 5663 needs a signal generator to be used as an LO input and these other two models do not? Besides, what are the differences between 5660 and 5661 as both operate on the same frequency range?

I needed an oscilloscope with  a 1GHz bandwidth and 2 channels, so the 5154 would be an ideal choice. I also want to use it to build a vector signal generator in the future. I guess that is what modular instruments are all about, being able to build new instruments with minor hardware changes and new software!

Can I build a vector signal analyzer using the 5154 oscilloscope together with the 5600 downconverter?

Thank you!

[Speedy_Badger · Accepted solution]

Hello,

The IF Frequency for the 5660 and 5661 (it is the same) is 15 MHz, with a 20 MHz instantaneous bandwidth. The difference between the 5660 and the 5661 lies within the accompanying digitizer. The 5660 uses the PXI-5620 digitizer which has a 64 MS/s sample rate and a digital downconverter limited to 1.25 MHz bandwidth. The 5661 uses the PXI-5142 digitizer, giving you a 100 MS/s rate and OSP circuitry allowing for digital downconversion and decimation of the full 20 MHz bandwidth.

The common comment in the above VSA's is the PXI-5600 RF downconverter which is a three stage super-heterodyne architecture. The LO's for the three stages of this module are internally self-supplied. The multi-stage architecture allows for improved image rejection and filtering at the expensive of a slightly higher noise floor due to the more complex signal path. It also has an onboard OCXO, giving it a more accurate time reference - reducing phase noise etc. The PXI-5600 by itself is three slots wide.

The PXIe-5601 downconverter from the PXIe-5663 is designed based on single stage downconverter and takes up a single slot. The single stage downconverter gives you improved noise floor characteristics and better dynamic range, with the expense of not having image rejection, simply since there is only one stage. The LO is provided by an external module in this case for several reasons. Having a separate external LO allows for more modularity in your system, as well as the opportunity to share a single LO generator between several VSAs. This opens up the possibility of MIMO applications. The internal LOs of the PXI-5600 are not shareable and thus cannot be synchronized between several PXI-5600s. The PXI-5663 (all three modules) takes up the same amount of slot space as a single PXI-5600 without a digitizer.

The PXI-5154 is indeed a powerful digitizer, given its 1 GHz instantaneous bandwidth. Keep in mind however that the ADC on this digitizer is 8-bit, compared to the 5622 which is 16 bits. Whether or not you need more resolution is of course entirely dependent on your application. The PXI-5600, like the PXIe-5601 is controllable as a standalone downconverter using the NI RFSA API. To use it with the  PXI-5154 you will need to program your application with both the NI Scope API and the RFSA API. Some other caveats to note would be that there is no OSP on the PXI-5154 so you cannot take advantage of equalization filters to correct for the frequency response of the PXI-5600. Also, as I mentioned above, the IF frequency of the PXI-5600 is 15 MHz with a 20 MHz bandwidth - having a 1 GHz bandwidth on your digitizer will be somewhat of an overkill for this IF signal.

While you are dead on with the advantage of modularity, I would take the time to really research the requirements of your application and make sure the various module choices and their combinations satisfy those needs.

Hope that helps!