downconverter external

[madd]

Hello

1 - i have a digitzer 5142 with a 14(16) bit resolution. can i change this digitization rate to 8 bit  or can i acquire a signal at 8bit using 5142 i  

     think i can using ni scope config . if yes Does it  also mean that i have  increased the  data rate from 100MBS to 200MBS (since i am only

     using 8 bit digitization). or the speed of transfer of data from my onboard memory to hardisk is increased only or both are applicable

2 -  I have been reading this 5142 specification when i came accross two things as i have circled in JPG attachments 

     a. anit-alisasing filter has a bandwidth of 40 Mhz and by default 5142 uses it  what does this means. Is it that i can use 

         some other external down converter insted of 5600 and which can acqiure 40Mhz bandwidth at any center frequency and i can fed it to

         5142 to get digitized and writre a file to my disk. .i can not use RFSA since i am using external downconverter. how to do it using ni

         scope/tuner vi's.

       b.  Another is real flat bandwidth =0.4*sample rate  and complex flat bandwidth= 0.8*sample rate who do we mean by these two. 

Regards

Madd

[Speedy_Badger]

Hi Madd,

1 - i have a digitzer 5142 with a 14(16) bit resolution. can i change this digitization rate to 8 bit  or can i acquire a signal at 8bit using 5142 i  

     think i can using ni scope config . if yes Does it  also mean that i have  increased the  data rate from 100MBS to 200MBS (since i am only

     using 8 bit digitization). or the speed of transfer of data from my onboard memory to hardisk is increased only or both are applicable

You can modify the vertical resolution of the digitized data and how it is stored on the 5142 by using the Binary Sample Width property under aquisition in the NI-Scope API. Search Binary Sample Width in the NI High-Speed Digitizers Help file for more information.

2 -  I have been reading this 5142 specification when i came accross two things as i have circled in JPG attachments

     a. anit-alisasing filter has a bandwidth of 40 Mhz and by default 5142 uses it  what does this means. Is it that i can use 

         some other external down converter insted of 5600 and which can acqiure 40Mhz bandwidth at any center frequency and i can fed it to

         5142 to get digitized and writre a file to my disk. .i can not use RFSA since i am using external downconverter. how to do it using ni

         scope/tuner vi's.

The PXI-5142 has a 100 MS/s rate (maximum) on the ADC giving us a primary nyquist zone from 0 to 50 MHz and a secondary Nyquist zone from 50 to 100 MHz. Since we generally acquire in the primary nyquist zone, we use the Antialias filter to prevent any higher frequency content from aliasing into the region and giving us false or distorted data. The filter itself is static in terms of the center frequency etc so you cannot simply choose a frequency region and place this filter around it. This is why we typically use an IF frequency of 15 MHz when using the 5142 with our 5600 downconverter as it is in the optimal flat passband of this antialias filter. The scope itself can tune its NCO when the DDC is enable to be something higher than 15 MHz, however with that filter on, you will have to make sure you are placing your signal correctly.

The 20 MHz limit is due to the physical filters on the PXI-5600 so you can conceivably have a higher bandwidth with another downconverter.You shoudl be careful in making sure your frequency plan and BW match up to the capabilities of the PXI-5142.

To control whether the antialias filter is on versus another option, use the the niScope Configure Chan Characteristics VI. The filter selection is based on the maximum frequency setting. For an example on how to use the 5142 on its own, I would look at the niScope EX OSP Quadrature Downconversion example shipped with the NI-Scope Driver API.

Third party downconverters are not supported by the NI-RFSA or NI-Tuner APIs.

       b.  Another is real flat bandwidth =0.4*sample rate  and complex flat bandwidth= 0.8*sample rate who do we mean by these two.

This refers to the FIR filtering in the OSP section of the PXI-5142. In the OSP there are two channels, I and Q, which are used to process and condition the inphase and quadrature portions of the complex waveform. Since each channel is real, we are using a real FIR filter which is defined in the hardware as 0.4*sample rate. Translating that into the complex domain, we can easily see that the bandwidth doubles, since each I and Q waveform corresponds to half the complex BW. This of course gives us the 0.8*sample rate for the complex flat bandwidth.

Keep in mind, when we are using the 5142 with the OSP and quadrature downconversion enabled, sample rate is analogous to IQ rate. With this in mind, we can look at an example:

If I set my NCO center frequency to 15 MHz in order to match up with the PXI-5600 IF frequency and I want my signal BW to be 20 MHz to match the PXI-5600 passband, my IQ rate will be set to 25 MS/s. This gives me a 25 MS/s * 0.8 = 20 MHz BW.

Hope that helps!

Message Edited by Speedy Badger on 02-19-2009 12:34 PM

[madd]

Hello

1-  Greateful for ur qick response. but i am still unclear about the signal bandwidth that i can provide 5142 with. Is it at the max 40Mhz and  the min 20Mhz. i think there must be some constrains on Bw as long as external downconverter is concerned. 

2- previously i was working with RFSA(5600+5142) so i have RFSA tool kit installed on my PC. offcourse this software has Ni scope driver and NI tuner driver vi's but should i install Ni scope driver seperatly on my PC to use 5142 without 5600. or the vi's installed with RFSA are the ones which will work fine in this condition.

3- using RFSA i dind't find the Example u have metioned niScope EX OSP Quadrature Downconversion

4- How will i check what is the minimum and maximum power of the input downconverted signal to the digitizer in case of external downconverter. so that i might not Blow up my 5142.... is it the same as using it with RFSA i.e +30dbm max...

Regards

Madd

[Speedy_Badger]

Hi Madd,

1-  Greateful for ur qick response. but i am still unclear about the signal bandwidth that i can provide 5142 with. Is it at the max 40Mhz and  the min 20Mhz. i think there must be some constrains on Bw as long as external downconverter is concerned. 

As I mentioned above, the BW that the PXI-5142 sees is dependent upon the IQ rate when the OSP and Digital Down Conversion (DDC) is enabled. For instance, if I set my NCO frequency of the DDC to 15 MHz, and my IQ rate to 25 MHz, my BW turns out to be 20 MHz due to the 0.8*IQ rate FIR filters in the OSP. This relates to your other question in the previous discussion. The IQ rates themselves are determined by the decimation factors supported by the 5142.

Decimation is the practice of downsampling from your overall ADC sample rate (typically 100 MS/s) to some smaller rate via integer values. The PXI-5142 is capable of downsampling via the following integers:

1, 2, 4, 6, 8, 10

12 to 4,096 (Multiples of 4)

4,096 to 8,192 (Multiples of 😎

8,192 to 16,384 (Multiples of 16)

To continue with my example above, I used a decimation integer of 4 to achive my 25 MS/s IQ rate and thus 20 MHz BW. Now, in regards to minimum and maximum BWs, lets look at the decimation integers of 2 and 16,384. If I decimate my ADC sample rate by a factor of 1, I will get 100 MS/s for an IQ rate. 100 MS/s x 0.8 (for the filters again) gives me a BW of 80 MHz maximum. Likewise, if I decimate my ADC sample rate by a factor of 16,384 I get a value of 6103.515625 S/s. Once again, using the 0.8 of the filters, my minimum BW will then be approximately 4.882 kHz.

It is important to take that Anti-aliasing filter into account, as well as the placement of your NCO frequency. The Anti-aliasing filter will start attenuating your signal at 40 MHz (-6 dB) and roll off from there, which means the maximum BW you will have is about 40 MHz. This translates directly to the decimation factor of 2 (100 MS/s /2 = 50 MS/s x 0.8 = 40 MHz).

2- previously i was working with RFSA(5600+5142) so i have RFSA tool kit installed on my PC. offcourse this software has Ni scope driver and NI tuner driver vi's but should i install Ni scope driver seperatly on my PC to use 5142 without 5600. or the vi's installed with RFSA are the ones which will work fine in this condition.

 The RFSA driver installs the full version of the NI-SCOPE API so you should be all set. The PXI-5142 is can be programmed via either API.

3- using RFSA i dind't find the Example u have metioned niScope EX OSP Quadrature Downconversion

This is an NI-SCOPE example, not an RFSA example. You can find it here: <Your LabVIEW Directory>\examples\instr\niScope.

What version of RFSA do you currently have instaled? I would recommend getting the latest driver version if possible. You can either upgrade RFSA or Scope by itself depending on what you want to do. Here are the links:

NI-RFSA

NI-SCOPE

4- How will i check what is the minimum and maximum power of the input downconverted signal to the digitizer in case of external downconverter. so that i might not Blow up my 5142.... is it the same as using it with RFSA i.e +30dbm max...

I believe this question was answered appropriately in this post.

Hope this helps!

Best Regards,

[madd]

Hello

Tnx for ur quick reply actually the reason was the reply I posted on I believe this question was answered appropriately in this post  was not responded quickly so I copy paste my reply on this post. But before the reply came on this post someone replied on that post.....

never the less Few more question regarding frequency translation...

1- Correct me if I am wrong. To the extent I have under stood frequency translation means that if I have acquired a signal at 50MHZ with a 40MHZ Bandwidth(F_high=70Mhz and F_low=30Mhz). From my downconverter. If I select frequency translation option enable and set  center frequency to 20MHZ (With DDC enable option as well) signal I have acquired is now translated to 20MHz(C.F), maximum frequency at 40Mhz and Minimum at 0Mhz. Is it so ?If this is correct then I think my spectrum will fulfill the requirement of nyquist spectrum and will not be attenuated..

2- Also mention that this decimation i can set is from property invoke node or from filter specification.

3- Another thing enabling  and disabling Noise/antialiasing filter. Maximum frequency set of  niscope configure characteristic.vi . If I set it to 20Mhz Noise filter is on now and when I set it to 35Mhz antialiasing filter is on. Is it so?

Regards

Madd

[Speedy_Badger · Accepted solution]

Hi Madd,

1- Correct me if I am wrong. To the extent I have under stood frequency translation means that if I have acquired a signal at 50MHZ with a 40MHZ Bandwidth(F_high=70Mhz and F_low=30Mhz). From my downconverter. If I select frequency translation option enable and set  center frequency to 20MHZ (With DDC enable option as well) signal I have acquired is now translated to 20MHz(C.F), maximum frequency at 40Mhz and Minimum at 0Mhz. Is it so ?If this is correct then I think my spectrum will fulfill the requirement of nyquist spectrum and will not be attenuated..

Not quite- Frequency translation is used to take your signal from some IF frequency to complex baseband (Centered around 0 Hz). If you set your center frequency to 20 MHz, your NCO will tune to 20 MHz and you will acquire some bandwidth, based upon IQ rate, around that center frequency. For instance, if you used 20 MHz bandwidth (IQ rate  = 25 MS/s) and a center frequency of 20 MHz, you will acquire from 10 MHz to 30 MHz in frequency and translate this to baseband. You will now have your frequency content at +/-10MHz.

To do what you seem to want to do, you would set your center frequency to 50 MHz and your IQ rate to 1.25xBW = 50 MS/s. This will translate your 40 MHz BW from 50 MHz down to complex baseband.

2- Also mention that this decimation i can set is from property invoke node or from filter specification.

Decimation is a propery of the OSP and clocking mechanism. Your Decimation is set by your IQ rate in relation to your Sample Clock timebase. For instance, by default we use a 100 MS/s timebase for the 5142. If I set my IQ rate to 2 MS/s, I will be using a decimation factor of 50. Our decimation table states however that in the range of 12 to 4,096 we can only use decimation integers which are multiples of four. Thus, we will end up being coerced in this case to 48 and a sample rate of 2.0833 MS/s. You can then perform resampling back to 2 MS/s in software.

3- Another thing enabling  and disabling Noise/antialiasing filter. Maximum frequency set of  niscope configure characteristic.vi . If I set it to 20Mhz Noise filter is on now and when I set it to 35Mhz antialiasing filter is on. Is it so?

That is correct - setting your maximum frequency to be within one of those filters will enable that path. If I set a max frequency of 19 MHz, the noise filter will be enabled. If I set a max frequency of 30 MHz, the Anti-aliasing filter will be enabled. 0 will enable the default, and -1 will enable full BW.

Regards,

[Chris_W.]

Hello Madd,

We have noticed many duplicates of this post on other threads. We would like to consolidate these posts to one thread. We will continue to address your issue on this post. 

Also, note that good information about your issue has been presented on this post as well.  This post has a good explanation of how the OSP works, using the block diagram as an aid. 

Thanks for keep your issue to this thread. 

Chris W