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2.7 GHz vs 6 GHz ... EVM Differences

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Dear NI RF HW Guys,

I have a 2.7 GHz RFSA as well as a 6 GHz RFSA ....

I see a huge difference in my EVM readings with the

same Labview VI and same DUT transmitting a predefined packet.

 

I am decoding my frames in 2.4 GHz band.

I get favourable results in the  2.7 GHz RFSA.

 

Can anyone explain why is this huge difference in EVM,

when I have not changed my Labview VI and the DUT transmit conditions Smiley Surprised

 

Regards,

Dharmendra

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Message 1 of 10
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Hi Dharmenda,

 

There are few things that I'd like more information on. When you say huge difference in your EVM readings , what does that really mean? Do you have any specific numbers?

The 2.7 GHz RFSA, is it a 5660(5600+5620) or 5661(5600+5142) ?

What is the modulation scheme used ? , and finally do you mind posting your VI?

 

Best Regards

Bueller
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Message 2 of 10
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Also, what power levels?

 

Regards,

Andy Hinde

National Instruments
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Mr.Bueller,

I have a 5661(5600+5142) for 2.7 GHz.

Why are you asking for HW Info specifically....

Shouldnt EVM be same irrespective of HW differences ?

Does the digitizer change make a difference when calculating EVM ?

 

I am using OQPSK modulation scheme... specific to the 802.15.4 standard.

 

Also answering Andy's query, I am transmitting at 0 dBm from my DUT

and at the RFSA end, my reference level is -10 dBm , with power trigger level of -30 dBm....

 

Andy, Just in case you were worried about signal saturation / clipping ....

I assure you i am capturing a clean signal on both 2.7 GHz and 6 GHz HW... 

The only difference seems to a differenct EVM value with the same VI used for calculation.

 

I suspect 6GHz HW because the same signal gave me similar readings

in NI 5661(5600+5142) for 2.7 GHz compared to EVM readings

from other Agilent N4010A & N8300A Analyzers used with Agilent  89600 VSA Software ... 

 

Regards,

Dharmendra

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Solution
Accepted by topic author RF_Buzzer

"Shouldnt EVM be same irrespective of HW differences ?"

 Not necessarily. Phase noise and amplitude accuracy have bearing on EVM and although I am not saying at this point that these are the explanations, HW difference makes a great impact on EVM performance in general. The digitizer difference between 5660 and 5661 most likely will not make a large difference in performance, and indeed you will notice the EVM specs for 5660 and 5661 are the same.

 

 "Andy, Just in case you were worried about signal saturation / clipping ...."

I am indeed worried about saturation. You're generating a 0 dBm signal into an analyzer whose max input level has been set at -10 dBm. How do things change when you use the proper reference level of 0 dBm? You say you are capturing a clean RF signal and I am curious as to how you determine this? Visually by looking at the spectrum? Signal distortion leads to amplitude errors and amplitude errors are one component of what EVM measures. Do you see a difference in EVM between reference level of 0 and -10 dBm?

 

 

Our EVM specs for QAM are listed in the 5663 specs and can be compared to 5660/5661 EVM specs. This data clearly shows the EVM performance of the 6.6 GHZ platform is better than the 2.7 GHz platform.

 

One difference  between the 2.7 GHz platfrom and the 6.6 GHz platform is the amount of 'headroom' between the stated reference level of the PXIe-5663 and the clipping level of the instrument. A -10 dBm reference level places the clipping/overload level @ 0 dBm on the 2.7 GHz platform. The same reference level would place the clipping level @ -4 dBm I believe (I could be off by 1 or 2 dB but not much more) on the 6 GHz platform. In any case, there is less headroom on the 6 GHz platform since the 6 GHz platform has 1 dB attenuation resolution (as opposed to 10 dB resolution on 2.7 GHz platform) and doesn't require as large an amount of headroom. This could very well be  your issue.

 

As a test you should run generator and analyzer examples for the Modulation Toolkit, QPSK, and see what happens. Also, perhaps we can provide more assistance if you provide your code and the ability toreproduce the behavior you are seeing. If we can reproduce it we can figure out why you are seeing differences. At the very least I recommend using a correct value for Reference Level.

 

Regards,

Andy Hinde

National Instruments
Message Edited by Andy Hinde on 06-11-2009 10:13 AM
Message Edited by Andy Hinde on 06-11-2009 10:14 AM
Message 5 of 10
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RF Buzzer,

Any word on this? Did raising the reference level help?

 

Regards,

Andy Hinde

National Instruments

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Message 6 of 10
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Hello RF Buzzer,

Did raising the reference level fix this?

 

Regards,

Andy Hinde

National Instruments

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Message 7 of 10
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Hello RF Buzzer,

Any updates?

 

Regards,

Andy Hinde

National Instruments

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Message 8 of 10
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Hi Andy,

Seems to me that indeed Signal Saturation was the problem which was causing high EVM.

Which brings up the next question...

 How do we verify whether Incoming RF signal is saturated ?

 

I check the Time domain waveform, looks fine..

I check the Spectrum, looks fine....

I give a safe Expected RF Power margin of around 10 dB [ ( in my case ) .. Tx @ 0 dBm .. Rx @ -10 dBm]

 

What else needs to be looked into ?

 

Regards,

Dharmendra 

 

 

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Hello Dharmendra,

You will receive a warning when the saturation is large enough to cause clipping of the digitizer.

 

With regards to your statement:

"I give a safe Expected RF Power margin of around 10 dB [ ( in my case ) .. Tx @ 0 dBm .. Rx @ -10 dBm] "

 

The headroom between the reference level and the digitizer clipping level is not 10 dB with the PXIe-5663.

 

I don't believe anything else needs to be looked into, as the hardware and software are operating as expected. In the future you need to use a reference level that is set equal to the expected total power present in your signal.

 

If your Tx power is 0 dBm, you should set the reference level of the PXIe-5663 to 0 dBm.

 

Regards,

Andy Hinde

National Instruments

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