sources of error in evm measurements

I was reading an article today about the tightened EVM requirements for
802.11-ac and that got me wondering about the topic. I would say the
obvious contributors would be power amplifier linearity and gain and
phase imbalance in I/Q modulators. What else?
-- 
Randy Yates
Digital Signal Labs
http://www.digitalsignallabs.com

Randy Yates  <yates@digitalsignallabs.com> wrote:

>I was reading an article today about the tightened EVM requirements for
>802.11-ac and that got me wondering about the topic. I would say the
>obvious contributors would be power amplifier linearity and gain and
>phase imbalance in I/Q modulators. What else?

Phase noise in the local oscillator, jitter in the DAC.

Steve

Randy, are you asking what degrades EVM or are you asking what contributes  to Error in the MEASUREMENT of EVM?

makolber@yahoo.com writes:

> Randy, are you asking what degrades EVM or are you asking what
> contributes  to Error in the MEASUREMENT of EVM?

The former. Let's assume the measurement is perfect. I see where
my wording may have been unclear.
-- 
Randy Yates
Digital Signal Labs
http://www.digitalsignallabs.com

On Sun, 22 Nov 2015 01:18:24 -0500, Randy Yates
<yates@digitalsignallabs.com> wrote:

>makolber@yahoo.com writes:
>
>> Randy, are you asking what degrades EVM or are you asking what
>> contributes  to Error in the MEASUREMENT of EVM?
>
>The former. Let's assume the measurement is perfect. I see where
>my wording may have been unclear.

Pretty much any component that touches the signal on its way out to
the antenna can contribute to making the signal less than ideal.

That being said, you already hit some of the big ones, PA
linearity/distortion, and phase and gain imblance.   Phase noise also
contributes, as does DAC linearity/distortion and quantization noise,
plus the DAC reconstruction filter characteristics as well as any IF
filter(s) that might be present.

Since 802.11ac signals can take up a lot of bandwidth, filter passband
ripple and flatness is important.

And we haven't even mentioned the potential distortions in the digital
part of the modulator yet.   Precision tradeoffs can matter as well as
the usual signal processing issues.


Eric Jacobsen
Anchor Hill Communications
http://www.anchorhill.com