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EVM measurement techniques - RF Cafe Forums
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yz
Post subject: EVM measurements Posted: Wed Aug 10, 2005 1:25
am
hello friends,
Is there any techniques to measure EVM
(error vector magnitude) with network analyzer or spectrum ?
(Instead
of the known technique with vector signal analyzer).
Thanks,
YZ
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Guest
Post subject: Posted: Wed Aug
10, 2005 9:11 am
NO
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guest
Post subject:
Posted: Wed Aug 31, 2005 4:54 am
Hi.I think you can use VNA to measure
the CW AM/AM and AM/PM. There is some relationship for them with the
EVM. And in ADS you can simulate it with the measured CW AM/AM and AM/PM
to get the simulared EVM value.
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IR
Post subject:
Posted: Thu Sep 01, 2005 2:49 pm
Site Admin
Joined:
Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,
[/b]This is partially true[b]
EVM is a function of more than
one parameter: Besides linearity, it is also depends on phase noise.
You can't get information about phase noise from AM-AM and AM-PM measurements.
These measurements solely meant to characterize the compression
characteristics of an amplifier/transmitter/system and to get information
about the linearity. You still need to get the phase noise information
(By doing that measurements with Spectrum Analyzer).
Then when
you have these quantities there are models to calculate the EVM. As
far as I know, the only method to measure EVM is with Vector Signal
Analyzer.
_________________
Best regards,
- IR
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Guest
Post subject: Posted: Sun Sep 04, 2005
11:35 pm
Hi...
I understand what you said because I often
encounter that I have good AM/AM and AM/PM charateristic but poor EVM
( I am designing the WLANPA now).
However, I found that usually
when I added some different large bypass capacitor, the EVM can be improved
with even the same AM/AM and AM/PM chracteristic. I don't know if it
is what you said the phase noise being reduced. I guess that could be
some kinds of low frequency noise. Could you please teach me more about
the phase noise and how to reduce it? The only way I know is to add
different kinds of large bypass capacitors.
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IR
Post subject: Posted: Sun Sep 04, 2005 11:55 pm
Site Admin
Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location:
Germany
Hello,
Different types of bypass capacitors can improve
many things in a PA circuit, like improve the stability. Concerning
improvement of EVM, I don't know if that can do much. What can do more
are the matching networks. You should stick to the recommendations of
the transistor's/device manufacturer by using the same values of external
components in your circuit. On top of that circuit, you can make additional
improvements. Use ADS or a similar tool to analyze your circuit (Provided
that you have the right model for the transistor), and see what you
can do to improve EVM.
Good luck!
_________________
Best regards,
- IR
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IR
Post subject:
Posted: Mon Sep 05, 2005 8:57 am
Site Admin
Joined:
Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,
One more thing I wanted to add: Bypass capacitors and bypass networks
of the PA (Which often consist from resistos, beads and capacitors)
can have an effect on a phenomenon called "Memory Effect", which is
a change in the AM-AM and AM-PM curves over temperature and frequency.
The "Memory Effect" is a distortion by itself and also affects the EVM.
Why won't you use some sort of linearizer? This will improve
your distortion by all means (EVM and other non-linearities)?
Good luck
_________________
Best regards,
- IR
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Guest
Post subject: Posted: Mon Sep 05, 2005
10:21 pm
Hi~~
Thank you for so good suggestion. I am a WLANPA
IC designer so I cannot use linearizer or specified bypass network because
I have to shrink my chip size as small as possible and also avoid external
matching components as few as possible. Also I cannot follow the suggested
external matching values on the datasheet because I will write the datasheet
^_^
Actually, I think the key point of good linearity is the
adjustment of on-chip active bias circuit beside the RF matching of
the whole circuit. However, both of the bias circuit adjustment and
the RF matching are the causes of AM/AM and AM/PM behavior what we will
get. But the phase noise as you said might not be related to them but
should be related to the bypass capacitors. In fact I ever encounter
one interesting thing. When I changed my 0.1uF bypass capacitor size
at Vcc from 0201 size to 0603 size, the EVM improve from 4% to 3% and
all other RF performance kept the same! Also I am interested in the
memory effect you said. I guess that can be concluded to the time variant
AM/AM and AM/PM behavior and will determine the temperature deviation
of the RF performances.
Top
IR
Post subject:
Posted: Mon Sep 05, 2005 11:44 pm
Site Admin
Joined:
Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello,
All those things are related to one another. The size of capacitor
changes its Q due to different resistance. You also change its resonance
frequency and its parasitic inductance. unsufficient bypassing can cause
AM or FM modulation over the signal - phase noise.
Active bias
is a good way to control over your AM-AM and AM-PM curvers. As you may
know, once you provide a low bias to the PA, you will get a curve with
as bigger dip (at the average region of the signal) and the amplifier
will enter more quickly into saturation. The opposite will happen with
a higher bias. Higher bias provides therefore a more linear and less
power-efficient amplifier and lower bias provide the opposite (per same
output power).
_________________
Best regards,
- IR
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Guest
Post subject: Posted: Tue Sep 06, 2005
12:49 am
Hi~~
Yes this kind of phenomena at low bias we call
it "EVM plateau" and usually it is caused by high AM/AM behavior. However,
recently it's found even we have high AM/AM behavior for CW operation,
the momentary AM/AM behavior for OFDM signal operation could be very
flat and the EVM plateau can keep very low. So, we can get very good
PAE but also very good EVM. I guess you may know it and it is called
" IMD cancellation". However, until now it is hard to simulate and design
it because if you take so high CW AM/AM into ADS simulation you will
get poor EVM performance. Actually I doubt that the CW AM/PM behavior
might help us to see if IMD cancellation happen but I've not yet
verified it.
Maybe in the future this kind of phenomena can be realized
more and we can easily design this kind of very linear and very high
efficient PA.
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SKChen
Post subject: Posted:
Tue Sep 06, 2005 1:14 am
Lieutenant
Joined: Tue Sep
06, 2005 12:58 am
Posts: 3
Location: Taiwan
Hi...
I
am the Guest as above and I decided to register an account. I am now
working for Japanese to design the WLAN PAIC. When I was in my graduate
school, I study the feedforward linearization method of PA. But when
I joined my company I found that many linearization method are not available
because we have very limited cost issue and size issue. So we have to
think about more for the on-chip active bias circuit. I've ever heard
that in Israel there is some company which can integrate the feedforward
linearization into a chip. I don't know if it is real because I think
it should be very difficult to make the coupler on chip.
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IR
Post subject: Posted: Tue Sep 06, 2005 2:26 am
Site Admin
Joined: Mon Jun 27, 2005 2:02 pm
Posts:
373
Location: Germany
Hi SKCHen,
First it is good that
you made an account!
Feedforward will always require an additional
PA (The Error PA) and also couplers and cancellation networks and I
don't think that it is achievable with today's technologies.
Nowadays,
Feedforward and more advanced methods are available at the module level
and not at the chip level.
There are however today ASICs that are
implementing Digital Pre-Distortion algorithms for RF PA's and also
chips that are doing Analog Pre-Distortion.
_________________
Best regards,
- IR
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SKChen
Post subject:
Posted: Tue Sep 06, 2005 5:09 am
Lieutenant
Joined:
Tue Sep 06, 2005 12:58 am
Posts: 3
Location: Taiwan
Hi IR:
Yes in the past years the Digital Predistortion PA research rapidly
grows up but unfortunately for us RF engineers it's difficult to enter
that field especially when we don't have the baseband talent in our
company. So, we have to try to find more tricks in our circuit itself.
But I guess that in the future we RF engineer may need to cooperate
more and more with the baseband engineer since I've heard that TI has
already combined the baseband part and RFIC part of GSM mobile phone
into a single chip by DSP. The only things remained outside for the
traditional RF engineers may be only PA, Switch, and antenna. But anyway,
some kinds of physical level things will not disappear. People still
cannot use digital circuit to realize the PA, switch, or antenna. Nevertheless,
we RF engineers have to estabilish our knowledge background more like
this topic, how is the EVM relationship with the traditional RF performance
items like AM/AM, AM/PM, phase noise as we know so far. That's why I
visit websites like here to find the answers because there is almost
no book that can tell us about topics like this.
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IR
Post subject: Posted: Tue Sep 06, 2005 5:47 am
Site
Admin
Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location:
Germany
Hello SKChen,
Yes that is true, the cooperation between
the RF Engineer and Baseband Engineer has to grow due to the interaction
of the linearization issue.
I have worked with several linearization
methods but all were intended for PA modules and not chips. Nevertheless,
I saw and seeing now (In my current job) the deep meaning and significance
of high-speed A/D and D/A converters, issues of phase noise, jitter,
dynamic range of A/D and D/A and their impact over the system performance.
If you would provide your email address I will be happy to write
you more.
Good luck.
_________________
Best regards,
- IR
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SKChen
Post subject: Posted: Tue
Sep 06, 2005 9:12 pm
Lieutenant
Joined: Tue Sep 06,
2005 12:58 am
Posts: 3
Location: Taiwan
Hi IR:
Yes
I also heard about that the high speed ADC or DAC help DSP people can
handle many things at baseband level which originally should be handled
at RF level. Embarrassingly I am very unfamiliar to that since in my
office we only desing PA and switch. However I still want to learn more.
I've made my mail address visible so if you have leisure please pass
me some information. I do appreciate your kindness.
Posted 11/12/2012
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