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S-Param Mag./Phase - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement


boon
Post subject: s-param mag./phase Posted: Sat Jul 15, 2006 2:02 pm

Captain

Joined: Tue Feb 21, 2006 5:19 pm
Posts: 17
hello all

this could be very stupid question....as far as s-param. related how is magnitude related to phase? meaning, if magnitude changes does the phase also change? meaning if you see a shift in dB curve of s11 does it necessarily mean the phase curve has to change to somewhat?

thanx
-B


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Stephen
Post subject: Posted: Sun Jul 16, 2006 1:27 am

Captain

Joined: Wed Jun 21, 2006 8:33 pm
Posts: 21
Location: Queen Creek, Arizona
Boon,
Actually, S-paramters account for both magintude and phase and can be described in complex numbers. Typically, you only see the magnitude portion in terms of dB, however, every S-paramter should also have an angular (phase) component reported with it, but typically is not.

For example, a circuit may have an S11 of -26dB. But it should also be reported that it has an angular component of -175 degrees or an almost 180 degree phase shift (which you would expect from a reflected compnent.).

Again, the problem is most of the time we read/discuss about s-paramters, it is almost always in terms of the magnitude and the phase is ignored. Just realize that S-params do account for phase and it should also be reported with the dB's.

I hope this helps.

_________________
CMOS RF and Analog ESD Specialist!
www.srftechnologies.com

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Peter Raynald
Post subject: Posted: Tue Jul 18, 2006 1:28 pm

Captain


Joined: Tue Sep 07, 2004 3:09 pm
Posts: 11
Most of the time the phase will change, but there are controlled situation where you can see either the mag or phase change while the other one does not.

I think of a phase shifter circuit, or a resitive pi attenuator on which you vary the resitances to change the loss.

If you create missmatch using a resistor, in serie or paralel, you will see the impedance change on the central axis of the smith chart, therefore you have no phase shift, but a magnitude change.


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boon
Post subject: Posted: Wed Aug 02, 2006 1:45 am

Captain

Joined: Tue Feb 21, 2006 5:19 pm
Posts: 17
hello all...

another question about phase. if i have a common emitter (CE) amplifier then we expect the phase to -180 degree ideally at the output. i have a case where the phase is approx. -125 degree but is very constant and then increases as you compress. in other words, its AM-PM curve is quite good. what can be said about the linearity of this circuit looking at phase (assuming AM-AM is good too)? is it good or bad? does not having -180 deg. mean/indicate bad linearity even though AM-PM is good or you just look at AM-PM & AM-AM curves?

thanx
-B


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IR
Post subject: Posted: Wed Aug 02, 2006 2:25 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello boon,

Besides the 180º shift which you should expect at the output of CE amplifier, the AM-AM and AM-PM can yield almost everything. The direction of the slope for these curves whether positive or negative depends on the technology of the device. I have seen (At the same device) both negative slopes for AM-AM and AM-PM and also positive
AM-PM and negative AM-AM. In general, you should expect that the slope of the AM-AM and AM-PM will increase (To whatever direction it has) when you reach compression.

_________________
Best regards,

- IR


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boon
Post subject: Posted: Wed Aug 02, 2006 2:42 pm

Captain

Joined: Tue Feb 21, 2006 5:19 pm
Posts: 17
hello IR,

thanks for replying. you are saying even though phase might not be exactly 180 degree out of phase, if AM-AM and AM-PM curves are good then it indicates linearity is good, regardless of if phase is 180 degree out or not?


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IR
Post subject: Posted: Wed Aug 02, 2006 2:48 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello again,

AM-AM and AM-PM curves are not an estimation for good or bad linearity. You should consider linearity by measuring the IM products. I was referring to the fact that the slopes of AM-AM and AM-PM can have different slopes at the same time i.e. negative slope and positive (Increasing) slope. The absolute value of the phase is not relevant.

_________________
Best regards,

- IR


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ben00i
Post subject: Posted: Sat Mar 24, 2007 11:28 am

Lieutenant


Joined: Wed Mar 21, 2007 7:08 am
Posts: 1
I am measuring Avago's MGA 83563 medium power amplifier at 2.45GHz. I got a negative AM-PM slope.

1) What does a positive / negative AM-PM slope tell me?

2) The absolute value as u mentioned is important. I am confused as to what value of change would constitute the PA device to having memory, quasi-memory or even memoryless. I think this is a confusing point as there are papers out there that state AM-PM measurement as a quasi-memoryless, memoryless or even a PA with memory.

Pls share. THanks.


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IR
Post subject: Posted: Sat Mar 24, 2007 11:52 am

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
The negative or positive slope of the AM-PM is related to the transistor's technology.

Memory effect can be caused (Most of) due to thermal effects and to impedance variations at the frequency of the modulating signal. The thermal effect causes the amplifier to act as a filter in certain frequencies.

Here is a link that can be useful for you which relates the causes and characteristics of memory effect based on IM3 measurements.

http://herkules.oulu.fi/isbn9514265149/html/x1900.html

Hope this helps.




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