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Systemview simulation using twta token from RF/Analog lib

Question asked by phuang on Feb 11, 2005
Latest reply on Feb 17, 2005 by mschif

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I'm having problems getting my simulation to run correctly when using the twta token . the sim works fine when i use the gain token.

the qpsk constellation looks strange when the twta token is used in place of the gain token.
attached are two files one that "works" (using the gain token) and another that doesn't work (using the twta token)

any ideas what i'm doing wrong?
thanks
pete

[file]dvb_sat_r_7_8_working_43mbps_filters_twta_token.svu[/file]
[file]dvb_sat_r_7_8_working_43mbps_filters_puregain.svu[/file]

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mschif Feb 14, 2005 6:38 AM
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HI,

Your TWTA token is NOT a TWTA model. it is a linear token. a twta model
the gain and phase are functions of the input power.

The TWTA token exits in SystemView take a look. Example TWTA-1 in the RF lib examples.

not sure where you got the G, phi model. Look at the gain, very strange.
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phuang Feb 14, 2005 10:54 AM
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Hi Maury,

Thanks for getting back so quickly.
I got the twt token from the SystemView Rf/analog library. It's the same as your token #22 in the TWTA-1.svu , in your Rf lib examples.

Token #339 in the dvb_sat_R_7_8_working_43mbps_filters_twta_token.svu file, is the "twt" token from the SystemView RF library.

token #330 in the dvb_sat_R_7_8_working_43mbps_filters_puregain.svu file, is the pure gain ampilifier from the SystemView Operator Library.

tokens 166 & 167 are custom filters that i created using measured filter data from actual filters that we have on one of our satellites.

My basic problem is that I can't get the file with the twt token to work.
The other file with the the pure gain token (#330) works.
thanks for any help you can give me.
pete
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mschif Feb 14, 2005 12:46 PM
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You are correct. The token mentioned is a TWTA token (my error)

But, are you aware that there is group delay through this token. The write up mentions the use of a Hilbert XFORm to provide I and Q. There is a group delay associated with this. Check it out , I think that this is the
source of your problem. All of your timing from the TWT on is now out of sync with what is need for data recovery.

Let me know!
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phuang Feb 15, 2005 5:59 AM
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Maury,

What writeup are you referring to that mentions the Hilbert xform? Where can I find it?
thanks,
pete
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mschif Feb 15, 2005 6:44 AM
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the write up on the token in the rf lib manuel. It is very terse. The TWT model changes amplitude and phase. In order to change the phase of a signal which is at RF is to take the HT which produces a 90 deg component. Then weight each component by the sine and cos of the desired phase, and finally add the components back together again.

The best way to find out what the delay is, is to enter a signal into the token such as a BPSK on carrier. The XCORR the input with the output.

Good luck!
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phuang Feb 16, 2005 2:29 PM
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maury,
wow, that looks like a handfull, i'll give it a try.

I've got another sim that has been giving me some problems. I have a working version running at 26.66 msps, i increase the symbol rate to 31.1 msps to try to quantify the degradation that i should get when by carrier bandwidth exceeds my channel bandwidth (channel BW is 36 MHz and a 31.1 msps carrier should take close to 42MHz).
The constellation diagram looks pretty bad when I increase the symbol rate to 31.1. But i'm not sure if it's just because i'm doing something wrong with systemview, or it's really showing me the truth. I just didn't think it would be that bad. attached are the working and non-working files with the 2 symbol rates.
thanks for any help you can give me.
pete

[file]dvb_sat_r_7_8_not_working_50mbps_filters.svu[/file]
[file]dvb_sat_r_7_8_working_43mbps_filterswo meta.svu[/file]
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- dvb_sat_r_7_8_working_43mbps_filterswo meta.svu134.7 KB
- dvb_sat_r_7_8_not_working_50mbps_filters.svu150.8 KB
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mschif Feb 17, 2005 5:48 AM
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Hi,

Took a quick look at problem. Nothing obvious. Sorry to say that I have
a day job and cannot devote the time to track down what is going on.

However, I am available for consulting. If this is an option for you,
contact me a numberdr@earthlink.net.

good luck
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Systemview simulation using twta token from RF/Analog lib

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