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cmosbjt
Joined: 25 Apr 2004
Posts: 231
Helped: 8
Location: Boston
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 18 Jan 2005 3:52 Receiver Front-End, "Phase Shifter"? |
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Hi, the above structure is the GPS downconverter IC CXA3355R from Sony:
http://www.sony.co.jp/~semicon/english/img/sony01/a6805199.pdf
They use a so call "Phase Shifter and Adder" to reject the image.
Is this Phase Shifter:
1. a polyphase filter
2. a 90deg phaseshift as the img below
3. or can be sth else?
Which is commonly used? 1, 2 or 3?
Thanks.
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Mazz
Joined: 03 Nov 2001
Posts: 592
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18 Jan 2005 10:56 Re: Receiver Front-End, "Phase Shifter"? |
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I think that last figure is a block diagram representation only. The implementation is using polyphase, that's the most commonly used architecture. The possible problem could be in the big values (and size) of R & C due to the low freq (1 MHz).
Also in R versus C there is a compromise:
Large R -> small C -> High noise figure
Large C -> small R -> Big area
After all analog design is to find the best compromise.....
I hope it can help.
Mazz
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DonJ
Joined: 27 Dec 2004
Posts: 127
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18 Jan 2005 11:13 Re: Receiver Front-End, "Phase Shifter"? |
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| The last figure is used in practice as well. You can consider it the simpleeest variant of polyphase filtering.
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cmosbjt
Joined: 25 Apr 2004
Posts: 231
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Location: Boston
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18 Jan 2005 11:23 Receiver Front-End, "Phase Shifter"? |
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| Thanks Mazz and DonJ.
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Mazz
Joined: 03 Nov 2001
Posts: 592
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18 Jan 2005 12:05 Receiver Front-End, "Phase Shifter"? |
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DonJ
in simple RC network, how's solved the problem of spread of parameters with the spec of minimum rejection (typically in the range of 20~30 dB)?
Mazz
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DonJ
Joined: 27 Dec 2004
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18 Jan 2005 12:31 Re: Receiver Front-End, "Phase Shifter"? |
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| Mazz wrote: |
DonJ
in simple RC network, how's solved the problem of spread of parameters with the spec of minimum rejection (typically in the range of 20~30 dB)?
Mazz |
Mazz 
The simple RC networks are, as you pointed out, not the most suitable one for high performance systems (high rejection).
People tend to incorporate careful layout for phase matching (+45 / -45 deg) and include some limiting amp. for amplitude balance in simple RC app's.
Don't misunderstand, I agree with you.
Don
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Mazz
Joined: 03 Nov 2001
Posts: 592
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18 Jan 2005 12:45 Receiver Front-End, "Phase Shifter"? |
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No misunderstanding, but I thoght that there could be some new informations for my designs...
Anyway, thanks.
Mazz
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DonJ
Joined: 27 Dec 2004
Posts: 127
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18 Jan 2005 23:50 Re: Receiver Front-End, "Phase Shifter"? |
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| Mazz wrote: |
No misunderstanding, but I thoght that there could be some new informations for my designs...
Anyway, thanks.
Mazz |
Well, actually you can improve the performance by calibration. A little bit messy but it works. The calibration complexity must obviously be considered against polyphase filter silicon area and signal attenuation.
A.T.B
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cmosbjt
Joined: 25 Apr 2004
Posts: 231
Helped: 8
Location: Boston
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19 Jan 2005 2:23 Receiver Front-End, "Phase Shifter"? |
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Hi DonJ,
Can you talk a little bit more about the calibration?
Thanks
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DonJ
Joined: 27 Dec 2004
Posts: 127
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19 Jan 2005 11:34 Re: Receiver Front-End, "Phase Shifter"? |
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| cmosbjt wrote: |
Hi DonJ,
Can you talk a little bit more about the calibration?
Thanks |
Hi
The principle is the same for image rejection calibration. You can find good papers describing it (e.g. IEEE).
It is basically compensation of the phase and amplitude errors in either a feedback/analog or pure digital signal manner.
Don
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vfone
Joined: 10 Oct 2001
Posts: 2284
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26 Jan 2005 20:23 Re: Receiver Front-End, "Phase Shifter"? |
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CXA3355 use a 90 deg Phase Shifter, and active low pass filters (point nr 2, in your enumeration).
Also some reviews said that this IC works better with external IF filter, instead of using the internal ones.
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dsjomo
Joined: 24 Oct 2003
Posts: 154
Helped: 15
Location: Taiwan
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27 Jan 2005 3:39 Re: Receiver Front-End, "Phase Shifter"? |
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There's a calibration mechanism proposed by B.Razavi in his very famous paper
"Design considerations for Direct-Conversion Receivers"
http://www.edaboard.com/viewtopic.php?t=93586
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