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Channel Filter vs. Matched Filter for FSK (Read 3469 times)
aaron_do
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Channel Filter vs. Matched Filter for FSK
Apr 02nd, 2009, 9:22pm
 
Hi all,


I have a question about direct-conversion FSK. Basically after down-conversion, I have a channel select filter which eases the linearity requirements of the limiting amplifier stage.

My question is does the channel-select filter usually have to double as a matched-filter, or do we normally design an additional matched filter after limiting amplification? My thinking is that having an additional matched filter would give better SNR since the limiting amplifier introduces broadband noise. On the other hand, i've seen some designs which do not seem to have the additional matched filter...


thanks,
Aaron
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Re: Channel Filter vs. Matched Filter for FSK
Reply #1 - Apr 5th, 2009, 4:42pm
 
Hmmm.... Havent done a FSK system in many years.... A lot of the architectures out there are probably legacy designs. Consequently I would suggest a "never been here before" approach on this. Develop and model at the system level before jumping into transistors.

Give some serious thought to direct down convert followed by a minimal BW limiting filter and then ADC, and put all your signal processing in the DSP. Might as well use a modern approach to an old modulation method.

Jerry
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Re: Channel Filter vs. Matched Filter for FSK
Reply #2 - Apr 6th, 2009, 12:36am
 
Thanks for the response, but seems like i'm left with more questions than answers Cheesy


what do you mean by Legacy design?

If I go for direct conversion followed by a minimum bandwidth limiting filter, I guess I will have problems with DC offset.

I'm trying to design a low-power receiver. Is ADC + DSP based filtering/detection going to be more power hungry than a analog filtering/detection? I'm actually totally unfamiliar with DSP based design at this stage in my career (the beginning)...

Also I selected wideband FSK as I thought the wide tone separation would make filtering (DC offset) as well as the modulation easier. Is there a good reason why not to go for FSK? I know there is an IEEE 802.15.4 (low power) standard out there which uses OQPSK + DSSS, but it seems kind of overkill for a low power low cost application...


thanks,
Aaron
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Re: Channel Filter vs. Matched Filter for FSK
Reply #3 - Apr 11th, 2009, 1:03am
 
Hi Pancho,


I'm only just starting to read up on the modulation/demodulation part of the system, so some of my questions may seem a bit nonsensical... anyway I've done some more reading and perhaps I can re-phrase...

Basically I'm looking at 2-FSK with wide tone separation (I guess you pointed me to your other post to illustrate that OQPSK and MSK have similar constellations, right?). Anyway I want the wide tone separation so that when I do direct down-conversion, I can do DC off-set filtering easily. So after my first mixer, I intend to have a channel-select filter which will ease the IIP3 requirements of the limiting amplifier (which follows). The limiting amplifier will have a high gain (maybe more than 60 dB). So after that, the signal needs to be converted into a baseband signal, right? This can be done by a quadrature detector, however, the quadrature detector needs to have more than around 10 dB SNR at the input (in accordance with the Threshold Effect). My question is, "is this threshold effect SNR determined by the noise bandwidth?". If so, is it beneficial to filter off the noise in between the mark and space with an additional filter prior to quadrature detection...


Second question is, "would a DSP based design be more power hungry?" i.e. after downconversion, have an anti-alising filter followed by ADC and then DSP...


hope my question makes more sense now.
thanks,
Aaron
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Re: Channel Filter vs. Matched Filter for FSK
Reply #4 - Apr 11th, 2009, 1:29am
 
Totally I can't understand meanings of your questions.

aaron_do wrote on Apr 2nd, 2009, 9:22pm:
My question is does the channel-select filter usually have to double as a matched-filter, or do we normally design an additional matched filter after limiting amplification? My thinking is that having an additional matched filter would give better SNR since the limiting amplifier introduces broadband noise. On the other hand, i've seen some designs which do not seem to have the additional matched filter...
It is almost impossible to realize matched filter as analog filter.

aaron_do wrote on Apr 11th, 2009, 1:03am:
I guess you pointed me to your other post to illustrate that OQPSK and MSK have similar constellations, right?
No.
They have completely same constellations.
MSK signal also can be generated as OQPSK with Halfsine pulse shaping.
http://www.designers-guide.org/Forum/YaBB.pl?num=1195226387

aaron_do wrote on Apr 11th, 2009, 1:03am:
My question is, "is this threshold effect SNR determined by the noise bandwidth?".
Right.

aaron_do wrote on Apr 11th, 2009, 1:03am:
If so, is it beneficial to filter off the noise in between the mark and space with an additional filter prior to quadrature detection...
I can't understand what you mean.

aaron_do wrote on Apr 11th, 2009, 1:03am:
Second question is, "would a DSP based design be more power hungry?"
i.e. after downconversion, have an anti-alising filter followed by ADC and then DSP...
I don't think so.
Again, it is almost impossible to realize practical matched filter as analog filter.

See paper on ZigBee Transceiver which is zero-IF architecture by ADI in ISSCC-2009.

[24.4] "A Highly Integrated Low-Power 2.4GHz Transceiver Using a Direct-Conversion Diversity Receiver in 0.18um CMOS for IEEE802.15.4 WPAN"
G.Retz1, H.Shanan1, K.Mulvaney1, S.O'Mahony1, M.Chanca2, P.Crowley3, C.Billon1, K.Khan1, P.Quinlan1
1Analog Devices, Cork, Ireland; 2Analog Devices, Valencia, Spain; 3Analog Devices, Limerick, Ireland

"A 0.18um CMOS RF sensor-network transceiver based on the 802.15.4-2.4GHz WPAN standard integrates a radio controller,
a direct-conversion diversity receiver and a transmitter based on direct VCO modulation.
The 5.85mm2 IC draws 16.8mA from 1.8V in receive mode and 18mA at 3dBm output power.
The receiver achieves 9.5dB NF, -96dBm sensitivity and -18dBm IIP3, with 54dB of IR at +/-5MHz offset."
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aaron_do
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Re: Channel Filter vs. Matched Filter for FSK
Reply #5 - Apr 11th, 2009, 2:23am
 
Thanks for the reply,


Quote:
Quote:
If so, is it beneficial to filter off the noise in between the mark and space with an additional filter prior to quadrature detection...

I can't understand what you mean.


The SNR at the input of the quadrature detector is affected by the noise bandwidth, and I intend to use FSK with wide tone separation. Suppose the data rate is 100 kB/s and the frequency deviation is 500 kHz giving a modulation index of 5. Now suppose the transmission BW is 1.2 MHz. In between the upper and lower frequencies of the 2-FSK signal there should be approximately 800 kHz of BW with a negligible amount of signal information but a non-negligible amount of noise. If I filter off this part of the spectrum, will I improve my overall SNR, which is the same SNR required for the threshold effect (about 10 dB)?


The reason i'm not sure is because this in-between noise is at a low frequency deviation and hence may not affect the FM detector at all. But i'm not sure whether it comes into play in determining the SNR for the threshold effect.


BTW ISSCC 2009 hasn't been uploaded onto IEEE xplore. I saw a ZigBee design in ISSCC 2008 but it was a low-IF architecture...


thanks a lot,
Aaron
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Re: Channel Filter vs. Matched Filter for FSK
Reply #6 - Apr 11th, 2009, 2:33am
 
Maybe you don't understand many isuues about communication theory correctly.

aaron_do wrote on Apr 11th, 2009, 2:23am:
In between the upper and lower frequencies of the 2-FSK signal there should be approximately 800 kHz of BW with a negligible amount of signal information but a non-negligible amount of noise. If I filter off this part of the spectrum, will I improve my overall SNR, which is the same SNR required for the threshold effect (about 10 dB)?
It could improve SNR. But how do you filter out ? Do you use band elimination filter ?
Here you have to vary elimination frequency as far as you want to put filter before I/Q-mixer.
Such variable band elimination filter is very difficult to realize.
Even though you can eliminate noise around center between two frequencies(mark and space) before I/Q-mixer, I/Q-mixer generate noise or offset around DC.

aaron_do wrote on Apr 11th, 2009, 2:23am:
BTW ISSCC 2009 hasn't been uploaded onto IEEE xplore. I saw a ZigBee design in ISSCC 2008 but it was a low-IF architecture...
I have DVD of ISSCC-2009.

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« Last Edit: Apr 11th, 2009, 7:10am by pancho_hideboo »  
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aaron_do
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Re: Channel Filter vs. Matched Filter for FSK
Reply #7 - Apr 11th, 2009, 2:34am
 
Just saw your last edit...The power consumption seems to be about 3 times higher than the design by Atmel (JSSC, can't remember when but also a ZigBee transceiver). Is there any significant advantage it provides?


thanks,
Aaron
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aaron_do
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Re: Channel Filter vs. Matched Filter for FSK
Reply #8 - Apr 11th, 2009, 2:43am
 
Quote:
Maybe you don't understand many isuues about communication theory correctly.


Clearly you can see the flaw in my logic. I would be grateful if you could point it out...


thanks,
Aaron


OK...i think i figured it out...kinda
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« Last Edit: Apr 11th, 2009, 4:07am by aaron_do »  

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Re: Channel Filter vs. Matched Filter for FSK
Reply #9 - Apr 11th, 2009, 2:53am
 
aaron_do wrote on Apr 11th, 2009, 2:34am:
Just saw your last edit...The power consumption seems to be about 3 times higher than the design by Atmel(JSSC, can't remember when but also a ZigBee transceiver). Is there any significant advantage it provides?

In Atmel transceiver, low IF architecture is used.
Remarkable feature of RX is that they use polyphase filter for RF signal path instead of LO signal path. Here complex image rejection filter which is also part of channel selection filter is used.

I don't know what design of Atmel you refer, but ADI's design in ISSCC2009 is far low-power than Atmel I know.
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Re: Channel Filter vs. Matched Filter for FSK
Reply #10 - Apr 11th, 2009, 4:17am
 
Quote:
16.8mA from 1.8V in receive mode and 18mA at 3dBm output power


Sorry I remembered wrongly. The Atmel design used 14.7 mA in receive mode (I forgot about the PLL + Digital) and 15.7 mA in receive mode. Still comparable to the paper you mentioned, but i get your point.


cheers,
Aaron
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Re: Channel Filter vs. Matched Filter for FSK
Reply #11 - Apr 11th, 2009, 4:19am
 
aaron_do wrote on Apr 11th, 2009, 4:17am:
Sorry I remembered wrongly. The Atmel design used 14.7 mA in receive mode (I forgot about the PLL + Digital) and 15.7 mA in receive mode.

ADI's design in ISSCC-2009 include much baseband than Atmel.
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Re: Channel Filter vs. Matched Filter for FSK
Reply #12 - Apr 11th, 2009, 4:28am
 
I see. I'll have a look when it is uploaded. Thanks.
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Re: Channel Filter vs. Matched Filter for FSK
Reply #13 - Apr 12th, 2009, 11:30pm
 
Just saw your recent edit...

Quote:
It could improve SNR. But how do you filter out ? Do you use band elimination filter ?
Here you have to vary elimination frequency as far as you want to put filter before I/Q-mixer.
Such variable band elimination filter is very difficult to realize.
Even though you can eliminate noise around center between two frequencies(mark and space) before I/Q-mixer, I/Q-mixer generate noise or offset around DC.


I was thinking of going for a direct-conversion FSK. So I can filter the spectrum in between the mark and space by using a bandpass filter centered at a frequency equal to the desired frequency deviation...i.e. the filter would have a center frequency of 500 kHz and BW of 200 kHz. The filtering is done at zero-IF (after the IQ mixers) so there shouldn't be any need for a variable band filter.

Does that sound right?

BTW, after the limiting amplifier, the quality of the signal is determined by the phase error right? Is there a way to simulate this phase error including the effects of thermal noise? I'm guessing that i need to run a PSS and Pnoise on the limiting amplifier and the rms phase error would be some kind of integration of the phase noise...references would be nice


thanks,
Aaron
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Re: Channel Filter vs. Matched Filter for FSK
Reply #14 - Apr 13th, 2009, 5:53pm
 
aaron_do wrote on Apr 12th, 2009, 11:30pm:
The filtering is done at zero-IF (after the IQ mixers) so there shouldn't be any need for a variable band filter.

aaron_do wrote on Apr 11th, 2009, 1:03am:
If so, is it beneficial to filter off the noise in between the mark and space with an additional filter prior to quadrature detection...
You mentioned filtering before I/Q-Mixer.
You mean frequency discriminator using I and Q signals by  "quadrature detection" ? Here I/Q-Mixer exists before "quadrature detection".

aaron_do wrote on Apr 12th, 2009, 11:30pm:
BTW, after the limiting amplifier, the quality of the signal is determined by the phase error right?
Right.

aaron_do wrote on Apr 12th, 2009, 11:30pm:
Is there a way to simulate this phase error including the effects of thermal noise? I'm guessing that i need to run a PSS and Pnoise on the limiting amplifier and the rms phase error would be some kind of integration of the phase noise...references would be nice
If SNR before limiter is fairly large, small signal noise analysis such as PSS/Pnoise is useful.
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