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Design >> RF Design >> purpose of off-chip band-select filter
https://designers-guide.org/forum/YaBB.pl?num=1215154188

Message started by aaron_do on Jul 3^rd, 2008, 11:49pm

Title: purpose of off-chip band-select filter
Post by aaron_do on Jul 3^rd, 2008, 11:49pm

Hi all,

just wondering, is the purpose of the off-chip band-select filter usually to suppress received interference or to suppress transmitted spurs generated by the PLL and PA?

I guess it depends on the application, but lets say ISM band applications for example...

thanks,
Aaron

Title: Re: purpose of off-chip band-select filter
Post by didac on Jul 4^th, 2008, 12:49am

Hi,
Both options are correct, it helps to filter out of band interferers and helps to avoid out of bands spurs generated by the transceiver.
Hope it helps,

Title: Re: purpose of off-chip band-select filter
Post by aaron_do on Jul 4^th, 2008, 1:14am

just wondering because i just read a paper where they avoided the use of an off-chip filter because the spurious transmissions were reduced some other way. no mention of incoming interference...

Title: Re: purpose of off-chip band-select filter
Post by didac on Jul 4^th, 2008, 6:32am

Hi,
Could you post the reference?, I suppose that if care it's taken the spurs will be below the emission limits. For the receiver part I can only think right now on two situations were you don't put a band select filter for out of band blockers: UWB systems that cover several GHz of bandwidth and don't make sense to use a filter(although I suppose that the could use something), this systems are usually limited by interferences not noise. The other systems I can think it's the "new" approach to Software Defined RadioReceivers(this transceiver with direct sampling) that are made with the intention of multistandard capability so Bandpass filtering it's not needed(in fact they arise from the need to reduce front ends and filtering), but for what I know they need high linearity front-ends.

Title: Re: purpose of off-chip band-select filter
Post by aaron_do on Jul 4^th, 2008, 5:54pm

Hi,

i'll post the reference as soon as I ge back to work. As far as i remember though, there was no other special reason for not needing an off-chip filter for the RX...

Title: Re: purpose of off-chip band-select filter
Post by aaron_do on Jul 6^th, 2008, 8:15pm

Hi didac,

sorry about the delay. The paper is "A Fully Integrated SOC for 802.11b in 0.18-μm CMOS" IEEE Journal of Solid-State Circuits, vol. 40, no. 12, Dec 2005. There are 22 authors, but the first is Shahla Khorram.

Its a good read and it appears to be designed by Broadcom since all the authors work there...

cheers,
Aaron

Title: Re: purpose of off-chip band-select filter
Post by didac on Jul 7^th, 2008, 2:07am

Hi,
Thanks for the reference and don't worry for the delay it was weekend. I've made a quick reading and I see that in fact they take care of transmitted spurs with internal filtering(section D) at the output of the LO generator and as they indicate they used a linear PA(AB)-they said clearly that a non-linear PA could cause problems with out of band spurs-. What I'm not sure if that they could go without band select filter for the receiver, I would try to search for their reference design for WLAN modules and see what they say.
Thanks for the reference,

Title: Re: purpose of off-chip band-select filter
Post by didac on Jul 7^th, 2008, 3:35am

Hi,
I didn't find a reference design for Broadcom modules(although a couple for other companies and they use the band select filter), but re-reading the paper it appears(I think) the clue for avoiding a band select filter in the input, they indicate that they use two RSSI one before the channel select filter that it's used to estimate the blocker level and adjust LNA gain dynamically so it seems that they can survive to blockers, the other RSSI it's just the typical one.

Title: Re: purpose of off-chip band-select filter
Post by aaron_do on Jul 7^th, 2008, 4:44am

Hi,

yeah I suppose that is at least part of the reason. I would think that the antenna has some selectivity too anyway right?

Aaron

Title: Re: purpose of off-chip band-select filter
Post by didac on Jul 7^th, 2008, 5:49am

Hi,
In principle it's true that antenna has a selectivity but in WLAN it's better to expect that the user made some strange things and be sure that the module itself is compliant... Look the attached WLAN antenna(that works and it's very easy to mount) or google for pringles,can antenna.

Title: Re: purpose of off-chip band-select filter
Post by aaron_do on Jul 7^th, 2008, 6:37am

haha...very nice.