An impedance matching problem of SAW filter

[GDF]

saw filter matching

I have a 1.5GHz SAW filter with 20MHz BW for channel selection, I have to match
it if I want to get correct filter response.

1) My question is do I need to do wideband matching which means the
50 ohms mathcing over wide frequecy range such as 0.5G~2.5GHz
or I only need to match it over narrow band 1.4G~1.6GHz?
2) If I do the narrow band matching, how do I guarantee the outband response,
for example the attenuation at 3GHz?

Anybody have the idea about this?

 

[khouly]

how to do matching of saw filter

u need just the matching circuit work in the band width of the filter

khouly

 

[John_li]

how to do matching of saw

Sure.u just the matching work in your interest bandwidth.

 

[biff44]

saw filter impedance matching

I think I see what you are asking. If you design a bandpass filter with low loss L's and C's, it will have a nice low loss passband, and a good rejection band ONLY if the load is a broadband 50 ohm match. If the load is no 50 ohm somewhere in the rejection band, you may not get the desired ammount of dB's of rejection. This is because the bandpass is constructed of a ladder network of reactances that transform the load impedance all over the place internal to the filter in order to reflect any incident power in the reject band. If the load is not 50 ohms, then the internal impedances transformed in the filter are not what they should be, and performance suffers. An obvious example of this is to put two bandpass filters in series with a low loss transmission line connection of some length. Where ever that length is some multiple of λ/2 at a specific frequency in the reject band, then the stopband attenuation goes almost to 0 dB!

BUT, a SAW filter does not work that way. It uses acoustic methods to filter the signal, not impedance transformation methods. As such, I think it is relatively immune from load impedance.

That said, I never did actually test a SAW filter to see if the above is true.

 

[DDavid]

saw filter, matching

Hi,

As biff said Saw filter it's not regular lumped filter!
So you must matched all you band!
HOw can you do it? You can do it with passive elements or active,
with active much easy
I used with amp my band was similat to you 800MHz to 2.2GHz
so I used with BGA427!

Another advise that I can give you it's GOOD LAYOUT!!! you must separate the ground plane between in & out!!!!

David

 

[GDF]

saw, matching

I think I see what you are asking. If you design a bandpass filter with low loss L's and C's, it will have a nice low loss passband, and a good rejection band ONLY if the load is a broadband 50 ohm match. If the load is no 50 ohm somewhere in the rejection band, you may not get the desired ammount of dB's of rejection. This is because the bandpass is constructed of a ladder network of reactances that transform the load impedance all over the place internal to the filter in order to reflect any incident power in the reject band. If the load is not 50 ohms, then the internal impedances transformed in the filter are not what they should be, and performance suffers. An obvious example of this is to put two bandpass filters in series with a low loss transmission line connection of some length. Where ever that length is some multiple of λ/2 at a specific frequency in the reject band, then the stopband attenuation goes almost to 0 dB!

BUT, a SAW filter does not work that way. It uses acoustic methods to filter the signal, not impedance transformation methods. As such, I think it is relatively immune from load impedance.

That said, I never did actually test a SAW filter to see if the above is true.

Thanks a lot, I think you got my point. In my case, I would like to do impedance
transformation to save the overall current consumption, but this would force the
system as a narrow band system. But it looks like OK to do this.