Saw filter input and output inductors

[harihk]

Hi All,

I recently saw a board schematic which has a sawfilter in it . There were 2 inductors one each at the input and output of the sawfilter. can someone please explain what is the purpose of these inductors? how do we determine the value of these inductors, say for example if the sawfilter has a bandwidth of 8MHz and center frequency of 44MHz?

Any help is greatly appreciated.

Thanks
hk

 

[toonafishy]

These inductors are used to match the SAW filter impedance to a more desirable value. I measure the S parameters of the SAW filter itself and then put the S parameters into a simulator and match it there.

 

[vfone]

**broken link removed**

 

[echo47]

Always refer to the manufacturer data sheet.
For example, this data sheet says 50 ohm termination and no impedance matching required:
https://www.triquint.com/company/divisions/sawtek/docs/856129/856129.pdf

I have a very similar filter from a different manufacturer, and it too uses no matching.

 

[harihk]

Hi toonafishy,vfone,echo47,

thanks for replying, your answers and pointers were very useful.

so, can I assume that the matching networks at the input and the output of a sawfilter donot have any direct relationship to its bandwidth and center frequency?

Thanks again,
hk

 

[toonafishy]

The biggest effect is on amplitude ripple in the passband response and amplitude/group delay tilt across the passband response. If the inductors are tuned to minimize ripple and to get a flat response across the passband, then all the other parameters will fall in line (assuming the filter designer did the job right).

 

[FANT]

The center frequency and bandwidth are given by the SAW filter itself.
You have very little effect on these parameters changing the in/out impedences.
The matching instead is very important for the ripple inside the bandwidth and the attenuation outside the passband, especially very close the edges.

You can also use RFSIM99 that is available free on the net ( just a simple search with google ).

Mandi