Matching of SAW Filter with Differential I/O

[crenyen]

saw filter matching

I have difficulties in matching 380MHz SAW Filter with 100 ohm balanced impedance. The circuit is attached.
Can anyone know how to well do the I/O matching.

 

[khouly]

differential saw filter

what is the output impedance of the chip and what also the input impedance

khouly

 

[crenyen]

how to do matching of saw

The I/O differential impedance of chip set is 50 ohm. While the I/O balanced impedance of saw filter is 100 ohm.

 

[BigBoss]

50 ohm differential match

Look at the pdf file..

 

[spikeyang]

saw, matching, differential

The rated 100Ohm could not be real impedance value of the SAW. It could be a resistance paralled with an inductor.

You better measure the input impedance using a network analyzer before do any matching work. The output port should be properly loaded.

How to measure the differential impedance correctly is another problem. Calibration of the connection cable is important. Or you will get wrong result which deviate a lot from the ADS theoretical simulation.

 

[vfone]

saw filter if differential

Differential matching is sensitive to the PCB layout and to the parasitic of the components and it is hard to get a good CAD simulation to find the values of the matching.
Remove the IC from PCB and place two small baluns on its terminals. Solder thin 50 ohms cables to the baluns and tune the circuit using a Network Analyzer.
SAW IF filters are sensitive also to the layout. Check the manufacturer recommendation for best grounding and vias placement between terminals. If the layout of the SAW is bad you cannot fix it playing with the matching components.

 

[crenyen]

balun saw filter

Trying to attach two baluns at SAW filter I/O port on my PCB is not easy.
My idea is that I modify this 4 ports SAW filter to 2 port SAW filter by grounding another two ports then I can match it just like a common two port circuit with network analyzer.
Does this method work?

 

[BigBoss]

cascading saw filter

Trying to attach two baluns at SAW filter I/O port on my PCB is not easy.
My idea is that I modify this 4 ports SAW filter to 2 port SAW filter by grounding another two ports then I can match it just like a common two port circuit with network analyzer.
Does this method work?

Frequency is not too high. You can try to add 1:1 balun whic is proper at your frequency to convert sigle ended to differential.If you embed the insertion loss of the baluns ( two times of one ) you can measure input impedance and loss of the saw filter.

The better way to realize a board that consists of two cascade connected ( single-diff-diff-single ) balun and measure the insertion loss ( calibrate) and then to insert the saw filter between these two balun. In this case the loss will be higher due to balun losses but you know this IL before and extract this from the last one.

Impedances are measured as ususal way... A VNA is sufficient.