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Impedance matching a SAW IF filter

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May 2, 2011
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I'm trying to make a radio receiver and encountered a little problem. I need to match the output of my mixer with the input of my SAW filter and then the output of the SAW with an IF input.

Now I know the output impedance of mixer and the input impedance of the IF stage but the impedance of the SAW filter is not listed in the data sheet.

The centre frequency of the SAW filter is 70 MHz with a 4 MHz BW. The mixer output impedance = 520 ohm ||3.6pF and the If input impedance is ~1.8k ohm ||7.8pF.

The SAW filter datasheet is linked below.
**broken link removed**

I would very much like to know how to make a matching network for the filter when input/output impedances are not listed.

Thanks in advance for any help.

some approx values can be found.

You can play with smith chart and given matching network under the hypotesis that the SAW is perfectly matched both IN/OUT and passives are lossless.

In fact under this condition, cutting the circuit in a section the impedance seen to the right side of the section is the complex conj of the impedance seen to the left side.
You can calculate the impedance seen from input pin toward the source an the one seen from the output pin toward the load.

Doing the exercise I have found:

52-J95 input impedance at 70 MHz
40-J24 output impedance at 70 MHz

Hope it can help.


p.s.: a phone call to RFM to have complete set of s-parameters will be easy to do...
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Filters are terminated not matched to.

If you believe the test load diagram in the spec. Loading pin 11 with 220 nH in series with 50 ohms means the filter looks like 237 ohms || 18.5 pF or series equivalent of 50.0 ohms + 23.5 pF.

Loading on pin 5 with 100 nH going to shunt 22 pF and 50 ohms means the filter pin 5 looks like 55.2 ohms || 24.8 pF or series equivalent of 21.8 ohms + 91.7pF.

At 70 MHz the fixture strays are not too bad. If you want to get picky there is likely about and additional 4 pF shunt in their fixture socket and connections.
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I try to calculate the actual Impedance using 70 Mhz however I am not getting your numbers (i deed I am soo far) can you open a little bit how did you come the that numbers?


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