This is about how to get started developing/testing RF matching sections for a 2-port device, when neither port has a working matching network, so the "second" port is not yet properly terminated.
INITIAL SPECIFICS:
The device in this case is a Tai Saw TA0245A SAW filter (139 MHz, PDF datasheet attached.) The ports are not natively 50 ohms (they're basically low impedance dominated by capacitive reactance.)
My questions revolve around this: Can the ports be matched separately? When the SAW filter is in passband (this device has insertion loss about 5 dB at center frequency), the getting-started condition is that neither port is matched. Does this complicate measurements "looking into" the one port when the other port is open/shorted/(what should I do here?)
ADDITIONAL DETAILS:
The datasheet indicates equivalent circuit for input port is 390 ohms || 300 pF.
The datasheet indicates equivalent circuit for output port is 1164 ohms || 374 pF.
The datasheet shows a representative input match section with 12 pF series followed by 56 nH shunt at the port.
The datasheet shows a representative input match section with 9 pF series followed by 56 nH shunt at the port.
I'm testing this on a small bit of double sided/plated thru FR4 (matching the device footprint, so the 6 remaining contact pads have individual vias to ground.) Input/Output paths are sized for 0603 components (shunt, series, shunt "pi" physical arrangement), but are otherwise very short (distance between the device and the match components are a few mm, then a few mm to SMA ports on the test module.)
My initial construction implemented the datasheet's suggested matching section. The filter S21 (insertion) resembles the datasheet, but insertion losses are several dB higher than expected (as I'm at the upper edge of the device passband, the added several dB there make a difference; group delay there won't be an issue.)
Using a VNA (calibrated at the SMA connectors of the small test module) at the device center frequency (139 MHz), with the datasheet matching sections installed, the input port yields 37 ohms || 78 pF; at the output port 16.3 ohms || 58 pF equivalence.
I have made a trial run (just on PC, not building anything yet) using
https://www.analog.com/en/resources/interactive-design-tools/rf-impedance-matching-calculator.htmlThis has input data options that exactly mirror the physical device requirements (device port is expressed as parallel RC combination, specifying R, C and F).
I'm in the "look before you leap" state right now; the analog.com page suggests match sections, but I want to be sure I'm considering (if necessary) the effect of the second device port properly (that is, within the active passband of the filter.)
Thank you for any comments, questions, or requests for clarification!
Dave