RFFC5071A RF Balun Matching Network

RF Solutions RF All Other (switches, controls, attenuators, ..)
1

Hi,

I have some questions regarding the input and output impedance of the RFFC5071A. In the datasheet section on impedance matching, the equivalent circuit model for the input impedance consists of an 85-ohm resistor, a 0.5pF shunt capacitor, and two 0.5nH series inductors.

In the recommended schematic for 3.7 GHz, two 15pF capacitors are used in series.

I simulated the recommended internal circuit using an ideal 1:1 transformer in ADS and compared its S11 with and without the 15pF series capacitor (as shown in the figure below). As you can see, adding the capacitor worsened the impedance matching instead of improving it.

Could you please check if there’s any mistake in my modeling or simulation? I’d like to ensure proper matching at other frequencies as well.

Thanks!

rffc5071_Mixer1_DC input_15pF
rffc5071_Mixer1_DC input_15pF1099×662 130 KB

2

This model is quite simplistic, and does probably not hold true at 3.7GHz. You’d need to use models or s-parameters of the passive components, and include in your simulation the microstrip line lengths that will be significant at this frequency. The 15pF caps are primarily for ac coupling, they weren’t intended as matching components. This 3.7GHz input circuit on the eval board has not been optimized for input return loss. The board was optimized for conversion gain. More useful info in this app note on matching and mixer performance.