 # Eddie Carle - Tech Blog

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## Ceramic Filter SPICE Model

##### October 1, 2015

As part of a larger project I’m working on, I’ve found myself needing to use a ceramic filter on a 10.7 MHz IF (intermediate frequency) signal. Specifically, I’m using a Murata CERAFIL SFECF10M7GA00-R0 to provide 230 kHz of bandwidth in a ~40 dB IF amplifier. To test the design, I opted to do a SPICE simulation. Murata does not publish a SPICE model of their filters so I had to approximate one myself. This post discusses that model.

### The Model

Unfortunately the manufacturer didn’t provide much in the way of details on how one might model a three terminal filter. The Cerafil Applications Manual, however, offers an “equivalent” circuit for the two terminal device. It is simplified to a series RLC circuit with a parallel capacitance as shown in figure 1.

The specification sheet for the filter specifies frequency independent input and output impedances of 330 Ω. I have trouble believing this but for my purposes it doesn’t really matter; so long as the impedance is as specified within the passband. Insertion loss is specified at 3.5 dB. From this I’ve extrapolated an “equivalent” circuit in figure 2 for the three terminal device.

The input and output impedances are modelled as simple resistors. The output is modelled by a VCVS (voltage controlled voltage source) that transforms the input voltage $V_i$ via $H(s)$ and applies a fixed gain $a$. $H(s)$ is a simple bandpass second order system defined by

while the gain, $a$, compensates for the insertion loss and the voltage division effect of the circuit. It is defined as

Thus the SPICE model is

.SUBCKT SFECF10M7GA00-R0 1 2 3

R1 1 3 330
R2 4 2 330
A1 %vd(1, 3) %vd(4, 3) filter
.MODEL filter s_xfer(
+num_coeff=[
+1.44513262065130e+006
+0.00000000000000e+000]
+den_coeff=[
+1.00000000000000e+000
+1.44513262065130e+006
+4.51988403152288e+015]
+gain=1.3366878)
.ENDS


### Results

In order to test the model, the circuit in figure 3 was used. Since the model contains no non-linear elements I only bothered to do AC tests and neglected the transients.