# Bandpass Filter Response

This example shows how to compute the time-domain response of a simple bandpass filter:

- Choose inductance and capacitance values using the classic image parameter design method.
- Use
`circuit`,`capacitor`, and`inductor`with the`add`function to programmatically construct a Butterworth circuit. - Use
`setports`to define the circuit as a 2-port network. - Use
`sparameters`to extract the S-parameters of the 2-port network over a wide frequency range. - Use
`s2tf`to compute the voltage transfer function from the input to the output. - Use
`rationalfit`to generate rational fits that capture the ideal RC circuit to a very high degree of accuracy. - Create a noisy input voltage waveform.
- Use
`timeresp`to compute the transient response to a noisy input voltage waveform.

## Contents

- Design a Bandpass Filter by Image Parameters
- Programmatically Construct the Circuit
- Extract S-Parameters From the 2-Port Network
- Fit the Transfer Function of the Circuit to a Rational Function
- Verify the Rational Fit Approximation
- Construct an Input Signal to Test the Band Pass Filter
- Compute the Transient Response to the Input Signal
- View Input Signal and Filter Response in the Time Domain
- View Input Signal and Filter Response in the Frequency Domain

## Design a Bandpass Filter by Image Parameters

The image parameter design method is a framework for analytically computing the values of the series and parallel components in passive filters. For more information on this method, see "Complete Wireless Design" by Cotter W. Sayre, McGraw-Hill 2008 p. 331.

**Figure 1:** A Butterworth bandpass filter built out of two half-sections.

The following MATLAB code generates component values for a bandpass filter with a lower 3-dB cutoff frequency of 2.4 GHz and an upper 3-dB cutoff frequency of 2.5 GHz.

Ro = 50; f1C = 2400e6; f2C = 2500e6; Ls = (Ro / (pi*(f2C - f1C)))/2; Cs = 2*(f2C - f1C)/(4*pi*Ro*f2C*f1C); Lp = 2*Ro*(f2C - f1C)/(4*pi*f2C*f1C); Cp = (1/(pi*Ro*(f2C - f1C)))/2;

## Programmatically Construct the Circuit

Before building the circuit using `inductor` and `capacitor` objects, we must number the nodes of the circuit shown in figure 1.

**Figure 2:** Node numbers added to the Butterworth bandpass filter.

Create a `circuit` object and populate it with `inductor` and `capacitor` objects using the `add` function.

```
ckt = circuit('butterworthBPF');
add(ckt,[3 2],inductor(Ls))
add(ckt,[4 3],capacitor(Cs))
add(ckt,[5 4],capacitor(Cs))
add(ckt,[6 5],inductor(Ls))
add(ckt,[4 1],capacitor(Cp))
add(ckt,[4 1],inductor(Lp))
add(ckt,[4 1],inductor(Lp))
add(ckt,[4 1],capacitor(Cp))
```

## Extract S-Parameters From the 2-Port Network

To extract S-parameters from the circuit object, first use the `setports` function to define the circuit as a 2-port network. Once the circuit has ports, use `sparameters` to extract the S-parameters at the frequencies of interest.

freq = linspace(2e9,3e9,101); setports(ckt,[2 1],[6 1]) S = sparameters(ckt,freq);

## Fit the Transfer Function of the Circuit to a Rational Function

Use the `s2tf` function to generate a transfer function from the S-parameter object. Then use `rationalfit` to fit the transfer function data to a rational function.

tfS = s2tf(S); fit = rationalfit(freq,tfS);

## Verify the Rational Fit Approximation

Use the `freqresp` function to verify that the rational fit approximation has reasonable behavior outside both sides of the fitted frequency range.

widerFreqs = linspace(2e8,5e9,1001); resp = freqresp(fit,widerFreqs); figure semilogy(freq,abs(tfS),widerFreqs,abs(resp),'--','LineWidth',2) xlabel('Frequency (Hz)') ylabel('Magnitude') legend('data','fit') title('The rational fit behaves well outside the fitted frequency range.')

## Construct an Input Signal to Test the Band Pass Filter

This bandpass filter should be able to recover a sinusoidal signal at 2.45 GHz that is made noisy by the inclusion of zero-mean random noise and a blocker at 2.35 GHz. The following MATLAB code constructs such a signal from 4096 samples.

fCenter = 2.45e9; fBlocker = 2.35e9; period = 1/fCenter; sampleTime = period/16; signalLen = 8192; t = (0:signalLen-1)'*sampleTime; % 256 periods input = sin(2*pi*fCenter*t); % Clean input signal rng('default') noise = randn(size(t)) + sin(2*pi*fBlocker*t); noisyInput = input + noise; % Noisy input signal

## Compute the Transient Response to the Input Signal

The `timeresp` function computes the analytic solution to the state-space equations defined by the rational fit and the input signal.

output = timeresp(fit,noisyInput,sampleTime);

## View Input Signal and Filter Response in the Time Domain

Plot the input signal, noisy input signal, and the band pass filter output in a figure window.

xmax = t(end)/8; figure subplot(3,1,1) plot(t,input) axis([0 xmax -1.5 1.5]) title('Input') subplot(3,1,2) plot(t,noisyInput) axis([0 xmax floor(min(noisyInput)) ceil(max(noisyInput))]) title('Noisy Input') ylabel('Amplitude (volts)') subplot(3,1,3) plot(t,output) axis([0 xmax -1.5 1.5]) title('Filter Output') xlabel('Time (sec)')

## View Input Signal and Filter Response in the Frequency Domain

Overlaying the noisy input and the filter response in the frequency domain explains why the filtering operation is successful. Both the blocker signal at 2.35 GHz and much of the noise is significantly attenuated.

NFFT = 2^nextpow2(signalLen); % Next power of 2 from length of y Y = fft(noisyInput,NFFT)/signalLen; samplingFreq = 1/sampleTime; f = samplingFreq/2*linspace(0,1,NFFT/2+1)'; O = fft(output,NFFT)/signalLen; figure subplot(2,1,1) plot(freq,abs(tfS),'b','LineWidth',2) axis([freq(1) freq(end) 0 1.1]) legend('filter transfer function') ylabel('Magnitude') subplot(2,1,2) plot(f,2*abs(Y(1:NFFT/2+1)),'g',f,2*abs(O(1:NFFT/2+1)),'r','LineWidth',2) axis([freq(1) freq(end) 0 1.1]) legend('input+noise','output') title('Filter characteristic and noisy input spectrum.') xlabel('Frequency (Hz)') ylabel('Magnitude (Volts)')

For an example of how to compute and display this bandpass filter response using RFCKT objects, go to: docid:rf_examples#example-rf_bpfDemo_rfckt.