What do we want to know about the filter?
For a two-port RF filter, a VNA gives us both transmission and reflection information. In a typical characterization we focus on four practical questions:
1. Connect and calibrate the measurement
For a band-pass filter, use a two-port measurement. Connect the DUT between Port 1 and Port 2 and perform a full two-port calibration such as SOLT, ECal or TRL. The calibration reference plane should be placed at the ends of the test cables, as close to the DUT as practical.
For the example in the RIGOL application note, the 4.26 GHz filter is measured with a 1–9 GHz sweep, 1001 points, 10 kHz IF bandwidth and −5 dBm source power. S11, S21 and S22 are displayed together.
2. Configure the VNA
Set the sweep
Choose a frequency span that covers the filter passband with enough margin to see the surrounding rejection.
Use enough sweep points
1001 points are used in the example. Narrow or high-Q responses may require more points or a reduced span.
Choose IF bandwidth
A narrower IFBW reduces the noise floor and improves dynamic range, but increases sweep time.
Display S11, S21 and S22
Use S21 for transmission / insertion loss and S11/S22 to evaluate input and output matching.
Practical point: once calibration is complete, keep cable routing stable. Moving or bending a cable can change its phase and amplitude response and reduce measurement repeatability.
3. Let Bandwidth Search do the repetitive work
On the DNA Series, Bandwidth Search can identify the two threshold crossings on the active S21 trace and calculate bandwidth, center frequency, insertion loss and Q factor automatically. For a standard filter measurement, the threshold is typically set to −3 dB relative to the reference peak.
4. Example measurement result
The filter used in the RIGOL passive-component application note produces the following typical readout:
| Parameter | Measured value |
|---|---|
| Center frequency | 4.26930 GHz |
| Lower −3 dB cutoff | 3.93091 GHz |
| Upper −3 dB cutoff | 4.60768 GHz |
| 3 dB bandwidth | 676.770 MHz |
| Loaded Q | 6.308 |
| Insertion loss | −4.217 dB |
How to read the result
S21 shows the transmission characteristic. The passband level provides the insertion-loss information, while the two −3 dB crossing points define the passband bandwidth.
S11 and S22 show how much signal is reflected at the input and output. They are useful for checking whether the filter is properly matched over the intended operating band.
Loaded Q is calculated directly from the measured center frequency and bandwidth. It is a convenient way to compare the selectivity of resonant or frequency-selective devices.