Filter Analysis

The Filter analysis mode automatically measures filter characteristics including center frequency, bandwidth, insertion loss, and roll-off rate for bandpass, lowpass, and highpass filters.

Enable Filter Mode

Connect filter between CH0 (input) and CH1 (output)
Set sweep to cover the filter passband and transition regions
Go to DISPLAY > MEASURE
Tap FILTER
Analysis results appear on screen

Test Setup

CH0 (Stimulus) ----[FILTER DUT]---- CH1 (Response)

Use appropriate adapters to match the filter’s connector type.

Display Output

The filter analysis shows:

S21 FILTER
f: 145.000 MHz (-2.3 dB)
Bw (-3 dB): 10.500 MHz
Bw (-6 dB): 12.800 MHz
Q: 13.8
        Low-side    High-side
f(-3):  139.750 MHz 150.250 MHz
f(-6):  138.600 MHz 151.400 MHz
Roll:   42 dB/dec   38 dB/dec
        12.6 dB/oct 11.4 dB/oct

Parameters Explained

Parameter	Description
f	Center frequency and peak insertion loss
Bw (-3 dB)	Bandwidth at -3 dB points
Bw (-6 dB)	Bandwidth at -6 dB points
Q	Quality factor (center frequency / bandwidth)
Low-side	Lower transition band data
High-side	Upper transition band data
f(-3)	-3 dB cutoff frequencies
f(-6)	-6 dB cutoff frequencies
Roll	Roll-off rate in dB/decade and dB/octave

Shell Commands

# Enable filter analysis mode
measure filter

# Disable measurement mode
measure none

Automatic Marker Placement

When filter mode is active, markers are automatically placed:

Marker 0: Peak (maximum transmission)
Marker 1: Lower -3 dB point
Marker 2: Upper -3 dB point

Filter Type Detection

The analysis automatically detects the filter type:

Bandpass Filter

Peak transmission between -3 dB points
Both low-side and high-side roll-off shown

Lowpass Filter

Peak at low frequency
Only high-side roll-off relevant
Low-side shows as “High-pass” in display

Highpass Filter

Peak at high frequency
Only low-side roll-off relevant
High-side shows as “Low-pass” in display

Measuring Different Filter Types

Bandpass Filter

Set center frequency to filter center
Set span to cover passband plus transition bands
Enable FILTER mode
Read center frequency, bandwidth, and Q

Lowpass Filter

Set start frequency well below cutoff
Set stop frequency at least one decade above cutoff
Enable FILTER mode
Read cutoff frequency and roll-off rate

Highpass Filter

Set start frequency at least one decade below cutoff
Set stop frequency well above cutoff
Enable FILTER mode
Read cutoff frequency and roll-off rate

Roll-Off Interpretation

The roll-off rate indicates filter order:

Roll-off (dB/decade)	Approximate Order
20 dB/dec	1st order
40 dB/dec	2nd order
60 dB/dec	3rd order
80 dB/dec	4th order

Or in octaves (1 octave = log10(2) decades):

Roll-off (dB/octave)	Approximate Order
6 dB/oct	1st order
12 dB/oct	2nd order
18 dB/oct	3rd order
24 dB/oct	4th order

Practical Examples

Characterize a Crystal Filter

Connect crystal filter between ports
Set sweep centered on filter frequency
Set span to show full passband and skirts
Enable FILTER mode
Read bandwidth, insertion loss, and shape factor

Verify a Commercial Filter

Compare measured center frequency to specification
Check that bandwidth matches datasheet
Verify insertion loss is within spec
Confirm roll-off meets requirements

Compare Filter Samples

Measure first filter, note all parameters
Measure second filter
Compare center frequencies (should match)
Compare bandwidths and insertion losses

Tips for Accurate Measurements

Threshold Detection

The filter analysis only works when:

Peak transmission is greater than -50 dB
Clear -3 dB and -6 dB points are detectable

If no filter is detected, check connections and ensure the filter passband is within the sweep range.