Characterizing a Filter

This tutorial covers complete characterization of RF filters using the NanoVNA-H, including bandwidth, insertion loss, shape factor, and return loss.

What You Will Learn

• Measuring filter passband and stopband
• Finding -3 dB and -6 dB bandwidth
• Calculating shape factor
• Measuring insertion loss and return loss
• Characterizing different filter types

Filter Parameters Explained

Parameter	Definition	Typical Values
Center frequency (Fc)	Middle of the passband	Depends on filter design
-3 dB bandwidth	Width where response drops 3 dB	Varies widely
-6 dB bandwidth	Width where response drops 6 dB	Wider than -3 dB
Shape factor	Ratio of -60 dB to -6 dB bandwidth	1.2 (sharp) to 5+ (gradual)
Insertion loss	Loss at passband center	0.5 dB to 6+ dB
Return loss	Input match in passband	-10 dB to -25 dB
Stopband rejection	Attenuation outside passband	-40 dB to -80+ dB

Basic Filter Measurement

1. Determine frequency range

   Set a sweep range that covers the passband plus stopband on each side.

   For a 10.7 MHz IF filter:

   • START: 10M (10 MHz)
   • STOP: 11.5M (11.5 MHz)

   For a 450 MHz low-pass filter:

   • START: 100M (100 MHz)
   • STOP: 900M (900 MHz)

2. Set appropriate IF bandwidth

   • Wide filters (MHz bandwidth): 1000 Hz IF BW
   • Narrow filters (kHz bandwidth): 100 Hz or 30 Hz IF BW
   • Crystal filters: 30 Hz or 10 Hz IF BW

3. Configure traces

   • Trace 1: S21 LOGMAG (transmission/insertion loss)
   • Trace 2: S11 LOGMAG (return loss/input match)

4. Calibrate

   Perform full SOLT calibration for accurate measurements:

   • OPEN, SHORT, LOAD on Port 1
   • THRU between Port 1 and Port 2
   • See Full Calibration

5. Connect the filter

   • Filter input to Port 1 (CH0)
   • Filter output to Port 2 (CH1)

Measuring Bandwidth

1. Find passband center

   Place a marker at the S21 peak (maximum transmission).

   If the passband is flat, place the marker at the center of the flat region.

2. Read insertion loss

   At the passband center, the S21 value (in negative dB) is the insertion loss.

   Example: S21 = -1.2 dB means 1.2 dB insertion loss

3. Find -3 dB points

   The -3 dB bandwidth is where S21 drops 3 dB below the passband peak.

   Using markers:

   • Place marker 1 at passband peak, note the value (e.g., -1.2 dB)
   • Calculate target: -1.2 - 3 = -4.2 dB
   • Move marker 2 to the lower frequency where S21 = -4.2 dB
   • Move marker 3 to the upper frequency where S21 = -4.2 dB
   • -3 dB bandwidth = marker 3 frequency - marker 2 frequency

4. Find -6 dB points

   Repeat for -6 dB below passband peak.

Tip

Use the marker search functions to find specific values automatically. Navigate to MARKER > SEARCH for options.

Calculating Shape Factor

Shape factor indicates how “square” the filter response is.

Shape Factor = (-60 dB bandwidth) / (-6 dB bandwidth)

1. Measure -6 dB bandwidth (as above)

2. Measure -60 dB bandwidth

   Find the frequencies where S21 = (passband) - 60 dB

   Note

   The NanoVNA-H has approximately 70 dB dynamic range below 300 MHz. For -60 dB measurements, ensure your noise floor is low enough.

3. Calculate shape factor

   Example:

   • -6 dB bandwidth: 15 kHz
   • -60 dB bandwidth: 30 kHz
   • Shape factor = 30 / 15 = 2.0

Shape Factor Interpretation

Shape Factor	Filter Characteristic
1.1 - 1.5	Very sharp (crystal, cavity)
1.5 - 2.5	Good (quality LC, ceramic)
2.5 - 4.0	Moderate (basic LC filters)
4.0+	Gradual rolloff

Filter Types and Expected Results

Low-Pass Filter

Characteristics:

• Flat response from DC to cutoff frequency
• Sharp rolloff above cutoff
• Insertion loss typically 0.5 - 2 dB

Measurements:

• -3 dB cutoff frequency
• Passband ripple (variation in passband)
• Stopband rejection at specific frequencies
• Return loss in passband

Example: 100 MHz Low-Pass

• Passband: DC - 100 MHz
• S21 < -40 dB at 200 MHz (stopband)

High-Pass Filter

Characteristics:

• Sharp rolloff below cutoff
• Flat response above cutoff
• Insertion loss typically 0.5 - 2 dB

Measurements:

• -3 dB cutoff frequency
• Stopband rejection at low frequencies
• Return loss in passband

Example: 100 MHz High-Pass

• Passband: 100 MHz and up
• S21 < -40 dB at 50 MHz (stopband)

Band-Pass Filter

Characteristics:

• Defined passband with center frequency
• Rolloff on both sides
• Insertion loss depends on Q and bandwidth

Measurements:

• Center frequency
• -3 dB and -6 dB bandwidth
• Shape factor
• Insertion loss at center
• Return loss in passband

Example: 10.7 MHz IF Filter

• Center: 10.7 MHz
• -6 dB bandwidth: 15 kHz
• Insertion loss: 3 dB

Band-Stop (Notch) Filter

Characteristics:

• Deep null at notch frequency
• Passband on both sides of notch
• Notch depth depends on design

Measurements:

• Notch frequency
• Notch depth (attenuation at center)
• Notch bandwidth (-3 dB from passband)
• Passband insertion loss

Example: 10 MHz Notch

• Notch at 10 MHz
• Notch depth: -50 dB
• Passband loss: 0.5 dB

Using the Filter Measurement Mode

The NanoVNA firmware includes a filter measurement function.

1. Enable filter measurement

   Go to MARKER > MEASURE > FILTER (S21)

2. The display shows:

   • Center frequency
   • -3 dB bandwidth
   • Insertion loss
   • Cutoff frequencies

3. Verify with manual markers

   The automatic measurement is convenient but verify critical measurements manually.

Measuring Return Loss

Poor input match causes reflections that can affect system performance.

1. Enable S11 trace

   Set a trace to FORMAT S11 (REFL) > LOGMAG

2. Check return loss in passband

   Good filters show return loss better than -10 dB (preferably -15 dB or better) in the passband.

3. Note frequency of worst return loss

   The worst match often occurs at passband edges.

Return Loss vs. VSWR

Return Loss	VSWR	Mismatch Loss
-10 dB	2.0:1	0.46 dB
-15 dB	1.4:1	0.18 dB
-20 dB	1.2:1	0.04 dB

Advanced: Phase and Group Delay

For filters used in communications systems, phase linearity matters.

1. Enable phase trace

   Set a trace to FORMAT S21 (THRU) > PHASE

2. Check for linearity

   A linear phase slope indicates constant group delay. Non-linear phase can cause signal distortion.

3. View group delay directly

   Set a trace to FORMAT S21 (THRU) > DELAY

   Constant delay across the passband indicates good phase linearity.

Documenting Filter Performance

Create a measurement report:

Parameter	Value
Filter type	_________
Center frequency	_________ MHz
-3 dB bandwidth	_________ kHz
-6 dB bandwidth	_________ kHz
Shape factor	_________
Insertion loss	_________ dB
Return loss (worst)	_________ dB
Stopband rejection	_________ dB at _________ MHz

Troubleshooting

Filter shows more loss than expected

• Check connector quality
• Verify calibration is current
• Ensure filter is properly terminated (50 ohms)
• The filter may be damaged or outside specifications

Passband ripple is excessive

• The filter may be improperly terminated
• Check that input and output impedance matches 50 ohms
• Some filter types (Chebyshev) have inherent ripple

Stopband shows insufficient rejection

• Ensure calibration dynamic range is adequate
• Check for coupling around the filter (shielding)
• The noise floor limits measurable rejection

Next Steps

• Your First S21 Measurement - Basic transmission measurements
• Measuring Crystals - Characterize crystal elements
• Full Calibration - Maximize measurement accuracy