Spurious Free Dynamic Range Calculator

Calculator Inputs

Input Mode

Enter carrier and largest spur using the selected unit system.

Fundamental Value

Largest Spur Value

Fundamental Frequency (MHz)

Largest Spur Frequency (MHz)

Impedance (Ohms)

Full Scale Reference (dBm)

Noise Floor (dBm)

2nd Harmonic Level (dBc)

3rd Harmonic Level (dBc)

Plot Floor (dBc)

Plot Margin (MHz)

Example Data Table

Scenario	Carrier (dBm)	Largest Spur (dBm)	Carrier Freq (MHz)	Spur Freq (MHz)	SFDR (dBc)
Wideband IF Chain	2.0	-82.0	100.0	100.8	84.0
RF Driver Stage	4.0	-70.0	245.0	246.2	74.0
Mixer Output	-1.0	-68.5	10.7	21.4	67.5
ADC Snapshot	0.5	-76.2	70.0	140.0	76.7
DDS Signal Path	1.2	-88.0	125.0	375.0	89.2

These sample rows help validate the form and provide realistic engineering reference points.

Formula Used

SFDR in dBc using power levels:
SFDR = P_carrier - P_{largest spur}

SFDR from watt values:
SFDR = 10 × log₁₀(P_carrier / P_spur)

SFDR from voltage values:
SFDR = 20 × log₁₀(V_carrier / V_spur)

Voltage from power and impedance:
V_rms = √(P × R)

Watts from voltage and impedance:
P = V_rms² / R

Estimated ENOB from SFDR:
ENOB ≈ (SFDR - 1.76) / 6.02

The calculator converts all entries into common units, then reports SFDR, ratios, noise margin, full scale referenced spur distance, and a relative spectrum plot.

How to Use This Calculator

Select the input mode that matches your lab data.
Enter the carrier level and the strongest spurious component.
Add carrier and spur frequencies in MHz for spectrum placement.
Set impedance when using voltage conversions or checking consistency.
Optionally include full scale, noise floor, and harmonic levels.
Press Calculate SFDR to show the result above the form.
Review the metrics table, chart, and derived equivalent bit estimate.
Use the CSV and PDF buttons for reporting or documentation.

Frequently Asked Questions

1. What does SFDR measure?

SFDR measures the level difference between the desired carrier and the strongest unwanted spur. Higher SFDR indicates a cleaner signal path and better spectral purity.

2. Why is SFDR shown in dBc?

dBc references the spur to the carrier, which makes comparisons easier across different absolute amplitudes. It focuses on signal purity instead of absolute output level alone.

3. When should I use watts mode?

Use watts mode when your instrument or simulation already reports absolute power. The calculator converts that power into dBm and Vrms to build the full result set.

4. When should I use Vrms mode?

Vrms mode is useful for bench measurements and circuit simulations where amplitudes are known as voltages. Impedance is required because voltage alone does not define power.

5. What is the difference between dBc and dBFS here?

dBc compares the spur to the carrier. dBFS compares the spur to the full scale reference. Both are helpful when checking converter headroom and spectral cleanliness.

6. Does noise floor change SFDR?

Noise floor does not define SFDR directly. SFDR only depends on the largest spur. The optional noise floor is included to show overall dynamic span and context.

7. Why estimate ENOB from SFDR?

ENOB derived from SFDR gives a practical indicator of usable converter linearity. It is not identical to SINAD based ENOB, but it is still informative.

8. What if the spur is stronger than the carrier?

The calculator still computes the numeric result, but a negative or very small SFDR usually indicates invalid setup, clipping, or a severely distorted measurement chain.