What is the dynamic range of a VNA and how does it limit my measurement accuracy?
VNA Dynamic Range Engineering
Understanding VNA dynamic range is essential for making accurate measurements, particularly of high-isolation devices (filters, switches) and high-gain devices (amplifiers).
| Parameter | SOLT Cal | TRL Cal | eCal |
|---|---|---|---|
| Accuracy | Good | Excellent | Good-very good |
| Standards Needed | 4 (S,O,L,T) | 3 (T,R,L) | 1 (module) |
| Bandwidth | Broadband | Band-limited | Broadband |
| Setup Time | 5-10 min | 10-20 min | 1-2 min |
| Best For | Coaxial, general | On-wafer, waveguide | Production, speed |
Calibration Procedure
(1) System dynamic range (SDR): the maximum measurable ratio between incident and received signals. SDR = P_source + receiver_sensitivity. For a typical mid-range VNA: P_source = +10 dBm, receiver sensitivity at IFBW = 10 Hz = -120 dBm. SDR = 130 dB. (2) Receiver compression: the receiver has a maximum input level (typically 0 to +20 dBm depending on the instrument). If the DUT has gain (amplifier), the received signal may exceed the receiver compression point, causing measurement errors. For an amplifier with 30 dB gain: source power must be reduced to avoid compressing the receiver: P_source < P_receiver_max - gain = 0 - 30 = -30 dBm. This reduces the SDR by 40 dB (from +10 dBm to -30 dBm source), leaving SDR = 130 - 40 = 90 dB. (3) The noise floor contribution to S21 measurement: the trace noise (random fluctuation in the measured S21) is approximately: sigma_S21 ≈ 10^(noise_floor - P_received)/20. At the noise floor: sigma = 1 (100% uncertainty). At 20 dB above noise: sigma = 0.01 (-40 dB, or 0.1 dB peak-to-peak ripple). At 40 dB above noise: sigma ≈ 10^-4 (< 0.001 dB ripple). Rule of thumb: for ±0.1 dB measurement accuracy, the received signal should be at least 20 dB above the noise floor. For ±0.01 dB: at least 40 dB above.
Error Sources
(1) Reduce IF bandwidth: IFBW directly sets the noise floor. Each 10× reduction in IFBW: 10 dB improvement in noise floor (and DR). IFBW = 1 kHz (general purpose, moderate speed). IFBW = 100 Hz (high DR, slow sweep). IFBW = 10 Hz (maximum DR, very slow sweep). IFBW = 1 Hz (extreme DR, impractical for most applications due to very long sweep time). (2) Increase source power: higher source power increases the received signal. But: limited by DUT power handling (for amplifier measurements, must stay below P1dB). For passive devices: use maximum available source power. (3) Averaging: sweep averaging reduces trace noise by sqrt(N_averages). 16 averages: 6 dB improvement. 100 averages: 10 dB improvement. For uncorrelated noise: the effective noise floor drops by 10×log10(N_avg). (4) Segmented sweep: measure different frequency segments with different IFBW and source power settings. In the passband of a filter: use high IFBW (fast) since the signal is strong. In the stopband: use low IFBW (better DR) since the signal is weak. This optimizes both speed and DR. (5) Use an external amplifier: for very low noise floor requirements, add a low-noise amplifier between the DUT output and the VNA receiver port. This reduces the effective receiver noise floor by the LNA gain (minus its noise figure).
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Fixture Considerations
VNA dynamic range varies with frequency: (1) Below 10 GHz: DR is typically at its maximum (120-140 dB at IFBW = 10 Hz). Source power is high, receiver sensitivity is good. (2) 10-50 GHz: DR decreases by 10-20 dB (source power drops, receiver noise increases). (3) Above 50 GHz: DR may decrease further if the VNA uses frequency multipliers/converters in the test ports (external mixers or frequency extenders). Typical DR at 67 GHz: 90-110 dB at IFBW = 10 Hz. (4) Above 110 GHz (with frequency extenders): DR = 70-100 dB depending on the extender type and frequency.
Frequently Asked Questions
Can I measure 100 dB of filter rejection on my VNA?
Yes, if your VNA has > 110 dB dynamic range. At IFBW = 10 Hz on a modern VNA (Keysight PNA-X, R&S ZVA): DR = 120-140 dB at frequencies below 26.5 GHz. This is sufficient for 100 dB filter rejection with 20-40 dB margin. At IFBW = 1 kHz: DR = 100-120 dB (marginal for 100 dB rejection — the stopband measurement will be noisy). To verify: set the source power to maximum, IFBW to 10-100 Hz, and measure the filter. The stopband should show a stable trace (not a noisy "grass" pattern). If the trace is in the noise: reduce IFBW further or add averaging.
How does dynamic range affect amplifier measurements?
For amplifier testing: the VNA source power is limited by the amplifier P1dB (must operate in the linear region). This reduces the available signal at the receiver: Received_power = P_source + gain - DUT_output_loss. But: the receiver may also compress: if the amplifier has 30 dB gain and the source is +10 dBm: the receiver sees +40 dBm (above the compression point of most VNA receivers). Solution: reduce source power or use a VNA with a built-in step attenuator on the receive port. The effective DR for amplifier measurements is typically 20-40 dB less than the DR for passive device measurements, depending on the amplifier gain and the VNA receiver maximum input power.
What is the difference between dynamic range and accuracy?
Dynamic range defines the measurement RANGE (the weakest signal you can see). Accuracy defines the measurement PRECISION within that range. At 20 dB above the noise floor: the measurement is accurate to ±0.1 dB. At 5 dB above the noise floor: the measurement shows the correct general trend but with ±1-2 dB uncertainty. At the noise floor: the measurement shows only noise (no useful information). Both DR and accuracy are important: DR determines whether you can see the signal. Accuracy determines how precise the reading is. A VNA with 140 dB DR and calibrated with poor standards may have only ±0.5 dB accuracy. A VNA with 100 dB DR and excellent calibration may have ±0.02 dB accuracy (within its DR range).