What is the proper procedure for measuring the noise figure of a mixer with a noise figure analyzer?
Mixer Noise Figure Measurement
Mixer noise figure measurement is one of the most commonly misunderstood RF measurements because of the DSB/SSB distinction and the interaction between conversion loss, noise temperature, and LO drive conditions.
| 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 |
Frequently Asked Questions
Why is the SSB noise figure always 3 dB worse than DSB?
The 3 dB difference comes from the noise bandwidth: DSB measurement: the noise source delivers noise at both the signal and image frequencies. Both contribute to the IF output. The effective noise power at the IF is doubled compared to a single-sideband measurement. Since the Y-factor sees twice the noise: the calculated NF is 3 dB lower (the mixer appears quieter because it is "collecting" noise from twice the bandwidth). SSB reality: in a receiver, the desired signal is at one frequency. The noise from the image sideband is pure interference (adds noise but not signal). The signal-to-noise ratio is 3 dB worse than the DSB measurement suggests. Hence: NF_SSB = NF_DSB + 3 dB.
How do I measure the NF of an active mixer?
Active mixers (Gilbert cell, FET-based) may have conversion gain (not loss), which simplifies the measurement: (1) The positive gain improves the cascade NF (reduces the contribution of downstream stages). (2) The measurement setup is similar: noise source → mixer RF → mixer IF → NF analyzer. Apply the correct LO drive and DC bias. (3) DSB/SSB correction still applies. (4) Active mixers have higher NF variation with LO drive and bias than passive mixers. Measure NF over the full operating range. (5) Active mixers may oscillate if the feedback path (LO to RF or IF to RF) has insufficient isolation. Check for oscillation by monitoring the IF output with a spectrum analyzer before applying the noise source.
Can I use a VNA to measure mixer noise figure?
Some modern VNAs (Keysight PNA-X) support mixer noise figure measurement using the cold-source method: (1) The VNA measures the mixer conversion loss (S21 magnitude and phase between RF and IF ports). (2) The VNA measures the output noise of the mixer with the RF input terminated in 50 ohms (cold source). (3) NF is calculated from the noise power and conversion loss. Advantages: no noise source needed. S-parameters and NF measured on the same setup. Mismatch correction is exact (the VNA knows the mixer port impedances). Limitation: the VNA must have a low enough noise floor to accurately measure the mixer output noise. For high-conversion-loss mixers (> 10 dB): the mixer output noise is very low, and the VNA internal noise may dominate. Add an IF preamplifier to improve sensitivity.