How do I measure the harmonic distortion of an amplifier using a spectrum analyzer?
Noise Figure Measurement
The Y-factor method relies on measuring the ratio of output noise power at two known input noise temperatures. The 'hot' temperature is provided by the noise source (approximately 10,000-30,000 K for a 15 dB ENR source) and the 'cold' temperature is the ambient temperature (approximately 290 K with the noise source off). The ratio Y = N_hot/N_cold is related to the noise figure by the formula F = ENR/(Y-1).
| 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
Can I use a spectrum analyzer for noise figure?
Yes, with the right software and noise source. Modern signal/spectrum analyzers include noise figure measurement applications. The analyzer operates as a calibrated noise power receiver. The accuracy is slightly less than a dedicated noise figure meter (±0.3-0.5 dB vs. ±0.15-0.3 dB) but is adequate for most applications and avoids the cost of a separate instrument.
What if my DUT has low gain?
For DUT gain < 20 dB: the measurement receiver's own noise contributes significantly to the measured output noise, degrading accuracy. Solution: add a low-noise pre-amplifier (known NF and gain) after the DUT to boost the noise level above the receiver's noise floor. Then correct the measured noise figure: NF_DUT = NF_measured - NF_preamp × (correction factor). Most noise figure meters automate this correction.
What about Y-factor versus cold source?
The cold source method uses an impedance-tunable noise source (tuner) to present different source impedances to the DUT, measuring NF at each impedance. This reveals the minimum NF and the optimal source impedance. The Y-factor method provides only the NF at 50 Ω. The cold source method is essential for LNA matching network design but requires more complex equipment (source tuner, VNA, noise source).