What is excess noise ratio and how is it used in noise figure measurements?
Understanding ENR in Noise Measurements
A noise source is a calibrated device that produces a known amount of broadband noise when energized. The excess noise ratio quantifies how much additional noise the source produces above the thermal background at 290 K. This calibrated value is the foundation of all Y-factor noise figure measurements.
| Parameter | Superheterodyne | Direct Conversion | Digital IF |
|---|---|---|---|
| Image Rejection | 60-90 dB (filter) | 30-50 dB (mismatch) | N/A (digital) |
| DC Offset | No issue | Major issue | No issue |
| LO Leakage | Low | High | Low |
| Integration | Difficult | Easy (single chip) | Moderate |
| Dynamic Range | 80-120 dB | 60-90 dB | 70-100 dB |
Frequently Asked Questions
Does ENR vary with frequency?
Yes. The noise power output of a noise diode is not perfectly flat across frequency. ENR typically decreases at higher frequencies. Always use the frequency-specific calibration values, not a single broadband ENR value.
How often should a noise source be recalibrated?
Annual recalibration is standard practice. More frequent calibration may be needed if the source is used heavily, dropped, or if connector wear is suspected. Compare measurements against a recently calibrated reference to verify.
What happens if I use the wrong ENR value?
The measured noise figure will be systematically biased. Using an ENR that is too high by 0.5 dB produces a measured NF that is approximately 0.5 dB too low. The ENR error transfers nearly 1:1 to the NF measurement error.