Measurement chain
Track signal power, connection limits, and cascaded noise through cables, pads, couplers, and amplifiers.
Matched ports at one operating frequency. Gains and losses are small-signal values. Power is one CW tone; output limits apply to that estimated signal power. Compression, reflections, and peak-envelope limits require separate checks.
Blank gain or loss means 0 dB (unity). Noise figure and temperatures are required; a blank limit means no limit. Order stages from the source to the receiver. Use a passive stage for cable loss, a pad, or a coupler branch. Name its output connection (for example, “DUT input”). Limits are optional, in dBm at that output.
| Connection | Gain / loss | Signal | Output limit | Headroom |
|---|
The last connection is the receiver port. Enter the analyzer's 0.1 dB compression level and its damage level, both from the datasheet, and the margin you want under compression. The page reports the headroom and the stock attenuator that would restore it.
Noise figure is referenced to 290 K. Passive-stage noise uses its physical temperature; amplifier noise figure is an input specification. Output noise uses the source temperature and equivalent noise bandwidth entered above.
| Stage output | Stage NF | Cumulative NF | Output noise |
|---|
Order-of-magnitude only
| Item | Thumb | Why it matters |
|---|---|---|
| Thermal noise at 290 K | −174 dBm/Hz | The floor every noise figure is added on top of |
| Noise in a bandwidth | +3 dB per doubling, +10 dB per decade | 1 MHz of bandwidth puts the floor at −114 dBm |
| Passive loss at ambient | noise figure equals the loss | A 3 dB pad is a 3 dB noise figure, exactly |
| A pad before an amplifier | adds its loss straight to the chain | Attenuate after the first gain stage, never before it |
| First stage | dominates once its gain passes ~10 dB | Later stages are divided by all the gain ahead of them |
| Receiver compression | stay 3 dB under the 0.1 dB point | Compression flattens the trace before it shows in the level; a pad at the last connection is the fix |
| Receiver damage | absolute, no margin applies | The datasheet figure is the level that breaks the receiver, not a place to operate near |
| Noise figure 3 dB | noise factor 2 | Below 3 dB the linear and decibel views diverge quickly |
Friis cascade: F = 1 + Σ (Fᵢ − 1)/Gbeforeᵢ, using linear power gain and noise factor. A passive loss L at temperature T has F = 1 + (L − 1)T/290 K. Noise output = kB[Tsource + 290 K(F − 1)]Gtotal.
Assumes matched stages, independent noise, and gain/noise figure constant over the chosen bandwidth. Coupler loss refers to the selected branch with other ports terminated. A filter's nominal bandwidth may differ from its equivalent noise bandwidth.
Calculations stay in your browser. Shared links and exports include entered values; use nonsensitive examples.