The BB60C Screen Is Full of Signals "That Shouldn't Exist" — How Do I Tell Real Signals From Instrument Artifacts?

The BB60C Screen Is Full of Signals "That Shouldn't Exist" — How Do I Tell Real Signals From Instrument Artifacts?

Symptom

Sweeps show regular comb teeth, ghosts that move as the span changes, fixed-frequency spikes, or signals appearing far outside the band under test (you tune to 2.4 GHz and a peak shows up near 1.2 GHz). The instinct is to declare the instrument broken. In practice the majority of these readings are legitimate measurement physics or genuine external signals.

Root Cause

Only a handful of mechanisms generate "false signals" on a spectrum analyzer: internal spurs (LO harmonics, mixing products, IF leakage — documented in the datasheet's internal-spur table), external intermodulation (two strong real signals mixing inside your own front end or in a rusty/damaged connector), overload artifacts (compression making honest signals lie about their level and spawn images), antenna/cable-received real interference, and display artifacts (aggressive averaging or video filters smearing honest noise into peak-like shapes).

The Discrimination Procedure

1. Change the span and watch the peak.

• Stays pinned at the same frequency → real signal, or an internal spur at a fixed frequency (check it against the datasheet spur table).

• Moves proportionally with span → an internal mixing product; it is an instrument artifact and you can route around it by re-centering the span.

• Appears only at certain center frequencies, disappearing when you retune → LO-related artifact.

2. Terminate the input. Remove the antenna/source and attach a 50Ω dummy load. Peaks that vanish were external; peaks that remain with nothing connected are internal spurs or noise-floor features — compare with the published spurious-response spec.

3. Attenuate and re-read. Add 10–20 dB of external attenuation. Real signals and real intermodulation drop by exactly the pad value. Compression-generated images often change shape or grow — a telltale sign your front end was overloaded.

4. Hunt for intermodulation sources. If the artifact only appears with the antenna connected and two strong broadcasts nearby, it is third-order IMD created inside the instrument (or in a corroded connector). A preselector filter, attenuator, or better connector cleaning removes it.

5. Verify display settings. Check the detector, averaging count, and RBW. Noise averaged over a very narrow RBW can look like a stable carrier; switch detectors and widen RBW to see whether the "signal" collapses into noise floor behavior.

6. Cross-check with a second method. The waterfall/persistence view distinguishes one-time events (a microwave oven, a door opener, intermittent transmitter) from continuous artifacts. Real emitters repeat on a schedule; internal spurs never blink.

When It Truly Is a Fault

A genuine internal-spur failure signature: a brand-new fixed-frequency spike that did not exist previously and that the datasheet spur table does not predict, usually accompanied by recent overload or ESD events, or an entire comb of new products. Such patterns mean front-end damage — see the overload and ESD articles, record the evidence, and send the unit out.

Documentation Tip

Before sending a unit in "spur" complaints, photograph the trace with full settings visible (center frequency, span, RBW, attenuation, detector). Service engineers triage these complaints in minutes when settings travel with the screenshot — and bounce them back when they don't.

*(Based on publicly available Signal Hound BB60C user manual and datasheet specifications.)

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