birdie

A birdie is unwanted electromagnetic energy produced by a device's own internal circuitry, rather than coming from an external signal source. In measurement and test instruments, birdies appear as spurious signals that contaminate data and can be mistaken for genuine signals under test. They arise from oscillators, switching power supplies, clock circuits, and other active components within the equipment itself.

The term comes from radio and radar work, where operators would hear these phantom signals as audible artifacts in audio outputs or see them as erratic deflections on oscilloscope traces. The name likely reflects the unpredictable, chirping nature of such interference patterns. Unlike broadband noise, birdies are often narrow-band and repeatable, appearing at specific frequencies that correspond to the device's internal clock rates or harmonic multiples.

Birdies become most problematic in sensitive measurement applications: spectrum analysis, phase noise testing, and low-level signal detection. A sensitive receiver might pick up its own local oscillator leakage, or a logic analyzer might generate EMI from its switching edges that couples back into high-impedance analog inputs. In high-frequency work above 1 GHz, birdies from even small clock frequencies and their harmonics can corrupt results across wide frequency spans.

Mitigation and identification

Identifying a birdie requires comparing measurements with and without the device under test in the signal path, or observing whether a signal persists after disconnecting external inputs. Shielding, filtering, and careful grounding of internal oscillators reduce but rarely eliminate birdies entirely. Quality test equipment includes internal filtering and uses low-phase-noise oscillators to keep birdie levels below the noise floor of actual measurements.

Birdies differ from images (unwanted mixer products) and feedthrough (direct coupling of a local oscillator to an output). A birdie is purely self-contained: it has no external source to heterodyne or mix with. This makes it consistent and reproducible, which is both a curse (hard to eliminate) and a help (easy to recognize and subtract if the device is stable enough).