What trigger coupling modes are available for RF signals?

Trigger coupling filters the signal before the trigger comparator. DC coupling: trigger on the full signal including DC offset; most common for general use. AC coupling: blocks DC, triggers only on the AC component; useful when a DC offset shifts the trigger level. HF Reject: low-pass filters (~50 kHz) the trigger path, removing high-frequency noise that causes false triggers. LF Reject: high-pass filters (~50 kHz) the trigger path, removing low-frequency components and hum. For RF signals, AC coupling or LF reject is typically used to eliminate DC bias and power supply noise from the trigger path.

What is trigger holdoff and when is it needed for RF?

Trigger holdoff is a programmable dead time after each trigger event during which the trigger circuit ignores new crossings. It prevents re-triggering on intermediate edges of complex or burst waveforms. For RF applications: burst signals (radar pulses, TDMA frames) need holdoff slightly less than the burst repetition period to trigger consistently on the first edge of each burst. AM-modulated signals may cross the trigger level multiple times per cycle at the envelope peaks; holdoff equal to one RF cycle prevents mid-cycle triggers. Modern oscilloscopes offer both time-based holdoff (ns to seconds) and event-based holdoff (trigger after N events).

RF Test & Measurement

Basic Trigger (Edge Trigger)

Q: How does edge triggering work?

The oscilloscope continuously samples the input signal. A comparator monitors the signal against the trigger level. When the signal crosses the trigger level in the specified direction (rising or falling), the trigger circuit generates a timing reference point. The acquisition system captures data before (pre-trigger) and after (post-trigger) this point. The trigger level must be set within the signal's amplitude range, and hysteresis prevents false triggers from noise. Modern oscilloscopes use hardware trigger circuits with <1 ps trigger jitter for stable RF measurements at GHz frequencies.

/BAY-sik TRIG-er/

The simplest oscilloscope trigger mode that initiates signal capture when the input crosses a specified voltage threshold (trigger level) with a defined slope (rising or falling edge). The foundation of all oscilloscope triggering. Enables stable, repeatable display of periodic signals. Modern oscilloscopes achieve <1 ps trigger jitter for stable RF measurements at GHz frequencies. Advanced modes (pulse width, runt, serial decode) build upon this concept.

Also called: Edge trigger

Jitter: <1 ps (modern)

Parameters: Level, slope, coupling

Understanding Basic Triggering

A stable oscilloscope display requires that each acquisition starts at exactly the same point on the waveform. Without triggering, successive traces would start at random points, creating an unstable, unreadable display. The basic edge trigger solves this by defining a precise voltage level and slope direction as the acquisition reference point.

For RF signals, triggering presents unique challenges. High frequencies require fast trigger circuits with minimal jitter. Modulated signals may have varying amplitude, making fixed-level triggering unstable. Burst signals have gaps where no trigger events occur. Understanding trigger coupling, holdoff, and sensitivity is essential for reliable RF waveform capture.

Trigger Parameters

Edge Trigger Conditions:
Trigger when: V_signal crosses V_level
Slope: rising (dV/dt > 0) or falling (dV/dt < 0)
Hysteresis: ΔV = noise margin (±2–5% of range)

Trigger Sensitivity:
Min signal for trigger: ~0.5–1 div (typical)
Max trigger frequency: = oscilloscope BW
Jitter: < 1 ps RMS (high-end), 5–20 ps (mid-range)

Holdoff for RF Bursts:
T_holdoff ≅ T_{burst_period} − T_{burst_width}
Radar 1 kHz PRF: holdoff ≅ 1 ms

Trigger Mode Comparison

Mode	Trigger Condition	Use Case
Edge (basic)	Level + slope	Periodic signals
Pulse width	Edge + timing	Glitches, runt pulses
Pattern	Logic combination	Digital/mixed-signal
Serial decode	Protocol-aware	SPI, I2C, UART, PCIe
RF power	Envelope level	Burst/pulsed RF

Common Questions

Frequently Asked Questions

How does edge triggering work?

Comparator monitors signal vs. trigger level. Crossing in specified direction generates timing reference. Acquires pre- and post-trigger data. Hysteresis prevents false triggers from noise. Modern: <1 ps jitter for GHz RF.

Trigger coupling for RF?

DC: full signal including offset. AC: blocks DC, triggers AC only. HF Reject: LP filter removes high-freq noise. LF Reject: HP filter removes hum/power supply. RF signals: AC coupling or LF reject typical.

What is trigger holdoff?

Dead time after each trigger event, ignoring new crossings. Prevents re-triggering on intermediate edges. Burst/radar: holdoff ≅ burst period. AM: holdoff = 1 RF cycle. Time-based (ns to s) or event-based (after N events).

Related Terms

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Test Equipment

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