To truly understand the differences between PEAK, QUASI-PEAK (QP), and AVERAGE (AV), we must start from the underlying principles of an EMI receiver.

1. The Essence of Detection

In EMC testing, a spectrum analyzer or EMI receiver collects RF energy from space or cables and down-converts it into an Intermediate Frequency (IF) signal. The detector’s role is to convert the fluctuating envelope of this IF signal into a recordable voltage reading.

• Charge Time Constant ($\tau_c$): The time required for the detector output voltage to reach 63% of its final value when a constant sinusoidal voltage is suddenly applied to the input. it determines how quickly the detector responds to impulsive signals.
• Discharge Time Constant ($\tau_d$): The time required for the detector output voltage to decay to 37% (i.e., $1/e$) of its initial value after the input signal is cut off. It determines the detector’s ability to maintain the pulse envelope.

2. Comparison of Three Detection Modes

2.1 PEAK (Peak Detection - PK)

• Logic: Ultra-fast charging (microsecond level), extremely slow discharging.
• Mechanism: As soon as an interference signal appears, regardless of its duration, the capacitor charges instantly and does not discharge for a long time. Therefore, it captures the absolute instantaneous maximum amplitude of the signal within a unit of time.
• Application: Pre-scanning and Military Standards. If the PK value of a product is lower than the specified QP or AV limits, it is directly judged as compliant.

2.2 QUASI-PEAK (Quasi-Peak Detection - QP)

• Logic: Fast charging (millisecond level), slow discharging (hundreds of milliseconds).
• Mechanism: This is the core metric for commercial EMC testing. The QP reading depends not only on the pulse amplitude but also on its repetition frequency.
  • For the same amplitude, denser pulses lead to continuous charging, making the QP value closer to the PK value. Sparser pulses allow more discharge time, resulting in a lower QP value.
• Design Intent: To simulate the subjective annoyance level of human ears to radio noise. High-frequency noise is irritating (High QP), while occasional noise is relatively tolerable (Low QP).

2.3 AVERAGE (Average Detection - AV)

• Logic: Symmetrical and relatively long charge/discharge time constants.
• Mechanism: It performs an integration operation on the signal, “smoothing out” the instantaneous sharp spikes to reflect only the average energy of the signal.
• Application: Detecting Continuous Wave (CW) interference. Specifically used to identify persistent signals such as crystal oscillators and PWM harmonics.

3. CISPR 16-1-1 Standard Quasi-Peak Parameters

The R-C parameters of the Quasi-Peak detector are strictly defined by the frequency range (Band) according to standards:

Band	Frequency Range	6dB Bandwidth (RBW)	Charge Constant ($\tau_c$)	Discharge Constant ($\tau_d$)	Meter Constant
Band A	9 kHz – 150 kHz	200 Hz	45 ms	500 ms	160 ms
Band B	150 kHz – 30 MHz	9 kHz	1 ms	160 ms	160 ms
Band C	30 MHz – 300 MHz	120 kHz	1 ms	550 ms	100 ms
Band D	300 MHz – 1 GHz	120 kHz	1 ms	550 ms	100 ms

Key Observations:

• Band B (Conducted Band) has the shortest discharge time (160ms), meaning this band offers a relatively larger “exemption” for pulse density.
• Band C/D (Radiated Band) has a significantly longer discharge time (550ms), providing stronger retention of high-frequency pulse envelopes.

4. Summary and Practical Insights

In any scenario, for the same interference signal, the values always follow: PK ≥ QP ≥ AV

• High PK but Low QP: Indicates the interference source consists of occasional transient pulses (e.g., relay switching, switching spikes).
• PK, QP, and AV nearly identical: Definitely narrowband continuous interference. Check crystal oscillators or high-speed buses.
• Test Strategy: First, spend a few minutes scanning the full frequency range with PK (fast) to identify dangerous points near the limit line. Then, perform precise dwell measurements with QP and AV (slow but accurate) at those specific points.