What is the video leakage of an RF switch and when does it matter?
Switch Video Leakage
Video leakage is often overlooked in switch specifications but can be a critical issue in sensitive receiver systems, pulsed radar, and precision measurement applications.
| Parameter | Option A | Option B | Option C |
|---|---|---|---|
| Performance | High | Medium | Low |
| Cost | High | Low | Medium |
| Complexity | High | Low | Medium |
| Bandwidth | Narrow | Wide | Moderate |
| Typical Use | Lab/military | Consumer | Industrial |
Technical Considerations
(1) FET switch: the FET has parasitic capacitances: C_gs (gate-source), C_gd (gate-drain), and C_ds (drain-source). When the gate voltage switches from ON to OFF (or vice versa): the gate voltage change (delta_V_g) couples through C_gd to the drain: delta_V_drain = delta_V_g × C_gd / (C_gd + C_ds + C_load). For C_gd = 0.05 pF, C_ds = 0.1 pF, and C_load = 0.1 pF (50 ohm at 1 GHz): delta_V_drain = 3.3 × 0.05 / (0.05 + 0.1 + 0.1) = 0.66 V. This is a significant spike (-1.6 dBm in 50 ohms). For stacked FETs (N FETs in series): the video leakage is reduced by approximately 1/N (each FET contributes approximately 1/N of the total coupling). An 8-stacked FET switch: video leakage ≈ 0.66/8 = 82 mV (-19 dBm). (2) PIN diode switch: when the bias transitions from forward to reverse: the stored minority carriers in the i-region must be extracted. This requires a reverse current surge: I_reverse = Q_stored / t_reverse, where Q_stored depends on the forward bias current and the carrier lifetime. For I_forward = 10 mA and carrier lifetime = 1 us: Q_stored = 10e-3 × 1e-6 = 10 nC. If extracted in 10 ns: I_reverse = 10e-9 / 10e-9 = 1 A (momentary). This current surge flows through the RF path and creates a large voltage transient.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Performance Analysis
(1) Apply the switch control signal (square wave at the intended switching rate). (2) No RF signal applied (the switch input is terminated in 50 ohms). (3) Measure the output with a wideband oscilloscope (bandwidth > 10/t_rise of the control signal). (4) The video leakage appears as a transient spike at each control signal transition. Record: peak voltage (mV or dBm), duration (ns), and polarity (positive and/or negative). (5) Specification: video leakage is specified as peak voltage (mV) or peak power (dBm) into 50 ohms, for a specified control signal edge rate. Typical values: GaAs FET SPDT: 10-50 mV (-33 to -23 dBm). SOI CMOS SP8T: 5-20 mV (-40 to -30 dBm). PIN diode: 50-500 mV (-23 to -3 dBm) (much higher due to stored charge).
Frequently Asked Questions
Does video leakage matter in a CW system?
In a CW (continuous wave) system where the switch changes state infrequently (e.g., band selection at startup): video leakage is not a concern because the transient occurs only once and the system settles before normal operation begins. In systems where the switch changes state during operation: (1) If the switching rate is slow (< 1 Hz) and the downstream receiver has a slow AGC: the video leakage creates a brief transient that the AGC absorbs. Minimal impact. (2) If the switching rate is fast (> 1 kHz) and the receiver bandwidth includes the switching frequency: the video leakage appears as periodic spikes in the receiver output, creating spurious signals at multiples of the switching rate. This can interfere with receiver processing. (3) In frequency-hopping systems: the LO switch changes state at each hop (potentially millions of hops per second). Video leakage creates a noise-like interference at the hopping rate and its harmonics.
How does video leakage affect radar minimum range?
In a pulsed radar: the T/R switch transitions from TX (ON) to RX mode immediately after the transmit pulse. The video leakage from this transition creates a large transient at the receiver input. The receiver must recover (the transient must settle below the receiver sensitivity level) before it can detect a target. The settling time determines the minimum detectable range: R_min = c × t_settle / 2. For t_settle = 200 ns: R_min = 3e8 × 200e-9 / 2 = 30 m. For t_settle = 1 us: R_min = 150 m. To reduce R_min: (1) Use a switch with lower video leakage. (2) Use a limiter circuit before the receiver to clip the transient (protecting the receiver while allowing faster recovery). (3) Use a separate receive antenna (eliminating the T/R switch entirely). This is common in CW and FMCW radars.
Is video leakage specified on switch datasheets?
Not always. Many switch datasheets specify only insertion loss, isolation, switching speed, and P1dB. Video leakage is specified on datasheets for: radar-grade T/R switches (where minimum range is critical), precision test equipment switches (where transient spikes affect measurement accuracy), and PIN diode switches (where stored charge makes the transient particularly large). If video leakage is not specified: request it from the manufacturer, or measure it yourself on an evaluation board. For GaAs FET switches: typical video leakage is 10-50 mV. For SOI CMOS: 5-20 mV (lower due to smaller gate capacitances).