AOD protector main

AOD Protector

The protection circuit for the acousto-optic deflectors used for tweezers.

This KiCad project contains the following parts:

• Main board (AOD_protector-main): the main board detecting and thresholding a single-channel rf signal
• Fault handler (AOD_protector-fault_handler): a board to output a LVTTL flag to indicate a triggered incident
• Power board (AOD_protector-power_board): the power regulating and distributing board (used in company with a switching AC-DC block)
• (optional) Bandpass filter (AOD_protector-BPF_testboard): bandpass filters to restrict the signal within a designated spectral band

Additionally, we made a custom CNC design based on Cheval's HS19AL1UD2 rack mount box. The design files can be found under rack-mount_box.

Main board

The board is designed to implement an rf switch controlled by a power detector so the circuit is broken when the rf power is above a threshold. The input signal goes through a sampling circuit and enters the rf switch (QPC6014TR7) before going to the output.

The sampling circuit diverts $3.85~%$ of the total power into the detection chip (ADL5511ACPZ-R7) which has a $250~\Omega$ input impedance. For low frequency (~ $100\textrm{ MHz}$) signals, the $V_{rms}$ output of the chip is $V_{rms}=1.92\overline{V_{in}}+11\textrm{ mV}$. The $\overline{V_{in}}$ is the averaged voltage of the input low-pass filtered over a corner frequency determined by the capacitor connected to pin FLT4: $f_{LP}=1/(800 \pi\ \Omega~(C_{FLT4}+20\textrm{ pF}))$. In our case with $C_{FLT4}=470\textrm{ pF}$, $f_{LP}=812\textrm{ kHz}$.

The output of the chip is fed into a comparator (MAX9141ESA+) whose inverting input is set by a voltage reference and a variable voltage divider. With all the components taken into consideration, we have $V_{th}\ge0.384\overline{V_{in}}+10.5\textrm{ mV}$, and the trimpot should allow us to tune the threshold of the rf power from $-7\textrm{ dBm}$ to $2\textrm{ dBm}$ measured at the power divider. The remaining circuit and the rf switch will contribute an additional insertion loss of around $0.8\textrm{ dB}$ at low rf frequency. It is worth noting that the estimated total insertion loss is around $0.98\textrm{ dB}$.

When the voltage goes beyond the threshold, the comparator will engage a latch, disable the rf switch, send a flag to the fault-handler board, and turn on the on-board red LED. The status of the comparator can be reset by the pushbutton manually.

Change the threshold range

The threshold's dynamic range can be adjusted by changing the resistors R3, R6, and the potentiometer RV1. The three resistors together form a voltage divider that outputs the desired $V_{th}$ from the $1.024~\textrm{V}$ voltage reference.

Together with the discussion about the rf power, $V_{th}$, and insertion loss in the previous paragraphs, the rf power threshold measured at the output port in dBm can be derived as follows:

$$20\log_{10}\left(10.24\frac{R_6+RV_1}{R_3+R_6+RV_1}-0.105\right)\textrm{ dBm}+0.52\textrm{ dBm}$$

Fault handler

The fault handler board collectively receives the flag from the 4 main boards with an OR logic. When there is an over-threshold incident occurs on one of the main boards, the anode voltage of the optocoupler (TLP2767) will be pulled down and a $3.3\textrm{ V}$ voltage will be sent to the output BNC connector.

Power board

The power board filters the input 5V DC power line from a switching AC-DC adapter and distributes it to multiple boards. The filtering circuit is adapted from the Jane project (for the KiCad version of the design, contact Alessandro Restelli).

Bandpass filter (optional)

Ideally we want to implement a bandpass filter to suppress signals outside the operational bandwidth of the AOD for safety. With that in mind I designed a 5-order Butterworth bandpass filter for the two kinds of AOD we use respectively. Please refer to the Qucs file and board design for the details.