LCVR Driver
A Liquid Crystal Variable Retarder (LCVR, also called a phase retarder or rotator) is a liquid crystal filled waveplate device placed in the light path of an optical system to allow its electronic modulation. It is filled with a solution of nematic liquid crystal (LC) molecules which rotate the plane of polarization of transmitted light. Two transparent conductive films1) allow an AC voltage to be applied across the optics cell. As the voltage is increased, the default orientation of the LC molecules is disrupted, changing the degree of rotation or phase retardation of transmitted polarized light.
LCVRs are compact, robust LC devices ideally suited as OEM components of analytical instrumentation. The voltage used to drive them is provided by a controller which applies a precisely generated 50% duty cycle square wave with zero DC component. While several LCVR manufacturers offer benchtop controllers, the LCVR Driver Wildcard is the only commercially available LC controller-driver suitable for embedding in a microcontroller-based OEM instrument.
LCVRs are used for various types of optical analysis including Polarimetry and Ellipsometry. The LCVR Driver Wildcard may be used to drive OEM LCVRs such as those available from Meadowlark or Thor Labs.
Nematic liquid crystal LCVRs are used for:
- electronic shutters,
- optical modulation,
- polarization rotation,
- variable phase retarders,
- as optical modulators or optical valves, and,
- liquid crystal variable waveplates.
LCVR controller specifications
The LCVR Driver Wildcard produces a 2 kHz symmetric square wave of programmable amplitude for driving Liquid Crystal Variable Retarders (LCVR) and other nematic liquid crystal (LC) devices. The output waveform has a precisely controlled 50% duty cycle with zero DC component. The output voltage is programmable, and the frequency may be changed if needed. This LCVR driver may be used with any of Mosaic's microcontroller boards.
The controller accepts an analog voltage input, Vin, in the range of 0=4.096V and converts it to a square wave magnitude of Vpeak = 2 Vin, or peak-to-peak magnitude of Vp-p = 4 Vin.
| Physical Specifications | |
|---|---|
| Weight | 18 gram |
| Footprint | Wildcard size, 2" × 2.5" × 0.365"2) |
| Power requirements | 5 VDC at 1 mA and 13.5 to 26 VDC at 12 mA from PDQ V+RAW, or, 5 VDC at 1 mA and 10.7 to 12.5 VDC at 12 mA from PDQ V+RAW (by installing R21) |
| Operating temperature | 0 to 50°C |
| Humidity | 5 - 95%RH noncondensing |
| Input characteristics | |
| Control Voltage | 0–2.048 or 0–4.096 FS from an Analog I/O Wildcard or other programmable voltage source |
| Input Impedance | 6 kΩ |
| Output characteristics | |
| Output form | Square wave with zero DC component |
| Output impedance | 5Ω |
| Frequency | Default 1953.125 Hz, resistor selectable to 3906.25 or 976.5625 Hz, or externally driven |
| Frequency stability | ±100ppm (-10°C to +70°C), ±5ppm/year |
| Duty cycle | 50% ± several ppm measured from zero crossings |
| Amplitude and resolution | Vp = 0 to ± 4.096 V FS in 0.001 V steps, or 0 to ± 8.192 V FS in 0.002 V steps |
| Maximum average output current | ±30 mA |
| Rise/fall times | Slew rate limited, 1 μsec max from zero crossing |
| Slew rate | Limited to 8V/μsec |
| DC offset | < 5 mV max (typically < 1mV) < 1mV when disabled |
| Overshoot | < 0.5V (1st peak), < 3 μsec ring-down (< 1.5 typ.) for Vp =1V, times measured from zero crossings |
| Maximum load capacitance | 100 nF |
| LCVR connector | PCB mounted, male contact, vertical SMB jack |
There are a few subtleties of these specifications:
- The DC offset is typically less than 1mV for voltages up to Vpeak=5V. For greater voltages, there is a slight asymmetry of the output waveform's rise and fall times accounting for an effective DC offset of at most ±Vpeak/1000. The output is DC coupled.
- The gain of the board is precisely 2, so that Vpeak = 2 Vin. The voltage accuracy of the output magnitude is determined wholly by the accuracy of the input voltage, which would normally be taken from a DAC output of an Analog I/O Wildcard.
Connecting and powering the LCVR controller
The board provides three connectors: a Wildcard bus connector for attaching to a Mosaic microcontroller, a right angle field header, and an SMB cable socket for connecting to an LCVR, as shown in the following diagram:
The right-angle field header provides the following signals:
| LCVR Driver Wildcard Field Header | |||
|---|---|---|---|
| Signal | Pins | Signal | |
| GND | – 1 | 2 – | +5V |
| Clock_Out | – 3 | 4 – | +12V Out |
| – 5 | 6 – | –12V Out | |
| – 7 | 8 – | Disable | |
| Clock_Input | – 9 | 10 – | |
| – 11 | 12 – | ||
| – 13 | 14 – | ||
| DACGND | – 15 | 16 – | DACGND |
| Input from DAC12 | – 17 | 18 – | |
| – 19 | 20 – | ||
| – 21 | 22 – | ||
| LCVR GND | – 23 | 24 – | LCVR Out |
| Note: Unlabeled pins are not connected. | |||
Because unlabeled pins are not connected it does no harm to provide external signals to the connector at those locations.
Mounting the Wildcard
The board plugs into a Wildcard bus at any location. It is not addressed, so it does not decrease the number of other Wildcards you may simultaneously use with your microcontroller. As the board is usually used in conjunction with an Analog I/O Wildcard, it is most easily wired if you mount the board on the Wildcard bus directly above the Analog I/O Wildcard.
Powering the Wildcard
The board uses onboard regulators to create the +12V and -12V supplies it needs, deriving them from the V+RAW voltage provided by the Wildcard bus, which in turn is provided by your system power supply or a Docking Panel. Depending on the value of V+RAW your system uses you must configure the board one of two ways:
- For
V+RAW= 13.5 to 30 VDC: For normal operation anyV+RAWin the range of 13.5 to 30 VDC will work well. In this case, the onboard regulators create +12V and -12V from the appliedV+RAWand you do not need to install a bypass resistor. - For
V+RAW= 10.7 to 12.5 VDC (nominally 12V): If you are providing a 12 volt supply to the system'sV+RAWyou can use that instead of the LCVR driver's onboard 12V supply. In that case you must solder a zero ohm resistor onto the Wildcard at location R20 for Rev 1 boards, or R21 for Rev 2 board. That resistor allows your supplied +12V to bypass the onboard 12V regulator. The supplied voltage must be 2.5V greater than the maximum square wave output magnitude, and less than 12.5V. For a maximum output magnitude of 8.2V your supplied 12V should be in the range of 10.7 to 12.5VDC. In no event should it exceed 13 volts or damage to the board may result.
Providing the input voltage
You should cable a DAC output voltage from an Analog I/O Wildcard to the LCVR Driver's field header, connecting the input voltage to pin 17 and the DAC ground from the Analog I/O Wildcard to either of pins 15 or 16. Do not connect any of the other Analog I/O Wildcard signals to the LCVR Driver field header, as the other signals are not compatible. It is most convenient to use the the Analog I/O Wildcard's DAC7 signal. In that case you can connect pins 16 and 17 (DACGND and DAC7) of the Analog I/O Wildcard field header to pins 16 and 17 (DACGND and Input) of the LCVR Driver field header.
Connecting an LCVR or other nematic LC device
You can plug an LCVR cable directly into a connector on the board. The board provides an SMB-socket to mate with an SMB-terminated cable.3) The cable end connector looks like this:
When connecting a right-angle SMB cable end be careful that it does not short against the Wildcard's field header. If there is a chance of shorting, take measures to prevent a short.
For convenience, you can also connect the LCVR to pins 23 (ground) and 24 (LCVR Out) of the field header.
Programming or setting the output voltage/magnitude
To use the LCVR driver you only need to output a voltage on a DAC channel of the Analog I/O Wildcard. You can use either of two DAC output ranges, corresponding to a constant passed to DAC's initialization routine, Init_Analog_IO:
- By passing the constant
INT_1V_DAC12toInit_Analog_IOthe DAC produces a full scale range of 0–2.048V and the LCVR Driver produces a square wave of magnitude 0-4.096 Vpeak with 1 millivolt resolution. To set the magnitude you pass an integer in the range 0–4095 to the routineTo_DAC12. The integer value programs the magnitude of the output waveform in millivolts. - By passing the constant
INT_2V_DAC12toInit_Analog_IOthe DAC produces a full scale range of 0–4.096V and the LCVR Driver produces a square wave of magnitude 0-8.192 Vpeak with 2 millivolt resolution. To set the magnitude you pass an integer in the range 0–4095 to the routineTo_DAC12. Twice the integer value gives you the peak magnitude of the output waveform in millivolts.
Accuracy of the output magnitude
The voltage accuracy of the output magnitude is determined by the accuracy of the Analog I/O Wildcard's DAC. Briefly, its zero offset error is typically a few millivolts, but may be as much as ±30 mV over temperature, and its full scale/reference/gain error is as much as ±1.5% for the 1.024V reference or ±2% for the 2.048V reference over temperature.
Changing the LCVR drive frequency
The frequency of the LCVR Driver's output is derived from a 16 MHz clock present on the Wildcard bus. In normal operation it is set to 1953.125 Hz. You can double or half that frequency by placing a zero ohm resistor at one of three locations, leaving the others blank:
| Zero ohm location | Frequency (Hz) |
|---|---|
| R1 | 1953.125 Hz |
| R2 | 976.5625 Hz |
| R3 | 3906.25 Hz |
| none | External clock input |
Alternately, you can remove the zero ohm shorts from R1, R2 and R3 and provide a clock signal to pin 9 of the field header. In that case, you must provide a 50% duty cycle logic level clock of the output frequency you want. You can easily generate a PWM signal of precisely 50% duty cycle at almost any frequency using the PDQ Board's PWM outputs. For help on doing that, see the PDQ Board User Guide's chapter on Pulse Width Modulated I/O.
Disabling the output
For testing or measuring the LCVR controller's DC offset you can disable the square wave output by inserting a jumper on the two pin header at location J1 on the board. The output may also be disabled by applying a logic level low to pin 8 of the field header. That is particularly useful if you need your application program to turn-off LCVR output using an available digital output pin.
You can also turn off the output by programming a zero voltage using the DAC output; however in that case, owing to the DAC's output offset errors of as much as 30 millivolt, you may not get a complete turn-off. On the other hand, applying a logic level low to pin 8 of the field header provides positive shutoff, setting the board's output to zero volts within the DC offset specification of less than 1 millivolt.