Astronomy 512 -- Spring 2006 -- Assignment 6 -- Due Tuesday May 2
- The output amplifier for the Triplespec HAWAII-2 array is
illustrated below (in the dashed box in the lower right of the diagram;
blown up with jumper options in the solid box in the upper right).
This amplifier provides a buffer between the (wimpy) on-chip array circuitry
and the outside world. Its voltage gain is close to 1.0 which may
seem useless, but its strength lies in the ability to drive that
signal through wires and into external circuitry with little or no
influence on the input signal. The input transistor is a J270 JFET.
These devices draw little or no current on their input gate (so they don't
load their voltage sources).
- Individually --
- Construct this circuit on the breadboard.
- Measure its gain and voltage offset by feeding a signal in and comparing the output signal on the oscilloscope.
- The circuit will transfer the signal from pixel
voltages on the HAWAII-2 array to the outside world. Sixteen of theses
circuits, each dedicated to 1/16th of two quadrants of the array, will
be used in practice. The pixels readout timescale is about 1
- Verify that the circuit will not exhibit
significant roll-off of frequency response up to and beyond this 1MHz
requirement. That is, see that the gain remains 1.0 for input signals
up to 10MHz.
- Explore the dependence of its gain and voltage offset on
- The temperature difference between the two FETs.
- different FETs in the circuit.
- In groups --
- The characteristics of the FET is strongly temperature dependent - particularly when changing from room temperature to their operating temperature of 77K.
- Devise a simple scheme to measure the drain current in a FET for variable gate-drain voltages.
- Locate the FET at the end of wires long enough to dip it in LN2 along with a simple diode temperature sensor.
- Construct a simple Labview VI that will monitor the drain current as the FET warms up for several different values of the gate-drain voltage.
- Use this VI to identify pairs of FET's with matching characteristics at LN2 temperature.
- Use a laser to determine the groove spacing and blaze angle of the
two diffractiong gratings that have been set out in the lab. Use the same technique to see if you can determine the physical density of the information stored on a CD vs. a DVD.