VEGAS: VErsatile GBT Astronomical Spectrometer
Analog IF Interface Control
Updated August 18, 2015: New IIc control scheme described.
Description
The VEGAS IF Interface has three selectable low pass filters. Selecting a filter is accomplished using eight control bits per pair of channels. The function of these control bits is shown in the table below.
Control Device
Control of the filter selection bits will be accomplished using an IIc port on the ROACH board. In the IF module will be an IIc 8-bit I/O expander. Bits 0-3 will control odd number channels, bits 4-7 will control even numbered channels.
A
PCF8574A I/O expander was chosen to go in the IF Interface. This particular expander uses an address in the x38 - x3F range and has no control registers requiring setting before use.
The
PCF8574A has all bits set to 1 when initially powered up. In operation some bits will be set to 0 for correct filter switching.
The values of bits 0 and 2 will be the opposite of those of bits 1 and 3, similarly for bits 4 and 6 versus bits 5 and 7.
Functions
|
Bit: |
0/4 |
1/5 |
2/6 |
3/7 |
Filter: |
950 MHz |
1 |
0 |
- |
- |
|
1150 MHz |
0 |
1 |
1 |
0 |
|
1450 MHz |
0 |
1 |
0 |
1 |
The ' - ' term signifies a 'don't care,' with the 0/4 bit set to 1 and the 1/5 bit set to 0 the 2/6 and 3/7 bits will not affect the filter setting.
Wiring
The physical location of pins associated with the control bits, the power supply and the ground are shown in IFmodulePinouts.bmp.
The wiring colors are denoted in the following table:
Pin/Bit |
Color |
Comment |
b0 |
98x |
|
b1 |
91x |
|
b2 |
97x |
|
b3 |
96x |
|
b4 |
95x |
|
b5 |
94x |
|
b6 |
93x |
|
b7 |
92x |
|
+5 |
3xx |
Pin at input jumpered to pin at output. |
GND |
0xx or 5xx |
Solder tab |