VEGAS: VErsatile GBT Astronomical Spectrometer

Analog IF Interface Control

Updated August 18, 2015: New IIc control scheme described.


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.


  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.


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

Topic attachments
I Attachment Action Size Date Who Comment
IFmodulePinouts.bmpbmp IFmodulePinouts.bmp manage 784 K 2015-09-16 - 10:36 GalenWatts IF Module Pinout
pcf8574a.pdfpdf pcf8574a.pdf manage 736 K 2011-11-30 - 13:00 GalenWatts PCF8574A I/O Expander data sheet.
Topic revision: r17 - 2015-09-16, GalenWatts
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