DAS Passband Mapping from IF to Output			RHF	14/9/98
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Default Configuration
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The following diagrams illustrate the way output passbands at the Band 
Splitter internal BU(aka. "X") and BL(aka. "Y") nodes map onto the IF input 
spectrum in the default configuration, ie.

	Band Splitter parameter	<4>		"I/P Spectrum"		= Natural(0)
					"		<11>	"BU O/P Spectrum"	= Natural(0)
					"		<12>	"BL O/P Spectrum"	= Natural(0)

An additional diagram illustrates the way output passbands at the Fine Tuner 
internal FU(X) and FL(Y) nodes map onto the selected output spectrum from the 
Band Splitter in the default configuration, ie.

	Fine Tuner parameter	<53>	"X Spectrum Flip"	= 0
							<54>	"Y Spectrum Flip"	= 0

Several points are of note:

1)  Group 1 filters <= 16MHz wide *duplicate* the data streams on the BU(X) 
and BL(Y) nodes.  B1*32 alone has *alternate* samples on BU(X) and BL(Y).

2)	Group 2, 4 and 6 filters map all the BU(X) responses on one side of input 
band centre and BL(Y) responses on the other.

3)  All output responses are shifted (aliased) to baseband.  Those marked "not 
inverted" appear with their lower frequency limit mapped to DC, while those 
marked "inverted" have their upper limit mapped to DC and therefore have their 
spectra inverted with respect to the input band.

4)  Spectral inversion may occur in two places.  When the IF is first sampled 
the spectrum of an odd numbered alias is inverted at baseband, which becomes 
the Band Splitter input.  Next when a band pass filter decimates the data rate 
along with signal bandwidth, any odd numbered aliases of the new sample rate 
are inverted at baseband.  The two processes are cumulative.  Thus it is 
possible to see directly that an 8MHz passband from 152MHz to 160MHz is the 
19th alias of 16MHz (the output sample rate) and will therefore appear 
inverted.  If it is desired to record it uninverted the channel's "flipper" 
must be set.


Input Convention
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In practice the IF input spectrum itself may already be inverted depending on 
the number of high-side conversions in the receiver.  The overall picture may 
be simplified by adopting the following convention:

	If for a particular receiver configuration the sky spectrum is inverted 
	when sampled to baseband, set <4> "I/P Spectrum" = Flipped(1).  If it is
	not inverted, do nothing.

Since <4> inverts the entire baseband spectrum, the net result is that the 
Band Splitter input is now 'normal' in all cases.  The mapping to BU and BL 
responses is as shown for baseband (0-64MHz) inputs and a consistent policy 
of inversions and output allocation etc. can be adopted.

Usually <30> "64M O/P" should also be set the same as <4>, so that 
recordings or correlations of unprocessed data also see a normal spectrum.

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