HSP50216_15 Datasheet, PDF (11/59 Pages) Intersil Corporation – Four-Channel Programmable Digital Downconverter

HSP50216
Backend Data Routing
PATH 0
AGC
LOOP
FILTER
MUX
MAG: I
dphi/dt: Q
I1
Q1
GAIN
FROM
CIC
(4:0)
M
FILTER
U
COMPUTE
X
ENGINE
PATH 1
FIFO/
TIMER
PATH 2
AGC
MULT
CART
TO
POLAR
DESTINATION BIT MAP
(BITS 28:18 OF FIR INSTRUCTIONS BIT FIELD)
EXT AGC
GAIN
SHIFT
d/dt
x1, x2
x4, x8
MAG
M
U
X
PHASE
I2
Q2
28 27 26 25 24 23 22 21 20 19 18
28
27
26, 25
24
23
22:18
AGC LOOP GAIN SELECT (PATH 01 ONLY)
UPDATE AGC LOOP (PATH 01 ONLY)
PATH 00 - - IMMEDIATE FILTER PROCESSOR FEEDBACK PATH
01 - - FIFO/AGC PATH
10 - - DIRECT OUT/CASCADE PATH
11 - - BOTH 00 AND 10 PATHS (FOR TEST)
STROBE OUTPUT SECTION (START SERIAL OUTPUT WITH THIS SAMPLE)
FEED MAG/PHASE BACK TO FILTER PROCESSOR
FILTER PROCESSOR SEQUENCE STEP NUMBER
A CIC filter has a gain of R^N, where R is the decimation factor
and N is the number of stages. Because the CIC filter gain
can become very large with decimation, an attenuator is
provided ahead of the CIC to prevent overflow. The 24 bits of
sample data are placed on the low 24 bits of a 69 bit bus
(width of the first CIC integrator) for a gain of 2^-45. A 32 bit
barrel shifter then provides a gain of 2⁰ to 2³¹ inclusive
before passing the data onto the CIC. The overall gain in the
pre-CIC attenuator can therefore be programmed to be any
one of 32 values from 2^-45 to 2^-14, inclusive (see IWA=*004,
bits 18:14). This shift factor is adjusted to keep the total
barrel shifter and CIC filter between 0.5 and 1.0. The
equation which should be used to compute the necessary
shift factor is:
Shift Factor = 45 - Ceiling(log₂(R^N)).
NOTE: With a CIC order of zero, the CIC shifter does not have
sufficient range to route more than 10 bits to the back end since the
maximum gain is 2^-14 (the least significant 14 bits are lost).
Back End Section
One back-end processing section is provided per channel.
Each back end section consists of a filter compute engine, a
FIFO/timer for evenly spacing samples (important when
implementing interpolation filters and resamplers), an AGC
and a cartesian-to-polar coordinate conversion block. A
block diagram showing the major functional blocks and data
routing is shown above. The data input to the back end
section is through the filter compute engine. There are two
other inputs to the filter compute engine, they are a data
recirculation path for cascading filters and a magnitude and
dφ/dt feedback path for AM and FM filtering. There are seven
outputs from each back end processing section. These are I
and Q directly out of the filter compute engine (I2, Q2), I and
Q passed through the FIFO and AGC multipliers (I1, Q1),
magnitude (MAG), phase (or dφ/dt), and the AGC gain
control value (GAIN). The I2/Q2 outputs are used when
cascading back end stages. The routing of signals within the
back end processing section is controlled by the filter
compute engine. The routing information is embedded in the
instruction bit fields used to define the digital filter being
implemented in the filter compute engine.
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FN4557.8
October 5, 2015