Wideband PFB

Purpose

The wpfb unit is a Wideband Poly Phase FilterBank which can process a datastream that is partly applied in serial and partly applied in parallel. It consists of a Poly Phase Filter (PPF) and a Fast Fourier Transform (FFT). The wpfb distinguishes from a single FFT by the PPF that functions as a window function in order to produce a flat response across the channel, but also provides excellent suppression of out-of-band signals.

Module Overview

An overview of the wpfb unit is shown in Figure 1. The wpfb is build out of two N-point wideband PPFs (one for the real input and one for the imaginary input) and a single N-point wideband FFT. All P outputs are connected to a subband statistic unit that calculates the power in each subband. The MM interface is used to read and write the filter coefficients and to read out the subband statistics (still to be done).

A control unit takes care of the correct composition of the control of the output streams regarding sop, eop, sync, bsn, err. The wpfb unit can handle a wideband factor \(>= 1\) or a narrowband factor \(>= 1\) (\(2^{nof\_chan}\)).

• For wideband factor = 1 the WPFB uses a pipelined FFT.

• For wideband factor > 1 the WPFB uses a wideband FFT.

• For wideband factor >= 1 the WPFB supports setting the number of channels >= 0, even though the concept of channels is typically not useful when wideband factor > 1.

• The WPFB does support the binary reordering of data after the FFT.

• The WPFB can perform a complex FFT or 2 Real to 2 Complex FFT.

• The WPFB does support input flow control using the extra control signals, with support for gaps in the time input.

Firmware Interface

Clock Domains

As with the wideband_fft, the WPFB has a mm_clk and a dp_clk though as with the wideband_fft, since no memory-mapping is supported as yet, the PFB only makes use of the dp_clk (200MHz).

Interface signals

Figure 2 shows the Simulink Wideband FFT block in its base configuration. In this form, it offers a minimal set of input/output ports that are comparable with what the CASPER FFT offers.

Figure 3 shows the Simulink Wideband FFT block in its expanded configuration which offers the bsn, sop, eop, error, empty and channel control signals. Their function is explained by the graph below.

The full set of signals available to the Simulink block are detailed in the table below.

Signal	Type	Size	Description
Reset	std_logic	1	Datapath reset.
Clock enable	std_logic	1	Clock enable signal (used by Xilinx black box).
Clock	std_logic	1	Datapath clock (used by Xilinx black box).
Sync	std_logic	1	In/out sync pulse, preceeds data by 1 clock cycle.
Valid	std_logic	1	In/out valid data signal. Goes high with first valid data sample.
Shiftreg	std_logic_vector	\(\log2(nof\_points)\)	Bit vector dictating at which stages to shift in an N-point FFT. A ‘1’ indicates a shift while a ‘0’ indicates no shift at that stage.
Ovflw	std_logic_vector	\(\log2(nof\_points)\)	Bit vector dictating at which stages overflow occured in an N-point FFT. A ‘1’ indicates an overflow while a ‘0’ indicates no overflow at that stage.
bsn	std_logic_vector	64	A timestamp identification port for the data.
sop	std_logic	1	A start-of-packet indicator (see figure 4 for detail).
eop	std_logic	1	An end-of-packet indicator (see figure 4 for detail).
Empty	std_logic_vector	16	Empty signal for the sosi data packet.
Error	std_logic_vector	32	Error indicator giving 32 different one-hot encoded errors.
Channels	std_logic_vector	32	An indicator for mapping of channels to streams.
Im	std_logic_vector	in_dat_w or out_dat_w	Data port for either one polarisation (when doing a dual pol FFT), or the imaginary part (when doing a complex FFT).
Re	std_logic_vector	in_dat_w or out_dat_w	Data port for either one polarisation (when doing a dual pol FFT), or the real part (when doing a complex FFT).

The wideband PFB extends the _WB FFT functionality to perform a complex or two real FFT. It further follows the same protocol for wb_factor control.

Parameters

Control over the PFB’s characteristics, data handling and implementation on the FPGA are tabulated below:

PFB parameters