Limiter.v

This module limits the magnitude of an I/Q data channel while preserving its phase. It takes as input a signed I, Q, -I, -Q sequence signal and outputs a signal whose magnitude is at most a given value (specified at compile time) while preserving its phase. It uses a serial Cordic rotator, so there is lag of about 1/3 μs.

Ports | Parameters | Notes | Example | Plots

Ports

clk	input	The clock input.
reset	input	The reset input.
en	input	The enable input.
go	input	Initiates operation of the limiter. The limiter retriggers itself after it is started.
quad	input	Two-bit quadrant counter.
in	input	The input data stream. Its width is given by the `width` parameter.
out	output	The output data stream. Its width is given by the `width` parameter.
busy	output	Indicates that a rotation is in progress.
done	output
active	output	Indicates that the limiter is actively limiting the output.
diag1	output	A floating diagnostic output.
diag2	output	A floating diagnostic output.

Parameters

width	16	Bit width (default 16) of the data input and output.
steps	11	The number of clock cycles through which the Cordic rotation runs. It must be -1 modulo 4 at this point (7, 11, 15, ...), but it need not be greater than the number of data bits.
mag	11319	The magnitude at which the I/Q data are limited. It must be in the range of the data.
msbs	4	Because the Cordic converter becomes unstable for small input signal intensities, it is disabled when the input intensity is below a threshold given by 2^-msbs smaller than full scale.

Notes

The magnitude is provided to the module as a parameter. The cosine-product normalization is incorporated into the mag parameter, so do not correct for it.

Example instantiation:

localparam w = 16;
wire			[w-1:0]	in, out;
Limiter
    #(.width(w), .mag(10720), .steps(w-5), .msbs(4))
    lim1 (
        .clk(clk),
        .reset(reset),		.en(1'b1),
        .go(startpulse),	.quad(quad),
        .in(in),		.out(out),
        .busy(busy),		.done(done)
    );

Plots (click to shrink/enlarge)

Plots of simulations.

Simulation

I/Q plots of limiter input and output run on the controller board. There is sinusoidal modulation applied.
Simulation