Baron Services DSSR-250C Plot Assisted Setups

RVP8 User’s Manual
October 2004

Plot–Assisted Setups

4. Plot-Assisted Setups

The IFD receiver module replaces virtually all of the IF components in a traditional analog
receiver. The alignment procedures for those analog components are usually very tedious, and
require continued maintenance even after they are first performed. Subtle drifts in component
specifications often go unnoticed until they become so severe that the radar’s data are
compromised.

The RVP8 makes a big improvement over this by providing an interactive graphical alignment
procedure for burst pulse detection, Tx/Rx phase locking, and calibration of the AFC feedback
loop. You may view the actual samples of the burst pulse and receiver waveform, examine their
frequency content, design an appropriate matched filter, and observe live operation of the AFC.
It is a simple matter to check the spectral purity of the transmitter on a regular basis, and to
discover the presence of any unwanted noise or harmonics. Moreover, the RVP8 is able to track
and modify the initial settings so that proper operation is maintained even with changes in
temperature and aging of the microwave components.

The Plot-Assisted Setups are accessed using the various “P” commands within the normal TTY
setup interface. These commands are described later in this chapter. The RVP8 supports
opcodes that allow the host computer to monitor the data being plotted. The dspx utility can
display these plots directly on the workstation screen, and thus, can carry out the graphical
checkup and alignment procedures remotely via a network.

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4.1 P+ — Plot Test Pattern

The RVP8 can produce a simple test pattern to verify that the display software is working
properly. From the TTY monitor enter the “P+” command. This will print the message
“Plotting Test Pattern...” on the TTY and then produce the plot shown in Figure 4–1.

This display is actually an overlay of six different strokes: 1) bottom line, 2) middle line, 3) top
line, 4) line sloping up, 5) line sloping down, and 6) the sine wave pattern. The later changes
phase with each plot so that, with a little imagination, it appears to be radiating from the left side
of the display.

Figure 4–1: The Test Pattern Display

When you are satisfied that the plot is being drawn correctly, type “Q” or hit ESC to return to
the TTY monitor.

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4.2 General Conventions Within the Plot Commands

The “Pb”, “Ps”, and “Pr” commands all have a similar structure to their TTY user interface.
Each command begins by printing a list of subcommands that are valid in that context. These
subcommands are single keystrokes that are executed immediately by the RVP8 as they are
typed. The “ENTER” key is not required. The available subcommands are different for each
plot command; but, as much as possible, each key has a similar meaning across all commands.

The working and measured parameters for each plot command are printed on the TTY as two
lines of information following the subcommand list. The first line contains settings that only
change when a subcommand is issued; but the second line is live and reflects the current status
of the burst input, the IF input, or the AFC output. The first line is printed just once, but the
second line is continually overprinted on top of itself. This makes it appear as a live status line
whose values always remain up to date. The ”Pb”, ”Ps”, and ”Pr” commands will report ”No
Trigger” on the TTY status line whenever the external trigger is expected but missing.

The TTY screen will scroll upward each time a new subcommand is executed, so that a history
of information lines and command activity can be seen on the screen. You may also use the
Carriage-Return key to scroll the display up at any time. If the initial list of subcommands
disappears off the top, you may type “?” to force a reprint. To exit the plot command entirely
and return to the TTY main menu type “Q” or ESC. These basic “help” and “exit” keystrokes
apply everywhere within the RVP8 setup menus. To save space and minimize clutter on the
TTY screen, they are not shown in the itemized list of subcommands.

Most commands have a lowercase and an uppercase version. If a lowercase command does
something, then its uppercase version does the same thing but even more so (or in reverse). For
example, if the “w” subcommand widens something by a little bit, then “W” would widen it a
lot. This simple convention reduces the number of different subcommand keys that are needed,
and makes the interface easier to memorize.

The graphical display and TTY status lines are continually updated with fresh data several times
per second. Occasionally it is useful to freeze a plot so that it can be studied in more detail, or
compared with earlier versions. To accomplish this, every plotting command supports a “Single
Step” mode that is accessed by typing the “.” (period) key. This key causes the display and TTY
status lines to freeze in their present state, and the message “Paused...” will be printed.
Subsequently, typing another “.” will single step to the next data update, but the plot and printout
will still remain frozen. Typing “Q” or ESC will exit the plot command entirely (as they
normally do). All other keys return the plot command to its normal live updating, but the key is
otherwise discarded (i.e., subcommand keys are not executed while exiting from single step
mode).

All of the plot commands support subcommands whose only purpose is to alter the appearance
of the display, e.g., zoom, stretch, etc. These subcommands make no changes to the actual
working RVP8 calibrations. However, the display settings are stored in nonvolatile RAM just
like all of the other setup parameters. This means that all previous display settings will be
restored whenever you restart each plot command. This is very convenient when alternating
among the various plots.

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The “Pb”, “Ps”, and “Pr” commands are intended to be used together for the combined purpose
of configuring the RVP8’s digital front end. You may, of course, run any of the commands at
any time; but the following procedure may be used as a guideline for first time setups. The full
procedure must be repeated for each individual pulsewidth that the radar supports.

1. Use Mb to set the system’s intermediate frequency (See Section 3.2.6).

2. Use Mt to choose the PRF and pulsewidth (See Section 3.2.4). Also, choose the

range resolution now, as it may constrain the design of the matched filter later.

3. Use Mt0, Mt1, etc., to set the relative timing of all RVP8 triggers that are used by

the the radar. Do not worry about the absolute values of the trigger start times.
Just setup their polarity and width, and their start times relative to each other (See
Section 3.2.5). Make an initial guess of FIR filter length as 1.5 times the
pulsewidth.

4. Use Pb to slew the start times of all triggers so that the burst pulse is properly

sampled (See Section 4.3). Refine the impulse response length if necessary so
that all samples easily fit within the display window.

5. Use Ps to design the matched FIR filter (See Section 4.4). Further refine the

impulse response length and passband width to achieve a filter that matches the
spectral width of the burst, and that has strong attenuation at DC. If the FIR
length is changed, return to Pb to verify that the burst is still being sampled
properly.

6. Continue using Ps and Mb to tune up the AFC feedback loop. The settings that

work for one pulsewidth should also work for all others.

7. Use Pr to verify that targets are being detected with good sensitivity (See Section

4.5).

Sometimes it is useful to run the Pb and Ps commands with samples from the IF-Input of the
IFD, rather than from the Burst-Input. Likewise, it is sometimes useful to view the Pr plots on
samples of Burst data. The top-level “~” command (See Section 3.3.3) allows you to do this
easily.

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4.3 Pb — Plot Burst Pulse Timing

For magnetron radars the RVP8 relies on samples of the transmit pulse to lock the phase of its
synthesized “I” and “Q” data, and to run the AFC feedback loop. The “Pb” command is used to
adjust the trigger timing and A/D sampling window so that the burst pulse is correctly measured.

4.3.1 Interpreting the Burst Timing Plot

The display plot will ultimately resemble Figure 4–2, which shows a successful capture of the
transmitter’s burst pulse. The horizontal axis of the display represents time, and the overall time
span from the left edge to the right edge is listed as “PlotSpan” on the TTY.

Figure 4–2: Successful Capture of the Transmit Burst

The upper portion of the plot shows the sampling window wherein the burst pulse is measured.
The duration of this window is determined by the impulse response length of the matched FIR
filter. This is because the same FIR coefficients that compute “I” and “Q” are also used to
compute the reference phase vectors for the burst pulses. The A/D samples of the RVP8/IFD’s
burst input are plotted (somewhat brighter) within the sample window.

The RVP8 computes the power-weighted center-of-mass (COM) of the burst pulse envelope.
This allows the processor to determine the location of the “middle” of the transmitted pulse
within the burst analysis window. The Pb plot displays small tick marks on the top and bottom
of the burst sample window to indicate the location of the COM. These markers are only
displayed when valid burst power is detected. A second “error bar” is drawn surrounding the
tick mark to indicate the uncertainty of the mark itself. This error interval is used by the burst
pulse tracking algorithm to decide when a timing change can be made with confidence.

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It is possible to independently choose a subinterval of burst pulse samples that are used by the
AFC frequency estimator. Thus, the AFC feedback loop is not constrained to use the same set of
samples that are chosen for the FIR filter window. The FIR window typically is longer than the
actual transmitted pulse, and thus, the samples contributing to the frequency estimate will
include the leading and trailing edges of the pulse. These edges tend to have severe chirps and
sidebands, compared to the more pure center portion of the pulse. The AFC frequency estimate
(which is power weighted) could be mislead by these edges and might not tune to the optimum
center frequency if they were included.

The lower portion of the plot shows the six triggers that are output by the RVP8. Trigger #0 is at
the top, and Trigger #5 is on the bottom. They are drawn in their correct polarity and timing
relative to each other, and relative to the burst sample window. Note that the sample window is
always drawn in the center of the overall time span. Thus, depending on the PlotSpan and
location of the six trigger’s edges, triggers that do not vary within the plotted time span will
appear simply as flat lines.

The RVP8 defines “Range Zero” to occur at the center of the burst sample window. This also
defines the zero reference point for the starting times of the six programmable triggers. For
example, a trigger whose starting time is zero will be plotted with its leading edge in the exact
horizontal center of the display. Knowing this convention makes the absolute value of the
trigger start times more meaningful.

4.3.2 Available Subcommands Within “Pb”

The list of subcommands is printed on the TTY:

Available Subcommands within ’Pb’:
I/i Impulse response length Up/Dn
A/a & S/s Aperture & Start of AFC window
L/l & R/r Shift triggers & RVP8/Tx waveform left/right,
T/t Plot time span Up/Dn
Z/z Amplitude zoom
B/b BP Tracking On/Off (temporary)
+ Hunt for missing burst
. Single Step

These subcommands change the matched filter’s impulse response length, shift the radar
triggers, and alter the format of the display.

I/i The “I” command increments or decrements the length of the matched

filter ’s impulse response. Each keystroke raises or lowers the FIR
length by one tap.

A/a & S/s These commands raise/lower the aperture/start of the subwindow of

burst pulse samples for AFC. If you never use these commands, then
the full FIR window will be used; however, shortening the AFC
interval will result in two sample windows being drawn on the plot.
The smaller AFC window should be positioned into the center portion
of the transmitted pulse, where the burst amplitude and frequency are
fairly stable.

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L/l & R/r These two commands shift the entire group of six RVP8 triggers left

or right (earlier or later in time). The lowercase commands shift in

0.025 msec steps, and the uppercase commands shift in 1.000 msec
steps (approximately). The reason for shifting all six triggers at once
is that the relative timing among the triggers remains preserved.
However, the absolute timing (relative to range zero) will vary, and
this will cause the burst pulse A/D samples to move within the sample
window.

T/t The “T” command increments or decrements the overall time span of

the plot. The available spans are 2, 5, 10, 20, 50, 100, 200, 500, 1000,
2000 and 5000 microseconds. The value is reported on the TTY as
“PlotSpan”.

Z/z The “Z” command zooms the amplitude of the burst pulse samples so

that they can be seen more easily. The value is reported on the TTY as
“Zoom”.

B/b These keys temporarily disable or re-enable the Burst Pulse Tracker.

The tracker must be disabled in order for the L/R keys to be used to
shift the nominal trigger timing. The “b” key disables tracking and
sets the trigger slew to zero; the “B” key re-enables tracking starting
from that zero value.

+ The “+” subcommand initiates a hunt for the burst pulse. Progress

messages are printed as successive AFC values are tried, and the
trigger slew and AFC level are set according to where the pulse was
found. If no burst pulse can be found, then the previous trigger slew
and AFC are not changed.

4.3.3 TTY Information Lines Within “Pb”

The TTY information lines will resemble:

Zoom:x2, PlotSpan:5 usec, FIR:1.36 usec (49 Taps)
Freq:27.817 MHz, Pwr:–53.9 dBm, DC:0.14%, Trig#1:–5.00, BPT:0.00

Zoom Indicates the magnification (in amplitude) of the plotted samples. A

zoom level of “x1” means that a full scale A/D waveform exactly fills
the height of the sample window. Generally, the signal strength of the
burst pulse will not be quite this high. Thus, use larger zoom levels to
see the weaker samples more clearly. You may zoom in powers of two
up to x128.

PlotSpan Indicates the overall time span in microseconds of the complete scope

display, from left edge to right edge.

FIR Indicates the length of the impulse response of the matched FIR filter,

and hence, the duration of the burst pulse sample window. The length
is reported both as a number of taps, and as a time duration in
microseconds.

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Freq Indicates the mean frequency of the burst, derived from a 4th order

correlation model. The control parameters for this model are set using
the “Mb” command (Section 3.2.6).

Pwr Indicates the mean power within the full window of burst samples.

DC offsets in the A/D converter do not affect the computation of the
power, i.e., the value shown truly represents the waveform’s
(Signal+Noise) energy.

DC Indicates the nominal DC offset of the burst pulse A/D converter. This

is of interest only as a check on the integrity of the front end analog
components. The value should be in the range 2.0%.

Trig#1 Indicates the starting time of the first (of six) RVP8 trigger outputs.

This number will vary as the “L” and “R” subcommands cause the
triggers to slew left and right. Note that if the radar transmitter is
directly fired by an external pretrigger, then the pretrigger delay (in
the form “PreDly:6.87”) will be printed instead.

BPT This shows the present value of timing slew (measured in

microseconds) being applied to track the burst. The slew will be zero
initially when the RVP8 is first powered up, meaning that the normal
trigger start times are all being used.

4.3.4 Recommended Adjustment Procedures

The burst pulse timing must be calibrated separately for each individual pulsewidth. Repeat the
following procedure for each pulsewidth that you plan to use. Each iteration is independent.

It is first necessary to setup the proper relative timing for all RVP8 triggers that are being used.
The six trigger output lines are completely interchangeable, and each may be assigned to any
function within the radar system. For example, Trigger #0 might be the transmitter pretrigger,
Triggers #2 and #3 might synchronize external displays, and Triggers #1, #4, and #5 might be
unused.

Choose an initial impulse response length of 1.5 times the transmit pulsewidth. This length will
be refined later when the matched filter is designed (See Section 4.4). Adjust the plot time span
to view a small region around the sample window, and set the initial amplitude zoom to x16.
This assures that the plotted waveform will still be noticeable even if the burst signal strength is
very weak.

Verify that the transmitter is radiating, and observe the burst pulse samples on the display. Use
the “L” and “R” commands to shift the timing of all six triggers relative to range zero. This has
the effect of moving the burst sampling window relative to the transmitted pulse. Depending on
whether the triggers are set properly, you may at first see nothing more than a flat line of
misplaced A/D samples. However, after a few moments of hunting, the burst pulse should
appear on the display screen. Fine tune the triggers so that the burst envelope is centered in the
window, and adjust the amplitude zoom for a comfortable size display.

The clean center portion of the burst pulse should then be isolated to a narrower subwindow of
the overall FIR interval. Use the”A” and “S” commands to change the aperture and start of the
narrowed region from which the AFC frequency estimator’s data will be derived.

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Check that the burst pulse signal strength is reasonably matched to the input span of the
RVP8/IFD’s A/D converter. The maximum analog signal level is +4dBm. Exceeding this level
produces distorted samples that would seriously degrade the algorithms for phase locking and
AFC. However, if the signal is too weak, then the upper bits of the A/D converter are wasted
and noise is unnecessarily introduced. We recommend a peak signal level between –6dBm and
+1dBm, i.e., a signal that might be viewed at x2 or x4 zoom. Take note of the burst energy level
reported on the TTY; it will help decide the minimum energy threshold for a valid burst pulse,
which is needed in Section 3.2.6.

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4.4 Ps — Plot Burst Spectra and AFC

Once the transmit burst pulse has been captured the next step is to analyze its frequency content
and to design a bandpass filter that is matched to the pulse. In a traditional analog receiver the
matched filters use discrete components that can not be adjusted, and the transmit spectrum can
not be viewed unless a spectrum analyzer is on hand. The RVP8 eliminates all of these
restrictions via its “Ps” command, which plots the burst spectrum, designs the bandpass filter,
plots its frequency response, and also helps with alignment of the AFC.

4.4.1 Interpreting the Burst Spectra Plots

An example of a plot from the Ps command is shown in Figure 4–3. The display screen is
divided into two independent areas. The major portion (the lower seven eighths) is devoted to
power spectrum plots of the burst pulse and/or the matched filter response. The top portion
(single line) serves as a visual indicator of the present AFC level.

Figure 4–3: Example of a Filter With Excellent DC Rejection

The horizontal axis of the spectrum plot represents frequency. The overall span from the left
edge to the right edge is 36MHz for the RVP8 CAT-5E IFD, and 18MHz when the legacy RVP7
FibreOptic IFD is used. The remainder of this chapter will refer only to the newer RVP8 unit.

The exact endpoints of the plot depend on which alias band the radar’s intermediate frequency
falls in. For example, a 30MHz IF would imply a horizontal axis range of DC to 36MHz,
whereas a 60MHz IF would make the range 36MHz to 72MHz. The frequency span is printed
on the TTY when the command is first entered. Since the left edge of the spectral plot always

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