37W-19638-0.pdf

Fundamentals

of Digital Phosphor ™ Technology

in Real-Time Spectrum Analyzers

Fundamentals of Digital Phosphor™ Technology in Real-Time Spectrum Analyzers

Primer

Table of Contents

Digital Phosphor™ Technology in Real-Time Spectrum Analyzers:

a Revolutionary Tool for Signal Discovery----------------------------------------------------------------------------------------------------------4

Digital Phosphor Display ----------------------------------------------------------------------------------------------------------------------------4 - 9

Application: Finding low-level signals beneath

a stronger signal ----------------------------------------------------------------------------------------------------------------------------------4 - 6

The DPX Display Engine ----------------------------------------------------------------------------------------------------------------------------6 - 7

Persistence ------------------------------------------------------------------------------------------------------------------------------------------8

Statistical Line Traces ------------------------------------------------------------------------------------------------------------------------------9

Super-Fast Spectral Updates ------------------------------------------------------------------------------------------------------------------------9 - 12

The DPX Transform Engine--------------------------------------------------------------------------------------------------------------------------10

Application: Guaranteed detection of

short, infrequent signals --------------------------------------------------------------------------------------------------------------------------10 - 11

Probability of Intercept ------------------------------------------------------------------------------------------------------------------------------11 - 12

Discover DPX------------------------------------------------------------------------------------------------------------------------------------------12

Getting the Most out of the DPX Spectrum Display ----------------------------------------------------------------------------------------------13 - 15

Adjustments for the Bitmap Display --------------------------------------------------------------------------------------------------------------13 - 14

Persistence ------------------------------------------------------------------------------------------------------------------------------------------13

Intensity --------------------------------------------------------------------------------------------------------------------------------------------13

Color Palettes --------------------------------------------------------------------------------------------------------------------------------------13

Color Scale ------------------------------------------------------------------------------------------------------------------------------------------14

Interaction with other RTSA Functions------------------------------------------------------------------------------------------------------------14 - 15

RBW ------------------------------------------------------------------------------------------------------------------------------------------------14

Span ------------------------------------------------------------------------------------------------------------------------------------------------15

Markers----------------------------------------------------------------------------------------------------------------------------------------------15

Frequency Mask Trigger ----------------------------------------------------------------------------------------------------------------------------15

Analysis Time----------------------------------------------------------------------------------------------------------------------------------------15

Power Level Triggering

----------------------------------------------------------------------------------------------------------------------------15

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Fundamentals of Digital Phosphor™ Technology in Real-Time Spectrum Analyzers

Primer

Digital Phosphor™ Technology in

Real-Time Spectrum Analyzers: a

Revolutionary Tool for Signal Discovery

Detection is the first step in characterizing, diagnosing,

understanding and resolving any problem relating to time-

variant signals. As more channels crowd into available

bandwidth, new applications utilize wireless transmission,

and RF systems become digital-based, engineers need

better tools to help them find and interpret complex

behaviors and interactions.

Tektronix’ patented Digital Phosphor technology, standard in

our next-generation RSA6100A Series Real-Time Spectrum

Analyzers (RTSAs), reveals signal details that are completely

missed by conventional spectrum analyzers and vector

signal analyzers. The full-motion DPX™ Spectrum’s live RF

display shows signals never seen before, giving users

instant insight and greatly accelerating discovery

and diagnosis.

However, the combination of phosphor coatings and vector

drawing in CRTs provided several valuable benefits.

Persistence: Phosphor continues to glow even after the

electron beam has passed by. Generally, the fluorescence

fades quickly enough that viewers don’t perceive it lingering,

but even a small amount of persistence will allow the human

eye to detect events that would otherwise be too short

to see.

Proportionality: The slower the electron beam passes

through a point on the phosphor-coated screen, the brighter

the resulting light. Brightness of a spot also increases as the

beam hits it more frequently. Users intuitively know how to

interpret this z-axis information: a bright section of the trace

indicates a frequent event or slow beam motion, and a dim

trace results from infrequent events or fast-moving beams.

In the DPX display, both color and brightness provide z-axis

emphasis.

Persistence and proportionality do not come naturally to

instruments with LCDs (or even raster CRTs) and a digital

signal path. Tektronix developed Digital Phosphor

technology so the analog benefits of a vector CRT could

be achieved, and even improved upon, in our industry-

leading digital oscilloscopes and now in our real-time

spectrum analyzers. Digital enhancements such as intensity

grading, selectable color schemes and statistical traces

communicate more information in less time.

Application: Finding low-level signals beneath a

stronger signal

This primer describes the DPX™ Spectrum display and how

it addresses situations involving brief, intermittent, complex

and/or coincident signals. Also covered are the methods for

achieving its key performance specifications:

– Detection and measurements on a signal as short as

24 microseconds

– 48,828 spectral transforms per second, compressed

into a display that is even easier to read than a

conventional spectrum trace

The RSA6100A Series DPX Spectrum display shows

multiple signals sharing the same frequency at different

times, not just the largest, smallest or average levels. An

example to illustrate the advantages of the DPX display over

traditional spectrum displays is a common WLAN

communications interchange between a PC and a network

access point (AP). With the laptop located one meter away

from the analyzers used for this demonstration, and the AP

at approximately thirty meters, the AP’s signal is almost 30

dB below that of the laptop.

Digital Phosphor Display

The name “Digital Phosphor” derives from the phosphor

coating on the inside of cathode ray tubes (CRTs) used as

displays in televisions, computer monitors and older test

equipment. When the phosphor is excited by an electron

beam, it fluoresces, lighting up the path drawn by the

stream of electrons.

Raster-scan displays (first CRT then LCD) replaced vector

CRTs in many applications due to their smaller depth and

lower power requirements, among other advantages.

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Fundamentals of Digital Phosphor™ Technology in Real-Time Spectrum Analyzers

Primer

Each analyzer is equipped with an identical antenna and

adjusted to show a detailed spectral display of the

communications signals. The 802.11 WLAN signal is

Complementary Code Keying (CCK), Time Division

Duplexed (TDD) and transmitting with intermittent RF bursts

at 2.46 GHz.

Two traces are configured in the swept spectrum analyzer’s

display because a spectrum trace formed by a single line

of points across the screen cannot represent multiple

amplitude values per frequency point. One trace is Max

Hold, to show the stronger intermittent signal from the

laptop. +Peak detection is selected for the other trace in

an attempt to capture the weaker but more frequent

AP signal (Fig 1).

After many sweeps, the conventional analyzer display

shows a rough envelope of the nearby laptop signal.

However, the trace has several rectangular notches that

don’t represent the true WLAN signal. These dropouts

show up in periods of the sweep that don’t happen to

coincide with the laptop’s transmit times. (“Probability of

Intercept” will be addressed in more detail later in this

paper.) If the signal remains active long enough, the

notches will fill in and the trace will assume a shape more

closely approximating the real signal.

Figure 1. Max Hold and Normal traces on a swept spectrum

analyzer, both using +Peak detection. The Max Hold trace shows

the laptop’s stronger signal, but neither trace shows the lower-

power access point transmissions.

The peak-detected trace, containing data from only the

most-recent sweep, was unable to capture the lower-power

AP signal. The bursts are very brief, so the likelihood of

seeing one in any particular sweep is small.

The DPX display (Fig 2) shows a very different picture of the

communications interchange. Since it is a bitmap image

instead of a line trace, you can distinguish many different

signals occurring at the same time and/or different versions

of the same signal varying over time. The live RF

appearance lets you see the signals varying over time.

Figure 2. The RSA6100A Series' DPX Spectrum display shows the

laptop transmissions, access point signal and background noise, all

in its live-motion bitmap trace.

demonstration (“Temperature”), the hot red color indicates a

signal that is much more frequent than signals shown in

cooler colors. The laptop signal, in yellow, green and blue,

has higher amplitude but doesn’t occur nearly as often as

the AP transmissions because the laptop was downloading

a file when this picture was taken.

The heavy band running straight across the lower third of

the graph is the noise background when neither the laptop

nor the AP is transmitting. The red energy lump in the

middle of the graph is the ON shape of the AP signal.

Finally, the more delicate spectrum above the others is the

laptop transmissions. In the color scheme used for this

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Fundamentals of Digital Phosphor™ Technology in Real-Time Spectrum Analyzers

Primer

Figure 3. Example 3-D Bitmap Database after 1 (left) and 9 (right) updates. Note that each column contains the same total number of “hits”.

Though these signals have duty factors and repetition rates

too low for conventional analyzers, they are well within the

basic capabilities of DPX technology. Variable persistence,

adjustable intensity and other advanced resources allow

the RSA6100A Series to handle signals even more elusive

than these.

The DPX Display Engine

Picture the bitmap database as a dense grid created by

dividing a spectrum graph into rows representing trace

amplitude values and columns for points on the frequency

axis. Each cell in this grid contains the count of how many

times it was hit by an incoming spectrum. Tracking these

counts is how Digital Phosphor implements proportionality,

so you can visually distinguish rare transients from normal

signals and background noise.

The actual 3-D database in an RSA6100A Series Real-Time

Spectrum Analyzer contains 501 columns and 201 rows,

but we will use an 11X10 matrix to illustrate the concept.

The picture on the left in Figure 3 shows what the database

cells might contain after a single spectrum is mapped into

it. Blank cells contain the value zero, meaning that no

points from a spectrum have fallen into them yet.

Compressing 1465 spectral measurements into one

screen update every 33 milliseconds is an oversimplified

description of the role DPX technology performs in the

RSA6100A Series. 48,828 acquisitions are taken and

transformed into spectrums every second. As we shall see

in a later section of this brief, this high transform rate is the

key to detecting infrequent events, but it is far too fast for

the liquid-crystal display to keep up with, and it is well

beyond what human eyes can perceive. So the incoming

spectrums are written into a bitmap database at full speed

then transferred to the screen at a viewable 30-Hz rate.

The grid on the right shows values that our simplified

database might contain after an additional eight spectral

transforms have been performed and their results stored in

the cells. One of the nine spectrums happened to be

computed at a time during which the signal was absent, as

you can see by the string of “1” values at the noise floor.

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