Please select To the mobile version | Continue to access the desktop computer version

Electronic Engineer Discuss

12Next
Return to list New
View: 12595|Reply: 12

Scope FFT and waveform math functions take on RF measurements

[Copy link]

58

Threads

1253

Posts

1333

Credits

金牌会员

Rank: 6Rank: 6

Credits
1333
Post time 2018-10-22 12:36:20 | Show all posts |Read mode
In the process of debugging and validating both digital and RF designs, the oscilloscope Fast Fourier Transform (FFT) function and a variety of other
math functions can prove valuable to designers moving beyond the prototype
stage and into production. For example, with digital designs, the FFT function
in an oscilloscope can quickly highlight the frequency content of signals that are
making their way onto power supply rails and further pinpoint the source of
such noise signals with that knowledge. That’s important because such signals
can translate into noise in other parts of the design, cutting signal margins and
potentially preventing the design from moving beyond the prototype stage until
the problem is fixed.


Reply

Use magic Report

58

Threads

1253

Posts

1333

Credits

金牌会员

Rank: 6Rank: 6

Credits
1333
 Author| Post time 2018-10-23 08:02:34 | Show all posts
An FFT spectral view also is helpful when looking at more complex, wide spectral signals to verify if the proper modulation is happening. Time-gated FFTs further
evaluate spectral components of a signal. Math functions such as a frequency trend
can quickly verify whether a classic modulation scheme is happening properly, like
a linear frequency modulation across pulses in a stream. This article will explore a
number of these examples and look at practical considerations for the measurements.



FFT measurement with an input sine wave

An oscilloscope that has a 1-GHz analog bandwidth and up to a 5-GS/s sample rate will

be used for measurements. These are both important specifications that will tie into

what kinds of measurement applications are possible. The first example measurement

is the capture of a 600-MHz, 632-mV (p-p), 0-dBm, 1-mW sine-wave signal into 50 ?

(orange) and resultant FFT (white) as shown in Figure 1.

Figure 1. Time-domain capture at 1 ns/div and FFT display from a 600- MHz sine-wave input



This post contains more resources

You have to Login for download or view attachment(s). No Account? Register

x

58

Threads

1253

Posts

1333

Credits

金牌会员

Rank: 6Rank: 6

Credits
1333
 Author| Post time 2018-10-24 09:15:09 | Show all posts

It’s important to understand how the oscilloscope sampling characteristics play

into the quality of this FFT measurement. The oscilloscope analog bandwidth,

sample rate, memory depth, and related time capture period all can have a

profound effect on the measurement result. This effect is heavily influenced by

the characteristics of the signal under test and how those signal characteristics

are related to the oscilloscope capture performance.

For example, in this simple illustration of measuring a single-tone 600-MHz sine-

wave signal and wanting to see the basic spectral characteristics of that signal,

the oscilloscope has to have enough analog bandwidth to minimally attenuate the

amplitude of the signal. Since this oscilloscope has a maximum 1-GHz analog

bandwidth, there is plenty of oscilloscope bandwidth to measure the 600-MHz

tone.



58

Threads

1253

Posts

1333

Credits

金牌会员

Rank: 6Rank: 6

Credits
1333
 Author| Post time 2018-10-25 08:04:31 | Show all posts

To avoid aliasing in the digitizing process, sampling must occur at a rate at least

twice the frequency of any appreciable frequencies present in the signal under test.

In this example, a 1.2-GHz sampling rate would be required. Clearly, if the scope is

sampling at its maximum 5-GS/s rate, that is more than sufficient. However, it will

be shown later that for certain scope time-base settings the sample rate (and

bandwidth) will decrease.

So what kind of quality is there in the FFT measurement made on the 600-MHz sine

wave? Referring back to the oscilloscope FFT measurement in Figure 1, notice the

main single frequency spike with a related measurement marker showing around a

600-MHz frequency and 0-dBm power. That matches expectations, but the FFT

response looks very wide for a single frequency input signal.



58

Threads

1253

Posts

1333

Credits

金牌会员

Rank: 6Rank: 6

Credits
1333
 Author| Post time 2018-10-27 08:00:11 | Show all posts

So what kind of quality is there in the FFT measurement made on the 600-MHz

sine wave? Referring back to the oscilloscope FFT measurement in Figure 1, notice

the main single frequency spike with a related measurement marker showing around

a 600-MHz frequency and 0-dBm power. That matches expectations, but the FFT

response looks very wide for a single frequency input signal.

The spacing between frequency spectrum lines in the FFT, or the width of frequency

buckets that signal energy is apportioned to, is called the frequency resolution. It is

based strictly on the time length of the acquired data and a factor for the FFT windowing

type selected. A rectangular window is used here with a factor of 1, so the frequency

resolution is simply the inverse of the record time. In this example:

Frequency Resolution = 1/(1 ns/div x 10 div) = 100 MHz



58

Threads

1253

Posts

1333

Credits

金牌会员

Rank: 6Rank: 6

Credits
1333
 Author| Post time 2018-11-1 08:09:28 | Show all posts

So this FFT could distinguish frequency components in the signal spectrum as close as

100 MHz, but any components closer than 100 MHz apart would merge together and

be indistinguishable. That’s actually a really coarse measurement.

How an increased time on screen enhances the FFT response

To demonstrate the importance of the record time upon FFT results, if the time/division

is panned to 200 ns/div, with a new record time of 2 μs across the screen, the frequency

resolution changes drastically to:

Frequency Resolution = 1/(200 ns/div x 10 div) = 500 kHz

The significant change in the FFT result can be seen in Figure 2 with a much finer display

of the 600-MHz frequency-domain spike. A trade-off is happening here. More time samples

are being processed, the calculated FFT has more spectral lines, and better frequency

resolution results. But the measurement runs slower than before to process more data—

10,000 samples instead of the original 50.


Figure 2. Time-domain capture at 200 ns/div and resultant FFT calculation with a 600-MHz sine-wave input




This post contains more resources

You have to Login for download or view attachment(s). No Account? Register

x

58

Threads

1253

Posts

1333

Credits

金牌会员

Rank: 6Rank: 6

Credits
1333
 Author| Post time 2018-11-5 09:22:37 | Show all posts
Start-frequency, stop-frequency, center-frequency, and span controls

An important capability in the FFT calculation and resultant view is to be able to

zoom into an area of interest for analysis. The first example had a wide span from

0 Hz to 2.5 GHz, so it was difficult to see any detail around the 600-MHz carrier.

Suppose there was suspected noise around the 600-MHz carrier frequency and a

desire to inspect that. The FFT controls can set a center frequency at 600 MHz and

a desired span, such as 100 MHz, around the 600-MHz carrier. A start frequency

of 550 MHz and stop frequency of 650 MHz also could have been selected with

the same result. An FFT measurement with these parameters can be seen in

Figure 3.

Figure 3. FFT of 600-MHz sine-wave input when FFT controls set for a 600-MHz center frequency and

100-MHz span



This post contains more resources

You have to Login for download or view attachment(s). No Account? Register

x

58

Threads

1253

Posts

1333

Credits

金牌会员

Rank: 6Rank: 6

Credits
1333
 Author| Post time 2018-11-6 08:43:54 | Show all posts
Wideband FFT analysis

An increasing number of today’s signals have modulation present that can increase

the spectral width to hundreds of megahertz or even multiple gigahertz. If spectral

widths of signals are beyond around 500 MHz, then spectrum analyzers or vector

signal analyzers available today do not have enough analysis bandwidth to make

meaningful measurements. In such cases, an oscilloscope or digitizer is required

that has enough analysis bandwidth for the application.

The carrier frequency of a signal of interest also is important. The carrier frequency

of the signal under test plus half the spectral width of that signal must be less than

or equal to the oscilloscope bandwidth for the oscilloscope to be used on its own for

the measurement. A wideband signal frequency domain measurement will now be

considered.

The signal under test is a 600-MHz RF pulse train, with 4-μs-wide RF pulses repeating

every 20 μs. There is a linear frequency modulation of the signal that chirps the carrier

frequency from 300 MHz at the start of the RF pulse envelope to 900 MHz at the end of

the pulse envelope.

To make a basic FFT measurement of the RF pulse, the first step is to get a clean time-

domain capture of a pulse from the signal on screen. The scope is reset to a known condition

by pressing Default Setup. Then Auto Scale is pressed, and the time/division setting is adjusted

to bring one main RF pulse on screen. The basic default rising-edge trigger is further qualified

with trigger holdoff. This ensures that a trigger doesn’t happen mid-pulse since that would create

instability in the captured trace. The trigger holdoff is set to something slightly longer than the

width of the RF pulse. The RF pulse is 4 μs wide so a trigger holdoff of 5 μs works well.



58

Threads

1253

Posts

1333

Credits

金牌会员

Rank: 6Rank: 6

Credits
1333
 Author| Post time 2018-11-7 08:15:20 | Show all posts
Next the FFT button is pressed to calculate a spectral view of the RF pulse train from the time-domain digitized signal on screen. There are FFT controls for start
and stop frequency or center frequency and span. A wide span is first chosen with
a start frequency of 0 Hz and a stop frequency of 2.5 GHz. Since this is a pulse
signal, and an entire pulse can be placed on screen with only noise on the left
and right side of the scope screen, a rectangular window is chosen for the FFT
calculation. FFT averaging with a count of eight also helps optimize the
measurement result. The FFT response that results is shown in Figure 4.




Figure 4. FFT of 4 μs-wide, 20-μs repeating linear FM chirp

This post contains more resources

You have to Login for download or view attachment(s). No Account? Register

x

58

Threads

1253

Posts

1333

Credits

金牌会员

Rank: 6Rank: 6

Credits
1333
 Author| Post time 2018-11-8 08:24:24 | Show all posts

Markers are placed on the FFT response, and it can be seen that this RF pulse does

have a wide spectral width, from 300 MHz to 900 MHz, or 600 MHz wide. What’s not

yet proven is that the frequency of the carrier shifts from 300 MHz to 900 MHz, linearly,

from the left side of the pulse across to the right side of the pulse.

The gated FFT math function

One way to quickly see some carrier frequency values across the pulse is to use the gated

FFT function. This is achieved by turning on the normal time-domain trace time gating

function. This function generates a normal trace view at the top half of screen and a

magnified view at the bottom of the screen. The time/division control expands and shrinks

the time-gate window placed on the upper normal trace, and the time delay control moves

the window along the trace. Whatever portion of the waveform is present in this window

shows up in the lower trace, but magnified.

An interesting measurement results from creating a small time-width window at the very

beginning of the pulse. The FFT is calculated from the data contained within the gated time

window as shown in Figure 5.


Figure 5. Time-gated FFT function observing the carrier at the beginning of the RF pulse



This post contains more resources

You have to Login for download or view attachment(s). No Account? Register

x
12Next
Return to list New
You have to log in before you can reply Login | Register

Points Rules

Dark room|Mobile|Archiver|Electronic Engineer Discuss

2024-3-29 19:23 GMT+8 , Processed in 0.178382 second(s), 22 queries .

Powered by Discuz! X3.2

© 2001-2013 Comsenz Inc.

Quick Reply To Top Return to the list