Introduction:   Low Noise Measurement System

A system for measuring noise spectral density consists of three important parts: SR785 Dynamic Signal Analyzer, EG&G 5184 Ultra Low Noise Voltage Preamplifier, and SR715 LCR meter. Function and characteristics of each individual part will be described next. A block diagram of the complete noise spectrum measurement system is shown in chapter 4.

SR785 Dynamic Signal Analyzer

SR785 Dynamic signal analyzer is a dual channel and makes use of Fast Fourier Transform (FFT) to obtain the spectral density of an input voltage signal. The resolution of the measurement is based on the display span and the number of spaces between the data points on the display. When the input is not periodic the FFT is computed on the basis of the highly distorted waveform at the ends of the time record. The signal analyzer solves this problem by using window functions. Other features of this signal analyzer is various types of averaging which is necessary in a given frequency range to reduce the variance of the final plot. Further more, the trace storage, retrieval, capture buffer and math function are used to store and subtract background noise and transformation from the final measurement.

EG&G Ultra Low-Noise Voltage Preamplifier

The excess noise generated by a resistance sample used in the experiment is in the rV to hV range. The sensitivity of a SR785 signal analyzer is limited to detect input noise of about 10 hV/ÖHz at 200 Hz. For this reason a very low noise amplifier is needed to amplify the noise signal to amplitude large enough to drive an input of the signal analyzer. The gain provided by the preamplifier is 60 dB (fixed).

SR715 LCR meter

The resistance of a sample changes when an electric current is flowing through it. In our measurement we have to keep track of the resistance when the current is flowing through it. LCR meter is capable of measuring the resistance, inductance and capacitance when current is flowing through the device.

In the coming sections we will briefly discuss about these instruments and instructions to use them.

Chapter 1

SR 785—100 kHz Two channel Dynamic Signal Analyzer

Getting started

The examples measurements in this chapter are designed to acquaint the first time user with the SR785 Dynamic Signal Analyzer. They provide the foundation for understanding how to use SR785. For a more complete overview of the instrument and its capabilities, refer to the SR785 operating and programming reference.

Some of these examples use the test filter. The filter is a simple twin-T 1 kHz passive notch filter. This filter provides an interesting frequency response for these measurements.

In This Section

·        General Overview

·        Front Panel Quick Start

·        Things To Watch Out For

·        Analyzer Basics

·        Analyzing a Sine Wave

·        Measuring a Frequency Response Function

·        Linking (Advanced operation)

·        User math Functions

Overview

Power switch

            The power switch is located on the rear panel. Depressing the upper half of the power switch turns on the SR785. The green power LED on the front panel indicates that the unit is powered. The video display may take a few seconds to warm up and become visible. The brightness is adjusted using the [Brighter] and [Dimmer] buttons below the softkeys. The firmware revision, serial number, memory size and calibration dates of the unit are displayed when the power is turned on.

Video display

            The monochrome video display is the user interface for data display and front panel programming operations. The brightness is adjusted using the [Brighter] and [Dimmer] buttons below the softkeys. The contrast is adjusted using [Alt][Dimmer]. To extend the life of the display, use the Screen Saver in the [System] <Preferences> menu.

Keypad

            The keypad consists of four groups of hardkeys (keys with printed labels).

            Hardkeys are referenced in braces like [Active Display] or [Input].

            The ENTRY keys are used to enter numeric parameters, which have been highlighted by a soft key. The MENU keys select a menu of soft keys. Pressing a menu key will change the menu boxes, which are displayed next to the soft keys. The CONTROL keys start and stop data acquisition, toggle the active display and link parameters and functions. The FUNCTION keys perform common functions such as Auto Scale and Auto Range. These keys can be accessed at any time.

Softkeys

            The SR785 has menu driven user interface. The MENU keys each display a menu of Softkeys. The softkeys are at the right of the video display and have different functions depending upon the displayed menu.

            Softkeys are referenced in brackets like <Span> or <Units>.

            There are three types of Softkeys – buttons, lists and numeric values. A button performs a function, such as <Full Span>.

A list presents a list of choices or options in the entry field (at the top of the screen). Use the knob to make a selection and press [Enter]. <Measurement> is an example of a list. A numeric value presents the current value in the entry field and awaits numeric entry. Enter a new value with the ENTRY keys and press [Enter]. <Start Freq> is an example of a numeric value.

            For complete description of the menu softkeys, refer operating manual.

Knob

            The knob normally moves the markers within the displays. If a parameter has been highlighted by its softkey, the knob adjusts the parameter. Parameters, which have a list of choices, are most easily modified with the knob. Numeric entry fields may also be adjusted with the knob.

Disk drive

            The 3.5” disk drive is used to store data and instrument settings. Double sided, high-density disks have a capacity of 1.44M bytes. The disk format is DOS compatible.

            Use the [Disk] menu to format a disk or access disk files.

Features

• DC to 102.4 kHz bandwidth
• 90 dB Dynamic range
• 16- bit A/D conversion
• Low Distortion Synthesized
• ANSI Standard Octave Measurements
• 145 dB Dynamic Range Swept Sine Mode
• Order Tracking
• 20 Pole/Zero Curve Fitting
• Time/Histogram
• 8 Msamples Transient capture memory
• Internal and external Triggering
• File Output to SDF, MAT, UFF, and ASCII
• Graphics Output to GIF, EPS, PCX, HPGL
• 3.5” MS-DOS Compatible Disk Drive
• GPIB and RS-232 Interfaces Included
• Windows Data Viewer Program Included

The SR785 two channels Dynamic Signal Analyzer is a precision, full-featured signal analyzer that offers state-of-art performance and a wide selection of features.

Standard measurement groups include FFT, order tracking, octave, swept sine wave, correlation, time capture and time/histogram. The SR785 brings the power of several instruments to an application: a spectrum analyzer, network analyzer, vibration analyzer, octave analyzer and oscilloscope. Unique measurement architecture allows the SR785 to function as a typical dual channel analyzer with measurements like cross spectrum, frequency response, coherence etc. Alternately, the instrument can be configured so that each input channel functions as a single channel analyzer with its own span, center frequency, resolution, and averaging. This allows you to view a wide band spectrum and simultaneously zoom in on spectrum details. The same advanced architecture provides storage of all measurement building blocks and averaging modes. Vector averaged, RMS averaging, unaveraged and peak hold versions of all measurements are simultaneously acquired and can be displayed without re-taking data.

Averaging

The SR785 comes equipped with a wide selection of averaging techniques to improve your signal to noise ratio. Choose RMS averaging to reduce signal fluctuations, vector averaging to actually eliminate noise from synchronous signals, or peak hold averaging.

In the order-tracking measurement group, time averaging is available. Both linear and exponential averaging is provided for each mode.

Time Capture

The SR785 comes with 2 Msamples of memory (8 Msamples optional). Analog waveforms can be captured at sampling rates 262 kHz or any binary sub multiple, allowing you to optimize sampling rate and storage for any application. For example, 8 Msamples of memory will capture 32 seconds of time domain data at maximum 262 kHz sample rate, or about 9 hours of data at a 256 Hz sample rate. Once captured, any portion of the signal can be played back in any of the sr785’s measurement groups except swept-sine.  The convenient Auto Pan feature lets you display measurement results synchronously with the corresponding portion of the capture buffer to identify important features.

Source

The SR785 comes standard with five precision source types. Generate low distortion (-80 dB) single or two-tone sine waves, white noise, pink noise, chirps, and arbitrary waveforms. The chirp and noise sources can be bursted to provide activity over a selected portion of the time record for FFT measurements. The digitally synthesized source produces output levels from 0.1 mV to 5 V and offsets from 0 to +5 V, and delivers up to 100 mA of current.

Arbitrary waveform capability is standard with the SR785. Use the arbitrary source to playback a section of a captured waveform, play a selected FFT time record or upload your own custom waveform from your computer.

User Math

We can create our own measurement in each of the SR785’s measurement groups using the math menu. Enter any equation involving RMS averaged, vector averaged or unaveraged time or frequency data, stored files, constants, or rich array supplied operations including arithmetic functions, FFT, inverse FFT, jw, d/dw, exp, ln, x, and many others.

Because all the averaging modes are available as user math operands, non-repeatable runout measurements (used in analyzing disk drives) can be performed in a single pass by entering the equation MAG (RMS<F1>)-MAG (Vec<F1>). Unlike many other analyzers, the SR785’s measurement rate is virtually unaffected when user math is selected.

Output

The SR785’s built-in 3.5” 1.44 Mb disk drive, GPIB and RS-232 computer interfaces and centronics printer port combine to allow almost unlimited flexibility in saving, printing, plotting or exporting your measurement data. Displays and setups can be printed or plotted to disk, GPIB, RS-232 or centronics port in PCX, GIF, PCL (LaserJet and DeskJet), dot matrix, Postscript, HPGL or ASCII formats. An annotation editor lets you add text, time and date, or filenames to any portion of the plot.

DataViewer

The SRS DataViewer is a Windows 95 program that allows you to quickly upload SR785 files into a graphical environment, perform simple editing and cut and paste into other applications. We can add pointers and text, change scaling, and perform simple math operations. Graphs can be saved in PCX, BMP or GIF formats.

Front Panel quick start

There are two types of front panel keys, which are referenced in this section. Hardkeys are those with labels printed on them. There function is defined  by label and does not change. Brackets like this – [Hardkey], reference hard keys. Softkeys are the ten gray keys along the right edge of the screen next to the key. Softkey functions change depending upon the menu and instrument configuration. Softkeys are referenced as the <Softkey>.

[Hardkeys]

            The keypad consists of four groups of hardkeys (keys with printed labels).

The ENTRY keys are used to enter numeric parameters, which have been highlighted by a softkey. The MENU keys select a menu of softkeys. Pressing a menu key will change the menu boxes, which are displayed next to the softkeys. Each menu presents a group of similar or related parameters and functions. The CONTROL keys start and stop data acquisition, toggle the active display and link parameters and functions. These keys are not in a menu since they are used frequently and within any menu. The FUNCTION keys perform common functions such as Auto Scale and Auto Range. These keys can be accessed at any time.

<Softkeys>

The sr785 has a menu driven user interface. The menu keys each display a menu of soft keys. The softkeys are at the right of the video display and have different functions depending upon the displayed menu.

There are three types of softkeys – buttons, lists and numeric values. A button performs a function such as <Full Span>. A list presents a list of choices or options in the entry field (at the top of the screen). Use the knob to make a selection and press [Enter]. <Measurement is an example of a list. A numeric value presents the current value in the entry field and awaits numeric entry. Enter a new value with the entry keys and press [Enter]. <Start Freq> is an example of a numeric value.

[ALT] Keys

The [ALT] key is a special key, which has no meaning by itself but instead modifies the meaning of another key press or knob turn. Pressing the [ALT] key toggles the state of the ALT indicator at the top of the screen. Pressing a control key while the ALT indicator is lit selects the “alternate” function labeled underneath the key instead of the normal function.

Knob

The knob normally moves the markers within the displays. If a parameter has been highlighted by its softkey, the knob adjusts the parameter. List parameters are most easily modified with the knob. Numeric parameters may also be adjusted with the knob.

Knob list selections are referenced in parenthesis like (Hanning).

Help

            Enter the on screen help system by pressing [Help/Local]. Help on any hardkey or softkey is available simply by pressing the key. Press [1] for the help index. Press [0] to exit the help system and return to normal operation.

Things to watch out for

If the analyzer is on but doesn’t seem to be taking data, there are a number of things to check.

Start

            Press the [start/reset] key to start the measurements. Make sure the run/pause indicator at the top of the screen displays ‘RUN’ instead of  ‘PAUSE’. Note that in many cases when settings are changed using the menus, the new settings will not take effect until [start/reset] is pressed.

Live display

            If the displays are showing recalled trace data, they are off-line and do not display the live measurement data. Set the Display to Live instead of Off-line (in the [Display Option] menu).

Narrow Span

            If the FFT span is narrow, the time record is very long( up to 1000’s of seconds). Completely new data is available only every time record. Change the Time Record Increment in the [Average] menu to display overlapped data more often.

Averaging

            Very long averaging times for any measurement may give the appearance that the display does not update. Check the FFT Number of Averages, the Octave Integration Time or the Swept Sine Integration Time.

            When Linear averaging is on, the measurement is paused after the average is completed. Press [start/reset] to another average.

Scaling and Ranging

            Check that the inputs are not completely overloaded by using [Auto Range Ch1] and [Auto Range Ch2].

            Scale the display to show the entire range of the data with [Auto Scale A] and [Auto Scale B].

Local

            Make sure that the analyzer is not in REMOTE state where the computer interfaces have setup the instrument and locked out the front panel. Press the [Local/Help] key to restore local control.

Reset

            If the analyzer still seems to be functioning improperly, turn the power off and turn it back on while holding down the  [<-](backspace) key. This will reset the analyzer into the default configuration. The analyzer should power on running and taking measurements.

Analyzer Basics

What is an FFT?

            An FFT analyzer takes a time varying input signal, as you would see on an oscilloscope trace, and computes its frequency spectrum. Fourier’s basic theorem states that any waveform in the time domain can be represented by the weighted sum of pure sine waves of all frequencies. If the signal in the time domain is periodic, then its spectrum is probably dominated by a single frequency component. The spectrum analyzer represents the time domain signal by its component.

Why look at a signal’s spectrum?

For one thing, some measurements that are very hard in the time domain are very hard in the time domain are very easy in the frequency domain. Take harmonic distortion. It is hard to qualify the distortion by looking at a good sine wave output from a function generator on an oscilloscope. When the same signal is displayed on a spectrum analyzer, the harmonic frequencies and amplitudes are displayed with amazing clarity. Another example is noise analysis. Looking at an amplifier’s output noise on an oscilloscope basically measures just the total noise amplitude. On a spectrum analyzer, the noise as a function of frequency is displayed. It may be that the amplifier has a problem only over certain frequency ranges. In the time domain it would be very hard to tell.

FFT Frequency Spans

Full span is the widest frequency span corresponding to the fastest available sampling rate. In the SR785, this is DC to 131 kHz using a sampling rate of 262 kHz. Because the signal passes through an anti-alaising filter at the input, the entire frequency span is not useable. The filter has a flat response from DC to 102.4 kHz and then rolls off steeply from 102.4 kHz to 156 kHz. The range between 102.4 kHz and 131 kHz is therefore not useable and the actual displayed frequency span stops at 102.4 kHz. 

Model 5184 Ultra Low Noise Amplifier

            The model 5184 is a medium-impedance (5 MW), AC-coupled, voltage preamplifier which features an ultra low-noise FET input stage (800 pV/Ö Hz at 1 kHz). It has a frequency range of 0.5 Hz to 1 MHz and fixed gain of 60 dB (´1000).

Introduction

The Model 5184 preamplifier can be powered from an external power source or from internal alkaline cells as selected from the front panel switch. In operation, the voltage to be measured is connected to the AC- coupled INPUT BNC connector. The preamplifier amplifies the signal voltage by a fixed 60 dB (1000 times), which is then output from the OUTPUT BNC connector.

Power Switch

The three position POWER switch allows EXTERNAL, OFF, or BATTERY to be selected. To operate the 5184 from internal batteries, set the switch to the down position. The status of the selected power source can be ascertained by pressing the POWER TEST button; the adjacent LED will light if the power source voltage, whether of the external power source or the 5184 internal batteries, is above the minimum required value for reliable operation.

Input

            The 5184 input circuit is of asymmetrical differential configuration. When the GROUND/ISOLATE switch is set to the down (GROUND) position, the screen of the BNC input socket is grounded and the input may be used in the conventional single-ended mode. When the GROUND/ISOLATE switch is set to the up (ISOLATE) position, the input stage is floated and the input may be used in the “pseudo-differential” mode. In “pseudo-differential” mode the signal voltages should be connected to the BNC INPUT socket and the signal ground to the GROUND terminal on the front panel. The maximum common mode input voltage is 300-mV peak to peak and the maximum voltage difference the shell of the BNC signals input connector and the front panel ground terminal is +600 mV.

Output

            The 5184 output can generate greater than 10 V peak to peak signals into loads greater than 100 kW.

Battery operation

            In battery powered operation, the 5184 requires four 9 V alkaline cells. Each battery fits into its own removable tray accessible from the rear panel. When they are exhausted, all four batteries should be replaced together. Care should be taken when exchanging the batteries to observe the correct polarity, which is marked on the inside of the battery trays and the top of the batteries. The batteries will provide up to eight hours operation at 1 V rms. output level.

SR 715/720 LCR meter

This section is designed to help the user begin making measurements with the SR715/720 LCR meter and to familiarize them with some of its features.

To operate the SR715/720, first verify that the correct line voltage has been selected on the power entry module on the rear panel. Verify that no parts are in the fixture; the units self test routine will fail if any components are in the test fixture. After connecting the line cord to the power entry module, switch on the power switch, located on the rear right hand side of the unit. The unit will display the ROM version on the left display and the serial number on the right display for about 3 seconds. Next the unit will begin its self-test procedure. If all the tests are OK, ‘tEST PASS’ will be displayed. Press [Recall] [0] [ENTER], to recall the default setup. This will set the unit as follows:

            Parameter                     AUTO

            Frequency                    1 kHz

            Drive Voltage               1.0 Vrms

            Bias                              OFF

            Measurement Rate        SLOW

            Averaging                     OFF

            Range Hold                  OFF

            Equivalent Circuit          SERIES

            Display             VALUE

            Trigger Mode               CONT

            Binning             OFF

At this point a part can be placed in the fixture and measurements can be made. The unit will automatically decide which type of component is in the fixture (R, C or L), change to the correct range, and display the part value with the correct units. If any of the test conditions need to be changed for a measurement, press the appropriate keys. Most functions are easy to change, but if more information is needed, refer to the Operating manual and programming reference.

The SR715/720 comes with a build in fixture for measuring radial leaded components, like most capacitors, and adapters for measuring axial leaded components, like most resistors. To measure a radial leaded component, remove any adapters and put one lead of the component in each side of the fixture. To measure an axial leaded component, first install the fixture adapters. Put the adapters on in the middle of each side of the fixture, and then slide them until the appropriate distance separates them. Insert the one lead of the component into each side of the fixture.

Note:  After changing the fixture configuration it is a good idea to perform an open and short circuit calibration. (See Operation section of the operating and reference manual)

Note:  if biased measurements are being made on capacitors, be certain that the part is installed with the correct polarity, as marked on the unit.

Typical measurement setups

The table below lists suggested test conditions for various types of components. This is by no means the only set of conditions for measuring, but is a good general “rule of thumb”.

Setups for measuring Typical Parts

Overview

This section gives the overall view of the SR715/720. For detail operating information on specific features, see the operating and programming reference.

Display

The led display shows values, entered parameters, instrument status, and user messages. There are two 5-digit LED displays and 25 indicators LEDs. When making normal measurements, the major parameter (L, C, or R) is shown on the left display and appropriate minor parameter (Q, D or R) is shown on the left display.

Fixture

A versatile test fixture is provided with the SR715/720 that provides a 4 wire Kelvin connection uses two wires to carry the test current to and from the device and two independent wires to sense the voltage across the device. This prevents the voltage drop in the current carrying wires from affecting the voltage measurement.

Radial leaded components (where the leads are on the same side) are simply inserted into the test fixture, one lead in each side.

Axial leaded devices (leads at opposite ends) require the use of the axial fixture adapters.

Surface mount (SMD) devices or components with large or unusually shaped leads can be measures with SMD tweezers or Kelvin clips. A BNC adapter is available for interfacing to remote fixtures.

If the device leads are dirty or coated with wax, clean them before inserting them in the fixture. See the maintenance section for information about cleaning the fixture.

The figures on the next page show the mounting of the component on the fixture.

Keypad

The keypad is used to select measurement conditions and to enter values. LEDs indicate the actual measurement condition.

Parameter

The [R+Q], [C+D], [C+R], and [AUTO] keys select the parameter being measured. The selected parameter pair is indicated above the major and minor parameters in the display. If AUTO is selected, the unit will select themost appropriate parameter pair and turn on the AUTO LED above the major parameter display.

Frequency

The [UP Arrow] and [DOWN Arrow] keys select one of the available frequencies.

Drive Volt

The [Select] keys cycles through the three output drive voltages.

Meas Rate

The [Rate] key selects a slow, medium or fast measurement. These rates corresponds to 2, 10, or 20 measurement frequencies of 1 kHz or higher.

Average

The [Average] key places unit in the averaging mode. Pressing it second time returns the unit to the non-averaged mode. The number of readings averaged is set to between 2 and 10 from the entry display.

Range Hold

[Range Hold] holds the unit its current measurement range. Pressing the key a second time returns the unit to autoranging or normal mode. The reange can also be entered with the numeric keys in the entry display.

Display

The [Display] key selects the parameter on the display. Pressing [Display] cycles through the available display types.

Store and Recall

The unit can store up to 9 complete instrument setups in non-volatile memory. To store the present configuration as setup #n, press [Store][n][ENTER] where n ranges from 1 through 9. to reacal an instrument configuration, press [Recall][n][ENTER]. Recall 0 returns the instrument to its default values.

Power switch

The power switch is located on the rear right side of the unit. Depressing the front side of the switch turns the unit on.

RS232 DB25 and IEEE-488 (GPIB) Connector

This connector allows for computer control of the unit over an RS232 interface. This connector allows for computer control of the unit over the GPIB or IEEE-488 interface.

Chapter 4

Noise Measurement Experiment:

After getting familiar with the individual instrument its time to use the capabilities of these equipment to measure the noise signal. Following are the steps to measure one over f noise and then store the signal in the memory and external floppy disk. Your experience from the three experiments discussed in chapter one will be useful here.

To measure background noise: the FFT of the background noise is obtained when the entire system is operating with no d.c. current passing through the thin film. This spectrum is captured and stored in the Dynamic Signal Analyzer.

Next, the voltage noise spectrum is obtained when d.c. current is applied to the test device, this spectrum is also captured and stored in the signal analyzer.

Using features (user Math Functions) of SR785 Dynamic Signal Analyzer, background noise spectral density is subtracted from total noise voltage spectral density i.e. Sv(total) – Sv(bgn) and the remainder is the excess noise produced by the thin film.

Before following the steps given below make the connections as given in the block diagram of the noise measurement system.

Block Diagram of noise measurement setup

Steps: Under No d.c. current flowing through the sample

1.                  Press [System]

2.                  Press<Preset>

3.                  [Enter] to confirm preset.

4.                  Press [Auto Range Ch1]

5.                  Press [Auto Range Ch2]

6.                  Press [Display Setup]

7.                  Press <Measurement>

8.                  Select (FFT ch1) with the knob and press [Enter].

9.                  Press [Active Display]

10.              Select (FFT ch2) with the knob and press [Enter]

11.              Press [Input]

12.              Press <Analyzer Config>

13.              Select (Independent Chan.)

14.              Press [Active Display]--- ch-1

15.              Press [Freq]

16.              Press <Span> (400 Hz.)

17.              Press <Start freq>-- 3 Hz.

18.              Press < Stop Freq >-- 400 Hz.

19.              Press [Active Display]

20.              Follow steps 22 to 25

21.              Press [Window]

22.              Select (Flattop) and Press [Enter]

23.              Press [Average]

24.              Press <Display Avg>

25.              Select (RMS) with the knob and press [Enter]

26.              Press <# Avgs>

27.              Press [Link]

28.              Select No. Of Averages—30

29.              Press [Active Display]

30.              Press[ Link]—Disp B is highlighted

31.              Select (Peak Hold) for ch-2, select same parameters for ch-2.

To SAVE and RECALL the waveform stored in the TRACE:

1.                  Press [Alt] [Print Screen]—to save current data in the active display to a trace.

2.                  Use the knob to select Trace 3 and press [Enter]

3.                  Press [Active Display]—Switch to Disp B

4.                  Do step 1 and 2 and store in Trace 4.

5.                  To Recall Press [Alt] [Help/Local]

6.                  Press[Link]

7.                  Press [Enter] to select Trace 3 on ch-1

8.                  Press[ Link ]

9.                  Select Trace 4 on ch-2.

10.              Put a blank floppy

11.              Press [Disk]

12.              Press <Disk Upkeep>

13.              Press<Format Floppy> and Press [Enter] to confirm

14.              Press <Return>

15.              Press [Active Display] to make ch-1 display active

16.              Press <File Name>

17.              Press [Alt]

18.              Enter a file name

19.              Press< Display to Disk >

20.              Press [Active Display]

21.              Press <File Name>

22.              Enter file name

23.              Press< Display to Disk>

24.              To recall from disk Press [Disk]

25.              Press <File Name>

26.              Turn the knob to display the file catalog

27.              Use the knob to select one of the disk files and Press [Enter]

28.              Press <Disk to Display>.

 Use SR785 to CAPTURE a signal and then analyze it from MEMORY.

1.                  Press [Capture]

2.                  Press <Allocate Memory>

3.                  Press <Waterfall Memory>

4.                  Press[0] and [Enter]—free memory occupied by other buffers

5.                  Press <Capture Memory>

6.                  Press [9][0][0] and [Enter]—select 900 blocks each block = 2000 points.

7.                  Press < Confirm Allocation > and [Enter]

8.                  Press <Capture Channels>

9.                  Use knob to select (Ch1) and Press [Enter]

10.              Press <Capture length>

11.              Press [1][8][0][0] and [Enter]

12.              Press [Start Capture] --   Takes 7.03 seconds

13.              Unplug the signal from the CH1 input

14.              Press [Input]

15.              Press <Input Source>

16.              Use knob to select (playback) and Press [Enter]

17.              Press <Playback Config>

18.              Press [1][8][0][0]

19.              Press <Playback Mode>

20.              Use the knob to select (Circular) and Press [Enter]

Note: Repeat the same same procedure for measuring total spectral density when d.c. current is flowing. Then make use of User Math Function( refer examples) to obtain the difference.