Leader Electronics Corp. LAG-125 User manual

LAG-125

LOW DISTORTION

AUDIO GENERATOR

INSTRUCTION MANUAL

CONTENTS

SECTION page

1GENERAL DESCRIPTION

1.1 Uses 2

1.2 SpeciHcations 2

1.3 Control Functions ^

1.3.1 Front Panel ^

1.3.2 Rear Panel ^

2OPERATION

2.1 Preliminary Notes 5

2.2 Output Connections ^

2.3 Interconnections g

2.4 Sine Wave Output g

2.4.1 Input/Output Characteristics g

2.4.2 Output Meter Roadings ^

2.4.3 Frequency Response 7

2.4.4 Harmonic Distortion Measurements g

2.5 Square Wave Output ^

2.6 Burst Output Signals jq

2.6.1 General jq

2.6.2 Loudspeaker Testing

2.6.3 Amplifier Testing 22

2.7 Use of the SYNC Connection 23

2.8 Notes on Output Connections 24

3MAINTENANCE

3.1 Fuse Replacement 24

3.2 Exposing the Chassis •24

3.3 AC Input Connections 25

3.4 Location of Adjusters 26

FUNCTIONAL BLOCK DIAGRAM

SCHEMATICS

(1)

1. GENERAL DESCRIPTION

1.1 Uses

The LAG-125 is particularly designed for measurements and tests of audio and supersonic

equipment and circuits. The three different types of output signals will find many applications in

testing high fidelity amplifiers, filter networks, and loudspeakers.

FEATURES

*Wide frequency range, lOHz to IMHz, with flat output response.

*Low distortion sine wave output.

*Clean-cut square waves for transient response testing and triggering pulse generators.

*Burst type signals for loudspeaker testing.

*Calibrated attenuator, and voltmeter for accurate output control.

*Pushbutton frequency ranging.

*Frequency synchronization possible from external source.

*Tilt-up stand for easy operation.

1.2 Specifications

Frequency Range

Frequency Accuracy

Output Functions

Sine Wave

Square Wave

Burst Signal

lOHz -IMHz in 5decade ranges.

Within 3% of dial marking.

Output voltage:

Distortion:

Output voltage:

Overshoot:

Sag;

Rise Time:

Output Voltage:

Gating:

Leakage:

3Vrms into 600J2.

0.03% :500Hz -20kHz

0.1% :lOOHz-lOOkHz

0.5% :50Hz- 500kHz

1% :lOHz -IMHz

3Vp-p into 600J2.

2% at maximum output.

5% at lOHz.

0.15/is (0.45jUs at floating output.)

1.5Vp-p into 600^2.

4cycles at 4and 12 cycle intervals; 8cycles at

8cycle interval.

Within 2% during off interval at 20kHz.

Frequency Synchronization

Control

Output Data

Output Impedance

Response

Approx. ±0.5% per volt rms.

600n ±3%; unbalanced and floating.

Flat within ±0.3dB into 600f2 at unbalanced output; to lOOkHz

at floating output.

Output Control

Output Meter

Power Supply

Size and Weight

Accessory, furnished

Total range: -50 to +10dBm (2.45mV -3.1Vrms) with lOdB

step attenuator and fine adjuster.

Voltage range: 0-1and 0-3Vrms.

Decibel range: -10 to +2dB (OdB =0.775V)

Accuracy within ±5% f.s.

100, 115 or 230V, as specified, 50/60Hz; approx. 12VA.

165(H) X205(W) X250(D) mm

61/2(H) X8(W) X97/8(D) IN

approx. 5.5kg.

12.1 LBS.

Output cord, w/plugs and clips 1ea.

1.3 Control Functions

1.3.1 FRONT PANEL, FIG. 1-1.

Fig. 1-1 Front panel.

(1) Frequency dial: Calibrated in Hz, 1to 10; actual output frequency depends on the setting

of the range selector.

(2) Fine frequency adjuster.

(3) FREQUENCY RANGE selector: Set the range multiplier for dial markings.

(4) FUNCTION switch: Selects the type of output signals, sine wave, square wave, and burst

[see (14)]

(3)

(5) 600r2 SHUNT switch: Connects a600n load resistor across the output.

(6) OUTPUT terminals: Black at left is for chassis ground; black at middle is the low

potential side of the output; red is the high potential side of the output.

(7) Shorting link: Connected across the black terminals at the unbalanced output condition

(not used when floating output is required).

(8) OUTPUT LEVEL switch: Adjusts the output in lOdB steps.

(9) VARIABLE control: For fine adjustment of the output between the lOdB steps.

(10) POWER switch: Push-push type for turning on the AC power.

(11) Pilot lamp: Indicates when the power is on.

(12) Output meter: Calibrated in rms volts with two scales, and dBm (OdB =0.775V).

(13) Mechanical zero adjuster for the meter.

1.3.2 REAR PANEL. FIG. 1-2.

®(@ ®

Fig. 1-2 Rear panel functions.

(14)

BURST signal selector: Set the mode of the burst output signals.

(1 5) SYNC terminals: For connection to an external frequency synchrnizing source.

(16) FUSE holder: For the AC line fuse.

(17) AC input cord.

(18) Label.

(19) AC cord hooks.

(4)

2. OPERATION

2.1 Preliminary Notes

1. When the output is connected to a circuit in which DC voltage is present, always connect a

blocking capacitor in series with the “hot” lead. This is to prevent damge to the output

circuit. The capacitor should have low reactance at the test frequency and suitable voltage

rating.

2. Use short leads for connections. The cord supplied will introduce aloss of less than 0.2dB at

IMHz. Along shielded cable will degrade the high frequency characteristics.

3. When the “floating” output connection is used, the frequency response of ±0.3dB is

applicable only up to lOOkHz. Further, the square wave rise time will be increased to 0.45iUs

maximum.

2.2 Output Connections

1. Unbalanced :

Connect the shorting link across the two black terminals. Insert the cord lead plugs -black

to black and red to red respectively.

The black lead should be used on the ground side on the chassis of the test circuit.

2. Floating:

Remove the shorting link on the black terminals.

Use the middle black terminal and red terminal for the output. If required, connect alead

between the left black terminal and ground side of the chassis.

2.3 Interconnections

The basic interconnections are shown in Fig. 2-1.

Fig. 2-1 Basic interconnections.

The specified load resistance, R, is connected across the output of the test circuit. It should be

non-inductive and have awattage rating at least twice the expected maximum power output.

In measuring the input/output voltages, an electronic voltmeter is required. The Leader

LMV-87A is recommended for the purpose.

(5)

An oscilloscope is required when measuring with use of square waves or burst signals. The

Leader LBO-301, or LBO-502, is most suited.

2.4 Sine Wave Output

In most amplifier measurements, sine wave signals are used. In this section, typical uses will be

described.

2.4.1 INPUT/OUTPUT CHARACTERISTIC

Control settings:

POWER switch at on.

FUNCTION at sine wave.

FREQ RANGE at xlk and dial at “1” for IkHz.

OUTPUT LEVEL, initially at 40 and fine adjuster set fully counterclockwise.

600n SHUNT switch at released position.

The input voltage to the amplifier is gradually increased by advancing the VARIABLE control

and lowering the attenuation with the switch towards OdB. The amplifier output will increase as

the input voltage is raised.

When the amplifier is overloaded, there will be no apparent increase in the output and

waveform distortion will be observed, i.e., flattening of one or both peaks of the trace.

By noting the input and output voltages, these values are plotted on log-log paper. In this

manner, the input voltage range of the amplifier can be easily determined.

When the ration Eout/E in is calculated, the voltage gain is given as follows:

Eout

VOLTAGE GAIN in dB =20 log £in

Reference should be made to adecibel table for the dB figures.

The results for voltage gain in dB can be plotted on semi-log paper, using the X-axis for Ej^^

and Y-axis for dB.

The power output is calculated from the relation -

Eout ^(volts)

POWER OUTPUT. Po in WATTS ^‘

R(ohms)

2.4.2 OUTPUT METER READINGS

The output meter is calibrated to indicate the dB level, or voltage, only at the sine wave

function into a60012 load.

The range depends on the setting of the OUTPUT LEVEL switch. Other data are shown in the

table below.

(6)

OUTPUT METER RANGES

OUTPUT LEVEL

SETTING VARIABLE

dB RANGE VARIABLE VOLTAGE

RANGE METER

SCALE

+10 0to +12 0.775 —3.1 Vrms 0-3

0-10 to +2 0.245 -1.0 0-1

-10 -20 to -8 0.0775 -0.31 0-3

-20 -30 to -18 0.0245 -0.10 0-1

-30 -40 to -28 0.00775 -0.031 0-3

(7.75mV —31mV)

-40 -50 to -38 0.00245 -0.010 0-1

(2.45mV -lOmV)

NOTES: a. At the +10dB setting, the pointer will go off scale when the load is

greater than 600J2.

b. For corrections when the load is different than 600f2, refer to Section 2.8.

2.4.3 FREQUENCY RESPONSE

The frequency response of an amplifier is determined by applying aconstant voltage to the

amplifier. This voltage is chosen so that the amplifier is operated below the overload point.

Set the reference frequency at IkHz, or 400Hz, and set the output controls for asuitable

output from the amplifier.

Note the input and output voltages.

Set the measuring frequencies with the FREQ RANGE switch and dial to lOHz, or higher if

required.

Since the generator output is practically constant at all frequencies, the input voltage will not

require any adjustment. However for the highest accuracy, the input at each frequency can be

adjusted to the predetermined value.

The output readings can be simplified by noting the output level in dB at the reference

frequency (1kHz or 400Hz). Then at each frequency the dB indication is noted and used in

plotting the response curve. (NOTE: Disregard the OdBm -0.775V, etc. in this case. The dB

readings can be read directly since the voltmeter is connected across aconstant impedance.)

The dB readings are added or subtracted from the IkHz reference level.

Example: Let “dB” at IkHz --2dB. Assume that the measured values are as in (A) in the

following data.

FREQ (Hz) 20 60 200 600 IK 2K 6K 20K

(A) dB

measured -6 -5 -2 -2 -2 -2 -1 -6

(B) dB -4 -3 0000+1 -4

(7)

The dB figures for (B) are used in plotting on semilog graph paper with the X-axis for fre-

quency and Y-axis for the relative response in dB.

4.4 HARMONIC DISTORTION MEASUREMENTS

The low output distortion characteristics of the LAG-125 make it possible to measure the

distortion in audio amplifiers.

For the measurements, aharmonic distortion meter, such as the LEADER LDM-170 and a

sensitive scope are required. The interconnections are shown in Fig. 2-2.

DISTORTION METER SCOPE LBO-502

Fig. 2-2 Harmonic distortion measurement.

Connectionst SCOPE output of the distortion meter to the horizontal scope input.

Test amplifier output to the distortion meter and also to vertical scope input.

With these connections, the scope will display the lissajous patterns for monitoring the

amplifier output. It will be possible to discriminate between the waveform distortion and the

noise components. Some examples of the display are shown in Fig. 2-3.

Co o<o

>2nd HARMONIC >3rd HARMONIC

HIGHER HARMONICS

Fig. 2-3 Lissajous patterns.

(8)

For accuracy in measurements, especially at low values, it is necessary at the test frequencies

and output levels to measure and note the distortion in the LAG-125 signal. This step is required

since the distortion meter indicates the total distortion, i.e., amplifier and oscillator outputs.

The distortion is then measured on the meter and noted. The actual amplifier distortion is

calculated from the relation -

(1)

where -Amplifier distortion in %

Dm -Distortion meter indication in %

DqSC ~Oscillator distortion in %

In simplified form, (1) may be expressed by -

The factor, K, for (2) is listed for convenience in the following table.

FACTOR “K” for (2)

^osd^M KDoSC^D,^ K

0.90 0.436 0.60 0.80

.85 .526 .50 .866

.80 .60 .40 .916

.75 .661 .30 .954

.70 .714 .20 .978

.65 .758 .10 .995

In general, when the oscillator distortion is less than 1/5 of the amplifier distortion, the meter

indication can be used without correction.

2.5 Square Wave Output

Square wave signals are useful in the rough determination of response characteristics of

amplifiers at the high and low frequency ends, and also response to signals with fast rise time.

The interconnections are identical with those for sine wave operation with the following

exceptions:

FUNCTION switch at square wave.

Use of agood scope, i.e., with fast rise time.

The chart given below shows the waveforms at the amplifier output under different

conditions.

NOTES: Output voltage settings are initially made with the OUTPUT LEVEL

(9)

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