Krohn-Hite 6900B Operating instructions

5Hz to 1MHz
WIDE RANGE FULLY AUTOMATIC
DISTORTION ANALYZER
MODEL 6900B SERIAL NO. __________
OPERATING AND MAINTENANCE
MANUAL
Unit 4, 15 Jonathan Drive, Brockton, MA 02301-5566
Tel: [phone]; Fax: [phone]
E-mail: [e-mail]; Web: www.krohn-hite.com

Model 6900B Table of Contents
TABLE OF CONTENTS
1.0 GENERAL DESCRIPTION....................................................................................................... 1-1
1.1 Introduction ................................................................................................................. 1-1
1.2 Specifications .............................................................................................................. 1-1
1.3 General Specification................................................................................................... 1-3
2.0 OPERATION........................................................................................................................... 2-1
2.1 Introductions................................................................................................................ 2-1
2.2 Power Requirements.................................................................................................... 2-1
2.3 Turn-on Procedure....................................................................................................... 2-1
2.4 Controls, Terminals, Indicators...................................................................................... 2-1
2.5 Operation .................................................................................................................... 2-2
2.6 Optional Features ........................................................................................................ 2-4
3.0 INCOMING ACCEPTANCE...................................................................................................... 3-1
3.1 Introduction ................................................................................................................. 3-1
3.2 Procedure................................................................................................................... 3-1
4.0 CIRCUIT DESCRIPTION.......................................................................................................... 4-1
4.1 Introduction ................................................................................................................. 4-1
4.2 Circuit Description........................................................................................................ 4-1
5.0 CALIBRATION........................................................................................................................ 5-1
5.1 Introduction ................................................................................................................. 5-1
5.2 Calibration Procedure................................................................................................... 5-1
5.3 Power Supply .............................................................................................................. 5-1
5.4 Voltmeter Calibration.................................................................................................... 5-2
5.5 Input Amplifier Calibration............................................................................................. 5-3
5.6 Stepping Amplifier Calibration....................................................................................... 5-3
5.7 Output Amplifier Calibration.......................................................................................... 5-3
5.8 Frequency Detector Calibration..................................................................................... 5-4
5.9 1kHz Oscillator Calibration............................................................................................ 5-4
5.10 1kHz Calibration .......................................................................................................... 5-4
5.11 Integrator Level Calibration........................................................................................... 5-4
5.12 Calibration Adjustments................................................................................................ 5-4
6.0 MAINTENANCE ...................................................................................................................... 6-1
6.1 Introduction ................................................................................................................. 6-1
6.2 Trouble-Shooting Procedure......................................................................................... 6-1
6.3 Step-by-Step Troubleshooting procedure....................................................................... 6-1
6.4 Power Supply .............................................................................................................. 6-6
6.5 ALC Amplifier .............................................................................................................. 6-6
6.6 Null Amplifier ............................................................................................................... 6-7
6.7 Meter Amplifier ............................................................................................................ 6-7
6.8 Operational Rectifier .................................................................................................... 6-7
6.9 Integrator..................................................................................................................... 6-8
6.10 Voltage Discriminator ................................................................................................... 6-8
6.11 Frequency Discriminator............................................................................................... 6-8
6.12 Oscillator..................................................................................................................... 6-8
7.0 PARTS LISTS, SCHEMATICS AND BOARD LAYOUTS ........................................................... 7-1
7.1 Ordering Information .................................................................................................... 7-1
7.2 Ordering Unlisted Parts ................................................................................................ 7-1

Model 6900B
Figure 1.1 Model 6900B

Model 6900B Section 1 –General Description
SECTION 1
GENERAL DESCRIPTION
1.1 INTRODUCTION
The Krohn-Hite Model 6900B, illustrated in Figure 1.1, is a Fully Automatic Distortion Analyzer designed or
ultra-low distortion and ac voltage measurements in the 5Hz to 1MHz frequency range. The Distortion
Analyzer will measure total harmonic distortion (THD) down to 0.005%, typically, with 0.001% resolution.
A unique Auto Set Level feature eliminates the need to manually preset the input reference level before
each measurement for any input voltage between 0.1-130 volts rms. An Auto-Null Network provides Auto
Nulling of the fundamental frequency over the entire frequency range. The Analyzer also measures ac
voltages from 10mV to 130 volts rms over the range of 5Hz to 1MHz.
The Auto-Ranging Digital Display Meter (DDM) reads directly in percent distortion or ac volts, and
eliminates the ambiguities commonly found in analog voltmeter. A 1kHz ultra-low distortion (0.003%)
sinewave output is provided for use as a stimulus to test circuit or system distortion characteristics, or for
self-checking of the Analyzer.
A switch selectable High-Pass filter reduces the effects of hum and low frequency noise. The 400Hz,
30kHz and 80kHz selectable filters are effective only in the DISTORTION MODE. An internal low-pass
tracking filter eliminates high frequency noise when making low distortion measurements.
A DISTORTION OUTPUT is also provided for visual inspection or spectral analysis of the input signal after
the fundamental has been filtered out.
An optional BCD output is available.
1.2 SPECIFICATIONS
1.2.1 DISTORTION METER SPECIFICATIONS
1.2.1.1 FUNDAMENTAL FREQUENCY RANGE: 5Hz to 1MHz automatically tuned over
the entire specified input frequency range. An intermittent display indicates an
out-of-range condition.
1.2.1.2 INPUT: No presetting required over input level from 100mV to 130 volts rms.
Impedance; 110k ohms shunted by 100pF.
1.2.1.3 DISTORTION RANGE: A digital panel meter with Auto-Ranging displays
percentage of total harmonic distortion (THD) to 19.9%.
Measurement (%) Resolution (%)
0.000 -.100 0.001
0.10 -2.00 0.01
2.0 -19.9 0.1
1.2.2 DISTORTION MEASUREMENT ACCURACY
1.2.2.1 Input volts: 0.1 –0.25
1.2.2.2 Distortion levels: >0.01%
10Hz to 50kHz: ±15% of reading or 0.01%, whichever is greater.
5Hz to 100kHz: ±15% of reading or 0.02%, whichever is greater.
100kHz to 500kHz: ±15% of reading or 0.1%, whichever is greater.
500kHz to 1MHz: ±25% of reading or 0.5%, whichever is greater. 1-1

Section 1 –General Description Model 6900B
1.2.2.3 Input volts: 0.25 –130
1.2.2.4 Distortion levels: >0.005%
10Hz to 50kHz: ±15% of reading or 0.005%, whichever is greater.
5Hz to 100kHz: ±15% of reading or 0.01%, whichever is greater.
100kHz to 500kHz: ±15% of reading or 0.05%, whichever is greater.
500kHz to 1MHz: ±25% of reading or 0.5%, whichever is greater.
1.2.3 RESIDUAL DISTORTION AND NOISE
1.2.3.1 Input: 0.1V –0.25V
5Hz to 1MHz; less than 0.03% from 5Hz to 50kHz rising to 0.5% at 1MHz.
1.2.3.2 Input: 0.25V –130V
50Hz to 20kHz; less than 0.01%
5Hz to 1MHz; less than 0.02% from 5Hz to 50kHz rising to 0.5% at 1MHz.
1.2.4 SETTLING TIME (to 0.1% THD): Typically less than 3 seconds from 50Hz to 1mHz.
Longer at lower frequencies.
1.2.5 FUNDAMENTAL REJECTION: Greater than 10dB below residual THD.
1.2.6 FILTERS
400Hz High Pass: -3dB at 400Hz ±10%; 40db/decade rolloff.
30kHz Low Pass: -3dB at 30kHz ±5%; 60db/decade rolloff.
80kHz Low Pass: -3dB at 80kHz ±5%; 60db/decade rolloff.
1.2.7 DISTORTION OUTPUT (Residual signal after fundamental is nulled): Voltage;
100mVrms/%THD. Impedance; less than 500 ohms.
1.2.8 ANALOG OUTPUT: 100mVdc/%THD. Impedance; less than 1k ohms.
1.2.9 DIGITAL DISPLAY METER (DDM): Three digit meter with Auto-Ranging.
1.2.10 AC VOLTMETER SPECIFICATIONS
1.2.10.1 Frequency Range: 5Hz to 1MHz.
1.2.10.2 Voltage Range: 0.01V to 130 volts rms.
Voltage Range Resolution (volts)
0.01 –1 0.001
1 –10 0.01
10 –130 0.1
1.2.10.3 Accuracy: ±2% ±1 digit from 10Hz to 500kHz, ±5% ±1 digit from 5Hz to 1MHz
over specified voltage range.
1.2.11 DIGITAL DISPLAY METER: Three digit meter with Auto-Ranging.
1.2.12 INPUT IMPEDANCE: 110k ohms shunted by 100pf.
1-2

Model 6900B Section 1 –General Description
1.2.13 OSCILLATOR OUTPUT
1.2.13.1 Frequency: 1kHz, fixed.
1.2.13.2 Output (rms): 5 volts at 3 milliamperes maximum.
1.2.13.3 Distortion: 0.003%.
1.2.13.4 Impedance: 600 ohms.
1.3 GENERAL SPECIFICATIONS
1.3.1 FRONT PANEL CONTROLS
MODE switch for selecting either voltmeter (VOLTS) or distortion (% DIST) operation.
FILTER switch for selecting 400Hz High Pass, 30kHz and 80kHz Low Pass operation.
POWER switch.
1.3.2 REAR PANELS
Switches for selecting 120V/240V ac operation and normal or low (NORM/LOW) as line
voltage.
1.3.3 CONNECTORS (BNC)
Front panel: INPUT, 1kHz OUTPUT and DISTORTION OUTPUT
Rear panel: INPUT, ANALOG OUTPUT
1.3.4 MAXIMUM DC COMPONENT: 100V.
1.3.5 ISOLATION TO CHASSIS: 500Vdc.
1.3.6 OPERATING TEMPERATURE RANGE: 0°C to 45°C.
Unless otherwise noted, specifications apply at 25°C ±5°C.
1.3.7 POWER REQUIREMENTS: Switch selectable, 90-110, 10-8-132, 180-220 or 216-264
volts, single phase, 50-400Hz, 15 watts.
1.3.8 DIMENSIONS AND WEIGHTS: 16.625”/42.3cm wide, 3.5”/9cm high, 14.375”/37.5cm
deep, 13.5lb/6.1kg. An optional rack-mounting kit, Part No. RK-319, is available for
installing the unit in a standard 19” rack.
1.3.9 BCD OUTPUT (optional): Provides 13 lines of parallel BCD output, plus 1 MODE output,
3 decimal point outputs and 4 separate 3-state control lines. Compatible with DTL, RTL
and TTL logic.
1-3

Section 1 –General Description Model 6900B
Figure 1.2 Operating Controls, Connectors and Display
1-4

Model 6900B Section 2 –Operation
SECTION 2
OPERATION
2.1 INTRODUCTION
This section describes the basic operation of the Model 6900B. It includes the proper ac power
requirements, the recommended turn-on procedure and a detailed explanation of all operating controls,
modes of operation and special optional features.
2.2 POWER REQUIREMENTS
The Model 6900B is designed to operate from a single phase, 50-400Hz ac power source of 90-110, 108-
132, 180-220, or 216-264 volts. Line switches allow the 6900B to be powered from one of the above 4
voltage ranges. The ac power receptacle is a standard 3-pin connector, and complies with the European
I.E.C. standard. A detachable 3-wire line cord is provided with the instrument. A 2/10A slow blow fuse
should be inserted in the fuse receptacle.
2.3 TURN-ON PROCEDURE
2.3.1 Set the line switches for the correct voltage range. For 90-110 volts, set the voltage range
to 120V and the NORM/LOW switch to Low. For 108-132 volts, set the voltage range to
120V and the NORM/LOW switch to Normal.
For 180-220 volts, set the voltage range to 240V and the NORM/LOW switch to Low.
For 216-264 volts, set the voltage range to 240V and the NORM/LOW switch to Normal.
2.3.2 Make sure the POWER switch is in the off position.
2.3.3 Plug the line cord into the unit and into an ac outlet.
WARNING!
The chassis of this instrument is connected to ground. For safety purposes,
connect the line cord to a grounded, 3 terminal ac outlet.
2.3.4 Turn the POWER switch on, and allow the unit to warm up for several minutes.
CAUTION!
Because of the potentially dangerous voltages that exist within the unit, the
cover of this instrument should not be removed when the instrument is
connected to an ac power source.
2.4 CONTROLS, TERMINAL, INDICATORS
2.4.1 FRONT PANEL:
2.4.1.1 INPUT: Input voltage; 100mV to 130 volts rms in PERCENT DISTORTION
MODE, 10mV-130V rms in VOLT MODE. Input impedance; 110k ohms shunted
by 100pf. Maximum dc; ±100V. Isolation to chassis; 500Vdc.
2.4.1.2 INPUT LEVEL: LOW LED indicates when the input, in the DISTORTION
MODE, is below the minimum level of 100m Vrms. 2.1

Section 2 –Operation Model 6900B
2.4.1.3 DIGITAL DISPLAY METER (DDM): Displays directly in percent distortion or ac
volts.
2.4.1.4 MODE: Pushbutton switch for mode of operation either percent distortion (%
DIST) or ac volts (VOLTS).
2.4.1.5 FILTER: There are three switch selectable filters: 40dB/decade, 400Hz, HIGH
PASS, for reducing the affects of line (to be used at input frequencies greater
than 1kHz); 60dB/decade, 30kHz, LOW PASS, which provides band limiting
required by FCC for proof of performance broadcast testing; 60dB/decade,
80kHz, LOW PASS, for reducing high frequency noise.
2.4.1.6 1kHz OUTPUT: 1kHz, fixed sinewave output, 5 volts rms at <0.003% distortion.
Output impedance; 600 ohms. Maximum output current; 3mA rms. May be
used as a standard to test linearity of components system distortion
characteristics or performance of the 6900B.
2.4.1.7 DISTORTION OUTPUT: AC voltage for visual inspection of the input signal with
the fundamental removed. Approximately 100mV rms/% THD.
2.4.1.8 POWER: Pushbutton on-off switch.
2.4.2 REAR PANEL:
2.4.2.1 ANALOG OUTPUT: DC voltage proportional to distortion; 100mV/%, BNC.
2.4.2.2 R761: Screwdriver null adjust to obtain maximum capability at low distortion
levels.
2.4.2.3 R786: Screwdriver null adjust to obtain maximum capability at low distortion
levels.
2.4.2.4 BCD OUTPUT (optional): Includes 13 lines of parallel BCD output, 1 MODE
output, 4 decimal point outputs, 4 separate 3-state control lines and 1
programming ground return for a total of 23 lines. An edge connector is
supplied.
2.4.2.5 3-wire ac receptacle.
2.4.2.6 LINE: Slide switches for 120 or 240 volt operation and normal or low line
conditions.
2.4.2.7 AC line fuse receptacle.
2.4.2.8 INPUT: In parallel with front panel INPUT.
2.5 OPERATION
2.5.1 CONDITIONS WHICH MAY AFFECT THE ACCURACY
OF THE DISTORTION OR VOLTAGE MEASUREMENT:
2.5.1.1 To minimize the effects of line voltage and low frequency noise when the
fundamental is greater than 1kHz, a switch selectable FILTER is provided, which
has a low frequency cutoff at approximately 700Hz, and an attenuation rate of
12dB/octave.
An internal low pass tracking filter eliminates the effects of higher order noise
when the total harmonic distortion being measured is less than 0.1%. The filter
has an attenuation rate of 6dB/octave. Its 3dB cutoff is approximately 10 times
the fundamental frequency. When the distortion level is less than 0.1%, the filter
is automatically switched in.
2-2

Model 6900B Section 2 –Operation
2.5.1.2 The presence of a spike, glitch or transient of the signal, will show up on the
distortion reading. In most cases, any discrepancies in distortion readings
between the Krohn-Hite Distortion Analyzer and another analyzer may be due to
the other analyzer’s inability to detect the spike or transient.
2.5.1.3 Another source of error is the proximity of other electronic equipment to the
Analyzer. When making distortion measurements, do not place or stack other
test equipment on top of the Analyzer. The equipment may generate an electro-
magnetic field of sufficient magnitude to create serious error in the reading at
low distortion levels.
2.5.1.4 Always use the Analyzer with its cover in place and secured. This not only
shields the Analyzer circuit from unwanted interference, but most importantly, it
also prevents injury from ac voltages within the unit.
2.5.2 DISTORTION MEASUREMENTS
The Model 6900B is designed to measure the total harmonic distortion (THD) of a
waveform with a fundamental frequency between 5Hz and 1MHz and amplitude between
100mV and 130 volts rms. The Distortion Analyzer will measure as low as 0.005%,
typically, and up to 19.5%. The THD is displayed directly as a percentage of the
fundamental by the front panel Digit Display Meter (DDM), which incorporates an Auto-
Ranging feature to provide increased resolution at low distortion levels.
The Analyzer is automatically tuned to the fundamental frequency by means of an Auto-Null
network.
2.5.2.1 To measure distortion, proceed as follows:
a. Set the MODE switch to “% DIST” position.
b. Connect the signal to be measured to the front panel BNC labeled INPUT.
There is no need to manually preset a reference level. The 6900B will
automatically preset its input reference level for any amplitude between
100m V and 130 volts rms. If the input amplitude falls below the required
level, the front panel LOW indicator will signal the out-of-range condition.
The input amplitude should then be adjusted until the indicator goes off.
c. The DDM will then display the total harmonic distortion directly in percent.
2.5.3 AC VOLTAGE MEASURMENTS
As an ac voltmeter, the 6900B will measure the rms voltage of an input wave form between
10 millivolts and 130 volts rms over the Analyzers frequency range of 5Hz to 1MHz.
2.5.3.1 To measure ac voltage:
a. Press the “VOLTS” button.
b. Connect the waveform to the front panel BNC labeled INPUT. There is no
manual presetting required.
c. The ac voltage is displayed in volts by the Auto-Ranging Digital Display
Meter.
2.5.4 DISTORTION OUTPUT
As an aid to further analysis, the distortion signal is made available at the front panel
DISTORTION OUTPUT and corresponds to approximately 100mV rms per 1% distortion.
The DISTORTION OUTPUT may be connected to an oscilloscope to create a Lissajous
patter, or may be connected to a Spectrum or Wave Analyzer for measurement of the
individual harmonic amplitudes. 2-3