Dr.-Ing. Artur Seibt Handbook of Oscilloscope Technology Circuitry – Accessories - Measurement – Selection Criteria - Service 2nd extended and updated edition 2006 2 Preface to the second edition 2005. After ten years from its first edition this handbook needed updating. HAMEG Instruments GmbH, a major manufacturer of both Combiscopes and analog scopes, a subsidiary of Rohde & Schwarz, sponsored this second edition. Repetitions are intentional, because this book is also intended for use as a reference to specific topics. The chapters about basics, analog circuitry, calibration, service and repair hints required only minor changes and additions; they remain important as there is a multitude of analog scopes in use, also many measurement tasks can not be fulfilled with any scope out of current production (e.g. 10 uV/cm). As there were few new accessories, the important chapter 10 is as valid as ever. Due to the predominance of DSOs meanwhile, the pertinent chapter 6 was considerably extended. As socalled Combiscopes were and remain the optimum choice, they were given extensive treatment in a new chapter 7 with special consideration to the HAMEG 1508. A greater selection of screen photos further assists the interested reader in his difficult task of making his choice Vienna, March 2006. 3 Table of Contents. 1. Oscilloscope families. 1.1 Introduction and principle. 1.2 Integrated and plug-in instruments 2. Oscilloscope displays 2.1 Cathode ray tubes (crt’s) 2.1.1 Electron optics basics 2.1.2 Beam generation and forming 2.1.3 Beam control, unblanking 2.1.4 Electrostatic deflection 2.1.5 Segmented deflection plates 2.1.6 Magnetic deflection 2.1.7 Methods of acceleration 2.1.8 Display distortions 2.1.9 Focus distortions 2.1.10 Types of phosphors 2.1.11 Writing rate 2.1.12 Graticule 2.1.13 Light filters. 2.2 Operation of cathode ray tubes 2.2.1 Generation of high voltage 2.2.2 Generation of auxiliary voltages 2.2.3 Unblanking 2.2.4 External interferences 2.2.5 Handling of crt’s 2.3 Storage cathode ray tubes. 2.3.1 Basics of storage 2.3.2 Bistable storage crt’s 2.3.3 Transmission storage crt’s 4 2.3.4 Transfer storage crt’s 2.3.5 Scan converter crt’s 2.4 Special cathode ray tubes. 2.4.1 Microchannel secondary electron multiplier crt 3. Analog oscilloscopes. 3.1 Block diagram of integrated instruments 3.2 Block diagrams of plug-in instruments. 3.3 Vertical channel. 3.3.1 Requirements 3.3.2 Properties of active components 3.3.3 Basic circuits 3.3.4 Properties of passive components 3.3.5 Input attenuators 3.3.6 Preamplifiers and their operating modes 3.3.7 Trigger signal take-off 3.3.8 Delay lines 3.3.9 Output stages 3.3.10 Adjustment of vertical amplifiers 3.4 Horizontal channel 3.4.1 Block diagram and operating modes 3.4.2 Interface to the vertical channel 3.4.3 Trigger circuits 3.4.4 Sawtooth generators 3.4.5 Horizontal output amplifiers 3.5 Additional features 3.5.1 Calibrators 3.5.2 Readout 3.5.3 t, Delta t, V, Delta V, f etc. 3.5.4 Auxiliary inputs and outputs 5 3.6 Power supplies 3.6.1 Linear regulators 3.6.2 SMPS 3.7 Operation and control 3.7.1 Direct operation 3.7.2 Indirect operation 3.7.3 Remote control 4. Analog storage oscilloscopes 4.1 General remarks 4.2 Bistable storage oscilloscopes 4.3 Transmission storage oscilloscopes 4.4. Transfer storage oscilloscopes 5. Sampling Oscilloscopes. 5.1 History. 5.2 Application of Sampling Oscilloscopes (SO’s) and DSO’s 5.2.1 Common characteristics. 5.2.2 SO’s 5.2.3 DSO’s 5.3 Basics of sampling 5.3.1 Common characteristics of the sampling methods 5.3.2 Real Time Sampling /RTS) 5.3.3 Single Event Sampling 5.3.4 Equivalent Time Sampling (ETS) 5.3.5 Random Sampling (RS) 5.3.6 False displays. 5.4 Vertical Channel 5.4.1 Sampling gate and sampling pulse generation 6 5.4.2 Sampling probes 5.4.3 Sampling gates with termination; „sampling heads“. 5.4.4 Sampling gates with trigger take-off, delay line with termination. 5.4.5 Feed-through samplers, reflectometer. 5.4.6 Hints for troubleshooting and adjustments. 5.4.7 Output stage. 5.5 Horizontal Channel. 5.5.1 Real Time Sampling (RTS) 5.5.2 Special circuits 5.5.3 Equivalent Time Sampling 5.5.4 Random Sampling (RS) 5.5.5 Hints for troubleshooting and adjustments Kapitel 6, zu überarbeiten und zu ergänzen, folgt Kapitel 7 wird gänzlich neu „Combiscopes“ mit dem Hameg 1508 als zentralem Gegenstand. Die Kapitel 8 .. 12 werden noch überarbeitet und folgen. 7 1. Oscilloscope families. 1.1 Introduction and Principle. The instruments covered here are called oscillo“scopes“ although they should rather be called oscillo“graphs“ because they do not „see“ but „write“ waveforms on the screen; however, this expression is standardized. Early oscilloscopes were by far no measuring instruments, waveforms could only be observed qualitatively. 1947 saw the birth of the oscilloscope as we know it today when two engineers (with some partners who later left) founded Tektronix Inc. in Portland OR, USA. This company presented the first calibrated oscilloscope, type 511 (10 MHz, 0.25 V/cm, 0.1 us/cm, $ 795, 50 lbs.). Already this first instrument contained an impressive number of achievements and innovations in circuitry to be found in every oscilloscope to this date witnessing a profound understanding of the fundamentals of electronics. The most important innovations were: - The principle of the enforced operating point: At that time there were only electron tubes available. Tubes are unequalled for linear low distortion amplification, especially, as they do not change their characteristics when driven and because they are immune to temperature. However, they age, also their characteristics depend on the heater voltage. The amplification of all active elements depends on the current; if the current is held constant the amplification will remain constant. This is the prerequisite for a calibrated instrument. Tektronix introduced the principle of enforced operating current into all stages which influence the calibration either by using current generators or approximating those by a large resistor returned to a large voltage. - Introduction of the difference amplifier. Making use of the principle of enforced operating current causes the complete loss of amplification in simple stages. Only by using difference amplifiers is it possible to keep the operating currents stable while retaining amplification. This is only one reason for its introduction, far and above the difference amplifier and especially its extended version as a cascode is the only dc-coupled wide band amplifier worth that designation. - Regulated power supplies. 8 - Perfect pulse response. Correct measurement of nonsinusoidal waveforms requires perfect pulse response - Triggered, calibrated time base. Former instruments had to be synchronized with the measuring signal in order to obtain a stable display, hence the time base could not be calibrated. The value of this new instrument was immediately recognized, the company grew enormously, its leadership was often challenged but rarely with success, and when only temporarily. Modern electronics is inconceivable without the modern oscilloscope, it remains hence its most important measuring instrument. Since then the oscilloscope remained the domain of American companies markedly proving their superiority in electronics. While later there appeared some Japanese instruments of partly impressive quality European firms never played any significant role regarding top performance instruments. The most important European company, a part of Philips, was sold to Fluke in 93. There is no room to describe the whole oscilloscope history. 1954 Tektronix introduced the first plug-in oscilloscope, the series 530. 1957 the 540 series followed (30 MHz), the work horse of the next decades. Both series used distributed tube amplifiers: input and output capacitances were built into associated delay lines eliminating them practically. The bandwidth of the amplifier, however, was not extended to the bandwidth of the delay lines because these do not exhibit Gaussian behaviour. The amplifications of the individual stages of a distributed amplifier only add as their currents add up at the output impedance. The delay line consisted of appr. 30 elements with an equal number of trim capacitors the alignment of which was more of an art. 1959 the first scope with a higher bandwidth (85 MHz, 0.1 V/cm, 10 ns/cm) appeared, the 581/5, still using distributed tube amplifiers throughout the vertical except for a few transistors at the input stage and also tubes in the other sections except for a tunnel diode/transistor trigger circuit. The crt had distributed deflection plates, the delay line was already a special cable. The aforementioned instruments although nearly completely equipped with tubes presented an extraordinarily high standard of quality, not surpassed to date, hardly touched. In order to reach certain performance levels the tubes in some stages (e.g. horizontal output) had to be heavily overstressed, causing tubes to fail after some time. So their ability to carry heavy 9 overloads for extended periods of time - in contrast to all semiconductors – gave rise to the unjustified opinion that tubes were less reliable than semiconductors. The first fully transistorized (except for input nuvistors) oscilloscope was the type 766 (25 MHz) by Fairchild/Dumont, however, of poor performance. 1962 Tektronix presented their first fully transistorized (except for nuvistors in the vertical, trigger inputs and the time bases) 647 (50 MHz, 10 mV/cm, 10 ns/cm) oscilloscope with no compromises in performance, a top product with some specifications never again achieved to this date like a working temperature spec of – 30 to + 65 degrees C, no fan.