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CHAPTER 6.1 The Oscilloscope • • • • • • • • The measuring systems discussed so far – Respond to the amplitude of the input – produce a numerical output proportional to the input • Analog on a scale readout; or • Digital on a numeric readout – do not give any time variation information about the input signal – Are limited in the frequency of the input signal to which they can respond To observe signal variation with time as well as amplitude another type of instrument is needed: – the cathode ray oscilloscope (CRO) The CRO displays a voltage vs. time waveform on the screen of a cathode ray tube (CRT) hence the name CRO. The CRO has the following main subsystems: – the display sub system - the CRT – the vertical deflection sub system – the horizontal subsystem – the trigger subsystem – the probes We will examine all of these in detail The CRT uses a beam of highly focussed electrons to produce an image on the CRT face Electrons have no weight and can therefore respond quickly to changes in input – high frequency response – > 1 GHz is available The CRT has 3 elements – the electron gun – the deflection system – the screen •Refer to Figure 8-2 in handout Observe the electron gun assembly • The gun consists of – a thermionic cathode; and • The cathode produces electrons when heated – accelerating and control electrodes • There are two controls – focus control – intensity control EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 1 © 2001 • • • • • • • • • • • The intensity control grid surrounds the cathode A small -ve voltage controls the number of electrons passing through the small hole – the INTENSITY control regulates this voltage A set of accelerating and focussing grids (Anodes) – accelerate the beam, ie increase the beam energy – focus the beam The anodes have voltages of 2 to 12kV – the FOCUS control is connected to one of them • the focus grid The deflection system consists of 2 sets of deflection plates – the vertical deflection plates • controls the y-axis movement – the horizontal deflection plates • controls the x-axis It is possible to deflect the beam in 2 ways – electrostatically • using electric charge – electromagnetically • using magnetic fields The CRO uses electrostatic deflection – smaller deflection angles are needed • note shape of CRT – higher frequency of operation In contrast TV uses electromagnetic deflection – The deflection yoke Figure 8-3 shows the screen end of the CRT. The voltages applied to the plates move the electron beam from the centre of the screen. The deflection D in cm from the centre of the CRT is given by: D • • • Ll Vd 2d Va Vd is the deflection voltage Va is the accelerating voltage The deflection sensitivity of the CRT is the deflection voltage needed to produce a particular deflection Vd /D – typical vertical sensitivity 10-20V/cm EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 2 © 2001 • • Recall from your labs that the vertical inputs can be as low as 20mV Hence the need for vertical and horizontal amplifiers to create these values of deflection voltage SCREEN • The inside of the CRT face (SCREEN) is coated with a PHOSPHOR • The phosphor is a compound which emits light when struck by high energy electrons – called FLUORESCENCE • When the electron beam is shut off, light is emitted for a short period after – called PHOSPHORESCENCE • The length of time the phosphor glows after being bombarded is called PERSISTENCE • Manufacturers vary the persistence of the phosphors to achieve certain levels of performance – shorter persistence is preferred for faster signal displays. • The electron beam creates a TRACE as it moves across the CRT face • The beam is shut off (BLANKING) and returned to the original position • the beam is then RETRACED • If the trace and retrace locations are identical, persistence will cause the display to appear stationary • HEAT is generated when the beam strikes the phosphor – only 10% electron energy converted to light • If intensity is high the screen could be burnt – Keep INTENSITY control low • The CRT screen has a grid etched on it – the GRATICULE – allows measurements to be taken – there is sometimes a SCALE ILLUMINATION control EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 3 © 2001 CHAPTER 6.2 Scope Subsystems Note: This handout is to be read in conjunction with the handout. Vertical Subsystem • Measurand is input here – via a PROBE • Can accept AC or DC signals • Contains an attenuator – set by the volts/division control – scales the input signal – analogous to the range selector on voltmeter – 2mV/div to 20V/div typical • Has the vertical amplifier • Provides proper signal levels to drive the vertical deflection plates Has the following controls: • Input coupling control – selects AC or DC coupling • AC coupling – capacitor inserted before attenuator – blocks DC, passes AC • DC coupling – bypasses capacitor – allows both AC and DC to be measured • GND – Disconnects source – grounds input amplifier – used to position trace on graticule • 50 – Used for impedance matching – puts accurate 50 load to ground • Vertical position – used to position the trace on the screen – applies a DC voltage to the vertical deflection plates Horizontal Subsystem • Generates the horizontal/time base signal – a sawtooth EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 4 © 2001 – This signal goes to the horizontal amplifier • • • Causes electron beam to sweep horizontally across CRT face Horizontal position of beam is proportional to elapsed time since start of sweep – Horizontal axis calibrated in units of time Horizontal amplifier provides proper signal levels to drive the horizontal deflection plates Has the following controls • Time Base – Sets the beam sweep rate – units are sec/div • Horizontal Position – Shifts the display along the x-axis • Horizontal Magnifier – Increases the resolution of x-axis Trigger Subsystem • Recall the CRT is displaying a trace of a vertical input (amplitude) with time • To produce a stable, usable display both vertical and horizontal sweeps must be synchronised • The Trigger section is responsible for this • It uses either the vertical input or an external signal to develop the trigger pulse • Pulse sent to horizontal section to initialise the sweep. Trigger controls • The LEVEL control – Used to select a specific point on either the rising or falling edge of the input signal that will be used for generating a trigger. – Useful in applications where the input signal may be corrupted by noise. – The LEVEL control selects a portion of the input signal that is not corrupted by noise for use as the trigger input. • The SLOPE control – determines which edge: rising or falling, of the input signal will be used for generating the trigger. • The MODE is a multiple position selector • AUTO – Selects an internal oscillator that will trigger a sweep in the absence of an external signal. – Allows a baseline trace to be established before applying an input. – Without the AUTO trigger a trace would not be produced on the screen. EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 5 © 2001 • • NORMAL ( or TRIGGERED) In this position the trigger is generated from one of 3 sources set by the SOURCE control – INTERNAL - based on the input signal – EXTERNAL - supplied by an external system for example a clock circuit • Used extensively in digital systems – AC LINE - derived from the AC power line frequency (60Hz locally) CHAPTER 6.3 Scope Probes • • The probe performs the following functions: – sensing the input signal; – transferring the value to the inputs The CRO probe may be modelled as follows: PROBE HEAD COAXIAL CABLE Contains sensing circuitry Passive Probes resistors, capacitors Active Probes powered devices: op-amps, FETs , transistors • • • TERMINATION An impedance, typical value 50 Inexpensive probes not terminated Probe head – Contains sensing circuitry – Passive • resistors, capacitors – Active • powered devices: op-amps, FETs , transistors Coaxial cable – A conductor with an external shield • Prevents noise pickup Termination – An impedance that may or may not be present EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 6 © 2001 – – – – Transmission lines have a characteristic impedance Maximum power transfer occurs when line is terminated by this impedance Typical value 50 Inexpensive probes not terminated CRO LOADING • The CRO is basically a voltmeter • It can therefore load a circuit like the typical voltmeter • We can model the input of the CRO as a resistance in parallel with a capacitance – Rin is typically 1M – Cin is typically 30 - 50 pF Typical scope input Vin • • • • Rin Cin At DC to low frequencies, the input sees mostly Rin As frequency increases the impedance of the Rin||Cin combination decreases – The loading effect thus increases with increasing frequency We can minimise the loading by using a compensated attenuating probe Consider the following: 9Rin PROBE Rin Cin SCOPE • • Here a probe of input resistance 9Rin has been connected to the CRO Therefore as far as a DC signal is concerned the Zin is 10Rin EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 7 © 2001 • • • V is however Vin/10 – it has been attenuated by a factor of 10 The degradation due to Cin still exists If the probe had an input capacitance as shown in the following modification: Cin/9 9Rin Rin Cin Figure 1 C1 Figure 2 • • Vin C2 V 2 The capacitors in Figure 1 are arranged as shown in Figure 2 Therefore: V2 C1 V1 C1 C2 If C1 C2 ; then 9 V2 1 V1 10 • • • • Note that this result is independent of f For an AC signal, V is again scaled by a factor of 10 – Total probe impedance 10Rin||Cin/10 This probe is called a 10:1 compensated attenuating probe – Compensated because it has adjusted for Rin and Cin – Attenuating because it scales the input signal down by a factor (of 10 in this case) The final commercial model has one final modification: EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 8 © 2001 Cin/9 9Rin Rin CC • • • • Cin The probe capacitance is adjustable. The coaxial cable used for the probe lead has some capacitance Cc we calculate CT the total capacitance as: We adjust the probe C1 to be CT/9 Again the total input capacitance is CT/10 CHAPTER 6.4 DUAL CHANNEL SCOPES NOTE: This handout is to be read in conjunction with the handout on oscilloscopes • • • The majority of lab scopes are dual channel – occasionally even more channels are provided This is accomplished in 2 ways – Using dual beams; or – Using dual traces We will examine both approaches DUAL BEAM CRO • Dual beam systems – uses 2 separate beams in CRT – separate vertical systems • one for each beam – may have separate or common time base systems – High performance – Very high cost DUAL TRACE CRO • Single beam system EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 9 © 2001 – Uses electronic switching to create the dual display • Has 2 additional controls – CHOP – ALT(ernate) A block diagram of the switching operation is as follows: Vert 1 Mux Vert Amp Vert 2 • • • • • • • • • • • CHOP – Rapidly switches between both channels – both channels appear to occur together – Preferred for slow signals • CHOP rate must be faster than the highest signal frequency ALT mode – Short for ALTERNATE Shows one complete sweep for 1st channel Switches to show one complete sweep for other channel Preferred for viewing high frequency signal If CHOP were used for a high frequency signal a series of dots would be seen – analogous to a sampled signal Dual trace CROs can – add signals – subtract signals – display an x-y plot X-Y plots – Time base is set to off One signal channel is mapped to the x-axis Other signal is mapped to the y-axis A common application of this is the Lissajous figure – Formed when sinusoidal signals drive both channels – Used for measuring phase differences in signals EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 10 © 2001 CHAPTER 6.5 STORAGE OSCILLOSCOPES • • • • • • • • A storage scope captures a waveform for later retrieval and analysis – One off events like clock or logic glitches – very slow moving signals Two types (like with meters) – Analog – Digital Analog Types – Variable persistence – Bistable storage Both types of scope ‘store’ the waveform in the CRT – An image of the waveform is created on the screen ( as normal) – The image is retained for a period of time depending on the mechanism used Both storage types require a CRT with 2 sets of electron guns: The WRITE gun – the standard electron gun discussed before The FLOOD guns – A set of low energy electron guns – produce collimated (parallel) low energy electrons – used to read the stored image Analog storage relies on secondary emission of electrons Variable Persistence Storage • Write gun and screen similar to conventional CRO • The CRT is constructed with – a collector mesh – a storage mesh coated with a dielectric material. – both being located behind the phosphor screen • See figure 8-26 in handout • Storage mesh is slightly -ve (-10V) • Collector mesh at +100V • High energy electrons from WRITE gun passes through collector and storage meshes • Secondary emission of electrons occurs on storage mesh • Small +ve charges therefore remain on the mesh where beam passed • The +ve region marks where the beam passed EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 11 © 2001 – an ‘image’ of the original beam is retained • The collector mesh collects the secondary electrons. Recalling Waveform • Flood gun produces electrons – low energy – wide parallel beam • Collector mesh attracts the electrons – provides slight acceleration to storage mesh • -ve voltage on storage mesh repels electrons except where +ve charges remain. – recall that these were created during the write cycle • Electrons pass through these points • High +ve potential of screen (20kV) accelerates these electrons – high energy electrons impact phosphor – Fluorescence and Phosphorescence occur – stored image is displayed • Image slowly fades as +ve regions in storage mesh are neutralized • Variable Persistence Scopes are useful for observing slow periodic signals Bistable Storage • See Figure 8-27 in handout • Does not use a storage mesh • Uses a special phosphor that has 2 stable states • Recall that phosphor glows when impacted by high energy electrons • During the write stage – energetic electrons pass through phosphor • image seen as before • After Phosphorescence stage phosphor does not return to original energy state – Returns to an intermediate second state – Secondary emission leaves a small +ve charge in Phosphor • Flood guns create a low energy collimated beam as before – electron energy is too low to cause primary emission, but high enough to cause phosphor to glow when they pass through the small +ve regions. – These electrons are collected by metallic film EE11A Handouts Chapter 6 Prepared by: Mr. Fasil Muddeen 12 © 2001