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Build a VGA digital oscilloscope How would you like a digital scope with a large screen? This one is based on a VGA monitor and displays two channels, one with a red trace and one with a green trace. At the same time, there is an electronically generated blue screen graticule to make measurements easy. PART 1: By JOHN CLARKE Many readers would like to have an oscilloscope but can’t quite raise the $1000 or so you need for a fairly basic scope these days. However, many readers do have a spare VGA monitor and this can be pressed into useful service with our new VGA Oscilloscope. Apart 26  Silicon Chip from the need for a VGA monitor, it’s a self-contained unit that doesn’t tie up your computer. No modifications are required to the VGA monitor itself – it just plugs into the VGA output on the test instrument which we’ll call the Scope Adaptor for convenience. When you turn both the VGA monitor and the Scope Adaptor on, the screen displays a large blue graticule with seven divisions horizontally and eight verti­ cally. Two traces are also displayed, one red and one green, for the two channel inputs. The Scope Adaptor has knobs for vertical sensitivity & trace position on both channels, plus timebase and trigger level controls. There are also toggle switches for trigger source selection, AC/DC cou­pling, timebase magnification and a few others which we will discuss later. Most importantly, the Scope Adaptor has two BNC sockets for the two channel inputs. These will accept normal 1:1 and 10:1 scope probes. Specifications Bandwidth ......................................useful up to 100kHz Timebase .......................................0.1s to 50µs per division in 11 ranges Sensitivity .......................................10V to 50mV per division in 8 ranges Resolution ......................................8-bit or 256 steps with 224 visible Linearity .........................................±1 LSB Calibration Accuracy ���������������������vertical <5%; horizontal <10% (50µs position uncalibrated) Input impedance .............................1MΩ This photo shows the unit displaying a sinewave in one channel and a square wave in the other. Timebase speeds range from 100ms to 50µs/ division. Using the VGA Oscilloscope is just like using any other scope. First, you connect the scope probes to the circuit to be measured and then adjust the vertical sensitivity controls on both channels to fill the screen. This done, you adjust the timebase control to give a reasonable number of signal cycles on the screen. Finally, you adjust the vertical position controls so that the two traces are comfortably separated (or overlapping if you wish). To obtain a stable display, you will probably need to adjust the trigger level control and also select positive or negative edge triggering with the Slope switch. Or you can select between a triggered or free-running display. As we said above, driving the VGA Oscilloscope is little different from any other scope – up to a point. Where the new VGA Oscilloscope does differ is that it also offers waveform storage, just like a digital storage oscilloscope – and that is exactly what it is. It works in much the same way as typical modern digital instruments such as the Hewlett-Packard HP 54600 series scopes or the Tektronix TDS 300 series. It con­verts the incoming analog signals into digital data and then stores them in RAM. The digital data is then processed in such a way as to generate a raster display on the VGA monitor. Mind you, this VGA Oscilloscope does not have the extremely wide bandwidth of the commercial oscilloscopes mentioned above; nor does it have their price. However, it can be used to monitor signals up to 100kHz, making it a useful instrument for lots of applications. And unlike the commercial scopes, it does have that large VGA screen. To top it off, the display is in full colour! Few commercial scopes can boast a colour screen. Some normal oscilloscope controls are not provided on the Scope Adaptor. No brightness controls are provided since these are on the VGA monitor. Focus is unnecessary since the trace thickness is set by the circuitry. A MAGnification control expands the trace out by either a factor of two or four. This feature can be useful for high fre­quency signals above 20kHz where it is difficult to see each waveform cycle in the x1 magnification. The screen is redrawn at a rate set by the UPDATE switch. The normal setting redraws the trace every second and this is seen on the screen as a momentary trace blanking. The other two positions of the switch are slow and fast. These are provided for low frequency signals and for displaying real time audio signals (music, speech, etc) respectively. Waveform storage A very useful feature of the VGA Oscilloscope is its ability to store a waveform and then display it indefinitely. This ena­ bles viewing of waveforms which cannot be readily seen on a normal oscilloscope. With the storage facility, you can capture momentary pulses in a circuit and view them at your leisure. As noted above, the VGA Oscilloscope does not tie up your computer Features • VGA display (no computer required) • • • Dual trace • Timebase magnification (x2, x4) • • • Free run and triggered display • • • Trigger level control 7 x 8 graticule Calibrated timebase and volts/ division Storage facility Triggering on + or - slope and Channel 1 or Channel 2 Vertical position for each trace AC/DC/GND input coupling July 1996  27 Fig.1: this block diagram shows the various signal processes inside the VGA Oscilloscope. since it directly drives the VGA monitor. It oper­ates best on a multisync monitor. With a standard VGA monitor, the extreme right hand graticule line may not displayed. This is, however, of little consequence. Block diagram Fig.1 shows the block diagram for the VGA Oscilloscope. In essence, signals for Channel 1 run along the top of the diagram while signals for Channel 2 run along the bottom of the diagram. Let’s talk about Channel 1, on the assumption that all operations will be duplicated in Channel 2. Channel 1 input signals are first passed into a switchable attenuator and amplifier (S1, S2, Q1, IC1 & IC2) which sets the amplitude to suit the following circuitry. The signals are then passed to an analog-to-digital (A-D) converter (IC3) which produc28  Silicon Chip es 8-bit data which is then stored in memory. Initially, the A-D conversion operation is triggered either by the free run oscillator which periodically retriggers the oscilloscope or by a trigger signal from CH1 or CH2. After each A-D conversion the new digital value is stored in the next memory address. The A-D conversion rate and memory address is under the control of the timebase oscillator (S5, 1C13, IC14, IC15) which clocks the 8-bit counter via the record switch in IC16. This counter increments the memory address. 256 memory locations are used to store all the data for one screen display. When all memory locations have been filled, the end of count signal (IC17, IC18) changes the chip select, read/ write block to switch the memory to read mode. It also deselects the A-D converter and switches IC16. During this conversion time the display trace is blanked. Note that the timebase oscillator frequency sets the rate of A-D conversions. At fast rates, high frequencies can be observed, while at slow conversion rates low frequency signals are accurately traced. If we want to store and view one complete cycle of the input waveform, the timebase must operate 256 times faster than the input frequency. If the timebase is slower than this then more cycles will be seen. Conversely, if the timebase is faster, then only a portion of the full waveform will be observed. When the memory is in the read mode, the 8-bit counter is clocked from the oscillator and line counter of the VGA timebase circuit. The display/ record switch, IC16, performs this function. This clocking rate is exactly what is required for the memory contents to be displayed on the screen. Screen display In order to understand how the information stored in memory is displayed on the VGA screen, let us look at Fig.2. The display on a VGA monitor is made up of 480 horizontal lines which are scanned by the red, green and blue electron beams. A dot will appear each time one of the beams is is turned on for an in­stant. The respective beams are turned on by the R, G or B signals on the VGA connector. For simplicity, Fig.2 only shows 11 horizontal scan lines instead of 480 but you get the general picture. The position of any dot on the screen is dependent upon how long after the line sync pulse the red, green or blue gun is turned on and on what line is being scanned at the time. Each horizontal line begins with a sync pulse and an entire set of lines is preceded by a frame sync pulse. This must be of sufficient duration for the electron beam to return from the bottom of the screen to the beginning of line 1. The time to scan one line is 32µs and to scan all 480 lines is 16.6ms. Thus, the horizontal scanning frequency is 31.25kHz and the frame rate is 60Hz. This last frequency is called the refresh rate. Making a picture So how do we get the dots on the screen in order to make a picture which means something? We have already stated that we have 256 memory locations which are scanned for each line of the screen. Each of these memory locations is 8-bits and therefore we can have a value stored in each location which ranges from 00000000 to 11111111; ie, 256 values. Let us consider that the top of the screen corresponds to 11111111 and the bottom of the screen is 00000000. So as each line is scanned by the beam of the VGA monitor, simultaneously the 256 memory locations are being scanned. Now imagine that the top line is being scanned and we come to memory location 6 (actually address 00000101 when scanned from left to right) and the value stored just happens to be 11111111. Yippee, we get a dot on the screen which corresponds to that memory location (or address). Next, consider line 2 and we scan across to memory location 5 and the value stored just happens to be 11111110. Again, we get a dot at that Fig.2: this diagram demonstrates the process of writing dots to the screen. There are 256 bytes of memory for each channel and each of the 256 lines on the screen corresponds to one of the possible values stored in each memory location. position. Further, as we scan further across the same line we come to location 7 and the value is also 11111110. Again, we got a dot on the screen. Now we could go on and on with this process and talk about all 256 lines and 256 memory locations but you should be starting to get the picture. This is shown in abbreviated form in Fig.2. This shows only 11 lines and 21 memory locations but the princi­ple is the same: if the value stored in a memory location corre­sponds with the line value we get a dot on the screen. That’s the principle but how is it done? Two magnitude comparators, IC5 & IC6, actually compare the data from each memory location with the line being scanned and its screen value; ie, the screen’s 8-bit address which comes from the line counter. If the line value equals the data value then a short pulse is produced from the output of the comparator and this is applied to the buffer (Q3, Q4 & Q5) which drives the green gun, for the Channel 1 trace. Exactly the same process occurs for Channel 2 input signals except that they are stored in another 256 byte (8July 1996  29 Most of the componentry inside the VGA Oscilloscope adaptor is readily available. The circuitry is mounted on three PC boards, with two small satellite boards used to accommodate the RAM chips. bit) memory. As the Channel 2 memory is clocked out, its values are compared by magnitude comparators IC11 and IC12 and dot signals are generat­ ed for the red gun, corresponding to the Channel 2 trace. To recap, the analog input signals are converted to digital data and clock­ ed into memory at a rate which is set by the timebase switch. The data is then read out of memory at a fixed rate, to suit the requirements of the VGA monitor. The remainder of Fig.1 is devoted to the generation of VGA timebase signals (ie, line and frame sync pulses) and the grati­cule signal which drives the blue gun. Next month The VGA Oscilloscope adaptor has most of the controls you would expect to find on a conventional oscilloscope. Vertical input sensitivity ranges from 50mV/div to 10V/div. 30  Silicon Chip So far, we’ve given you the overall picture of how the VGA Oscilloscope works. The circuit details are just a teensy bit more complicated, as you might expect. We will discuss these next month and also publish the SC parts list.