LCS-1M - A Low-Cost Hobby Oscilloscope
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Power Supply, PC Interface
The connection between the scope and the PC is a standard
RS-232 serial connection. A Maxim MAX232 level converter
converts between the CMOS levels to/from the Microcontroller
and the RS-232 levels to/from the PC, and also adds a layer of
protection between scope and PC. The data is sent at 19200
baud, which according to my tests is about as fast as the
microcontroller can read the data from the memory, meaning
an increase in baud rate does not translate into an increase in
screen update rate.
RS-232 serial connection. A Maxim MAX232 level converter
converts between the CMOS levels to/from the Microcontroller
and the RS-232 levels to/from the PC, and also adds a layer of
protection between scope and PC. The data is sent at 19200
baud, which according to my tests is about as fast as the
microcontroller can read the data from the memory, meaning
an increase in baud rate does not translate into an increase in
screen update rate.
| Click on the image for a larger view |
Note: The latest revision of the scope replaces the MAX232 with a simpler - and lower noise - resistor
interface. More to that in the "Upgrade" section.
As for the power supply, the scope runs from any generic DC power supply capable of supplying at
least 300mA (the scope really only uses about 150mA, but it is better to have at least some
headroom) at a voltage between 9V and 20V. This voltage is regulated down to a very constant 5V with
the venerable (and easy-to-obtain and inexpensive) 7805 linear regulator.
You may wonder why there are two identical regulators in the circuit. The reason is that digital logic
chips (like the microcontroller or the sample logic) cause large spikes in current whenever they are
switching (basically whenever the clock toggles). This in turn produces sudden drops in the supply
voltage. While this is usually of little consequence for the digital circuits as long as it doesn't get
excessive, it impacts the accuracy of the analog circuits in the scope and would introduce noise in the
measured signal.
To avoid this situation, I chose a very common supply arrangement where one regulator supplies all
digital logic, while the second regulator supplies the analog portion of the circuit (op-amps,
digital-to-analog converters supplying offset voltages, analog section of the analog-to-digital
converters). That way compared to my original breadboard version of the scope I was able to reduce
the noise on the analog power supply by about a factor of three, to less than one LSB of the 8-bit
sampler. The analog supply voltage also doubles as reference voltage (against which the signal level
gets measured), which again asks for a very stable supply because any changes of noise would
show up as changes or noise in the measured signal.
Four 100uF electrolytic capacitors (one before each of the regulators, and one after each one) buffer
supply current changes. In addition, a 0.1uF ceramic capacitor close to each IC (distributed over the
rest of the schematic pages) buffers fast current spikes caused by digital switching.
interface. More to that in the "Upgrade" section.
As for the power supply, the scope runs from any generic DC power supply capable of supplying at
least 300mA (the scope really only uses about 150mA, but it is better to have at least some
headroom) at a voltage between 9V and 20V. This voltage is regulated down to a very constant 5V with
the venerable (and easy-to-obtain and inexpensive) 7805 linear regulator.
You may wonder why there are two identical regulators in the circuit. The reason is that digital logic
chips (like the microcontroller or the sample logic) cause large spikes in current whenever they are
switching (basically whenever the clock toggles). This in turn produces sudden drops in the supply
voltage. While this is usually of little consequence for the digital circuits as long as it doesn't get
excessive, it impacts the accuracy of the analog circuits in the scope and would introduce noise in the
measured signal.
To avoid this situation, I chose a very common supply arrangement where one regulator supplies all
digital logic, while the second regulator supplies the analog portion of the circuit (op-amps,
digital-to-analog converters supplying offset voltages, analog section of the analog-to-digital
converters). That way compared to my original breadboard version of the scope I was able to reduce
the noise on the analog power supply by about a factor of three, to less than one LSB of the 8-bit
sampler. The analog supply voltage also doubles as reference voltage (against which the signal level
gets measured), which again asks for a very stable supply because any changes of noise would
show up as changes or noise in the measured signal.
Four 100uF electrolytic capacitors (one before each of the regulators, and one after each one) buffer
supply current changes. In addition, a 0.1uF ceramic capacitor close to each IC (distributed over the
rest of the schematic pages) buffers fast current spikes caused by digital switching.
