New Oscilloscope: Instek GDS-820C 6
If you know me on a more personal basis, then you know that I have been experiencing some issues with several projects/articles in the works for this site. Up until now, I did not have my own oscilloscope. Usually I am able to make due, but recently it has been making things harder than it is worth.
After some research I happened to find a deal on a new Instek GDS-820C DSO. The offer was impossible to refuse and cheaper than the GDS-805C. Besides the obvious increase in sample rate, the GDS-820C comes standard with USB and parallel ports along with the RS-232 port, but GBIP is still only an option. These digital storage options will allow me to post pretty pictures on the blog!
This oscilloscope samples at 100MSamp/sec and is capable of 150mHz operation. They claim an ‘effective’ sampling rate of 25GSamp/sec when viewing repetitive signals. In comparison, Tektronix base model samples at rates up to 1GSamp/sec.
At 150mHz it will be hard to see much if the signals are not repetitive! If you know anything about Shannon’s Sampling Theorem, the effective sampling rate makes 150mHz operation seem… possible. Without this hack, the scope would be limited to half the sampling frequency or 50mHz. I could have paid more for a Tektronix scope, but the hardware would have been limited to 40mHz, mono anyway. This will suit my needs.
Update: If you view a signal sampled at over 100MSamp/sec, then you can tell it is making due with the effective sampling rate. I am not sure if this really matters because I do not have access to signal over a MHz at the moment.
Sound card based signal generators 4
Previously, E-DSP visited the possibility of using your sound card as a signal/function generator. I was curious about the results, but did not have a Windows machine close by to test it. After some searching, I found a Linux alternative and was able to test the limitations of my Sound Blaster Live!
SigGEN: A Linux signal generator
SigGEN is a fairly advanced signal generator designed specifically for Linux. It is able to generate sine, cosine, triangle, sawtooth, square, and pulse waves as well as white noise and frequency sweeps. It also supports separate waves on each stereo channel and setting phase differences between them.
Digital Signal Generator (Windows)
I was also able to find a Windows-based solution more advanced than E-DSP’s. It is able to generate sine, square, and triangle waves as well as chirp signals and noise.
Reproduced wave accuracy
Modern computers have no problem accurately calculating a reproducing small signals. Therefore, any limitations will lie in the sound cards specs. Most modern sound cards have a 16-bit digital to analog converter (DAC), and 24-bit DACs are becoming more popular. Even 16 bits of accuracy is far better than the 10 bits common on most embedded microcontrollers.
The major limiting factor in wave reproduction is the sound cards sampling frequency. It is limited to 48kHz. Hence, any frequencies near or above 24kHz are extremely hard (if not impossible) to reproduce without additional hardware. This limit is rather low and limits the generator usefulness as a high frequency signal generator.
Maximum deliverable voltage
I am sure that different sound cards are capable of delivering different voltages. My Sound Blaster Live! card was able to deliver four volts as seen above.
Maximum current and power
A sound card is also limited in the amount of power it can deliver. Once a voltage source reaches its power limit, it begins acting as a constant current source. It simply delivers less voltage to compensate for the limited current.
In order to test the sound card’s limit, a potentiometer is connected between the generated signal and ground. The resistance is then decreased until the current peaks and the voltage starts to decrease.
Once we find the point of maximum power, the potentiometer’s resistance is measured, and Ohm’s Law is used to find the current delivered. In this case, the resistance is 24.8K ohms. V = I*R tells us 0.16mA is being delivered. P = V*I = 0.6mW maximum power.
This is not a lot of current or power. This is why E-DSP reccommends an amplifier.
Project photo gallery
Sound cards as Signal Generators 3
Function generators are a huge investment. Even the simplest ones
Unfortunately, their software signal generator can only generate two functions, a sine wave or a square wave. Don’t forget your sound card is limited to 16 bits and 24khz. Therefore its reproduction of sine waves is not exactly perfect. Square waves should be reproduced near perfectly, though.
Also note that most PCI based sound cards have a limit of 2V, but you can always amplify this signal.
This has really sparked my interest so you can expect a Linux software signal generator later today!
Cheap workbench power supply
Recently, I’ve been scouring the web for a cheap power supply. Building my own was definitely a consideration, but I never considered ripping one straight from a computer. Even my rusty old 386dx can supply ±5V, ±12V, and GND.
My journey led me to eHow’s wiki page that describes how to convert an ATX power supply into a workbench power supply. The instructions are extremely thorough. Unfortunately there is no 3.3V tap, but you can figure out how to add that on your own!
There is a similar conversion that explains the engineering and need for a power resistor in greater detail. Basically, the “cheap” circuitry in the ATX supply requires some (significant) current in order to “turn on.”