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Showing posts from May, 2010

Poor student's oscilloscope

Enterprising undergrad engineering students can achieve a lot if given the right set of tools. Once you level up to designing some complex circuits, a signal generator and an oscilloscope would definitely go a long way in helping you in debugging your projects. I bought my first oscilloscope ( Acute DS1202 200MHz PC based) for INR 37,500 after accumulating my salary for 4 months at my first job. But I always wished I has an access to an oscilloscope as a student. It seems that now days there is a low cost option for students to use: the PCs sound card. Most modern motherboards have an onboard Audio Codec Chip which is used to accept and output sound, you can find atleast 3 audio jacks labeled Microphone, Line In and Line Out on the back side of your CPU Cabinet. You can then use specialized freeware/shareware applications which can turn your PC into an Oscilloscope/Signal Generator. Here are screen shots of some of these: Oscilloscope 2.51 (aka Winscope) - the captured wavefor

GSM 217Hz TDMA noise

Bring a GSM cellphone near some low grade audio equipment and you can hear some very irritating noises. GSM or Global System for Mobiles uses FDMA, TDMA and SDMA for providing network access to multiple users. This each achieves as follows: Base Transceiver Stations within Cells (a fundamental unit of geographical areacovered by the GSM service provider) surrounding a given cell will all use frequencies different from each other (Frequency Division Multiple Access). The maximum amount of power transmitted by BTS as well as mobile units within a given cell is limited and so cells separated by a considerable geographical area can both reuse the same frequency for transmission (Space Division Multiple Access). And finally within a given cell, mobile units will use Time Division Multiple Access so that man users can be served by the same Base Transceiver Station. Of course different frequencies are used for uplink and downlink. Depending on the country, GSM operates in 800MHz to 900MHz or

RF Frequency Modulation

Visha Electronics carries a very low cost FM transmitter kit based around BF494 NPN Bipolar Junction Transistor. The kit has a condenser microphone to pickup sound. A variable capacitor is also present to tune the transmission frequency anywhere in the 88-108 MHz range. The transmitted signals can be picked up by any stand radio receiver. A simple receiver circuit can also be built, like the one shown here . Please visit for details on more such kits. Students are invited for workshops/courses at our LARE Learning Center for an opportunity to assemble and experiment with such kits.

Serial Peripheral Interface

SPI is a serial link standard used between chips. Now days most microcontrollers have an on board SPI block. SPI link is made of atleast one master and one slave. The link is composed of four wires: SCLK - Serial Clock, always provided by the Master MOSI - Master Out Slave In, Data (from master to slave) MISO - Master In Slave Out, Data (from slave to master, clock still has to be provided by master) nSS - Slave Select, usually the Chip Select signal for the slave. Sensors, ADC/DAC, memory and many other devices had slave SPI interfaces. Some examples of SPI compatible ICs/Devices:- Accelerometers (ADIS16201) 4-20mA current control (AD420) Flash Memory (EN25T80) Nokia 6610 Colour Graphical STN LCD (PCF8833) Nokia 3310 Monochrome Graphical LCD (PCD8544) EEPROM (25AA640) SD Card 10BASET Ethernet Controller (ENC28J60) Many microcontrollers - like AT89S52 and all the AVR devices available from Atmel - are programmed via SPI. Here is a screen shot of SPI signals captured

Binary Phase Shift Keying

Here are some photos and waveforms from a PSK generation lab practicals. All undergraduate students pursuing engineering in electronics and its allied fields get a chance to do it at least once during their four years. The process of PSK generation in a lab is as follows: Generate a sinusoidal carrier wave from a signal generator. Call it Signal A Use an operational amplifier to shift the phase of Signal A by a fixed amount : usually 90 degrees in case of Binay Phase Shift Keying. Call this Signal B. Apply Signal A and Signal B to the two inputs of a 2:1 analog multiplexer. Apply the data signal to the select line of the analog switch. The output of the analog switch will be the required PSK signal. The data signal is a simple digital signal with two states 1 and 0 i.e. 0 and +5V. This can be emulated using a 555 Timer wired to generate a simple square wave. The analog switch will select Signal A or Signal B depending on the logic level of this data signal thereby outputting a sine wav

Highway Addressable Remote Transducer Protocol

HART is a digital industrial automation protocol which has been in use for quite some time now. It uses " Continuous Phase " Frequency Shift Keying to transmit data over the same wires (a pair of wires - one for each direction of flow of current) which had been used for 4-20 mA current loop signaling. bit 1 is represented by a sinusoid at 1200 Hz and bit 0 is represented by a sinusoid at 2200 Hz. The signal has an amplitude of 1mA peak to peak (current signaling): Top: HART Signal Below: Data Transmitted The HART Packet consists of a sequence of fields each consisting of varying number of bytes: Various fields of HART Packet sent from Master to Slave Each byte is it self a standard UART frame sent out at 1200 bps in 8-O-1 format. So a PC equipped with a serial port can act as a master simply by using a HART modem and appropriate application: A HART Modem (black device with white sticker on right) from MACTek. The PCB shown here is used to sniff the data being sent from

Square Wave: Zooming in!

Oscilloscopes are fun. And I like collecting stuff. So inevitably this led me to start collecting "waveforms" of common signals propagating in and between the various electronic equipment that run our lives. First up is a simple square wave captured on a Tektronix TPS2024 oscilloscope. The square wave is taken from the oscilloscope itself and is actually meant for probe calibration/compensation . The square wave is 1KHz 5volt peak to peak. The animated GIF file below shows the square wave at each step as the time division knob is moved . I always wanted to make such an animation - sort of satisfies our inherent kiddish desires to play with knobs and buttons and move them/bang on them randomly madly! Here is an FFT ( Hanning window ) of the above waveform: Measuring some characteristics of the square wave using oscilloscope's built-in features: Transient analysis (rise time measurement) of the above square wave: