Monday, October 4, 2010

50 Hz AC Mains

So the waveform collection wouldnt be complete without the 50Hz AC Mains. Since the oscilloscope I am currently using is USB powered PC based, it can't handle high voltages such as the 230 Volts AC/Hz available in India.


So I stuck my probes onto the secondary of a AC/DC step down adapter..


..connected the oscilloscope to my laptop and observed the waveform and more specifically, its frequency:


Just to be safe, I operated the laptop on battery and disconnected the laptop charger from the mains.




Next I loaded the AC/DC adapter with a 47 ohms resistor...


.. and observed the signal (note the distortion):




If you are wondering if (and why) the utility frequency ever changes then the following articles would interest you:

Sunday, October 3, 2010

10 Mbps Ethernet

Waveforms of signal on a 10 Mbps Ethernet CAT5 cable (captured differentially on the RX+ and RX- wires):





For the info regarding how I managed to captured this waveform have a look at the usenet discussion here and the supporting info here.

Click on the image below to know how I forced my network card to operate in 10 Mbps mode only:

Dual-Tone Multi-Frequency

Whenever we press the keys on our touchtone landline phones, it transmits two sine waves simultaneously. The interface at your telephone exchange recognizes the frequencies of both of these waves and hence identifies the key pressed. This is known as Dual-tone multi-frequency signaling. The various combinations of the frequencies of each of the keys are as below:

DTMF Frequencies in Hertz
     
KEY  FREQ1    FREQ2
       
1    697    1209
2    697    1336
3    697    1477
A    697    1633
       
4    770    1209
5    770    1336
6    770    1477
B    770    1633
       
7    852    1209
8    852    1336
9    852    1477
C    852    1633
       
*    941    1209
0    941    1336
#    941    1477
D    941    1633

Here are how the waveform of DTMF signals corresponding to various key presses look like:

Digit 0
Digit 1
Digit 2
Digit 3
Digit 4
Digit 5
Digit 6
Digit 7
Digit 8
Digit 9
Hash (#)
Asterix (*)


And here is what the FFT of DTMF signal for Digit 5 looks like - note the peaks near 770 Hertz and 1336 Hertz:

Here is the 20Hz Ringing Signal which appears on the line to indicate an incoming call:


And finally this is a capture of a waveform during a conversation:

RS485

Here is a printout of screen capture of A and B signals of an RS485 Bus as captured on an oscilloscope (Agilent 54621A). The RS485 Bus usually consists of a twisted pair cable connected to an RS485 Transceiver. In the case, below, a twisted pair cable has not been used - the signals were directly observed at the A and B pins of DS75176 loaded by a 120 ohm resistor and biased by two 560 ohm resistors. The bus is bein operated in half-duplex mode. DS75176 (which is equivalent to MAX485) was connected to the UART of Atmega8 AVR Microcontroller. The data being transmitted were the characters: 'a' '0' '1' '2' 'K' 'Q' - in that order. The UART frame format is 8-N-1 and the baud rate is 38.4 kbps. The signals A and B were captured on two different channels both referenced to the common system ground.

This waveform was captured while me and my classmates were working on our final year project at the end of our undergraduate engineering course. The reports of this project titled "Multi-Utility Sensor Network" are here.



Related Links:

Spectrogram

How to observe Spectrograms of various signals or latest pop/techno songs:
  1. Buy a Nokia phone - specially the ones like Nokia 5800 or Nokia X6 or any other which are labelled "comes with music" or "music unlimited"
  2. Install the nokia support software on your computer - especially the Ovi Player
  3. Validate your music coupon and download a few songs from using Ovi Player. This is completely legal.
  4. Install Winamp 2.95 (older version). You can get it from oldversion.com here.
  5. Start Winamp, go to preferences, set the visualization plug-in to "Spectrum Analyzer + Voiceprint" which is present in the "Nullsoft Tiny Fullscreen 2001" Library
  6. Load the song in winamp, play it and press "Ctrl+Shift+K" to watch the live spectrogram of the song. The Spectrogram is updated from left to right. Press "Esc" key to get out of fullscreen mode.
  7. If you want to see the voiceprint of your voice, "add a URL" to playlist and type "linein://" into the textbox asking for location. After this is done, double click on the entry in the playlist which says "Line in" and set the recording control to Microphone, switch back to winamp, press "Ctrl+Shift"K" and speak or whistle into the Microphone to view the spectrogram. Instead of the microphone, you can set the recording control to "Line in" and apply any signal to your sound card's "Line in" and observe its spectrogram. If you are worried about frying your computer's sound chip, try this.


Setting up the preferences for visualisation in Winamp

Spectrogram of a song. The instantaneous spectrom is displayed using the bars at the bottom of the screen. This spectrum is translated to pixels of varying colours depending on the amplitude at the frequency and displayed as a vertical line in the voiceprint (or spectrogram). Within the spectrogram, the frequency increases vertically from bottom to top.

Setting the recording control to microphone for "linein://"

Whistling into the microphone and observing the Spectrogram

Frequency Modulation

Here's a GIF Animation of the modulating signal (top) and the corresponding frequency modulated signal (bottom) captured on an analog Cathode Ray Oscilloscope.
The modulating signal is a very low frequency sine wave fed from signal generator to an ICL8038 FSK trainer kit. You can see the frequency of the carrier wave change as the voltage level of the modulating signal changes slowly.

The video was shot using a digital camera. The location is VESIT, Mumbai's Communications Laboratory.




Here's how the GIF animation was created on a Windows based PC using free tools:
  1. The video was originally in .MOV (Quicktime) format. It was converted to uncompressed AVI using Rad Tools (free version of QuickTime must be installed on your system before you can convert the MOV to AVI).
  2. The uncompressed AVI was then loaded into VirtualDub and Image Sequence of a selected duration of video was exported. This particular time period of the video consisted of a sequence of 14 frames. The frame sequences were selected such that the waveform in the start frame and the end frame were almost in phase (i.e. looked the same) so that when the 14 images when displayed one after another should give an impression of a continuous loop.
  3. The exported images (or frames) were converted in a batch job from BMP to GIF using IrfanView.
  4. The GIF animation was created using Microsoft GIF Animator.The GIF images were imported into it, the animation was set to loop forever and the delay between each image was set to 3/100 seconds since the video was originally shot at 30 frames per second.
  5. The GIF animation was saved and ulpoaded to Picasa which now supports GIF images with animations.
The above steps can be used to convert any AVI file to GIF Animation. You do not need to used RAD Tools in case your video is not in MOV format as VirtualDub can directly import video files encoded with popular Codecs like MPEG or DivX or XviD. Now days are many websites which help you create a GIF by uploading individual images for each frame. Some of these are:
GIFup (can import images from flickr) 
gifninja (allows conversion of video to GIF)