Sunday, January 18, 2015

Low power FM stereo transmitter

BA1404 is quiet famous FM stereo transmitter IC manufactured by ROHM Semiconductor. This 18pin chip contain FM stereo multiplexer, 19 kHz pilot tone generator and RF output stage with oscillator. The FM transmitter in this article is also based on BA1404 and 78L33 voltage regulator.

The FM transmitter in this article is specifically designed to transmit audio signals generated by PC/Laptop sound cards. To archive this goal, this transmitter is design to drive using 5V DC power source.


Except Q1 (38 kHz crystal) and L2 (oscillator inductor), this transmitter uses commonly available electronic components. 38 kHz crystal is vital to generate 19 kHz pilot tone and this crystal is available to purchase in several web stores.

For L2, wind 3 turns of 22 S.W.G enameled copper wire on 5mm bobbin with adjustable ferrite core.  This variable inductor is used to select the output frequency of the transmitter.

To archive good quality output and frequency stability it is necessary to construct this transmitter on PCB.

As per our tests the maximum range of this transmitter is 20m - 25m with obstacles and 50m - 60m in open area. In shorter distances this transmitter performs very well and produces really good audio output. Due to legal reasons we didn't test this transmitter beyond 50m.

PCB design for BA1404 FM stereo transmitter.

As per the time of writing, BA1404 is discontinued product, but there are lots of places to buy this chip in either DIP or SOP form.

The schematic and PCB of this transmitter is available to download at elect.wikispaces.com.

Friday, January 9, 2015

100W RF dummy load

This is quick post about 100W RF dummy load which we build to use with HF and VHF transmitters and transceivers. This dummy load uses two non-inductive 100Ω 50W thick film power resisters manufactured by Vishay Sfernice and both these resisters are connected in parallel to archive 50Ω impedance.

Final version of 100W RF dummy load.

Outer-casing (which also act as heatsink) of dummy load is constructed using four bonded fin 50mm × 115mm × 20mm heatsinks.

During our testing we got flat 1:1.2 SWR with this dummy load.

Internal view of 100W RF dummy load.

Above mentioned 100Ω 50W power resisters are obtained through Farnell element14.

Monday, January 5, 2015

ImageHide: Digital image based steganography tool

Steganography is method of hiding information and one of the techniques is hide an image inside a "cover image" by without distorting the supplied "cover image". ImageHide is command-line driven console application to perform this task and it is capable to hide/embed colored or gray-scale image inside another color or gray-scale cover image and/or extract embedded image from the supplied "cover image". 

This application is developed using FPC/Lazarus and it can execute on any FPC supported operating system which including Linux, Windows, Mac OS, etc. ImageHide is an open source project and its source code is available at https://github.com/dilshan/image-hide under the terms of GNU General Public License version 3.0. Compiled binaries of ImageHide are also available for both Linux and Windows operating systems. As an input this application accept bitmap, PNG or JPEG image formats and it can export processed image as bitmap or PNG file. Because of lossy compression techniques, JPEG exports are not supported by ImageHide.

ImageHide documentation and examples are available at https://github.com/dilshan/image-hide.

Saturday, January 3, 2015

Homebrew Audio Signal Generator

Audio Signal Generator (or AF Signal Generator) is often used in RF and AF base designing and experiments. In this article we introduce less complex AF signal generator based on commonly available electronic components. The heart of this AF signal generator is Intersil ICL8038 precision waveform generator/voltage controlled oscillator IC. ICL8038 is a function generator capable of producing sine, square and sawtooth waveforms at same time. In this project we combine ICL8038 with inverting and non-inverting amplifiers to get above mentioned 3 waveforms with 0° and 180° phase differences at the same time. Those inverting and non-inverting outputs help to use this AF signal generator with balanced audio systems.

Final version of AF signal generator.

The given schematic and PCB design of this AF signal generator contains ICL8038 waveform generator, inverting and non-inverting amplifier stage, output buffer and 12V regulated power supply. All the above mentioned modules are integrated into 65mm × 110mm PCB with standard through-hole type components.

This AF signal generator is design to work with 18V 500mA AC input. To increase the stability, we recommended using "audio grade coaxial cables" for all external connections except to power indicator LED.

This given design is capable to generate 200Hz to 10kHz sine, square sawtooth waveforms. It is possible to have some slight frequency drifts due to temperature variations in the operating environment.

Generating 1.415kHz sawtooth waveform using AF signal generator.

As per our measurements and observations warm-up time of this AF signal generator is around 3 to 5minutes, but this can be varied based on the type of enclosure, component arrangements, external temperature, etc.

Schematic, PCB design and wiring diagrams of this AF signal generator is available to download at elect.wikispaces.com. All these design files of this project are distributed under the terms of Creative Commons Attribution 4.0 International License (CC-BY-4.0). High resolutions photographs of this project are available at this flickr page.