One of the simplest digital modulation schemes in current use is the Frequency-shift keying (FSK). FSK is similar to Frequency Modulation or FM except that the modulating signal is a binary pulse stream that varies between two discrete voltage levels rather than a continuously changing analog waveform. In FSK, two discrete frequencies are used to represent the binary digits 0 and 1.
The heart of the circuit consists of two Wien-bridge oscillators built using a dual op amp LM1458, for the two frequencies. The two frequencies are enabled corresponding to digital data using two switches in HEF4016BP. The control lines of these switches are logically inverted with respect to each other using one of the switches in HEF4016BP as an inverter, so as to enable only one oscillator output at a time. The digital bit stream is used to control the analog switches as shown. Since the switching frequency limit of HEF4016BP is 40 MHz, high-data rates can be easily accommodated. This method comes in handy when expensive FSK generator chips are not readily available; also, the components used in this circuit are easily available off the shelf and are quite cheap.
FSK was originally used to transmit teleprinter messages by radio (RTTY) but can be used for most other types of radio and landline digital telegraph. Currently, FSK is commonly used in Caller ID and remote metering applications.
Low-cost FSK Generator – [Link]
By David Szondy @ gizmag.com:
A global economy brings many benefits, but it also makes international terrorism extremely difficult to combat. With more goods passing through the world’s shipping terminals and airports than ever before, hunting explosives with large, static detectors or teams of inspectors armed with detecting devices and reagents is a bottleneck that increases the chances of evasion. To help US counterterrorism efforts, GE has developed RFID stickers that act as wireless, battery-free explosives detectors that can be placed almost anywhere.
GE RFID tech turns stickers into explosives detectors - [Link]
An active bandpass filter can be wide-band or narrow-band as needed. If the higher -3dB frequency divided by the lower -3dB frequency is greater than 1.5 then a wide-band filter is needed. An effective and easily understood active wide-band filter can consist of merely a low-pass and high-pass filter in series. The high-pass and low-pass filters are share the same design, with resistors and capacitors in the same positions but reversed for each type of filter.
Bandpass Filter - [Link]
Kenneth made a 2m low pass filter and wrote a post on his blog detailing its assembly:
I’ve been playing with the DRA818V modules that have been making quite a stir in the amateur radio world at the moment. I haven’t gotten one on a spectrum analyzer yet, but I have reason to believe that it will require a low pass filter to be RF legal. I’ll write more about that once I get a look at it, but figured I’d first built myself a low pass filter in case I need it (if not for these modules, but some other VHF project in the future).
My process for building a low pass filter went as follows:
Select the type of filter and cutoff frequency desired
Look up normalized coefficients in the ARRL Handbook
Divide these coefficients by the cutoff frequency
Convert the inductances into turns on some core and capacitors into the nearest values
Build the filter
Designing and building a 2m low pass filter - [Link]
Phillipe Cantin writes:
So you want to two HC-05 modules to automatically connect together, as soon as they’re powered up and with zero code? Well this is your lucky day since this can be done using the AT+BIND command.
Let’s do this thing!
For this, you will need:
1 Arduino (I’m using UNO)
2 HC-05 modules
Arduino IDE (I’m using version 1.0.5-r2)
HC-05 Bluetooth link with zero code - [Link]
Dilshan Jayakody published a new build, a VHF Aircraft radio communications receiver:
This is simple radio receiver project which is capable to receive aviation radio voice signals which are transmit between 118MHz to 138MHz in AM (A3E). We got initial idea of this project by reading Sverre Holm’s (LA3ZA) article about “cheap VHF receiver project”. Like LA3ZA’s original article this project is also based on Samsung’s KA22429 FM radio receiver IC.
KA22429 is low voltage FM radio receiver system developed by Samsung for portable radio receivers and as described by Holm’s it can push into VHF region by changing few values in its original circuit. In this project we redesign the schematic and PCB with some of those changes and now it can directly tune into aviation band with minimum number of adjustments.
VHF Aircraft radio communications receiver - [Link]
I’ve for some time now wanted to do more RF design. Although I have taken some RF design courses, I haven’t actually made a single RF design before. But you can’t learn without doing and inspired by the MIT coffee can radar designed by Gregory Charvat, I figured that building a radar should be a doable project that would offer some challenge while also having some real world use.
The simplest radar is a continuous wave Doppler radar, which continuously transmit a constant frequency signal. This signal reflects from a moving target and Doppler shift causes reflected signal to change frequency. This reflected signal is then received and mixed with the transmitted signal. Mixer product is the difference of the frequencies which is proportional to the speed of the target. This kind of radar is very simple to make, in fact there are even some children’s toys. Unfortunately it can’t detect the range of the target and isn’t that exiting.
6 GHz frequency modulated radar - [Link]
by Colin Jeffrey @ gizmag.com:
In the world of electronic components, there are many devices out there that do their job well and reliably, but are almost never heard of – even though they may be vital to equipment that plays a role in our technology-driven lives. The radio frequency (RF) circulator is just such a device: it has simply done its job as a nondescript box of gubbins buried in radio communications systems, quietly directing radio frequency signals to the places they should go. Now researchers at the University of Texas have given the RF circulator a makeover. Not only is the new prototype smaller, lighter, and cheaper, it’s also claimed to be easily adapted to different frequencies on the fly, which is something the old style circulator cannot do.
New RF circulator to run rings around old technology - [Link]
by Sam Freeman and Wynter Woods @ makezine.com:
This simple hack turns your Raspberry Pi into a powerful FM transmitter! It has enough range to cover your home, DIY drive-in movie, a high school ball game, or even a bike parade (depending on the stragglers).
PiFM software not only boldly enhances the capability of your Pi, but does so with nothing more than a single length of wire. This hack starts with the absolute minimum you need to run a Raspberry Pi — an SD card, a power source, and the board itself — and adds one piece of wire. It’s the coolest Pi device we’ve ever seen with so few materials.
Raspberry Pirate Radio - [Link]
RaysHobby build a project called RFToy:
it’s an Arduino-compatible microcontroller board for interfacing with radio frequency (RF) modules, such as the popular 433/315MHz transmitter/receiver, and the nRF24L01 transceiver. The RFToy has a built-in ATmega328, USB-serial converter (CH340G), 128×64 OLED display, three buttons, and a coin battery holder. Programming is done in Arduino through the on-board mini-USB port. It has three sets of pin headers to directly fit RF modules, and an audio jack to output RF receiver signals to a computer’s sound card. Using RFToy you can build a variety of projects involving RF modules, such as remote control and wireless sensors.
Introducing RFToy, an Arduino-compatible gadget for radio frequency modules - [Link]