Implementation of Simple Wireless

Network

Nikolas Simon

5/11/15

EE-4980 Wireless and Mobile Networks

Dr. Wierer

The purpose of this project is to interface different existing systems together in order to provide a

platform to implement a simple wireless network. A Morse code audio signal is provided by a computer,

it is then processed so the Arduino can pass the data. Pulse Width Modulation is used to convert the

analog input to a digital signal and a very simple decode/encode method is applied, inverting the output

of the transmitter and then inverting the input of the receiver to change it back to the original signal.

Figure 1: Simple Wireless System High Level Block Diagram

System Discussion:

A significant amount of time was spent on trying to get a simple FM receiver to function. Several

designs including different numbers of transistors and different inductances were attempted however

none would produce an output. I suspect the main cause of this problem to be the very specific

inductances that are required for these simple FM circuits. Most designs call for exact wire gauges, wire

types, wire spacing, wire turns, and turn diameter. Also the majority of designs researched did not give

an exact inductance in Henrys, only physical wire information. At least one design did not work because

the only specific gauge wire available was braided core when the design called for solid core. I also

wanted to avoid looking at multistage receivers due to the increased complexity and exotic components

required (center tapped inductors/variable inductors). In fact the only simple transmitter design I found

that worked did not have an air core inductor.

To overcome the receiver issue I started to consider premade FM radios however with a surface mount

consumer product it would be difficult to isolate discrete components and make alterations. This is

when I decided to purchase an inexpensive radio kit. I use an Elenco Model FM-88K which is a much

more complicated super heterodyne design as opposed to the super regenerative designs I was

considering.

The technique to be able to pass audio through an Arduino Uno requires the amplifier/DC offset stage

and a modification to the microcontroller registers in order to continuously monitor one analog input in

order to increase the sampling rate. If this technique was not used, a higher end microcontroller would

be required to implement the system. Even with this method, the sampling rate is slightly under what

audio should be sampled at, around 38.5KHz. (Ghassaei 2)

Several of the simple FM TX/RX designs researched call for a medium frequency NPN transistor, namely

the BF199. One of these transistors is used in the transmitter design that I selected. The transmitter is

somewhat weak in its operation most likely because it lacks an amplification stage, however the

broadcast is still clear at close ranges. One interesting point of observation during testing the

transmitter was that sometimes the audio broadcast would overlap with a radio station broadcasting on

the same frequency. When this occurred both transmissions were audible at the same time. I found

two possible solutions to this. One was to use a variable capacitor to attempt to tune the desired

broadcast away from the interfering broadcast and the second option was to isolate the system so no

outside broadcasts can interfere. I attempted to construct a Faraday Cage out of aluminum foil and this

somewhat helped in isolating the desired frequency when the interfering broadcast happened to be on

the same frequency.

Figure 2: DC offset and amplifier circuit

This amplification and DC offset stage is necessary to provide proper signal input to the Arduino. Audio

output from the computer at full volume produces approximately a 500mV signal varying between

positive and negative voltages. The Arduino Uno can only process signals from 0 to 5V so a 10uF

capacitor is used to offset the voltage so that the signal only contains positive voltage. An op amp in a

non-inverting configuration with a potentiometer is used to amplify the input signal and a voltage

divider is used to achieve a voltage close to 2.5V where 5 ∗ ... ≈ 2.5 (Ghassaei 2)

Figure 3: FM Super Regenerative Transmitter

The air core inductor is replaced with 5 inches of 50 Ω coaxial cable with the inner and outer conductors

of one end soldered together. This component serves as an inductive monopole antenna, perpendicular

to the ground plane. (Simple Coil Less FM Transmitter 1)

Theory:

A regenerative receiver amplifies a signal several times with transistors placed in positive feedback

(regeneration is another term for positive feedback). A super regenerative configuration amplifies a

signal more than a regenerative circuit.

Super regenerative circuits are widely used today mostly for remote keyless access applications. The

designs are generally simpler therefore power consumption is less than more complicated receivers.

This means that other possible applications can involve requiring a low number of parts to fit in smaller

devices. Some of the disadvantages of super regenerative circuits are poor selectivity frequency

stability, and interference from adjacent channels (what was experienced when testing the transmitter

circuit). Some factors that affect range in super regenerative circuits are transmitter power to the

antenna and antenna efficiency. Again this agrees with the observation that range in the transmitter is

limited, most likely due to the fact that no amplification stage was added. (7)

Regenerative circuits were eventually superseded by super heterodyne circuits. Super heterodyne

utilizes mixing. The mixer multiplies two frequencies creating a difference and a sum of the inputs.

Compared to the regenerative circuits the super heterodyne circuits are more sensitive and more

accurate.

Figures 3 and 4:

Super-hedrodyne reciever block diagram (left) Super-Regenerative reciever block diagram (right)

The super heterodyne circuit requires system feedback and it incorporates automatic frequency control

(AFC). (8)

Bibliography:

1) "Simple Coil Less FM Transmitter." Simple Coil Less FM Transmitter. Accessed May 11, 2015.

http://electronics-diy.com/simple-coil-less-fm-transmitter.php

2) Ghassaei, Amanda. "Arduino Audio Input." Instructables.com. Accessed May 11, 2015.

http://www.instructables.com/id/Arduino-Audio-Input/ .

3) "Simple Fm Receiver." Simple Fm Receiver forum thread discussion. Accessed May 11, 2015.

http://www.electronicspoint.com/threads/simple-fm-receiver.268913/ .

4) "BF199 Data Sheet." Accessed May 11, 2015. http://www.farnell.com/datasheets/200356.pdf .

5) Insam, Eddie. "Designing Superregenerative Receivers." Designing Superregenerative Receivers.

Accessed May 11, 2015. http://www.eix.co.uk/Articles/Radio/Welcome.htm .

6) "50 Ohm Coax Data Sheet." Accessed May 11, 2015.

http://www.belden.com/techdatas/english/8262.pdf .

7) Rumley, Stuart. "Superregenerative Receivers for Remote Keyless Access Applications."

Accessed May 11, 2015. http://www.valontechnology.com/images/REGEN.PDF .

8) "Superheterodyne Receivers." Introduction to Naval Weapons Engineering. Accessed May 11,

2015. http://fas.org/man/dod-101/navy/docs/es310/superhet.htm .

9) A.K. Poddar, U.L. Rohde. "Super-Regenerative Receiver." IEEE Xplore. Accessed May 11, 2015.

http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4520077 .

10) "Frequency Modulation (FM)." - National Instruments. Accessed May 11, 2015.

http://www.ni.com/white-paper/3361/en/ .