The fundamental aim of this project was to design a miniaturised low powered, multi-channel
FM transmitter for modulation across the commercial Band. This final section shall begin
with a detailed discussion of the main topics from initial approach to final design and
implementation. Also in this section a number of conclusions shall be drawn from the
approach that I have taken and finally recommendations on how what should have been
done.
When considering a design for frequency modulation, a number of key elements have to be
considered, such as a good understanding of the concept of modulation schemes and the
electronic circuitry that goes into creating the scheme.
In sections 1 Frequency Modulation Background and Electronic Components and their
properties of this report the theory of frequency modulation was covered, then a broadview
view of electronic components and their various properties were considered. Section 3
Basic Building blocks for an FM transmitter introduced the various building blocks and
considered their possible use in the final design. In section 4 Designs Under consideration a
quantitative overview of some of the possible designs that were considered in the progress
of the project was given. A detailed description of how each design works is given here,
along with why the design was or was not chosen. The final design was discussed in full,
in section 5 Final Design , Construction and Assembly along with details of construction
and assembly, pictures of the final circuit enclosed in a handheld plastic casing are shown
here to give the reader of the report a general feel for what the final transmitter looks
like.
Finally Section 6 Test and Results gives the results of the various tests used on the
design.
The design chosen was miniature, low powered and tuneable to different frequencies.
The parts used are very common and the circuit is very easily constructed. The circuit was
firsts build on vero-board and worked rather well without applying any real effective RF
techniques, all that had to be adhered to was to keep the leads short and compact the
circuitry as much as possible. The PCB, as expected performed exceedingly well, but more of
an better attempt had to be made in matching the antenna (section: 3.12 Impedance matching
& 5.6 Antenna Considerations) and shielding the RF section from the output as the PCB
layout was a lot more efficient in radiating power out. Unwanted Electro-magnetic radiation
had to be stopped from destructively interfering with the carrier modulation. To keep the
design simple and easy to construct it was decided to just wrap household cling-film
electrically protecting the circuitry from the Aluminium foil that was used to electro-magnetically
shield the RF stage.
The effective range of the transmitter was 80 feet in a household environment and about 50
feet in a lab environment, which makes the transmitter ideal aiding Tourists hear a Tour
Guide within a densely packed room. The Antenna co-axial extension lead helps in raising
the antenna over people’s heads and transmitting to their FM receivers (otherwise the water
content in the human body would bounce the electromagnetic waves). Because of the 14 hours
of effective transmission given by the PP3 and the power consumption of the battery, the
device could be used as a baby monitoring device within the home, with a range of about 80
feet, it reaches out even to the garden.
The Miniaturised FM transmitter is essentially a Design and Implementation project.
To approach a project like this a parallel path has to be taken in regards to the Theory
and the practical circuitry, for a successful conclusion in any project the path’s must
meet, and this only happens when they are fully understood. This is why a good grounding
in the basics of Communication theory and Analogue design must be achieved before ever
approaching a project like this. To start off looking at block diagrams of basic transmitter
was a must, even if it seemed abstract and obscure the underlying meaning of each block can
be found out one by one. Which is what made the overall project challenging and rewarding.
The design use for this project is essentially quite a simple one, and it is this
simplicity which partly brings it down when it comes to the overall reliable performance.
The main area of instability is in the oscillator part of the circuit. Shielding
(section 5.5 Construction and assembly) the oscillator helped in part to counteract
this.
After learning a lot from this project, there would have been a few things that could have
been done to the final design to improve it’s performance.
- Use negative temperature coefficients to compensate for typically positive-temperature-coefficient tuned circuits.
- Follow the oscillator with a buffer amplifier to reduce the effects of load changes.
