6. FM tuning: A unique solution
This invention required innovation in design, materials application and manufacturing processes. The resulting mechanism is unique and a good example of a miniature electro mechanical mechanism. Although a patent specification was drawn up, SRL never proceeded with the patent for unknown reasons however there can be no doubt that the invention was patentable at the time.
A radio receiver must be able to tune into a range of radio stations and discriminate between them. The FM frequency spectrum covers 88-108 MHz and the tuner must be able to adjust the frequency smoothly and repeatable within this band.
The conventional tuning circuit consists of a parallel connection between a capacitor and a coil, also known as an LC circuit with the L being inductance and the C being capacitance. This circuit is connected in series between the aerial/antenna and earth. Tuning is in principle a variation of the resonance frequency of the circuit and this is conventionally achieved through the varying of the capacitors capacitance value whilst the coil inductance remains constant. Variable capacitors for this purpose are manufactured in a wide range of types. The tuned signal is then taken from the two circuit junctions and rectified through a diode, then amplified.
At the time of development of the FM Radio Watch there existed no variable capacitors small enough to be built into the available space volume of the watch. This was a very serious problem as the watch had to have a tuner capable of tuning the FM frequency spectrum. The invention of a tuner mechanism was a critical path item.
As a boy I had built many simple radio circuits and I had always wondered if the tuning could be achieved through other means than those traditionally used. Would it be possible to vary the inductance of the coil instead of the capacitance of the capacitor?
The solution which emerged was to develop a novel and unique tuning circuit in which the inductive element or coil inductance could be tuned instead of the capacitance of the capacitor, the overall variation in resonance being equivalent to varying the capacitor in the conventional way. This was theoretically sound, however the practical execution represented a major inventive step as nothing like this had ever been achieved on the miniature scale required for the FM Radio Watch.
The Design challenges which faced me in this task can be summarised as follows:
Design a tuning mechanism for the FM
frequency spectrum of 88-108 MHz where the inductive element is
varied instead of the capacitor.
The whole tuning mechanism must be
compatible with the electronic tuning and FM Radio circuits and
physically small enough to fit inside the FM Watch This meant it had
to be smaller than existing variable capacitors – a tough
Design and manufacture of a suitable
coil, which had to be compatible with thick film hybrid technology
manufacturing processes. This meant that the coil needed to be
assembled onto the ceramic substrate using normal pick and place
robotics technology and be compatible with the vapour phase re-flow
soldering process running at 205 degrees Celsius.
Design a method of varying the inductance
of the coil and design a mechanism which would allow a human being
to carry out the tuning process.
- Design all components of this mechanism in such a way that they can be successfully manufactured through conventional manufacturing processes and assembled into the volume/space constraints of the FM Radio Watch.
Electrical Principle Design
A test circuit was designed and built using a fixed capacitor and a fixed wire wound coil. The circuit was connected to a frequency generator and a frequency analyser. By experimentation it was found that the resonance frequency of the circuit could be varied by presenting a triangular shaped piece of aluminium at the open end of the coil and moving the triangle slowly across the coil in such a manner as to vary the amount of aluminium covering the end of the coil between minimum and maximum to achieve full coverage of the 88-108 MHz FM band. Thus the tuning of the watch was achieved by varying the inductive L component rather than the capacitive C component. This was a unique solution and necessary as we would not have had room for a variable capacitor in the watch.
Through calculation and further experimentation it was established what the values of L and C needed to be and the shape of the triangle required to achieve the tuning function. Graham Beasley and I collaborated closely on this development.
Initially a conventional wire wound coil was considered and this would have been possible. However due to the small size of the coil and its fragility in manufacture and operation, it quickly became apparent that a coil produced by flexible printed circuit board technology would be better, more repeatable and lend itself much better to handling and processing during manufacture. The flexi circuit is double sided, with the two halves of the coil circuit connected via a plated through hole in the middle.
Coil Pillar Design, Coil Assembly, Pick & Place robotics and Vapour Phase Reflow Soldering
The coil needed to be presented at a certain height above the thick film substrate in order for the tuning band to avoid clashing with any of the chip components on the substrate. To achieve this and a rigid positioning of the coil, a plastic pillar was designed. The pillar had to be carefully dimensioned and asymmetrically formed to miss neighbouring chip components and their relatively wide tolerance bands.
The coil was wrapped round the pillar and secured on the underside by four headed brass pins. The pins acted as mechanical and electrical connectors. This produced a robust assembly which could be handled by the pick and place robot and once soldered in position present the coil in a position suitable for the tuning mechanism. In order to allow for the assembly to pass through the Vapour Phase Reflow Soldering Process running at a constant 205 degrees Centigrade, it was necessary to specify a plastic material which could withstand this. Poly Phenylene Sulphide (PPS) can handle 225-235 degrees C and is inexpensive and was chosen for the pillar. The whole assembly performed extremely well during manufacturing. I worked with plastic moulders in Essex, Dowty Circuits and CorinTech Circuits (www.corintech.co.uk) in Fordingbridge to implement this.
Thetuning mechanism relied on a metal taper shape moving linearly across the coil to effect the change in tuning required. To achieve this it was necessary to produce an endless band containing the metal triangle. In practise I worked with Timex Feltham to design a triple lamination of polyester film/aluminium/polyester film. The polyester overlap for bonding when formed into an endless band and this arrangement proved satisfactory.
With the functional principles established and an outline design for the mechanism, a draft design for the tuning frame was sketched out and immediately modelled in 3D. This component was machined from ABS on my Aciera F1 Watch maker’s milling machine. On assembly it worked right first time and a second prototype was drawn with some more detail and sent for prototype machining. This prototype proved entirely satisfactory both mechanically and electronically and was subsequently drawn up as a final design and sent for toolmaking and manufacture.
These consisted of two precision turned brass shafts, one with a secondary op milled spade end to allow for driving and two PVC extruded sleeves for the tuning band to engage with. The drive depended on the characteristic of polyester adhering closely to PVC by the exclusion of air and this effect was utilised very satisfactorily to drive the tuning band.