Friday 24/05/24

Balanced Oscillator, Patented

The Siemens "GOLDplus" chipset utilises a balanced local oscillator. The oscillator comprises two on-chip transistors with associated biasing components, and was intended to be used with a "Colpits" style of external resonator. At the time, this was a relatively common technology, with several other Siemens chips using a similar circuit configuration.

During the development phase of the Gold chip demo platform, it became apparent that there were a number of disadvantages inherent in this type of circuit. Those disadvantages began with the component count.

The oscillator was of course phase locked, and hence needed to be tunable. The lineup of external components thus included a number of decoupling components and a dual varactor diode, in addition to the inductor and capacitors required for the resonator. We were short of space, and a new approach was required.

Many hours (days, in fact) were spent with the circuit simulator, exploring other possibilities for circuit configurations that required fewer components.

This work paid off.

Abex UK was already very familiar with the design of oscillators and VCO's, and with crystal oscillators and VCXO's. We have a long history of involvement with this area of circuit design. It was known to us that the often ignored "Hartley" configuration can deliver a number of advantages over the significantly more popular "Colpits" style circuits, and so we set about finding a way to construct a balanced Hartley oscillator.

The circuit simulator in use was the EEsof "Series IV", which included harmonic balance with the ability to analyse oscillator phase noise. This facillity had not previously existed in this class of simulator, and a significant number of CPU hours were clocked up on a Sun SparcStation 10.

The resulting design was patented by Siemens, and subsequently cited as a reference in numerous other patents on the subject of balanced and "push-pull" oscillators. Naturally, Siemens own the rights to this patent because the work was carried out for them, but the design was provided by, and is entirely the original work of Abex UK working under contract to Symbionics Ltd of Cambridge. The entire patent may be viewed online by searching for "US5831487". Alternatively, follow this link.

Patent Schematic

Schematic, Fixed Frequency Version

This schematic drawing, as represented in the patent, shows the basic elements that make the design "new" and demonstrate the "inventive step" that is required in a patent. A practical version of the design, for use with a frequency synthesiser, must of course be tunable. A varactor diode would be added to achieve this.

Patent Layout

Layout Example

This layout example taken from the patent shows an example of the basic non-tunable circuit as shown above.

The patent references the chip "S 042" as an example. In fact a number of other chips were also applicable, and of course, the PMB2240 GOLDplus.


GOLDplus, Reverse

The GOLDplus Design

These images show the version of the oscillator that was used on the GOLDplus demonstration platform.

The top image shows the PMB2240 chip, with the dual varactor diode in the foreground. Between the varactor and the PMB2240, part of the printed balanced inductor is visible.

In the lower image, the passive components associated with the oscillator are visible.

The remainder of the printed inductor is not visible because it is on an internal layer of the 4-layer PCB.

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