Circuit board layout drama: swept frequency VCO

Article By : John Dunn

This swept frequency voltage-controlled oscillator was designed for a signal generator that featured a swept RF output.

The project was a signal generator that featured a swept RF output that could go from 1 MHz to 40 MHz. We had no way to make a single sweepable signal source that could cover a 40-to-1 range of frequencies, so the following approach was taken instead.

Figure 1 Swept VCO frequency plan

A swept frequency voltage-controlled oscillator (VCO) was designed to work from 71 MHz to 110 MHz, which was a mere 1.55 to 1 frequency range. That oscillator’s output was heterodyned against a fixed frequency 70 MHz local oscillator and the difference frequency was extracted using a lowpass filter.

Following the little bit of trauma, about to be described, this approach worked very nicely.

I worked up a schematic for what I was calling the “mixer card” and gave that schematic to the design and drafting department in that particular company, where a circuit board layout guy was assigned to configure the artwork. What came out was something like the following sketch.

Figure 2 An abysmal and unusable layout

The layout guy placed all of the resistors in one corner, all of the capacitors in a second corner, the several transistors in a third corner, and the ring mixer and a broadband transformer in the remaining fourth corner. He then interconnected all of those components with thin lines of copper foil. The end result very much resembled a freshly cooked pot of vermicelli, although perhaps a little less orderly.

When I confronted this fellow about his unacceptable work, his response was “Hey!! I’ve been doing this for 25 years!!!!”. Since I would estimate his age at the time to be somewhere around 30, the implication might be that he had been a kindergarten prodigy although I do have my doubts.

I rejected his work. Instead, I provided and insisted that he follow the layout I’d used in making my breadboard, which clearly I should have supplied in the first place.

I was prompted to recall this incident when a somewhat distraught colleague recently described a similar situation that had befallen him. If at some juncture this should happen to you, don’t despair, take heart and stand firm. You’ll get through it.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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