In the late 1960s I was part of a small product development team whose goal was the design of a frequency-synthesized radio receiver. This included a phase-locked frequency synthesizer whose output could be set to anything from 156 to 186 Mhz in 100 Hz steps. It used two VCOs (Voltage Controlled Oscillators — their output frequencies vary as a function of steering voltage). The output frequency of each VCO was determined by a phase-locked loop. It was the first time that our employer had developed a phase-locked loop frequency synthesized product. The frequency synthesizer’s high-speed VCO covered 30 MHz in steps of 1 MHz. We distributed the 30 MHz range amongst three oscillators, each of which covered a 10 MHz range. While prototyping, we discovered low-frequency noise sidebands on the VCO’s output signal. We needed to remove these noise sidebands, or they’d appear in the receiver’s output. On a spectrum analyzer, the noise sidebands appeared to be 20 to 400 Hz each side of the output signal and 15 dB below output signal level.
After selectively freezing circuit components while observing the noise sidebands, we learned that the carbon composition resistors in the loop filter were (one of) the culprits. That was a surprise: I had always thought of resistors as discrete inert lumps. I learned that carbon composition resistors consist of tiny grains of carbon bound to tiny grains of ceramic. We were seeing noise caused by random molecular motion. These voltage spikes, though mere microvolts in amplitude, were modulating the VCOs, resulting in noisy sidebands on the VCO’s output signal.
We substituted carbon film resistors, and discovered that the noise sidebands dropped substantially. I learned that carbon film resistors can dissipate less power, but offer a more contiguous medium than ordinary carbon composition resistors. Once we’d peeled back this layer, we discovered that the ceramic feedthru filter capacitors that fed each oscillator with DC power were also modulating the high-speed oscillator.
Instrumenting this circuit was interesting: we couldn’t load the phase locked loop itself with instrumentation or the phase lock would fail; we could only examine the noise sidebands that were an effect of noisy phase locked loop components. Since those days, I’ve learned that often we can’t directly examine phenomena — we can only see indirect effects. I suppose that this is how astrophysicists have discovered dark matter. Dark matter doesn’t reflect or refract electromagnetic energy; it’s detectable only indirectly because it’s affected by gravity.
What do we ever know about anything?
B.F. Skinner was alternately praised and despised because his brand of psychology, which became known as behaviorism, admitted that it’s impossible to know the inner workings and hidden mechanisms of humans; we can only observe behavior. He had a point: are humans less mysterious than electron movements or dark matter? In the end, what do we know about anything or anybody other than its behavior?
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© Russ Bellew · Fort Lauderdale, Florida, USA · phone 954 873-4695