July 1972 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Phased-locked loop (PLL) oscillator circuits are the standard implementation for tunable frequency sources these days, as opposed to LC tank circuits with either mechanical (variable capacitor and/or inductor) or electrical (varactor) control elements. Crystal oscillators have been around for many decades, nearly a century in fact, but it was not until the advent of PLL circuitry using solid state digital ICs that such stability and accuracy was available across a wide range of frequencies. As author Klaus Peter states, using a huge bank of fixed frequency crystals with selector switches, even when using double and triple conversion schemes, is totally impractical from physical implementation, production, and cost perspectives. These early PLL designs did not have the convenience of single-IC circuits as are widely available today and at a very low price. The plethora of USB-interfaced signal generators on the market now is testimony to that.
New Tuner Uses Single Crystal and Digital Frequency Synthesis
By Klaus J. Peter
Fig. 1 - Simple phase-locked loop circuit locks oscillator to reference.
An ideal tuner must be sensitive and selective, have good spurious and image rejection, have good capture ratio, and provide recovered audio with low distortion. Since low distortion is achieved only when the r-f signal is properly tuned, low drift is essential to maintain this condition. For operating convenience, a method of station pre-selection as well as the ability to scan should also be available. The readout accuracy should be such that there is absolutely no doubt as to which frequency or channel is being received. Manual fine tuning to reach the center or low distortion point of a signal should be eliminated because mistuning is probably the greatest source of distortion in FM reception.
The most obvious and simplest solution to eliminating fine tuning of the oscillator is to use a crystal-controlled oscillator. Since the oscillator frequency is always 10.7 MHz above the receiving frequency, 100 crystals covering from 98.8 MHz, to 118.6 MHz in 200-kHz steps would be required for all channels. A 100-position rotary selector switch would serve as a tuning knob and provide a mechanical or electrical readout determined by its shaft position. This system effectively eliminates the need for fine tuning but is far too expensive since the quartz crystals alone amount to about $200 in parts cost. Using frequency mixing techniques, it is possible to bring the number of crystals down to 20 for dual conversion and 15 for triple conversion but the additional switchable filter requirements also make this approach expensive. Furthermore, multi-conversion designs may be troubled by spurious response and poor image rejection performance due to the nonlinearity of the mixer circuits.
The Crystal Oscillator. The oscillator of the Scott 433 digital FM tuner is crystal controlled on every frequency but uses only a single quartz crystal as a reference standard. This is accomplished by making the oscillator part of a digital phase locked loop (PLL) circuit. In order to understand this principle, let us look at the simplest PLL circuit which locks an oscillator to a reference frequency.
When the system shown in Fig. 1 is first turned on, the voltage-controlled oscillator frequency will not be exactly the same as the reference frequency. The output of the frequency-phase comparator is an error voltage which tunes the VCO in a direction to minimize the error until phase-lock condition is established and fosc = fref. Since the control voltage for the VCO is ideally a dc voltage, the low-pass filter is used to remove any high frequency components which might be present at the output of the comparator.
Scott 433 digital FM tuner circuitry is described in text by its designer.
In order to generate a large number of frequencies from a single referenc