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This section describes how to select an oscillator circuit for a particular application. It also summarizes the most important oscillator design characteristics to aid those who would like to design their own oscillator circuits.
13.1. SELECTING A CIRCUIT
What circuit should be selected depends on the application. If the best short-term stability without regard to other circuit characteristics is needed, then the modified Meacham circuit is the best choice. But it will be necessary to put up with the Meacham's drawbacks of circuit complexity, the difficulty of designing a stable circuit, and a high parts count.
(An alternative circuit that is easier to design and gives performance equal to the modified Meacham is the RLC half-bridge, which is de scribed in the Appendix.) If very good short-term stability is required, but other circuit characteristics such as simplicity and ease of design are also important, then the Pierce is a good choice. The Pierce can be used at any frequency from the highest to the lowest, fundamental or harmonic, as shown by the assortment of Pierce schematics in this guide.
Or, if harmonic operation above 20 MHz, with above average performance and reasonably few parts is needed, the Butler emitter follower circuit is the one to pick. If the bare minimum number of parts is a requisite and performance requirements can be relaxed a little, then the Colpitts, either fundamental or harmonic, is the one to pick, A discrete transistor circuit will usually give better performance than an IC circuit. This is because it is easier to control the crystal's source and load resistances, the gain, and signal amplitude in a discrete transistor circuit. The discrete transistor amplifier usually has less time delay in it, as well, since it normally has only one or two transistors.
On the other hand, oscillators using ICs are frequently cheaper when assembly labor costs are included. They also interface more easily with digital circuitry. And although their performance is generally less than that of a discrete transistor circuit, it is still better than what is needed for many types of digital circuitry.
In CMOS, the Pierce inverter circuit works well over the 1 kHz-2 MHz frequency range and requires only two inverters. From 4 kHz-10 MHz, the 7209 special oscillator IC, which also uses the Pierce inverter circuit, can be used..
In TTL below 3 MHz, the series-resonant two-inverters circuit gives good performance over the 100 kHz-3 MHz frequency range as long as the circuit's spurious oscillation problem with the crystal out of the circuit can be tolerated. In TTL above 3 MHz, either the series resonant circuit using an ECL line receiver or the Pierce-ECL circuit would be a good choice. A TTL buffer circuit similar to those shown in the oscillator schematics is required for these above-3 MHz circuits.
In ECL below 20 MHz, the series-resonant line receiver circuit or the Pierce-ECL circuit would be a good selection, covering a frequency range of 100 kHz-20 MHz. In ECL above 20 MHz, the Butler emitter follower circuit (harmonic) using the 10216 line receiver would be a good circuit to use.
13.2. CIRCUIT DESIGN
The design of a crystal oscillator circuit is dominated by the crystal's internal series resistance R,, far more than by any other factor. For a typical quartz crystal in a gas-filled container, the crystal's internal series resistance varies from a high of 200K ohm at 1 kHz to a low of 10 ohm at 20 MHz. For the best short-term frequency stability, the equivalent series load resistance seen by the crystal should be equal to or somewhat less than the crystal's internal series resistance R,. To meet this crystal need, the circuit's impedance level has to vary over a wide range-high impedance at low frequencies, and low impedance at high frequencies.
It is this characteristic that leads to such a wide variety of circuits over the frequency range.
In calculating the crystal's load, the crystal's shunt terminal capacitance C, of about 5 pF should be included as part of that load rather than as part of the frequency determining L, and C, elements in the crystal.
Since the terminal capacitance C,, is in parallel with the actual load resistance (or capacitance), a parallel-to-series network conversion is per formed to find the equivalent series load resistance that is to be minimized. Good in-circuit Q can be obtained with either a high or low value of parallel load resistance across the crystal terminals. For the best Q, this parallel load resistance should be two to three orders of magnitude greater than or less than the impedance of the total shunt capacitance across the crystal terminals. Figure 6.4 shows graphically these minimum and maximum parallel load resistances that will give good in circuit Q as a function of frequency. The graph in Fig. 6.4 assumes that the crystal's total shunt capacitance is just the shunt capacitance C, of the crystal itself (about 5 pF), which is the usual case.
The oscillator circuit should contain some means of ensuring that the amplifier is biased "on" at startup and is not in a biased "off' state, which would prevent oscillation from starting when power is applied.
This is an essential element in any good oscillator circuit.
If the crystal is removed from the circuit or becomes defective, the circuit should not oscillate. Some circuits, like the TTL two-inverters circuit, continue to oscillate (spuriously) when the crystal is removed; this is poor design.
In general, the time delay in the oscillator's amplifier should be minimized, since it causes frequency shifts when the amplifier's temperature changes. The greater the amplifier's time delay as a percentage of the period of oscillation, the greater the frequency shifts with tempera ture changes. The square wave driving the crystal should be a good square wave, with a 50/50 on-off ratio and a rise time that is not too slow or too fast. The amplifier's frequency response and rise time should not be way beyond the oscillation frequency either. If they are, then multiple on-off switchings due to noise will occur in the amplifier at each zero crossing of the sine wave out of the crystal. A desirable delay and rise and fall time seems to be about 2 percent of the total period of oscillation.
It is worth repeating that both sine and square wave outputs are avail able from most oscillator circuits. The square wave is on the driving side of the crystal, and the sine wave is on the output side.