1. Oscillation circuit
The crystal resonator is a passive component, and is therefore affected by the power voltage, ambient temperature, circuit configuration, circuit constant and substrate wiring pattern, etc. Operation is roughly divided into normal operation and abnormal operation. In the design of the oscillation circuit, therefore, one of the preconditions is determining how to make the crystal resonator oscillate stably and securely. Only after this determination is made can the subsequent items such as the frequency accuracy, frequency variation, degree of modulation, oscillation start time and oscillation waveform be discussed.
(1)Roles of components & reference values
In the design of the oscillation circuit, it is necessary to recognize the role of individual components. Roles are described in Table 1 by taking, as an example, an oscillation circuit (Fig. 1) using a general-purpose C-MOS IC (74HCUo4AP by Toshiba).
When the feedback resistor (Rf) is not mounted in the oscillation circuit, as shown in Table 1, the resonator does not start oscillation even if power is applied to the oscillation circuit. Unless a resistor of appropriate value is connected, oscillation does not start in the formal mode of the crystal resonator, but overtone oscillation or fundamental oscillation starts instead. In the case of the fundamental resonator (MHz band) feedback resistance is usually 1 MΩ. In the case of the overtone resonator (MHz band), values depend on IC and frequency characteristics, but are in the range of several kΩ ∼ tens of kΩ. In the case of tuning fork type resonators (kHz band), it is necessary to connect a resistor of 10 MΩ or more.
Appropriate values of control resistors (Rd) depend on the type of resonator, frequency band and the value of the outfit capacitor (C1, C2). Accurate values are determined by measuring the characteristics of the oscillation circuit (including negative resistance and drive level). Reference values for the AT cut resonator (MHz band) are in the range of several Ω∼ several kΩ. For tuning fork type resonators (kHz band) the reference values are in the range of 100 kΩ ∼ several kΩ.
Appropriate values of outfit capacitors depend on the type of resonator, frequency band, value of control resistors and the order of oscillation, and are in the range of 3 pF ∼ 33 pF or so, for reference purposes.
(2)Methods of examining oscillation circuit
1.Measurement of oscillating frequency
It is essential to measure true values, as far as possible, of the oscillating frequency of the resonator mounted on the circuit, using correct methods. In the measurement of oscillating frequency, probes and frequency counters are usually used. However, the goal is to measure by limiting the influence by measuring tools on the oscillation circuit itself.
Three patterns of measuring frequencies are available, as shown in the following drawings (Figs. 2, 3 and 4). The most accurate method of measurement is realized by using any spectrum analyzer capable of accurate measurement without coming into contact with the oscillation circuit.
|Measurement method 1|
Probes do not affect Fig. 2 because the output from the buffer is measured by entering the output of the oscillation circuit into the inverter in the next stage.
|Measurement method 2|
Probes do not affect Fig. 3 because buffer outputs (1/1, 1/2, etc.) are measured at IC.
|Measurement method 3|
Fig. 4 illustrates a case of no buffer output reception
from the IC, whereby the effect of probes is minimized by measuring via the
small capacitor of 3 pF or less between the output point (XTAL OUT terminal
of IC) and the probe.
It should be noted, however, that output waveform is smaller using this method, and measurements cannot be made that depend on the sensitivity of the frequency counter even if the oscilloscope can check the oscillating waveform. In such a case, use amplifiers to measure.
2.Measurement of negative resistance
This is the measurement used to determine the margin
of oscillation of the oscillation circuit, and is used to predict the stability
of oscillation from obtained values.
As shown in Fig. 5, connect the resistor (R) in series to the resonator and increase the value gradually. The resonator will then stop oscillating at a certain value of resistance. This resistance value just before the resonator stops oscillation is the value of negative resistance.
- Using an oscilloscope at the measurement point of oscillating frequency, make a decision based on the presence/absence of oscillation waveform.
- Check if negative resistance is secured sufficiently even by the minimum value in the operating voltage range.
- Check negative resistance by changing temperature for the measurement
result at room temperature.
(Be sure to use dryers, quenching agents, constant temperature tanks, etc. according to the operating temperature range.)
- Reference must be made as to whether the negative resistance (10 times
or more as large as the CI regulations of the resonator) to be used can be
secured in the vehicle-mounted units and safety units related to human life
and whether the negative resistance (5 times or more as large) can be secured
in other applications.
*These reference values are subject to change without prior notice.
3.Measurement of excitation levels
In the oscillation circuit used, measure the power consumed by an operating resonator.
As shown in Fig. 6, measure the current (i) that flows into the resonator by using a current probe.
(We use P6022/Tektronix)
Other measurements may be available. In the method using probe contacts, however, true value cannot be obtained because current flows into the GND from the probe.
Max. value to operate resonator normally depends on the shape of resonator and oscillation mode. Please contact us for details.
4.Check of abnormal oscillation
In the oscillation circuit used, check for the possibility of resonator oscillation in modes other than the formal oscillation mode.
– Simple method of checking –
When the oscillation mode of the resonator is fundamental, as shown in Fig. 7, abnormal oscillation can be tested by grabbing the lead (2 pcs) of the resonator with slightly wet fingers and detaching the fingers while turning the power from OFF to ON. If the oscilloscope waveform is not in the 3rd overtone oscillation, there is no possibility of abnormal oscillation. On the other hand, if the oscilloscope waveform is in the 3rd overtone waveform while the fingers are detached, there is a possibility of abnormal oscillation.
Checking by using wet fingers facilitates oscillation in high frequency bands by forming a small resistance between resonator terminals and reducing Rf as shown in the circuit diagram in Fig. 7.
2.Nonconforming conditions caused by oscillation circuit & sample contermeasures
In the design of the crystal oscillation circuit, various
nonconforming conditions may occur.
- Frequency cannot be contained within the required range.
- Frequency cannot be adjusted.
- About 1/3 , or 3 times larger than formal frequency is given.
- In the activation of power, resonator start time of oscillation is greatly delayed.
- Resonator does not oscillate or start of oscillation is far delayed.
To suppress such symptoms, therefore, it is essential,
at least, to examine the basic items of oscillation circuit described above.
If, here, the specification of resonator need not be changed to solve nonconforming conditions, it is comparatively easy to implement countermeasures in the circuit side (See Table 2).
However, many characteristics of the oscillation circuit may not be met even if the conforming conditions are improved, and still other nonconforming conditions could occur. Therefore, the circuit should be examined at our plant. Also, in case the problems cannot be solved even by implementing the sample countermeasures, the rumbling will be examined at our plant if requested. Please contact our marketing personnel.
|Table2 Nonconforming conditions & example countermeasures|
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