CERAMIC RESONATOR APPLICATIONS
Typical Oscillation Circuit: The most common oscillator circuit for a ceramic resonator is a Colpitts circuit. The design of the circuit varies with the application and the IC to be used, etc. Although the basic configuration of the circuit is the same as that of a crystal controlled oscillator, the difference in mechanical Q results from a difference in circuit constants. Some typical examples follow.
Design Considerations: It is becoming more common to configure the oscillation circuit with a digital IC, using an inverter gate. Fig.3.1 on the following page shows the configuration of a basic oscillation circuit with a CMOS inverter.
CMOS Inverter: A CMOS inverter can be used as the inverting amplifier; the one-stage type of the 4069 CMOS group is most useful. Because of excessive gain, ring oscillation or CR oscillation is a typical problem when using the three-stage buffer type inverter, such as the 4049 group. ECS employs the RCA CD4O69UBE as a CMOS standard curcuit, as shown in Fig. 3.2.
HCMOS Inverter Circuit: Recently, the high speed CMOS (HCMOS) is increasingly being used for circuits allowing high speed and low power consumption for microprocessors.
There are two types HCMOS inverters: the un-buffered 74HCU series and the 74HC series with buffers. The 74HCU system is optimum for ceramic resonators. See Fig.3.3
INV.1 operates as an inverting amplifier for the oscillating circuit. INV.2 is used as a waveform shaper and also acts as a buffer for the output.
The feedback resistance Rf provides negative feedback around the inverter so that oscillation will start when power is applied.
If the value of Rf is too large and the insulation resistance of the input inverter is low, then oscillation will stop due to the loss of loop gain. Also, if Rf is too great, noise from other circuits can be introduced into the oscillation circuit. Obviously, if Rf is too small, loop gain will be decreased. An Rf of 1M is generally used with a ceramic resonator.
Damping resistor Rd has the following function although it is sometimes omitted. It makes the coupling between the inverter and the feedback circuit loose; thereby, decreasing the load on the output side of the inverter. In addition, the phase of the feedback circuit is stabilized. It also provides a means of reducing the gain at higher frequencies, thus preventing the possibility of spurious oscillation.
TTL Inverter Circuit: The value of load capacitance CL1 and CL2 should be greater than those of CMOS due to impedance matching. In addition, the feedback resistance Rf should be as small as several K. Note that the bias resistance Rd is required to properly determine the DC operating point.
Frequency Correlation: The oscillator circuits shown on the following page are ECS standard test circuits. The inverters used in these circuits are widely accepted as industry standard because their characteristics are representative of those found in microprocessors within the same family (CMOS/HCMOS/TTL). Naturally, applications will differ in what IC is used, and as can be expected, oscillator circuit characteristics will vary from IC to IC.
Usually, this variation is negligible and a ceramic resonator part number can be selected simply by classifying the processor as CMOS, HCMOS or TTL.
Loading Capacitance: Load capacitance CL1 and CL2 provide a phase lag of 180˚. These values should be properly selected depending on the application, the IC used, and the frequency. If CL1 and CL2 are lower values than necessary, the loop gain at high frequencies is increased, which in turn increases the probability of spurious oscillation. This is particularly likely around 4-5MHz where the thickness vibration mode lies.
Oscillation frequency (fOSC by the following equation.
) in this circuit is expressed approximately
Given that the standard ECS ceramic resonators are 100% frequency sorted to the test circuits on the following page, it is relatively easy to correlate the frequency of oscillation of our standard circuit to that of a customer specified circuit.
For example, if the microprocessor being used is a Motorola 6805at a frequency of 4MHz, then the correct ECS part number would be ZTA4.OMG (frequency sorted to the CD4O69UBE CMOS test circuit). Circuit parameters should be selected as below:
1 + ( C 1 / C
C L )
Where, fr: Resonance frequency of the ceramic resonator. C1: Equivalent series capacitance of the ceramic resonator. C0: Equivalent parallel capacitance of the ceramic resonator. CL =CL1 • CL2/CL1 +CL2
This clearly shows that the oscillation frequency is influenced by the loading capacitance. Caution should be taken in defining its value when a tight tolerance for oscillation frequency is required.
C1 = 30pF C2 = 30pF R1 = 1M
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