Basic Oscillators 101

Oscillators may seem like simple devices, until you have to design one into your system. That's when you may discover there is more to these critical timing devices than meets the eye.

Following is a list of the minimum information required to provide a quotation:

The key word here is "minimum." When discussing a customer's requirements, it is often helpful for the application engineer to obtain more than just the bare essentials. To ensure accuracy, as much of the following information as possible should be provided:

Providing most of the above information will ensure that you get the right oscillator for your application. No problem, assuming you understand the terminology. If you don't, it can be a frustrating, time consuming exercise. To make sure that doesn't happen, we have compiled a handy reference guide to oscillator terminology.

1.0 Oscillator Types

1.1 Clock
A clock is an uncompensated crystal oscillator. Stability is usually rather loose as shown in Table 1. The construction of OFC's clocks is typically hybrid design versus discrete design. Hybrid design combines open ICs (dies), uncanned crystals (blanks), substrates, wire bonds, etc., into a functioning unit. Discrete design combines ICs, canned crystals, printed circuit boards (PCB), etc., into a functioning unit.

1.2 TCXO
A Temperature Compensated Crystal Oscillator (TCXO) typically contains a temperature compensation network to sense the ambient temperature and pull the crystal frequency to prevent the frequency drift over the temperature range. Figure 1 shows an example of this.

Figure 1: Example of TCXO Compensation

1.3 OCXO
An Oven Controlled Crystal Oscillator (OCXO) usually contains an oven block where the temperature sensor, heating element, oven circuitry, and insulation function to maintain a stable temperature. By maintaining the temperature of the crystal, great improvements in oscillator performance are realized over other forms of crystal compensation. OCXOs can use either AT-, SC-, or IT- cut crystals depending on temperature range and aging performance.

Note: These numbers are dependent on size, frequency, cut of crystal.

Table 1: Best Stability Each Oscillator Type Can Hold

1.4 VCXO and VCO
A Voltage Controlled Crystal Oscillator (VCXO) is an uncompensated clock that is pullable. Pullable means the nominal frequency is adjustable. A Voltage Controlled Oscillator (VCO) is similar to a VCXO except it contains no crystal.

2.0 Frequency Stability

2.1 Units

ppm: parts per million (1 ppm = 1 x 10^-6 = 0.0001%)
example: 1 ppm equals 1 million Hertz (MHz) changing 1 Hertz (Hz)
ppb: parts per billion (1 ppb = 1 x 10⁹)
example: 1 ppb equals 1 million Hertz (MHz) changing 0.001 Hertz (Hz)
%: the percent change in frequency

Table 2 shows the relationship between units:

Note on exponents: add 1 when moving left, subtract 1 when moving right

Table 2: Stability Chart

2.2 Stability Calculation
(f_measured - f_nominal) / f_nominal = difference

2.3 Temperature Stability
How the unit changes frequency over the temperature range.

2.4 Initial Set Tolerance
Initial Set Tolerance, a.k.a. Initial Accuracy, is where the frequency is set close to the nominal frequency at room temperature. The closeness can differ from ±100 ppm to ±0.25 ppm.

2.5 Aging
Aging is the frequency shift of the crystal over a certain time period. Table 3 shows typical first year aging for 10 MHz clocks, TCXOs, and OCXOs.

Table 3: Typical Aging Performance

2.6 Load Variation
Load variation is the frequency shift with change in load

2.7 Supply Variation
Supply variation is the frequency shift with change in supply.

3.0 Output

3.1 Frequency
Frequency is how fast the output signal is changing, measured in Hertz (Hz). One Hertz corresponds to one complete cycle of a waveform occurring in one second. The waveform is periodic, which means it repeats the same pattern indefinitely. Examples of waveforms are squarewave, sinewave, and triangle. The output of oscillators at OFC is either squarewave or sinewave.

3.2 Squarewave
Figure 2 shows what a squarewave is. Following Figure 2 are miscellaneous terms used in conjunction with squarewave.

Figure 2: Squarewave Waveform

3.3 Sinewave
Figure 3 shows what a sinewave looks like. Following Figure 3 are miscellaneous terms used in conjunction with sinewave.

Figure 3: Sinewave Waveform

V_P: peak voltage

V_P-p: peak to peak voltage
V_rms: root mean square voltage = 0.707 * V_P
dB: decibel - a ratio of one level compared to a specified reference level
dBm: a ratio of output voltage and load (typically 50 ohm) compared to 1 mW.
The formula for converting V_rms to dBm is dBm = 10 log {[(V_rms)²/load]/1.0 x 10^-3}.

Table 4 shows some typical voltage to power conversions:

Table 4: Voltage to Power Conversions

3.4 Miscellaneous
Harmonics: multiples of the main frequency (sinewave only typically)

For example, the fundamental (main) frequency is 10 MHz, the second harmonic is 20 MHz, the third harmonic is 30 MHz
Spurious: noise not related to the multiples of the main frequency

4.0 Voltage - Current - Power

4.1 Units
Power (P): W (watts),or mW (milli Watt = 0.001 W)
Current (I): A (amps),or mA (milliamps = 0.001 A)
Voltage (V): V (volts)

4.2 Formulas
P = I * V (power = current times voltage)
V = I * R (voltage = current times resistance)

4.3 Warm-up and Steady State
Warm-up a.k.a. stabilization is the amount of time for the crystal to return to close to its nominal frequency. It is typically on OCXO specs only. TCXOs occasionally have a warm-up or stabilization spec.

Steady state is when the oven in the oscillator has reached its operating temperature.

4.4 Miscellaneous
Power consumption for OCXOs is typically around 5W at warm-up and 1.5W at steady state, depending on size.

5.0 Mechanical Trim or EFC

5.1 Mechanical Trim
Mechanical trim allows the frequency to be adjusted via an internal potentiometer (pot). The pot is accessed through a sealed or unsealed hole.

5.2 EFC
EFC (electrical frequency control) requires an external circuit to adjust the frequency. The external circuit usually consists of a pot or DAC. The power for this circuit can be applied via an external voltage source supplied by the customer or an internal reference voltage supplied by the manufacturer.

5.3 Linearity
Linearity is the change from a straight path.

Figure 5: Linearity

5.4 Slope
Slope is the direction the frequency changes with respect to the voltage. Positive means the frequency is increasing with increasing voltage. Negative means that the frequency is increasing with decreasing voltage.

Figure 6: Slope

6.0 Phase Noise

Phase noise is a small fraction of undesirable frequency near the output frequency. Phase noise is dependent mostly on the crystal with the circuitry making up the unit playing a small role. The measurement is commonly in the 1 Hz bandwidth. The description of phase noise is "at x Hz offset it is y dBc/Hz". Table 5 lists some typical OCXO and TCXO phase noise numbers.

Table 5: Typical Phase Noise @ 10 MHz

Some situations that make phase noise worse are vibration and frequency multiplication.

Vibration - sinusoidal or random - causes spurious sidebands at the output. These sidebands offset the desired output frequency by the frequency of the vibration. These sidebands are dependent on the frequency and magnitude of the vibration, the output frequency of the oscillator, the acceleration sensitivity of the crystal, and the mechanical performance of the crystal and oscillator.

When frequency multiplication is used to multiply the crystal frequency to the required output frequency, the phase noise degrades by

20 log (multiplication factor)

which equates to about 6 db less for a multiplication factor of 2, about 10 dB less for a multiplication factor of 3, and about 20 dB for a multiplication factor of 10.

7.0 Allan Variance

Allan Variance a.k.a. short-term stability is similar to phase noise except that it is based in the time domain instead of the frequency domain. Typical numbers for OCXOs are shown in Table 6.

Table 6: Allan Variance for a 10 MHz OCXO

8.0 Environmental

8.1 Temperature

Military: -55 °C to +125 °C

Industrial: -40 °C to +85 °C

Commercial: 0 °C to +70 °C

Handy formulas for converting temperature:
°F = 32 (°C * 1.8)

°C = (°F - 32)/1.8

8.2 Shock
Shock is defined as a sudden powerful blow. A typical shock number for TCXOs is 100g.

8.3 Vibration
Vibration is a rapid motion back and forth. There are two types of vibration for oscillators. They are sine vibration and random vibrations. Sine vibrations concentrates all the energy at a single frequency. The units are g PEAK. Random vibration is distributed over the entire spectrum. The units are g²/Hz.

For commercial units, vibration numbers should be limited to a 0.06" peak to peak amplitude change in a frequency range of 10 Hz to 55 Hz for OCXOs. For TCXOs, typical numbers are 5 to 10 g for sine vibration up to 500 Hz.

8.4 Acceleration
Acceleration is an increase in speed. Acceleration sensitivity, also known as g-sensitivity, is the frequency shift caused by subjecting the crystal to a constant acceleration. Typical acceleration numbers are summarized in Table 7.

Table 7: Typical g-Sensitivity Numbers

Summary
Armed with this basic knowledge of oscillator terminology, you should have a much easier time discussing your requirements with an application engineer and obtaining the device best suited to your application.

Oak Frequency Control Group, 100 Watts Street, Mt. Holly Springs, PA 17065. Tel: (717) 486-6014; Fax: (717) 486-5920.