SLLA549 July 2021 TCAN4550 , TCAN4550-Q1 , TCAN4551-Q1

5 Measure the Oscillation Frequency

Figure 5-1 Oscillation Frequency Test Setup

Check for the motional parameters C0, Cm, resonant frequency (fs) and ESR on the data sheet of the crystal manufacturer. Calculate the resonant frequency of the crystal and of the crystal with the load capacitance.

Where the resonant frequency and motional inductance of the crystal is:

Equation 8.

f_{s} = \frac{1}{2 π \times \sqrt{L_{m} \times C_{m}}} \to L_{m} = \frac{1}{{4 π}^{2} \times f_{s} \times C_{m}}

Where the resonant frequency of the crystal and capacitive load is:

Equation 9.

f_{l} = f_{s} \times \sqrt{1 + \frac{C_{m}}{C_{0} + C_{L o a d}}}

Because the oscillator frequency is determined by load capacitance which is a product of the crystal’s shunt capacitance C0, the crystal’s Motional capacitance Cm, and the load capacitance of the board and external capacitors C_Load, it will shift according to the following formula:

Equation 10.

\frac{d F}{F} = \frac{- C_{m}}{2 (C_{0} + C_{L o a d})}

Measure the oscillation frequency using an E-Field probe, sometimes called either a sniffer probe or antennae probe, connected to a RF Amplifier to strengthen the signal detected by the oscillation of the crystal, and a Spectrum Analyzer or Frequency Counter to display the frequency of the signal detected. This method does not place a direct load on the oscillation circuit that will alter the frequency of oscillation through additional resistance or capacitance inserted into the circuit by a directly connected voltage probe.

If an E-Field probe is not available, one can be made by exposing the conductor on one end of a coax cable that will act as an antenna, insulating it with heat shrink tubing to prevent electrical contact with any components on the board, and placing the insulated conductor directly on top of the crystal. The shield of the coax cable should be electrically connected to a ground location near the crystal to improve the signal quality and provide a reference for the test equipment.

The signal detected will likely have low amplitude and may need to be amplified with an RF Amplifier for a more reliable measurement by the spectrum analyzer. The amplitude of the signal is not important and it simply needs to be large enough for the spectrum analyzer to measure the frequency.

Tune the spectrum analyzer with a center frequency at the nominal crystal frequency and with a fairly narrow span such as 100 kHz so that there is enough resolution to capture the oscillation signal and accurately measure the center frequency.

If a spectrum analyzer is not available, a High Resolution Frequency Counter with a minimum of 7 to 8 digits of precision may be used if the signal has enough amplitude and is free of enough noise to return an accurate frequency count.

An alternative measurement can be done with an oscilloscope through a very low capacitance and high impedance active probe. The probe will add an additional capacitive/resistive load on the circuit and will slightly alter the oscillation frequency. Less than 1pF of probe capacitance is needed to minimize the frequency shift caused by the probe.

Oscilloscope measurements may not have enough resolution to get a high precision measurement of the exact oscillation frequency. A more accurate measurement can be obtained by using a High Resolution Frequency Counter.

Verify the frequency of oscillation is within the required specification and adjust the external capacitors to shift the frequency if needed. Increasing the capacitance will lower the frequency, and likewise reducing the capacitors will increase the frequency.

Re-calculate the load capacitance if the external capacitor values have been adjusted.