Crystal Oscillator Parameter Calibration

During the batchcompute machine testing, some prototype color problems such as no color or colored bars were encountered. Except for the ISP's own register settings and the output peripheral parameters of the TV decoder, most of the problems in the application circuit are caused by the frequency deviation of the crystal oscillator. The following describes how to match the parameters of the crystal oscillator network and the actual calibration cases.

 

1. Follow Parameters

(1) normal temperature offset ± 10ppm (25 ℃)

(2) temperature offset ± 30ppm (-10 ~ + 70 ℃)

(3) load capacitor 15pf

(4) Resonant Resistance <40 Ω

The most important parameter of the crystal oscillator is the frequency offset and load capacitance. The c109 and c110 capacitance values can be determined through the load capacitance.

(A) typical Circuits

 

Figure (a) parameters in a typical circuit are described as follows:

(1) c109 and c110 are additional load capacitors;

(2) r116 is the return resistor, about KB ~ 1 MΩ;
(3) r117 is a traffic limiting resistor. Generally, the loop does not need to be connected to this resistor. If the loop amplitude is too large, it can be added. The value is about 470 Ω ~ 1 k Ω;

 

Ii. Load capacitance Calculation
Ct = (c109 * c110)/(c109 + c110)
CL = CT + CS

CS: stray capacitance distributed on the line

CL: load value, which refers to the load capacitor (PF) at a specific load frequency)

The parameter of the selected crystal oscillator is listed in the product specifications of the crystal oscillator manufacturer. With known parameters, we can calculate the value of the additional load capacitor. Generally, we take c109 and c110 to get the same value, therefore:

Ct = Cl-cs = 15p-cs;

Generally, CS is 3 ~ 7pf: The Experience Value is 5 PF. Therefore, the matching value of c109 and c110 is 20pf.

The actual value requires the actual measurement of each board. Because of this, calibration is available, especially for the application circuit that is extremely sensitive to frequency, just as the CVBS composite analog signal output in the ccdboard machine, the color of the modulated Brightness Signal is fixed frequency (3.58 MHz/4.43 MHz), and the difference between the color and reference Color Burst is used for color. Therefore, after the frequency is inaccurate, in order to have many color-related problems.

 

 

Iii. Actual matching capacitor Adjustment

1. load capacitor selection value:

(1) Select 10pf Capacitor

1> c109 = c110 = 10pf

2> nominal frequency F1 = 28.375000 MHz

3> tested frequency F2 = 28.377137 MHz

4> frequency error e1 = 2137Hz

5> the actual frequency deviation is equal to (E1*10 ^ 6)/F1 = + 75.3ppm

(2) Select 22pf Capacitor

1> c109 = c110 = 22pf

2> nominal frequency F1 = 28.375000 MHz

3> tested frequency F2 = 28.374912 MHz

4> frequency error e1 =-88Hz

5> the actual frequency deviation is equal to (E1*10 ^ 6)/F1 =-3.1ppm

We can see from the above that we should increase the matching capacitor to reduce the actual frequency. Through multiple replacement tests of different capacitors, The 22pf capacitor is always used, and the frequency offset can meet the requirements.

The actual capacitor is 22pf. According to the calculation formula, the Cs of the machine is about 4pf.

 

2. Negative impedance measurement:

"Negative impedance" is an indicator used to evaluate the quality of the oscillating loop (q. In some cases (aging, temperature change, voltage change... Etc.), the oscillating circuit will fail, and the circuit may not be able to withstand vibration, so the confirmation of the negative impedance (negative resistance,-R) of the IC becomes relatively important. Stable oscillator circuit. The negative impedance (-R) of the oscillator IC is at least five times of the oscillator impedance, namely |-r |> 5rr.

The negative impedance is measured as follows:

1> series resistance r118 to crystal output end (xout)
2> adjust the r118 value to enable crystal to start from vibration to stop oscillation.
3> when the circuit starts from vibration to stop oscillation, The r119 value is measured.
4> obtain negative impedance |-r | = r119 + RR. The RR is the Resonant Resistance of the crystal oscillator.

Note: stray capacitance of the Loop will affect the accuracy of the above results.

The above steps show that the r119 value is greater than 1 K, which meets the requirements of negative impedance.

If the measured results do not meet the negative impedance, The c109 capacitor value should be reduced and c110 should be fine-tuned until the above two conditions are met. If the values of c109 and c110 must be consistent, then, adjust the r118 value to a value greater than 5R (200r), and then determine whether the vibration starts.

Postscript:

Board manufacturer suggestions: Recommended crystal oscillator encapsulation specifications in board machine application solutionsSMDOr49usEncapsulation, in addition to good drop resistance, the most important is the low temperature frequency deviation, more suitable for outdoor high and low temperature environment applications.

The crystal oscillator manufacturer suggests that, even if r117 is not used in the crystal oscillator matching network, the 0r resistance must be reserved and welded. The frequency amplitude can be adjusted through this resistance to avoid re-layout.

Crystal Oscillator Parameter Calibration