Clock crystal knowledge

1. Why does the Clock crystal use such a special frequency point as 32.768KHz

Because 2 to the 15th is 32,768

So 32.768KHz=32768Hz, the clock circuit uses 32.768KHz frequency division 15 times, that is, 1Hz

In addition, compared with the high frequency crystal oscillator, the low frequency 32.768KHz power consumption is low, and the anti-interference is good.

2. As a clock crystal, how many seconds is the daily error, monthly error and annual error of 1ppm

1ppm=1/1000000

Daily error: 1*24 hours *3600 seconds /1000000=0.0864 seconds

Monthly error: 1*30 days *24 hours *3600 seconds / 1,000,000 =2.592 seconds

Annual error: 1*365 days *24 hours *3600 seconds /1000000=31.536 seconds

3. When the clock crystal is tested with the network instrument card, it will encounter the situation of "unstable". Why is this?

After the crystal is inserted into the fixture, it will take more than 0.01 seconds to stabilize the oscillation, so our network instrument card

When testing the crystal, it is generally scanned twice, whichever is second.

However, when the error of the frequency FL of the first and second scans exceeds 0.5ppm, the instrument is judged to be unstable.

At this time, we will use a manual test machine to verify, if normal, to determine that the product is still qualified.

4. What causes the 'instability' of surface crystals? How to solve it?

Due to the domestic fine-tuning process, mechanical grinding wheels are used to adjust, after adjustment can not be cleaned, so there will be a little dust left on the tuning fork, when oscillating, there will be a change of about 1ppm, so it will occur 'unstable' situation.

In addition, if the vacuum in the shell is not good, the solder paste is not dry, and the tin point is virtual welding, instability will also occur.

Knowing the reasons, then the solution can be targeted:

1) The use of non-mechanical frequency modulation, such as laser modulation, will not produce dust, stability will be greatly improved

2) When sealing, ensure that the shell is clean and vacuum

3) The tin point should be dry and welded firmly

5. Why do you use tuning fork chips for crystal production, rather than AT-cut chips

The study of piezoelectric crystal found that different vibration modes can only produce a certain frequency band. Previous studies have found that the tuning fork vibration mode has the most stable oscillation in the KHz segment (currently known as 20-300KHz) to achieve the best quality.

The AT-cut vibration, generally in the MHz segment, can be the most stable and achieve the best quality.

6. What is the TC curve of the crystal? What's the formula?

The TC (temperature characteristic) curve of the surface crystal is a parabola with a downward opening at the vertex at 25 ° C, as shown in the figure below.

The TC curve of the tuning fork can be calculated theoretically by the formula. The specific formula is:

Temperature frequency difference (ppm) = temperature coefficient * (temperature point -25) ^2

The temperature coefficient is generally: -0.03 or, -0.036 or, -0.042

For example, if the coefficient is -0.042, the theoretical temperature frequency difference at 85 degrees is:

-0.042*(85-25)*(85-25) = -151.2ppm

Note: As long as the temperature is different from the vertex temperature, whether the temperature is high or low, the frequency will decrease, that is, the change in ppm will be negative.

7. How much will the frequency change with direct contact with the surface of the crystal?

Human body temperature is generally 36-37 degrees, according to the TC curve, contact the surface of the crystal with the hand for a long time (more than 5 seconds), the frequency difference is: -0.042* (36-25) * (36-25) = -5.08ppm

That is, the frequency will drop by 5.08ppm. Therefore, the test should avoid direct human contact as far as possible. Even if the contact is less than 5 seconds, the frequency will be biased.

8. What determines the frequency of tuning fork when making crystal?

When the tuning fork is making the watch crystal, the fork depth is mainly determined after fixing the width of the fork finger, the thickness of the chip and the interval of the fork finger. Inversely proportional, that is, the smaller the fork depth, the higher the frequency.

The mechanical grinding process adopts the upfrequency method, that is, the frequency is first set below the nominal point, and then gradually adjusted higher.

Correspondingly, we are currently using the ATcut frequency reduction method. Through the silver layer covering, increase the load, to achieve the purpose of frequency reduction.

9. Assembly method of printed circuit board

When the tuning fork vibrator is placed horizontally in reverse, secure it fully to the circuit board. Especially the vibration part, as shown in the figure in the circuit board and vibration between the buffer material, or with a good elastic adhesive (silica gel, etc.) to fix. In addition, please avoid applying adhesive to the glass of the base.

When the vibrator is used upright, the vibrator is separated from the circuit board by more than 3mm for type 3×8 and more than 2mm for type 2×6.