Brilliant brace up, it is the important electronic element in circuit, controlling the metre that the system moves. Be based on different applied setting, brilliant brace up have a variety of kinds, passive brilliant brace up be among them low-cost and a application is wide kind. Passive brilliant is measured in use oscillograph brace up when outputting frequency, often can discover brilliant brace up output does not have signal, brilliant brace up do not rise brace up wait for unusual situation. The article at this point the circumstance talks about just a little slightly.

 

 

 

 

Zhen Jian of 1. passive brilliant uprights

 

Passive brilliant brace up, accurate for should call Crystal(crystal) , active brilliant brace up call Oscillator(oscillator) . Passive brilliant brace up be in quartz the electrode on the two end plating of chip and into, its two pin do not have polarity. Passive brilliant brace up oneself cannot concussion, tie-in periphery circuit needs when the job. Below certain condition, chip can produce quartz piezoelectric effect: The electric field of two end and mechanical deformation can change chip each other. When adscititious hand in change when the natural frequency of the frequency of voltage and chip is equal, the vibration that crystal produces and electric field strength are the greatest, this calls the resonance that suppress report, similar the resonance with LC loop.

 

Graph 1 quartz is the circuit symbol of crystal, equivalent character of circuit, reactance and periphery circuit graph

 

Because crystal is passive parts of an apparatus, its are relatively sensitive to the parameter of periphery circuit, it is laden capacitance especially. According to the manual of crystal, we are informed a test have in circuit recommend capacitance, whether does this capacitance rise to crystal brace up big concerned couplet:

 

 

Cg, Cg calls a distribution the look, it is to receive in brilliant brace up the right capacitance on two feet, its action adjusts namely laden capacitance makes its and brilliant brace up demand bearing is consistent, those who need an attention is the total electric capacity after Cg, Cg establishs ties value (
 

 

) just be significant laden capacitance portion. Cic: Chip brings crural distributinging capacitance and chip interior electric capacity. △ C: PCB takes line distributinging electric capacity, experience value is 3 to 5pF.

 

 

 

The passive any crystalline substance of a 32.768kHz that uses on some project brace up, laden capacitance recommends a value to be 12.5pF in manual. This value is relatively petty it is thus clear that, tiny metabolic enough affects circuit character.

 

The influence of 2. probe

 

Probe, follow oscillograph actually same, it is the one part that measures a system, its are used correctly very big impact is worn test result. When the bougie of probe clicks measurement point, of probe receive initiate to be opposite the circuit that be measured causes an effect, this is called the laden effect of probe. This kind of laden effect simplifies commonly the paralell connection that is resistor and capacitance. In the oscillograph under bandwidth 500MHz, average bid matchs is 1 times attenuation or the passive probe of 10 times attenuation, the attenuation of certain probe is compared but the hand uses an option. The probe that different attenuation compares is above capacitance of resistor of bandwidth, input, input have difference:
 

 

Graph 2 ZP1025SA 1 times, the parameter difference when 10 times attenuation

 

See the input capacitance of probe, the laden capacitance that compares crystal manual wants big. Of probe intervene, sure affect greatly former already the circuit that parameter has optimized, affect crystal circuit badly thereby case brace up. Two kill photograph authority to take its light, measure passive brilliant brace up when should preferential choose probe of 10 times attenuation. If the parasitism parameter of probe of 10 times attenuation still is too big, can consider to choose active high pressure to differ minute of probe, its load parameter is optimized very smally, be like the ZP1000 probe of Lecroy, input impedance can amount to ohm of 0.9pF, 1M.

 

3. chooses appropriate measurement point

 

Since two end are not crystal,Chang Min feels, do not go up at receiving probe is measured, that can change a kind of train of thought, this signal is measured in other place.

 

Certain clock chip takes Clock Out function, this function is the signal of Buffer crystal, the output of its pin has very powerful drive capacity, accordingly OK and direct use probe is measured.

 

The clock input that crystal gives out arrives in processor, can use clock to make scale down handle to this signal, output the signal after scale down pin next. Such signal after we need to measure scale down only, can calculate the frequency that gives original clock.

 

The method of this kind of indirect test, can check the frequency of crystal only, cannot measure crystal to output the extent of signal. If can check the accuracy of its frequency in the operating mode limits of equipment, the job of that crystal circuit is OK.

 

Graph the amortize function of 3 use chip, tally function will measure

 

If signal drive capability is very strong, can consider to be not test program of the contact: Near field probe. Near field probe cooperates spectrum appearance, or the FFT of oscillograph analyses a function, can measure the frequency that gets part of peak value voltage. Because be blame contact plan, the laden effect of nonexistent probe, need to notice the frequency resolution that right now spectrum appearance, FFT analyses nevertheless, the stage that this meeting influence measures an outcome is entered, precision.

 

4.Tip: How to measure frequency

 

After the output signal that takes crystal, how should measure its frequency? In the ZDS series oscillograph that manages in me, can choose parameter of hardware frequency meter, frequency to measure, ascendant edge parameter is measured wait for a method.

 

When hardware frequency meter is coming true, have measure cycle and the algorithm that measure pulse number. These two kinds of different test methods, it is the frequency size that can input signal because of answering and of the choice, in order to expect measured value is exacter. When signal frequency hour, can choose the method that measures cycle, had measured the cycle of signal, periodic reciprocal is frequency, the frequency; that source of this method error depends on measuring periodic timing clock is become when signal frequency is big, can choose the method that measures pulse number, a number of a signal ascendant edge is measured inside standard time, the error of this method is the problem of make choice of inside standard time.

 

In the time parameter that measures in parameter, have this test of “ frequency ” . This test it is to get the time difference between two ascendant edges, beg reciprocal to get frequency again. This test the error depends on ascendant edge decide with periodic timing frequency, get the sampling rate that current sampling nods be confined to.

 

In the statistical parameter that measures in parameter, have the method of ” of computation of “ ascendant edge, its principle is a number that measures ascendant edge. In the test, can will measure limits to choose cursor area, and cursor limits is set for 200ms, the ascendant edge that measures so is multiplied with 5, it is signal frequency namely.

 

Graph computation of 4 frequency, ascendant edge surveys interfacial plan

 

Choose a signal generator to output the signal of different frequency, use 3 kinds of afore-mentioned methods to be measured so that frequency shows 1 times like the watch.

 

Express 1 different measurement technique to measure the frequency contrast that go out to express

 

 

 

 

Visible 3 person gauge result difference not quite, the resolution of hardware frequency meter is higher, and parameter is measured in effective digit has 5 only. This signal generator outputs the accuracy of frequency to be ±1ppm, oscillograph interior brilliant brace up frequency accuracy is ±2ppm, in frequency meter of afore-mentioned 24MHz hardware, metrical accuracy is 80Hz/24MHz=3.33ppm, be in basically the measurement of the instrument inside precision. Parameter measured value appears more adjacent and true below certain circumstance value, because its are effective,this is digit is insufficient and the reason of round, accuracy is taller still is hardware frequency meter.

 

5. brief summary

 

The passive brilliant with sensitive to load article brace up the measurement of signal did contracted analysis, elaborated the receives laden to circuit effect effect of probe, this kind of influence also applies to the circuit with output very big impedance likewise.