Quartz is a so-called passive electronic component, which has the particularity of vibrating (resonating) at a very specific and very stable frequency. It is involved in the production of oscillators, clocks, counters, frequency meters, and in general any equipment for which time precision is important. Quartz can also be used in the production of rejection filters with a narrow bandwidth and a high rejection (attenuation) rate.
- Uses And Applications:
Quartz is used in all type CB transmission equipment, FM transmitters, quality wireless microphones, TalkyWalky (for example, a 27.125 MHz quartz which corresponds to channel 14 of the CB band), in computers. Also in some video equipment, such as PAL or SECAM encoders, where a 4.43 MHz quartz is used for the color (chroma) subcarrier. You can also find crystals in audio equipment providing analog to digital conversion (or vice versa).
The oscillation frequency of the quartz is not necessarily used directly, it can be divided to obtain a value more easily. For example, a wireless microphone might incorporate a frequency synthesizer, which only uses a single crystal, but can still offer 1440 different channels, each providing frequency stability that depends on that of the crystal. In an analog to digital audio converter, a single crystal can very well be used to generate a 32 kHz, 44.1 kHz, or 48 kHz clock.
For equipment requiring better stability than that offered by quartz (television transmitter for example), VCXO is used.
A practical and less “technical” example: take the example of your quartz watch. It contains a quartz (generally 32.768 kHz) which allows precise timing. Divide successively by 2 the value of this quartz, until the result is no longer an integer (division by 2 is very easy to achieve in electronics).
=>>Applications :
Quartz is designed to vibrate at frequencies ranging from tens of kilohertz to a few tens of megahertz. The global production of electronic quartz is two billion (2 × 109) each year. Most are intended for quartz watches, and for providing a time base in electronic circuits. Crystals are found in test and measurement equipment, such as meters, low frequency signal generators, high frequency oscillators or oscilloscopes. Quartz is also widely used in radiocommunication systems, for frequency references, but also for making narrow band filters.
- Values :
The resonant frequency of quartz that is easily encountered is between a few tens of kHz and a few 100 MHz. There are quartz manufacturers who can custom cut a quartz to make it resonate at any frequency you want. And as you can imagine, having a quartz cut to measure for a single use can present a significant price constraint, which the amateur cannot necessarily afford.
- Quartz operation :
A quartz is a mechanical element that has characteristics that allow it to vibrate at one or more well-determined frequencies. When we say vibrate, it is indeed a mechanical vibration that we are referring to. This mechanical vibration can be initiated when you tap (gently) the quartz but cannot remain maintained: it disappears quickly once the shock has passed. In order for the quartz to continue to vibrate (to oscillate), it must be subjected to an electric current which stimulates it.
And if the electronic circuit in which it is placed meets “sufficient” criteria, the quartz resonates (it begins to oscillate) and the rest. The principle of a quartz oscillator is precisely to provide the energy necessary for the quartz to enter into oscillation and remain so.
- Stability and precision of the oscillation frequency:
The precision of a quartz represents the maximum difference between the value of the real resonant frequency and that which is expected (which is written on the case). Its stability represents its capacity to oscillate at the same frequency whatever the environmental conditions, in particular the ambient temperature. Quartz manufacturers have developed several manufacturing techniques to improve the stability of their components, but the result is much higher selling prices for high precision components. Which of course will not surprise anyone. If the stability of a quartz is good from an instant point of view and at a fixed ambient temperature, the same cannot be said any more for a quartz subjected to wide variations in temperature. At an ambient temperature of (+25 ° C), the stability of the quartz is estimated to be around (+/- 20 ppm (ppm = part per million). On the practical side and for a real time clock, this corresponds to an accuracy of approximately 1.7 seconds per day, or a little more than 10 minutes per year. For my part, I do not consider this precision to be catastrophic, especially if we compare it to that of certain poorly adjusted watches that must be reset. hour every week. But remember that we are talking here about the precision at an ambient temperature of (+25 ° C), and that this precision decreases sharply when the temperature decreases or increases in large proportions (for example -10 ° C to +45 ° C), and yet we are not even talking about certain industrial conditions where the operating range can be between (-40 ° C and +85 ° C). At extreme temperatures, precision passes from (+/- 20 ppm to more than +/- 100 ppm and can even go beyond +/- 150 ppm). At this stage, we achieve a precision of +/- 13 seconds a day or more than an hour a year is not the same thing at all.
- Modelization :
A quartz can be modeled as an electrical circuit having two resonant frequencies close to each other, one at low impedance (series), and the other at high impedance (parallel). The impedance of the circuit can be written:
où :
is the complex frequency (
) ,
is the series resonance pulsation (
) ,
is the parallel resonance pulsation (
) .
=> Notes: The Co capacity depends on the physical configuration and the type of crystal size. For a quartz of size AT, this is practically the capacity formed by the metallizations of the quartz, and it is of the order of a few pF (pico Farad). The series resonant frequency (given by L1-C1) does not depend on Co. At this frequency, the quartz is practically equivalent to the resistance R1, which is from a few ohms to a few tens of ohms. The elements L1 and C1 are fictitious elements which model the resonator. At series resonance, the impedance of C1 is equal in modulus to the impedance of L1. This impedance is equal to Q times R1. As the surge coefficient Q is several thousand, we can easily see that this impedance is very large: the capacitor C1 is in femtofarads, and the choke L1 is in millihenrys.The addition of a bypass capacitor, therefore in parallel to Co, will cause a decrease in the parallel resonant frequency of the quartz. This phenomenon can be used to adjust the frequency as needed. Manufacturers take this into account when cutting quartz to have the correct frequency for a given load. For example, a 32.768 kHz – 6 pF crystal will only operate at this frequency if used with a circuit with a capacitance of 6 pF.
- Housing :
Several types of cases are available, from the very small watchmaker type, which can be found in quartz watches (32.768 KHz), in formats just as widespread as the HC18, HC25, HC33, HC38 or even HC49 …
