Frequently Asked Questions
- What is the Piezoelectric effect?
Quartz crystals depend upon the piezoelectric effect in order to function. It is based upon a quartz resonator coated with an electrode on both faces then mounted in a sealed holder. Application of an external AC voltage to these electrodes gives rise to a mechanical vibration.
- What is the minimum information needed to order a crystal?
We need the answers to the following questions:
- Frequency in kHz or MHz – preferably to six decimal places, anything after the significant digits will be taken as zero unless indicated.
- Holder style
- Adjustment (sometimes called Calibration) tolerance (in ppm)
- Temperature stability over operating temperature range (in ppm)
- Operating temperature range in degrees centigrade
- Load capacitance either parallel resonant in pF or series resonant (SR)
- What is load capacitance?
This is the capacitance of the oscillator circuit as presented to the quartz crystal resonator. The most common type of crystal is parallel resonant where the crystal operates at a frequency above its series resonant frequency (point of lowest impedance).
- Will the crystal operate outside its specified temperature range?
Yes! If for example a crystal is specified over –10 to +60°C it will perform without any issue over –40 to +85°C but it is possible that it will fall outside its specified stability. This may not matter if the application needs just a stable frequency rather than an accurate frequency.
- Why is it important to specify the operating temperature range?
See 4. Above. Also if the temperature goes significantly outside the specified temperature range it is possible that damage to the crystal may be caused.
- Explain the differences between fundamental crystals and overtone crystals
A fundamental crystal oscillates at the frequency determined by the dimensions of the quartz blank. Overtone crystals operate at either the 3rd, 5th or 7th multiple of the fundamental. The crystal is specially designed to operate in these modes.
- Where do I get specifications of the IQD products?
These appear in PDF format on the IQD website www.iqdfrequencyproducts.com If this information is not sufficient for your needs please contact our Sales office.
- Who do I call for technical assistance at IQD?
Please contact the Sales office and ask for the Applications Support department who will be able to assist you.
- I am thinking of changing an existing design from using leaded crystals to SMD crystals – what do I need to consider?
This depends upon the application. If for example the crystal is pullable i.e. the frequency can be changed by varying the load capacitance of the crystal by electrical means, then this may be difficult to achieve. For example strip blank SMD crystals have a very much lower pullability than the HC49 style round blank package.
This exercise should not be undertaken lightly and if you are in any doubt, contact IQD's Applications Support department for further advice.
- Is it preferable to design-in a crystal or an oscillator for my application?
This depends upon the application. If you are designing a discrete circuit and have little or no experience of oscillator design then it may be preferable to use a packaged oscillator since this will remove any tolerance issues. It is "easy" to design a "one off" for lab use but if you have to manufacture large numbers in volume this may give rise to all sorts of issues. If you are using a chipset which requires a crystal to drive it then this decision is much simpler since the on board oscillator circuit should have been optimised.
- What are the differences between SPXO's/ VCXO's / TCXO's and OCXO's
An SPXO or Clock Oscillator is the basic type of oscillator and consists of a crystal and a basic drive circuit. Since there is no form of compensation the frequency/ temperature stability is essentially that of the crystal itself – typically ±50ppm.
An oscillator which relies on the inherent pullability of the quartz crystal in order to allow the output frequency of the oscillator output to be changed by application of an external voltage. This change is limited to a few tens of ppm, typically ±100ppm. As with SPXO's the frequency/ temperature stability is that of the crystal itself.
Where the stability of the crystal is not sufficient it may be necessary to use a TCXO or Temperature Compensated Crystal Oscillator. This type of device is used where the stability of a basic quartz crystal is insufficient. Typically TCXO's can achieve a stability of less than 1ppm as opposed to a typical crystal of 30ppm.
The "ultimate" piezoelectric product is the OCXO or Oven Controlled Crystal Oscillator. Where a very high stability is required then this type of product should be considered. These types of devices offer performances of typically 3E10-9.
- Are products still available that were previously in the Data Book or on the website but do not appear currently?
Often this is the case but not in all situations. If a product has become obsolete then please discuss with Applications Support; in most cases a suitable alternative product can be offered.
- Does IQD offer a quick turnaround on oscillators?
Yes; if the device that you are looking for is not a stock part then in some cases a programmable version may be available, these are compatible with the fixed frequency device and can often be supplied in a few working days – a custom fixed frequency may take at least 6 weeks to produce. Programmable oscillators particularly lend themselves to prototyping where time is critical.
- Are IQD's products available through Distribution?
In most cases the answer is yes. Please look under the heading Distributors and Reps on our website to select the most appropriate Distributor for your company.
- Does IQD offer products which are not on its website or in its Data Book?
Yes; our engineering department is constantly developing our product range and indeed much of its capability is in customisation. In any event please contact us!
- Why are crystal packages getting smaller?
This situation is being driven by the electronics industry in general and the need for more portability in particular. There is a downside to this however which is that the smaller the crystal blank the higher the starting frequency. For example, the lowest frequency in a 7x5mm package is approximately 8MHz whereas it is approximately16MHz in a 2.5x2mm package with lower frequencies being achieved by external frequency division which increases circuit complexity.
- What are the most cost effective crystal and oscillator packages to consider for new designs?
For quartz crystals consider the HC49/4HSMX and for oscillators (SPXO's) look at the CFPS-72/73 series. For other products consult IQD as this can be performance dependant.
- Can I obtain the test results for my products?
Samples are automatically despatched with the appropriate test results. Production quantities are tested to the appropriate AQL level and the results retained within our company records. For further information please contact IQD.
- What is the most common cut of Quartz Crystal?
AT cut quartz accounts for the majority of crystals produced in the 1 to 200MHz range. This just refers to the nominal angle with respect to the Z axis of the quartz bar. The angle of cut determines the frequency/temperature performance and is usually selected by the crystal manufacturer.