PARTNERFOCUS Typical ESR values in production batches


7x5mm Fundamental Crystal Frequency 14-20MHz 20-40MHz


7x5mm 3rd Overtone Crystal Frequency 30-40MHz 40-50MHz


Spread


ESR 10-15Ω ESR 5-10Ω


Spread 50-70Ω 50-68Ω


3.2x2.5mm Fundamental Crystal Frequency 12-16MHz 16-20MHz 20-40MHz


Spread 60Ω


30-40Ω 10-20Ω


done after a series of tests with preselected ESR values to establish a maximum operational ESR for the circuit. For example, referring to the table above, if


the 30 to 40MHz third overtone 7x5mmcrystal mentioned is considered and the optimum ESR range for the circuitmay be between 40 and 60Ω, this thenmeans that some of the batchmay fail and a factory selection of 40Ω maximummay, for example, be required. However, this is only cost effective if a high enough yield can be achieved within a standard production batch of crystals. Most good suppliers will be able to advise what production spread of ESRs is achieved and, it is important to stress, for a specific frequency. Whilst 10Ω may not seem significant, it is worth bearing in mind that these are typical figures and most specifications only state a maximum figure. A good, engineering-led, supplier would not supply crystals close to the maximum ESR limit, but obviously this could potentially offer a wider spread of ESR. It is also worthy of note that the correct


load capacitance should be selected as the crystal will be tuned to operate within a circuit exhibiting the specified load capacitance. Therefore, a crystal with a stated 12pF load capacitance for instance will only work to its optimum tolerance within a circuit that is tuned to 12pF. Any variation of capacitance acting on the crystal will retune the frequency of oscillation and the issue then becomes a question of what tolerance limits are acceptable in the design.


Specifying quartz crystals When specifying quartz crystals, there are generally eight principal parameters that should be stated, although it is usual for only four or five to be provided by most purchasers. 1. Frequency - in MHz but can be in kHz 2. Package Style - Dictates the maximum size of the crystal and available specifications


SEPTEMBER 2021 | ELECTRONICS TODAY 49


Spec Max 40Ω Max 40Ω Max


Spec Max 100Ω Max 80Ω Max


Spec Max 100Ω Max 100Ω Max 60Ω Max


3. Frequency Adjustment Tolerance –@25°C in ppm. This is the tolerance towhich the final frequency is set at the factory


4. Frequency Stability over Temp range – in ppm. This gives frequency performance limits over the operating temperature range


5. Operating Temperature Range – in °C and is the range over which frequency stability will be guaranteed. It does not necessarily mean that the crystal will cease working outside these temperature limits


6. Load capacitance or condition – this is vital for the tuning of the final oscillator circuit. The performance of the crystal will be optimised at the final adjustment stage of the manufacturing process for this parameter


7. Equivalent Series Resistance or ESR – in Ω and determines the drive level required for stable oscillation


8. Drive Level Dependency or DLD – specifies the minimum drive level required by the crystal before oscillation occurs. This is a parameter most manufacturers monitor as part of the manufacturing process


Comment


Tight Spread of values Tight Spread of values


Comment


Better to use fundamental types Better to use fundamental types


Comment Std frequencies tuned for ESR


9. Any special selections/requirements – such as ESR selection if practicable


In general, the more intolerant the circuit design is to minor changes in impedance and frequency the more difficult the task of crystal substitution becomes. Problems may arise when reducing the size


of a crystal at a specific frequency, as many designs provide a low drive source for the quartz crystals, which may mean the crystal will not oscillate due to increased ESR characteristics. This requires the circuit to deliver a higher drive level to the crystal and in this age of low current circuits may offer a real obstacle. It is important for designers to ensure that


the oscillator circuit has a suitable crystal specified and that this has been tested over the operational temperature range. It is also important that the tuned circuit matches the load capacitance of the crystal for the optimum performance throughout the operating range. The general rule is that the larger the crystal the smaller the ESR as this is derived


from the physical properties of the quartz. Euroquartz Group www.euroquartz.co.uk


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