Frequently Asked Questions

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5.0V Clock required - Use 3.3V CMOS Clock

The 5V CMOS clock oscillators are declining in availability. This is a solution to use the readily available 3.3V clock oscillators.

The Cardinal CC065L and CC137 families of 3.3V oscillators can be used with this circuit to fulfill the need for a 5.0V clock oscillator. The resulting signal characteristics will be equal to or better than the older 5V clock oscillators.

 

Converting Clipped Sinewave Output to CMOS

Clipped Sinewave output signal from many TCXO’s and other oscillators is easily converted to a CMOS output signal.

This simple circuit can be driven by the oscillators rated for 0.8Vpp clipped sinewave output specified to have a maximum load of 10K ohm in parallel with 10pF. The output will have a CMOS square wave with a rail to rail level and close to a 50% duty cycle.

This circuit will work from 1.65V to 5.5V and to over 70MHz at the lowest supply voltage.

Deterministic Jitter

The predictable component of jitter is called deterministic jitter. For non multiplied quartz crystal oscillators the deterministic jitter is typically less than 0.01pS. Additional information on jitter can be found in our Helpful Documents from Cardinal section..

Fine Leak Testing Criteria

Test Condition:

Helium Bombing Soak time : 2hrs

Bombing pressure : 75psig

Reject Criteria:

>5.00-8 atm.cc/sec

LVDS driving mutliple inputs
LVDS oscillators can drive mutliple inputs. There is an excellent application note from Xilinx about how to do this for their FPGAs.

http://www.xilinx.com/support/documentation/application_notes/xapp231.pdf

Phase Noise to Jitter Conversion

Oscillator jitter performance is often measured in the frequency domain as phase noise because of the increased accuracy and resolution of that method. Maxim Semiconductor has done an excellent application note that describes the conversion of phase noise measurements to jitter values. This includes a simplified version that is easy to implement in EXCEL.

http://pdfserv.maxim-ic.com/en/an/AN3359.pdf

Shock level and broken crystals causing failures
One of the most common failures of quartz crystals and quartz crystal based oscillator is broken crystals.

The crystal inside the package is suspended in free space inside the package, suspended in dry nitrogen or in an evacuated space. A high shock can cause the the precision timing element to fracture.

Most of the Pletronics products are rated to 1500Gs, this level of shock can be easily reached by dropping the device on the floor.

This chart shows the G force reached when dropping an object on a hardwood or a vinyl type floor surface. The forces are much higher on a concrete floor.

Why do TCXOs have clipped sinewave outputs?

Many ceramic LCC packaged TCXOs are only available with clipped sinewave outputs. There are many reasons why this is done, even though it frustrates many of the circuit designers who want CMOS output levels.

* CMOS output would add significant power dissipation to the TCXO IC. The resulting temperature gradients on the IC and in the package would limit the ability to achieve the best TCXO compensation.

* The wide range of possible CMOS loads would alter the compensation design as internal thermal gradients result in the TCXO not meeting specifications.

* CMOS outputs result in power supply and ground transients when the output is changing levels. This noise would adversely affect the phase noise performance of the TCXO.

* CMOS outputs generate much larger EMI/RFI signals that can result in difficulties meeting the USA FCC and other countries radiated energy limits from the final system. The clipped sinewave, being both low amplitude and mostly sinusoiddal, results in low signal levels and less harmonics.

* Many of the applications where these TCXOs are used have ASIC inputs that accept the clipped sinewave and convert to CMOS logic levels internally.

See our FAQ about converting clipped sinewave levels to CMOS.

Why does a crystal have a lower frequency limit?

The thickness of the crystal determines the frequency. For an AT cut crystal 0.001″ (0.0254mm) thick is about a 63MHz fundamental mode resonator. If the thickness is doubled then the resonate frequency would be 0.5 * 63MHz or 31.5MHz.

An 8MHz crystal would be 0.0078″ (0.200mm) thick. A 4MHz crystal would be 0.0157″ (0.400mm) thick.

Two conditions are encountered that limit the lowest frequency in a particular size of package:

1) The simple condition, the newer low profile ceramic LCC packages will not accept the thicker crystals. Depending on the package, the lower frequency limits range from 8MHz to 12MHz and downward.

2) Remember that a quartz crystal is a vibrating mechanical device. For the best designs, those with lower ESR (CI) and no perturbations (undesired modes of vibration that cause frequency jumps) the vibrating area is intended to be in the center area of the crystal.

When the crystal frequency is lower and the crystal is thicker, the vibrating area is more mechanically coupled to the edges. In this case the perturbations (unwanted resonances) become difficult to nearly impossible to control. The solution is to bevel or contour the crystal (make the crystal look more like a magnifying glass concave shape). This thinning of the edges restricts the vibrating area to the crystals center and allows control of the perturbations.

Unfortunately, restricting the vibrating area increases the ESR significantly. For example at 12MHz:

CX5 7x5mm crystal ESR maximum is 50 ohms

CX532 5×3.2mm crystal ESR maximum is 80 ohms