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Frequency Control

Figure 3

Design challenges IQD’s current oscillator design is based on a 3rd overtone SC-Cut crystal. To move to higher frequencies using a 5th overtone SC-Cut crystal, our circuit design has to be changed completely to a different circuit concept because there must be a different frequency selection part of the circuit to avoid the oscillation on the 3rd overtone instead of the 5th. Also because of the higher resistance of the crystal the gain has to be improved to guarantee a start-up ability under all causes.

For this next generation design the Phase Noise performance also has to be improved. The goal should be to reach the values near the carrier in the range of our current 10MHz IQOV-90-series and improve the noise floor value (near the carrier means at offset frequencies up to around 100Hz away from the carrier). In this frequency range the Phase Noise is determined by the crystal, so a very good crystal with high Q-factor will help to reach similar values. Far away the Phase Noise is determined by the power supply and the output stage. So with the current power supply and the filtering of the noise we should reach similar values. For the output stage we have to improve the noise, which will reduce the noise floor to less than -160dBc/Hz. The physical limits are shown above, but it is not so easy to find a suitable design with enough output power without affecting

the noise. For the temperature stability the main issue is to have a very good thermal coupling between the crystal and the heating circuit and also with the temperature sensor and this should work over a wide range of ambient temperatures. For an OCXO the internal heating temperature must be 10 to 20°C higher than the maximum ambient temperature to guarantee stable working conditions at high temperatures. This is due to the unregulated part of the power dissipation caused by the oscillator circuit, power supply and output stage. Ultimately, all of the components and the material must be suitable for these high temperatures. To satisfy the requirements of future communication systems, OCXOs for reference clocks will have to deliver tighter stabilities, better Phase Noise and Jitter performance, higher frequencies and wider temperature ranges, all in smaller SMD package sizes. With our in-house crystal design and manufacturing and more than 35 years of experience in OCXO development and production, IQD is well placed to meet the challenges of future OCXO performance.

IQD Frequency Products |

Uwe Schweickert is R&D Manager at IQD FOQ GmbH

Figure 4 Components in Electronics February 2012 17

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