Tech Note: Telecommunications

Mobile Basestations

Mobile communications applications require extremely accurate synchronization of the signal frequency throughout the network. Without absolute frequency stability typically within ±0.05 ppm at all points, cut-outs and failed calls will occur. To minimize frequency discrepancies, each timing device within the network is locked to an external reference source, e.g. GPS in the case of CDMA. This imposes two key requirements on each timing device: it must be able to synchronize adequately to the reference clock over its entire lifetime of ten years or more, and it must be able to hold its frequency stable for a period of 24 hours or more in the event of loss of the external reference signal.

While high-stability rubidium clocks are usually specified to meet the strict timing requirements of mobile basestations, they are significantly bulkier and more costly than quartz-based devices. However, our CFPO-SC1 range of self-calibrating, oven controlled quartz crystal oscillators (OCXOs) can in many aspects outperform traditional rubidium clocks.

The self-calibrating CFPO-SC1 OCXO compensates for crystal ageing effects that normally require re-calibration of an oscillator after it has been installed in the customer's system. Because of this, it can offer free-run frequency stability to within ±0.016 ppm over a 15-year lifetime, even without an external reference. With short-term stability of ±0.00015 ppm and a power requirement after warm-up of less than 3.0 W, the CFPO-SC1 is more than adequate for use in mobile basestations yet comes in a standard six-pin hermetically sealed metal package measuring just 67 x 60 x 40 mm and costs less than rubidium-based alternatives. Standard output frequencies include 4.096, 5.0, 8.192, 10.0 and 10.24 MHz.

Less tightly specified but even more competitively priced than the CFPO-SC1, the CFPO-DO3 is a single-oven OCXO that surpasses the performance of double-oven devices but in a significantly smaller package. Through precise optimization of the thermal design, complementary adaptation of the components, and highly controlled in-house manufacturing, we have achieved free-run holdover stability of around ±0.0002 ppm over 24 hours in the event of reference clock failure, and an overall lifetime stability of ±0.06 ppm within a 51.0 x 41.0 x 19.0 mm package. This is suitable for certain mobile basestation applications.

Both the CFPO-SC1 and CFPO-DO3 have excellent phase noise characteristics.


IQD has a strong track record in providing timing devices conforming to SDH/SONET Stratum requirements, including devices approved by chipset vendors Semtech and Zarlink.