Frequency Control
Stepping up to the mark
New system requirements necessitate new solutions. Uwe Schweickert looks at some of the key trends impacting on high specification oscillators
A
ll the systems in our modern world need a clock either as a reference or for synchronisation. Wireless
communication is a good example; these digital systems consist of a number of radio cells and the users are normally mobile, so they move from one cell to another. Users don’t want to lose their connection when they move, so there must be a connection between the cells and also a synchronisation to handover the connections.
Responsibility for this synchronisation is a high precision crystal oscillator, normally an ovenised (oven controlled) crystal oscillator commonly known as an OCXO. Also for broadcasting transmitters it is very important to have a highly stable clock which is used for reference and for synchronisation.
could only send documents on paper via telephone lines. The standard today is different kinds of ‘Digital Subscriber Line’ (DSL) for wired data communication with transfer rates up to 50Mbit/s on standard copper lines. For wireless communication we use either the ‘Global System for Mobile Communications’ (GSM) or the ‘Universal Mobile Telecommunications System’ (UMTS) which allows data transfer rates via ‘High-Speed Downlink Packet Access’ (HSDPA) for download up to 7.2Mbit/s and network coverage is nearly everywhere.
In the near future we will have much Figure 1 For these fields of application, OCXOs in
the frequency range from 10 to 40 MHz are typically used. Depending on the requirement of the system performance, high performance TCXOs can also be used or for very high performance, a GPS based synchronisation module can be used; this includes an OCXO which is locked to a very accurate reference signal via GPS.
Communication system requirements
If we look back some years ago, the requirement for such systems was much lower. We had analogue telephone lines with a frequency range of 300Hz to 3.4kHz, for mobile phones we had huge and very expensive systems which could be installed in a car, but it was not possible to buy a hand-held unit. For data transmission there was Fax but with this system we
16 February 2012
more interaction and linking of people, home and vehicle for communication, internet and multimedia. New standards like ‘3GPP Long Term Evolution’ (LTE), ‘Next-Generation Network (NGN) and ‘Fiber to Home’ (FTH) will help to cope with the rapid increase of data volume and higher transfer rates. The broadcasting industry has also undergone big changes. Before Satellite TV was introduced, there were for example in Germany, only 3 analogue programs available via a terrestrial antenna. The quality was poor, because of the refresh rate of only 50Hz. Today the standard is High Definition (HD) digital broadcasting via cable or satellite, with many more channels based on new digital modulation techniques which allow much more data to be transmitted compared with the old analogue modulation techniques. Also in the future we will have many more functions such as 3D without special glasses and much more interaction between the users and the broadcast stations. This will require much more data, so the requirements for the modulation techniques will increase and also the requirements for the reference clocks and synchronisation modules.
New demands These new system requirements necessitate new solutions from the oscillator industry as several performance parameters have to be improved. Due to the significant increase in users in the future, more transfer channels or wider bandwidth is needed. But the available frequency range for the different
Components in Electronics
kinds of technologies is limited, so one thing is to reduce the tolerances. With tighter tolerances the gap between the channels can be reduced, so the bandwidth for each channel can be increased or with the same bandwidth, more channels can be defined in the same frequency range. Higher Transfer Rates require reduced Bit
Error Rates. For this requirement the stability of the clock source must be better, so the Jitter has to be reduced and the Phase Noise performance has to be improved.
Most of the communication systems need higher reference clocks like high performance crystal oscillators can deliver, so the system needs technologies to generate higher frequencies from a low frequency clock. This can be realised such as by using a ‘Phase Locked Loop’ (PLL), but this will decrease the performance of the system. So to get a higher resolution one approach is to use higher reference clock oscillators. For higher frequencies and to reduce
board space it is necessary to use small and SMD-package sizes. The technique goes to more and more outdoor units all over the world, so the temperature range will increase. To realise these new requirements there are some parameters which have to be considered.
Importance of the Crystal The most important parameter for the stability of an OCXO is the Crystal. For high performance OCXOs a SC-cut crystal is used; for 10MHz a 3rd Overtone crystal is typical. The oscillation mode 3rd Overtone is preferred compared to a Fundamental mode because of its higher stability in all cases because of the blank thickness. The blank thickness is inversely proportional to the crystal frequency, so a highly stable crystal should have thick blank to reach the best stability.
When we talk about stability this has to be defined in a more accurate way. One part of stability is defined over a long time period like days, months or years which is called the Ageing. Typical values for a 10MHz OCXO are 50ppb per year, real good values are in the range of 20 to 30ppb per Year. This parameter is very important for the definition of the system stability over a long time of operation. For only a short time period like 1 sec up to 100 sec Short Term stability is a key factor. Short term stability can be described in the time domain as Allan Variance or in the frequency domain as Phase Noise. These effects are caused by instabilities of
the crystal. To reach good short term stability a crystal with a high Quality Factor (Q-Factor) is necessary. The Q-factor depends on the crystal mode, frequency, package and some other parameters defined during the crystal production process like roughness of the surface, material of the electrodes and the quality of the raw material from which the crystal blank is cut. A 3rd overtone crystal reaches higher Q-factors compared with fundamental mode at the same frequency. For a 5th overtone at the same frequency the Q-factor is also better but also the resistance will increase. Therefore it is very complicated to produce low frequency crystals in 5th overtone. Also the high resistance of the crystal can cause problems in the oscillator circuit to guarantee a stable oscillation under all cases.
Figure 2
For higher frequencies in the range above 50MHz, 5th overtone SC-Cut crystals are the best choice to reach a high stability because for 3rd Overtone crystals the Q-factor decreases and also the ageing will get worse compared with a 5th overtone.
So typically 5th overtone crystals are generally the best choice for higher frequencies but this needs a lot of research and development in the crystal design and crystal production to produce these crystals with really tight tolerances. To use higher overtones like 7th or 9th is theoretically possible, but it is very hard to build and measure these crystals and also the oscillator design is very complicated because of the high resistance and the low pullability of these crystals.
Another important parameter is the temperature stability. Because we talk about OCXOs, this parameter is mainly defined by the heating circuit and heating control of the oscillator circuit. For the crystal it is very important to have a tight adjustment tolerance at the turnover point because the pullability of a 5th overtone crystal is less compared with a 3rd overtone.
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