Choosing TCXOs for high-frequency apps

There are numerous benefits and risks to weigh with either a PLL or analog multiplying scheme

BY T. WICKARD and B. YUEN
Vectron International, Hudson, NH
http://www.vectron.com

There are many factors to consider when selecting a TCXO for UHF frequency system requirements. The first decision is the standard one: make versus buy. The availability of digital PLLs makes it tempting to generate the necessary UHF at the system level.

Other factors range from the signal type required to the stability necessary to achieve the mission of the system. All these factors are related, and it is difficult to separate one from the rest. The physics of the quartz resonator used in the TCXO will dominate, while the circuit design and method used to achieve the output frequency will likewise determine many of the characteristics of the TCXO.

Size does matter

Real estate is a premium in many system designs. To save space and costs, in recent years system components that once filled a complete rack have been reduced to a single slot in a backplane. Therefore package size, both area and height, continues to be the primary concern when selecting any frequency control product.

UHF TCXO products are not different. It was not long ago that one needed a 2-in.2 package to achieve the same performance that is available today in a 9 x 7-mm package. However, a number of the methods used to reach the UHF and higher frequency range will consume board space and grow the overall package volume.

The most common method for high-performance TCXO products is simple analog frequency multiplication, which takes advantage of the harmonics produced in the oscillator stage. By using a simple tuned circuit, it is possible to select and subsequently amplify these harmonics to produce the necessary frequency.

Another method is the use of a PLL. Both approaches can increase overall unit size, which may result in the loss of a system slot if the height increases or may require the area of the system to grow. If the aim is to minimize part count and reduce size, several options are available when specifying a UHF TCXO.

First, the use of high-frequency fundamental bulk-acoustic-wave (BAW) crystals may be used. With BAW crystals, the fundamental mode of the quartz crystal can be produced into the 200-MHz range.

Also, TCXOs using the third overtone permits the frequency range to push into the 700-MHz region without multiplication. Similarly, surface-acoustic-wave (SAW) resonators can be employed in TCXO products to provide a small and economical TCXO alternative. Because SAW resonators can be built to operate directly in the UHF range there is no need for the use of multiplication. The result is a much simpler oscillator design that requires the equivalent board space of a fundamental BAW device.

Frequency vs. temperature

The frequency versus temperature stability of a TCXO product will always be tightly coupled to the performance of the quartz resonator. The accompanying table includes a comparison of package size and temperature performance for high frequency TCXOs.

Frequency stability over the required operating temperature range determines what is necessary for the system requirements. Over-specifying the stability versus temperature range will result in higher costs. In the most basic terms a TCXO works by generating an inverse polynomial to the quartz resonator frequency versus temperature characteristic.

Today high-order polynomials are used to compensate for the deviation from the third or fourth order describing the quartz resonator. Coupled-modes (sometimes referred to as perturbations) result in deviations from the “ideal” polynomial and even the highest order polynomial generator can not compensate these and thus the performance over temperature is compromised.

Keeping the operating temperature range as narrow as possible will permit increased performance since the resonator can be designed to move coupled-modes outside the required range. The optimal performance of UHF TCXO will be obtained using precision BAW quartz resonators.

Other sources of frequency instabilities such as aging, hysteresis, retrace, and supply or load sensitivities should be defined in accordance with the total error budget permitted for the system. Over specifying any of these will drive costs.

Supply voltage andavailable power

Often the system will define the supply voltage and available power. A UHF TCXO design which employs PLL to achieve the UHF will often require a higher supply current. Multiplication and overtone BAW resonators will require the lowest supply current.

Finally, the supply voltage and current will impact the phase noise performance. In general phase noise performance will be better at higher supply voltages (8 V or higher) and supply current will be higher.

Subharmonics andoutput power

System tolerance for subharmonics is an important factor to consider when specifying a UHF TCXO. While sub-harmonics of less than –30 dBc is achievable, the cost will increase to achieve that level.

The output waveform can be Sine, PECL, HCMOS and others. In general, sine into 50 Ω at 0 dBm on a 3.3-V supply is preferred because it allows the oscillator to operate in the linear region and allow for filtering losses which in turn minimizes harmonics.

Higher output levels can also be a cost driver caused by the additional amplification required to achieve outputs of 5 dBm or greater. The system designer also needs to remember the available supply voltage often dictates the upper limits. PECL, in some cases, is easier to implement than the higher sine levels.

Phase noiseperformance

It is very important that the user define the phase noise performance required by the system as early in the specification process as possible, to account for the variables of phase noise degradation with multiplication, PLL noise levels, and crystal phase noise performance. For a UHF TCXO, it is ultimately the quartz that determines the device’s starting frequency, because quartz resonator frequency increases the sensitivity of the device to factors influencing the frequency stability.

The minimum thickness one can typically achieve with a BAW quartz resonator is approximately 0.001 in. using mechanical processing. Add chemical processing and the thickness can be reduced to 0.00033 in. These thicknesses equate to roughly 60 and 200 MHz, respectively.

If the system requires higher frequency, one needs to use a multiplier stage(s). However, each multiple, N, degrades the phase noise by the relation 20*log(N) so that tripling a 100-MHz fundamental will degrade the phase noise by nearly 10 dB.

One approach to eliminate the degradation is the use of a PLL. A TCXO based on a PLL design approach can provide the phase noise performance of a lower-frequency device at UHF.

Another approach is to use a SAW device to provide the necessary frequency without multiplication. The SAW approach will permit a low noise floor to be maintained but the compromise is the performance closer to the carrier (see Fig. 1 ).

Fig. 1. The above graph shows a comparison of the performance trade-off of BAW vs. SAW devices at various frequencies.

Environmental considerations

UHF TCXOs are frequently used in systems such as mobile instrumentation or military applications where the performance requirements must be maintained during shock, vibration, high temperature, and slew rates. The performance of a TCXO is greatly dependent on the environment, so in these applications the construction of the oscillator is important in determining whether the device will be robust enough to withstand the harsh environmental conditions.

Generally speaking, a TCXO is an ideal option for applications requiring a very wide operating temperature range. However, if operation at 105°C or higher for extended time is expected, special processing should be included to minimize the long term aging.

Quartz resonator designs using two-point crystal attachments tend to be more susceptible to vibration and shock versus three-point or four-point crystal attachment methods. With typical acceleration sensitivities of UHF TCXO products in the 1 to 5-ppb/G level, the impact of acceleration on performance is generally dismissed.

However, if the application has high acceleration or the system is highly sensitive, close attention is required to optimize the resonator design and these requirements need to be communicated. ■