Wideband VCXO Using Resonators Of Quartz Alternatives
by Bernd Neubig, AXTAL Products

VCXO usually employ quartz crystal units as the frequency determining component. By applying a DC-voltage at the EFC- (Electronic Frequency Control) input of the oscillator, the output frequency can be varied within certain limits. The frequency pulling range of quartz crystal based VCXOs is typically ±50 to ±150 ppm for a control voltage range of 0.5 to 4.5 V.

Figure 2: Gallium phosphate crystal
(Courtesy AVL List http://www.gapo4.com/)

Figure 3: Langasite crystal
(Courtesy Fomos)

The recent progress in the industrial production of new piezoelectric materials such as: Berlinite (aluminium phosphate AlPO4), Gallium phosphate (GaPO4) or Langasite (“LGS”, Lanthanum-Gallium silicate La3Ga5SiO14) allows the manufacture of resonators with an essentially higher pullability than quartz crystals. With these new resonators it is possible to realize wideband voltage controlled oscillators with ten times the pulling range of quartz-based VCXOs.

Figure 1: Electrical circuit diagram
of a piezoelectric resonator

Quartz crystal units offer good frequency stability and a high Q-factor. Their low dynamic capacitance C1 referred to their shunt capacitance C0 as a severe disadvantage for VCXO and filter applications. In VCXOs it is the limiting factor for the frequency pulling range.

The capacitance ratio r

is a measure for the pullability of a resonator. The higher the r, the smaller is its pulling range. AT-fundamental mode quartz crystals possess a r value of approx. 200, in the third overtone mode r is about 1800. The capacitance ratio is determined by the piezoelectric coupling factor k

which is dependent on the crystal material and the crystal cut angle.

Besides quartz there are a number of other crystals which are piezoelectric. Only those materials can be considered for oscillator application, where single rotated crystal cuts with a low temperature coefficient exist. The higher the piezoelectric coupling factor k is, the higher is the realizable pulling range.

So-called quartz-homeotypes, i.e., crystal materials belonging to the same crystal class 32 as quartz, have a big practical advantage. The most prominent quartz alternatives are Berlinite (aluminium phosphate AlPO4) and Gallium phosphate (GaPO4). Research and development of these crystals started already in the 80’s. In recent years Langasite (“LGS”, Lanthanum-Gallium Silicate La3Ga5SiO14) became more and more important as a new piezoelectric material for resonators. The actual research focuses on other lanthanum compounds such as Langatate (“LGT”, La3Ga5TaO14), and Langanite (“LGN”, La3Ga5NbO14). Today most of these materials are commercially available as synthetically grown crystals in different sizes.

As shown in Table 1, all mentioned new piezoelectric materials have a significantly higher coupling factor than quartz and can therefore be used to realize VCXOs with a very wide pulling range.

Figure 4: Coupling factor of GaPO4 as a function of the cutting angle (1)

Figure 4 shows how the coupling factor varies with the cutting angle . The maximum k is in the vicinity of the so-called Y-cut, i.e. = 0°. The optimum cut angle for wideband-VCXO applications is the locus, where a high coupling constant is combined with zero-temperature coefficient of frequency.

In the following two realized examples at 5 MHz and 10 MHz will be introduced, which are offered by AXTAL Products under part number AXIS30 (see http://www.axtal.com/).

Wideband VCXO at 10 MHz
For these VCXOs resonators manufactured in the enclosure HC-52/U, the following typical data were achieved:

dynamic capacitance C1 = 112 fF
shunt capacitance C0 = 5.2 pF
resonance resistance Rr = 2.5 Ohm
Q-factor Q = 70.000

The capacitance ratio r is 46, which is five times smaller than a quartz crystal of same frequency and size.

Figure 5: Frequency vs. temperature response of a resonator

The frequency vs. temperature response is shown in Figure 5. It has the shape of a 2nd order parabola with a 2nd order coefficient of superior than -2·10-8 / K2. The turn-over point can be oriented in the center of the operating temperature range, e.g. at 22°C, by choosing a suitable cutting angle. This leads to a frequency stability of ± 20 ppm in the temperature range from -20° to +70°C. In an extended range of -40° to +85°C a temperature stability of ± 40 ppm can be achieved.

With this resonator a VCXO is realized in a small SMD package of the size 9 x 14 mm. It has a pulling range of more than 2500 ppm for an EFC range between 0.25 and 4.75 V (see Figure 6). This is about 10 times the pulling range of a conventional quartz VCXO!

Figure 6: Pulling characteristics of the 10 MHz VCXO

To achieve a pulling range in excess of ± 100 ppm, quartz-VCXOs require additional inductors in the frequency determining circuit. The larger the inductance, the more the frequency stability is governed by the stability of the coil rather than the crystal.

VCXOs made from quartz alternatives do not show this disadvantage.

Wideband VCXO at 5 MHz
5 MHz of Quartz crystal units in a small enclosure like HC-52/U are designed as convex or contoured plates, which leads to a very low C1 value and a large capacitance ratio r.

The quartz alternative has a thinner resonator plate, which allows the use of a plano-parallel resonator plate, and therefore yields a more favorable capacitance ratio than a convex plate.

The 5 MHz resonators in HC-52/U have the following typical parameters:

dynamic capacitance C1 = 58 fF
shunt capacitance C0 = 2.8 pF
resonance resistance Rr = 6 Ohm
Q-factor Q = 90.000

The capacitance ratio r is about 48. Quartz crystals of equal frequency in the same size have a r value, which is about 7 times higher. The temperature response has the same parabolic shape as depicted in Figure 5.

Figure 7: Pulling characteristics of the 5 MHz VCXO

VCXOs were realized with these resonators in the same SMD package 9 x 14 mm. The pulling characteristics shown in Figure 7 shows a pulling range of ± 700 ppm for an EFC voltage range of 0.25 to 4.75 V.

Conclusions
Resonators made from quartz alternatives can be successfully applied to manufacture stable wideband VCXOs, which have 10 times the pulling range of conventional quartz crystal oscillators. As these alternative materials are now available in industrial quality, such wideband VCXOs can now be offered as professional products.

The essential parameters of the AXTAL model AXIS30 are summarized in Table 2.
For more details about wideband VCXOs, please refer to the website http://www.axtal.com/.

References
1 “Temperature compensated orientations in BAW Resonators”; AVL paper, Aug. 2002, http://www.gapo4.com/

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