Quartz-Based Resonators for Short-Distance Radio Applications

By Christoph Steinebrunner (Product Marketing Manager SAW Components)

Page Contents:   [ Featured Products | Datasheets | Application Notes | Buy Now ]

Rugged Resonators in Mini Format
EPCOS has developed a new miniaturized SAW filter and resonator series for industrial applications, which, thanks to their ceramic packages, are extremely rugged.

Center frequency:	315MHz or 433.9MHz
Effective bandwidth:	320kHz
Insertion loss:	2.4dB
Footprint:	3 x 3mm2

More and more radio based systems are currently being developed to enhance comfort in the home. Typical examples such as radio headphones, garage door openers and lighting in and around the home can now be conveniently operated without wires at the press of a button. In future, even the consumption data of electricity, water, gas and heating energy will be read off by the utility companies via radio. Radio based systems are also finding an ever broader range of applications in the industrial and security sectors. They include access controls, fire and intrusion alarm systems as well as the transmission of data from production installations or the control of cranes.

All these systems must be manufactured cost effectively and as far as possible have minimum dimensions and weight. But they must also be rugged and operate reliably even under harsh environmental conditions such as rapid temperature changes, shock and vibrations. These requirements naturally also apply to the components used in them such as surface acoustic wave (SAW) resonators.

Since the beginning of 2006, EPCOS has been offering quartz filters and two-port resonators in a rugged SMT ceramic package measuring only 3 x 3mm². This means an area reduction by 38 percent compared with the predecessor types with a footprint of 3.8 x 3.8mm². The two port resonators even represent a 64% reduction from the previous 5 x 5mm² package. The insertion heights of the components have also been significantly reduced: the filters have shrunk from a previous height of 1.5mm to only 1mm, and the two port resonators from 1.35mm also to 1mm. With the introduction of a new generation of quartz filters and the two port resonators, all types of SAW components for short-distance radio applications are now available in 3 x 3mm² packages: they even satisfy the qualification to AEC-Q200 criteria. Table 1 shows key data of the new quartz filters compared to the previous larger types. Figure 1 shows the attenuation curve of the new B3730 filter (in blue) and its predecessor type B3760 (in red).

 

 

 

Due to the higher speed of propagation of surface acoustic waves on lithium tantalate substrates, these filters could previously be implemented at identical frequencies with smaller active structures on the surfaces than quartz-based filters. However, new design techniques now also allow the benefits of quartz filters to be offered for the first time in a 3 x 3mm² package and at similarly attractive prices as lithium tantalate systems. Table 2 shows the key differentiation criteria between quartz (Q) and lithium tantalate (LT) filters of the same dimensions.

 

 

Maintaining Useful Bandwidth

The first representatives were developed for the two key center frequencies of 433.92MHz (Europe) and 315MHz (USA, Asia) of the ISM (Industrial, Scientific, Medical) bands. Table 1 shows a comparison with filters in the larger 3.8 x 3.8mm² package, the B3761 (315MHz) and B3760 (433.92MHz) series. The comparative series in the 3 x 3mm² package have the designations B3731 and B3730. It is initially striking that such low losses are recorded for the near end and far end selectivity. However, the insertion loss is increased by 0.5dB. The useful bandwidth was maintained at around 300kHz over the temperature range from -40°C to +95°C that is usual for motor vehicles. To keep the impedance rise as low as possible constituted a particular challenge, as it usually rises strongly in the case of miniaturization. It could be kept almost constant in the B3730 by using the design know-how developed over many years. In the B3731, with its more critical lower center frequency, an impedance of only 610Ω was reached, considerably less than the target maximum of 1000Ω.

SAW resonators in the 1-gate version have already been offered for several months with narrow or broader initial tolerances for transmitters with the very reliable frequency modulation or simpler amplitude modulation in a 3 x 3mm² package. The two port variants are now also available in these dimensions. This means that the two key benefits of miniaturization can be used for even more applications:

• More compact design or higher level of integration
• Implementation of simple, reliable and particularly cost effective transmitters and local oscillators in receivers

 

 

In addition to applications in motor vehicles, these benefits are also very important for diverse non-automotive applications. Thus, the broader toleranced low-cost SAW 1-gate resonator variants in the 3 x 3mm² package can replace the previously less expensive LC oscillators with lower frequency accuracy that are still used in their millions in non-automobile applications. The price advantage is shifting in favor of the use of low-cost resonators against the background of the higher cost of tuning in the manufacture of the LC oscillators. In addition, two port resonators can be used to implement a Pierce oscillator. This is more stable than the Colpitz oscillator usual for 1-gate resonators, especially at higher frequencies (>800MHz). The Pierce oscillator is used especially as a local oscillator in the receiver circuit of radio headphones due to its low power consumption compared with a PLL-based oscillator. The rechargeable batteries then have longer intervals between charges.

 

Benefits of Two-Port Resonators

The component area was successfully reduced by more than 60 percent compared with the 5 x 5mm² package version – with no significant performance loss. In particular, the initial tolerance was retained and the insertion loss was increased by only 0.5dB. Table 3 shows the key data of the new two port resonator and compares it with the larger predecessor version. This component also shows good resonance behavior, making it equally suitable for new, more highly integrated and more cost effective Pierce oscillators.

 

 

 Featured Products

Part Number		Description	Data Sheet	App. Notes
B39321-B3731-H110-MR		SAW Filter, 315MHz, for Short Range Devices
B39321-B3731-H110-MR		SAW Filter - Short Range Device 315MHz
B39321-R981-U410		SAW Resonator, 315MHz, for Short Range Devices
B39431-B3730-H110-MR		SAW Filter, 433.92MHz, for Short Range Devices
B39431-B3730-H110-MR		SAW Filter - Short Range Device 433.92MHz
B39431-R960-H110-MR		SAW Resonator, 433.92MHz, for Short Range Devices
B39921-B3588-U410-MR		SAW Filter, 915MHz, for Short Range Devices
B39921-R2906-H110		SAW Resonator, 915MHz, for Short Range Devices

refers to New Product Introduction