The increased use of multi-carrier radios in cellular infrastructure systems is driving the requirement for high linearity components, especially in the power amplifier chain. Isolators and circulators have historically been known to impact the overall intermodulation distortion (IMD) performance of a wireless transmission system and are therefore critical to specify. Engineering teams from Skyworks Solutions and Trans-Tech (a subsidiary of Skyworks Solutions) have successfully developed new materials/device designs for isolators and circulators that are optimized for IMD performance and offer the industry very low IMD specifications. The new SKYFR-000700 circulator has IMD performance better than -90 dBc with two +47 dBm tones.
IMD is a nonlinear effect of two or more signals mixing within a device to produce undesirable higher-order products. These unwanted signals may fall within the transmitting or receiving bands, causing interference as a result. These relationships are illustrated in Figure 1, where f1 and f2 are two frequency tones.
Figure 1 IMD products for a two tone signal with frequencies f1 and f2.
As the odd-numbered products occur at frequencies close to the fundamentals, they are the ones of greatest concern, with the third-order being the most dominant. An industry-standard test to measure the IMD performance of a device uses two continuous wave (CW) tones, f1 and f2. These signals, separated by 5 MHz, are combined and input to the device. At the output, the third-order and fifth-order products are measured. The third-order products are at 2xf1 – f2 and 2xf2 – f1 and the fifth-order products at 3xf1 – 2xf2 and 3xf2 – 2xf1.
Figure 2 SKYFR-0007000 image.
The SKYFR-000700 is a single junction circulator in a 25 × 25 mm housing (see Figure 2), designed to operate in the standard GSM band of 925 to 960 MHz. This circulator can achieve IMD performance of better than -90 dBc with two CW tones of +47 dBm, spaced 5 MHz apart. Two test conditions were used, f1 = 925 MHz and f2 = 930 MHz, and f1 = 960 MHz and f2 = 965 MHz, with each frequency tone at +47 dBm.
The IMD performance of the SKYFR-000700 is compared to a competitive unit. The competitor's unit is designed for maximum IMD performance using industry-standard magnetic material. The competitive unit achieves IMD performance of -73 dBc versus the Skyworks unit that is better than -90 dBc. The actual performance is shown in Figure 3.
Figure 3 SKYFR-000700 two tone IMD measurement at 925 MHz.
The significant improvement, around 10 dB, in IMD performance of the SKYFR-000700 is the result of unique material selection and improved magnetic and electrical design in the circulator junction. This new approach to non linearity allows the circulator designer to achieve IMD performance of more than 10 dB improvement over existing products. It is expected that additional improvements will be offered in the future by further optimizing combinations of device and materials.