A Combined 360° Phase Shifter and Attenuator for Feedforward Amplifiers and Predistorters
Micro-Precision Technologies Inc.
Feedforward amplifiers and predistorters used for PCS base station transmitters characteristically use electrically variable attenuators and phase shifters in their various signal cancellation and control loops to adaptively adjust the phase and amplitude of the signals involved. Typically, these components are built using discrete components on a PCB. Phase shifters usually consist of a quadrature hybrid, varactor diodes and miscellaneous biasing and bypass components. A single-section phase shifter of this type will typically yield approximately 120° of phase shift. If greater range is desired, sections are cascaded. Attenuators are typically built with multiple PIN diodes and may be configured as p- or T-section designs, or they may be reflective designs using quadrature hybrids. Usually, a single attenuator section will yield sufficient attenuation range for these applications. For applications that require a full 360° of phase shift range as well as 20 dB of attenuation range, the entire circuit will require as many as four surface-mount quadrature hybrids, 12 varactor diodes, four PIN diodes and a combined total of approximately 50 capacitors, resistors and inductors for biasing, bypassing and DC blocking purposes. Although the combined cost of these components is not excessive (approximately $25 in volume), the amount of real estate necessary on a PCB and the electrical delay (the bane of feedforward designs) are large. Typically, internal padding is also necessary to minimize delay ripple. This padding further complicates matters by requiring additional gain elsewhere in the circuit. All of these factors combine to drive up cost, reduce performance and increase power consumption and size.
The model MPT1820APS combined phase shifter and attenuator has been introduced to address these problems. This new device uses the same circuit techniques noted previously to provide 360° of phase shift range and 20 dB of attenuation range over the 1800 to 2000 MHz frequency range in a single surface-mount package that measures 1.00" x 1.00" x 0.17", including the leads and required functionality. The only external components required are bypass capacitors on the four control inputs. The resulting size reduction compared to a typical PCB implementation is dramatic. In addition, the nominal delay is very low — approximately 2 ns. This low delay translates directly to a lower value delay line in feedforward amplifiers, resulting in correspondingly lower loss. Internal padding is still used to minimize delay ripple but, due to the hybrid construction, this padding can be minimized. Overall, the minimum circuit loss is approximately 14 dB.
For typical PCS bandwidths (1930 to 1990 MHz, for example), delay variation for different phase shifter and attenuator settings is approximately ±100 ps. This level corresponds to a phase flatness of approximately 3° peak to peak. Amplitude flatness is held to approximately 0.35 dB peak to peak. The input third-order intercept is greater than 32 dBm for all control settings and increases for any attenuator setting above minimum. Overall, these data represent superb performance for a phase shifter and attenuator combination and would be difficult to match with a PCB implementation (even if board space were not an issue).
Figure 1 shows the device’s attenuation vs. control voltage. (Note the nearly linear attenuation vs. voltage characteristic.) The MPT1820APS unit has separate inputs for each of its three phase shifter sections. The individual phase shifters can be controlled separately as desired for certain applications using open-loop temperature compensation, or they can be ganged together to yield more than 360° of combined phase shift range as shown in Figure 2 . (Note the very linear phase shift vs. control voltage characteristic.) Input and output return losses are shown in Figure 3 .
The MPT1820APS phase shifter and attenuator is intended for applications in the 2 GHz PCS band. A companion unit — the model MPT810APS — is being developed for 800 to 1000 MHz applications. Similar devices could be developed for any band up to approximately 3 GHz. The phase shifter and attenuator family is suitable for those designers preferring separate amplitude and phase control. Certain applications, such as predistortion schemes requiring r-q control or various temperature-compensation schemes, require the independent control. Some designers prefer the independent control for all applications due to the intrinsic simplicity associated with it.
For those designers who desire it, a family of vector attenuators designed for similar applications is also available. Unlike phase shifter and attenuator combinations such as the MPT1820APS, vector attenuators do not offer independent control. They achieve 360° phase adjust capability by splitting the signal into in-phase and quadrature components, attenuating each separately and then recombining them. In this way, endless phase capability and moderate attenuation range are achieved.
Typical vector attenuators have very nonlinear attenuation vs. control voltage characteristics and, even when driven with sine and cosine control inputs, the amplitude cannot be controlled independent of phase and vice versa. For many applications such as closed-loop cancellation systems in some feedforward amplifiers, this restriction is not an issue. For these applications, vector attenuators offer real performance advantages. Vector attenuators typically offer lower delay and delay variation, flatter amplitude and comparable performance in other categories at a somewhat lower cost. The lower cost may be offset by the need to provide external circuitry that offers some degree of linearization of the attenuation vs. control voltage characteristic. In the end, the choice between a vector attenuator and a phase shifter and attenuator combination is largely a matter of preference.
All of the company’s phase shifter and attenuator combinations and vector attenuators are manufactured using an advanced etchable thick-film process for thin-film-like performance at thick-film prices. This process yields high performance circuits in the smallest possible package size. The packages feature compliant leads with ±2 mil coplanarity for reliable reflow solder attachment, and the circuits are sealed to withstand normal circuit board cleaning methods.
The MPT1820APS phase shifter and attenuator represents a low cost, small-size, surface-mount device for PCS feedforward amplifier applications that will not sacrifice performance. The unit provides the circuit designer with a compact solution to the ever-increasing demand to reduce PCB size and eliminate components.
Micro-Precision Technologies Inc. (MPT),