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A Vector Attenuator for Feedforward Amplifier and RF Predistortion Use
Micro-Precision Technologies Inc. (MPT) Salem, NH
Feedforward amplifiers and RF predistortion circuits used in cellular radio base stations require gain and phase adjustment in their cancellation loops. The classical approach to this requirement has been through the use of variable attenuators and phase shifters. However, these components exhibit several characteristics that make their use less than ideal. In particular, variable phase shifters have several limitations, including limited range. While a 360° range may be adequate in principle, consider a situation in which the phase shifter has been adjusted to the 360° maximum and a change of one degree is required. The control system driving the attenuator is required to return to 1° by changing 359° rather than the one degree change that is desired. Even if the control system has the intelligence to recognize this situation and handle it correctly, a transient condition will occur as the phase shifter is adjusted to the proper value. In practice, a situation such as this is unacceptable.
Even if the wraparound characteristic of a phase shifter does not present a problem, the limited practical range of the phase shifter may. A single-section varactor phase shifter will usually have no more than 120° of range. This limited range may be increased by adding series inductance to the varactors, effectively building a tuned circuit with greater change in reactance for a given change in varactor capacitance. The problem with this approach is the introduction of group delay distortion. Ideally, the phase shifter should have a linear phase vs. frequency characteristic. A varactor phase shifter will exhibit variable group delay (nonlinear phase), which will only worsen if the varactors are resonated to increase the phase change range. Typically, if 360° of range is necessary, from a group delay standpoint it is preferable to cascade three single-section phase shifters of 120° each. Even this approach will have significant group delay distortion. The extent to which this distortion can be tolerated depends on the application. The greater the signal bandwidth and the greater the required cancellation depth, the more this distortion is a problem. In this regard, the distortion cancellation loops in feed-forward amplifiers and RF predistortion circuits are particularly sensitive.
Another limitation of varactor phase shifters that is often encountered is poor intermodulation performance. Since the purpose of feed-forward amplifiers and RF predistortion circuits is to minimize intermodulation products, this limitation is an important consideration.
Variable attenuators also display variable phase as well as group delay with changes in attenuation. Although the companion phase shifter can be used to null out the attenuator's phase change, this correction introduces cross coupling between phase and gain, which is undesirable from a control system standpoint. The phase shifter cannot be used to correct for any group delay change. Since cancellation loops must be adjusted for delay as well as phase, this variability is a problem if cancellation over a large bandwidth is required.
All of these difficulties can be avoided or minimized by using a vector attenuator. Architecturally, a vector attenuator looks like a vector modulator. The fundamental difference is that attenuators rather than modulators (or mixers) are used in the signal paths. The input signal is split into in-phase (I) and quadrature (Q) signals, as shown in Figure 1 , that are applied to biphase attenuators. The attenuator outputs are combined in phase. By controlling the attenuators, any phase and amplitude combination of the output signal can be achieved. A control system controls the I and Q attenuator inputs rather than the phase and attenuation of a phase shifter and attenuator. The vector attenuator displays negligible group delay distortion and virtually no change in absolute delay with changes in control inputs. The advantages are obvious: Any phase can be achieved without the wraparound phase shifter problem and the cancellation depth can be maximized due to the lack of group delay distortion or delay change.
The recently developed model MPT 810VA vector attenuator uses PIN diodes as the controlling elements, producing good intermodulation performance. A vector attenuator has a minimum theoretical loss of 6 dB. The MPT 810VA device exhibits a minimum typical loss of 10 dB. The maximum loss depends on the amplitude and phase flatness specifications at that loss value. Figure 2 shows the vector attenuator's transmission loss and phase vs. I and Q current (fixed delay and phase removed). The amplitude and phase flatness at a loss of 20 dB are sufficient to yield a cancellation depth in excess of 35 dB for typical cellular radio bandwidths. Figure 3 shows the unit's input and output return losses.
The vector attenuator is built in a 10-pin surface-mount package measuring 0.8" x 0.8". The unit operates from 800 to 1000 MHz and is functionally equivalent to the combination of an endless phase shifter and attenuator. It can be used to control a signal's amplitude without introducing intermodulation distortion, dispersion or group delay variation. The vector attenuator has no limitation on phase change, that is, the phase can increase or decrease continuously without reaching an end point. The unit can also transition directly from any attenuation and phase, such as from minimum attenuation at 0° to minimum attenuation at 180°, without a continuous phase transition from 0° to 90° to 180°. This characteristic makes the MPT 810VA particularly useful in signal cancellation systems where the phase and amplitude of one signal must be adjusted to completely cancel another signal of arbitrary amplitude and phase.
While it is possible to build a vector attenuator using discrete components on a printed circuit board, significant performance limitations will result due largely to circuit parasitics. The MPT 810VA vector attenuator maximizes performance and minimizes circuit size with thin-film construction, producing a cost-effective solution for phase and gain control in feedforward amplifiers and RF predistortion circuits.
Micro-Precision Technologies Inc. (MPT), Salem, NH (603) 893-7600.
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