A Digitally Compensated Phase Shifter

A Digitally Compensated Phase Shifter

COMSAT RSI,
Anghel Laboratories
Rockaway, NJ

Developing a phase shifter that produces a constant or flat phase response with respect to frequency is difficult. Networks that exhibit this type of behavior are narrowband and must be optimized for each specific frequency and desired phase delay. Fortunately, many applications that require phase shifters have frequency information associated with them, that is, although the application may require a broad frequency range, it is only used over a narrow, predetermined portion of that band at any given time. Taking advantage of this application information, a new digitally compensated phase shifter has been developed that receives frequency and phase shift information on its inputs and generates the required phase for that frequency by accessing a precisely calibrated look-up table.

The Microwave Design

The approach to the phase shifter's microwave design is shown in Figure 1 . It combines the good stability and repeatability of a digital PIN diode switched-line phase shifter with the high resolution and flexibility of a varactor-controlled analog phase shifter. The resultant combination produces stable phase shifts with a small step size and high accuracy. For extremely wide bandwidth applications, a bandswitch option is installed so that the analog portion is effective over the full band.

Control Circuits

To control the microwave portion of the device, control circuitry, shown in Figure 2 , is used. PIN diode drivers control the switched lines and bandswitch. A digital-to-analog (D/A) converter drives the varactor phase shifters. These functions are driven from an electrically erasable programmable read-only memory (EEPROM) look-up table that is programmed to the specific needs of the user's requirement, including input pin assignments for phase and frequency. The entire circuit is built as a single assembly using surface-mount components. Access to the board for programming is achieved through a custom test connector that mates with a printed pattern on the board.

Software

The capability of the microwave and control hardware is practically limitless. To harness this capability and direct it into a useful product requires a powerful software bundle that interfaces on one end to the user requirements and on the other end to the automated test equipment (ATE) that calibrates the unit and records data. On the user end, the entire product specification is defined, including user name, model number, supply voltages, connector pinout, frequency sub-band definition, phase resolution and tolerances. The data file generated with this specification is exported to the ATE station where the unit is programmed automatically and tested to the specification limits.

Because the software is flexible, the 12 user inputs can be allocated exactly as required with no limitation on how many or which inputs are allocated to the phase or frequency function. Although the phase is restricted to binary weighting, the frequency band allocation is completely open, permitting noncontiguous or overlapping bands of varying sizes and relative positions. The frequency input coding can be binary, binary-coded decimal or otherwise user defined. A major key to the success of the software (and of the product in general) is the flexibility of design that requires little or no manual intervention. Figures 3 and 4 show examples of the phase shifter's worst-case phase error and worst-case insertion loss, respectively.

Packaging

The microwave and control circuits are packaged in an aluminum housing that measures 1.00" x 3.25" x 5.00". The power and control connector is a 20-pin, dual-row header with a self-locking body and the RF connectors are SMA. The microwave structure is designed to accommodate a family of microstrip configurations that cover 1 to 20 GHz in overlapping octave increments. Thus, the same housing and control assemblies are used for all phase shifters. (Only the microstrip assembly is changed to accommodate the specific octave bands.) This capability permits the manufacturer to stock units with much of the assembly complete, shortening delivery times and minimizing charges on special orders.

Additional Features

To integrate the digitally compensated phase shifter into a variety of applications more easily, the 12 digital input lines are equipped with latching buffers. The input threshold is 2.5 V, making the assembly compatible with most logic families as well as open collectors and contact closures. The inputs are rated at ±50 V (max) for durability in a variety of operating environments. The input latches also are available to the customer with a ±10 V (max) input. In addition to the functions of phase and frequency, the user may designate one of the 12 input lines as an extinguish control that places the unit in a high insertion loss state when it is asserted. Table 1 lists the digitally compensated phase shifter's general specifications; Table 2 lists the unit's typical bandwidth/resolution trade-offs.

The digitally compensated phase shifter is not able to satisfy all applications. The basic premise upon which the unit is built renders it unsuitable for spread spectrum applications. In addition, the analog section typically limits the amount of RF input power to +10 dBm and the switching speed to 2 ms. However, in cases where these limitations are not significant, the phase shifter's flexibility and capability permit special orders to be satisfied in only a short period of time as well as extremely repeatable production units that require essentially no manual tuning or testing.

COMSAT RSI,
Anghel Laboratories,
Rockaway, NJ (201) 627-5981.