A multistage resonant phase shifter can be considered a transmission line with a variable phase velocity. To obtain larger phase shifts, several resonators can be concatenated into a multistage resonant phase shifter. In the case of a microwave phase shifter, all of the resonator components are already present as varactor chip and package parasitics as well as external reactive components used to tune out the parasitics. If an amplitude variation of 3 dB is allowed, a phase shift variation of 90° can be obtained. As the circuit is tuned around its resonant frequency, the introduced phase shift is changed. The principle of operation of a single-stage resonant phase shifter is straightforward, as shown in Figure 1. It is possible to build suitable resonant phase shifters for the STM64 clock frequency with a limited number of inexpensive surface-mount device (SMD) plastic-packaged silicon varactors. In particular, resonant phase shifters were found to be both simple and efficient. In addition, the microstrip circuit board area may become quite large due to the many quadrature hybrids and reactive components used to tune out varactor parasitics.ĭifferent phase shifter circuits were developed and tested to minimize the circuit board area and number of varactors used. The insertion loss of such a complex circuit as well as insertion loss variation with the varactor control voltage may become considerable. Due to the limited phase shift of available varactors, the concatenation of several phase-shifting stages is required. If chip and package parasitics are tuned out by additional reactive components at a particular frequency, circuit losses due to poor varactor Q will further increase.Ĭlock phase shifters require a range of at least 360° and some overlap is desired on both ends. The Q of silicon varactors may be less than 10 at X-band frequencies while GaAs varactors are just slightly better. Furthermore, varactors are rather lossy capacitors at microwave frequencies. The quadrature hybrid requires two identical varactors.1 However, the phase shift of available varactors becomes quite limited at microwave frequencies due to chip and package parasitics. The adjustable reflection coefficient of a varactor is transformed into a variable phase shift with a circulator or quadrature hybrid. In addition, some jitter may be removed from the clock signal by narrowband phase-shifting devices.Įlectronically controlled phase shifters are usually built with varactors when a continuous phase adjustment is required.
One side advantage of this technique is that distortion of the data signal due to reflections is minimized. Therefore, a practical solution is to feed the data line directly with no adjustable delays while the phases of all required clock signals are made adjustable. Fortunately, in most electro-optical systems only the relative phase between data and clock is important. If the delay must be adjusted frequently while changing some other components in the system or the drifting delay of some electro-optical devices must be compensated automatically, mechanical devices should be replaced, preferably by electrically adjustable delay devices.Įlectrically adjustable phase shifters are usually narrowband devices that can be used only to adjust the delays of clock signals. Since the bandwidth of stretchable coaxial (TEM) lines extends from DC up to the cutoff frequency of other waveguide modes, adjustable coaxial delay lines can be used both for data and clock applications. To compensate for (electrical) cable-length tolerances, mechanically stretchable coaxial lines are usually used in optical line-terminal equipment. This article describes the particular problem of providing a continuously adjustable delay for the STM64 (OC192)-specified clock frequency of 9.95328 GHz and its integer multiples and submultiples to control different electro-optical devices in optical-fiber line-terminal equipment.
There are several possible designs for adjustable phase shifters (or adjustable delay lines) depending on the operating frequency, required bandwidth, power handling and technology used (mechanical, ferrite, PIN diode, varactor or I/Q modulator).
0 kommentar(er)
