High Frequency Prescalers for Advanced Synthesizer Applications
Modern digital communications, instrumentation, electronic warfare and radar systems require high frequency and/or broadband synthesizers that are stable in frequency and exhibit low phase noise at frequencies up to 50 GHz. Prescalers are typically used in phase-locked loop circuits for these synthesizer applications. A prescaler converts the RF input signal to a low frequency signal and phase locks it to a stable source. Prescalers reduce the phase noise of the RF signal by approximately 20log(n), where n is the dividing ratio, as long as the added phase noise of the prescalers is negligible relative to that of the RF signal. Where prescalers are not available (for advanced synthesizers with a maximum operating frequency greater than 18 GHz, for example), frequency conversion can be performed with a subharmonic mixer and a local oscillator (LO). In this case, however, the phase noise of the LO directly adds to the phase noise of the RF input signal and degrades its quality.
A new class of high frequency prescalers has recently been introduced based on advanced indium phosphide heterojunction bipolar transistor (InP HBT) technology. These divide-by-two and divide-by-four prescalers are available at three maximum operating frequencies — DC to 13 GHz, DC to 25 GHz and DC to 50 GHz. A divide-by-two prescaler has also been demonstrated with an operating frequency as high as 92 GHz,1 which paves the way for future products operating at frequencies up to 100 GHz. In this article, product performance, packaging options and system applications are discussed.
Product Performance and Packaging Options
Inphi’s prescaler products are listed in Table 1. These devices operate from a single power supply of either –3.3 or +3.3 V. The 13 and 25 GHz prescalers are available in a 7 x 7 mm2 surface-mount land grid array (LGA) ceramic package or in die form, and the 50 GHz prescalers are available in a metal package with V connectors or in die form. Evaluation boards with SMA connectors are also available for the 13 and 25 GHz versions. The 13668DV in an LGA package and the 50668DV in a metal package are depicted in the product photos.
These prescalers demonstrate high input sensitivity and constant output power across a wide frequency range. DC power consumption is extremely low, ranging from 230 mW (divide-by-two) to 360 mW (divide-by-four) for the 13 GHz products and from 520 mW (divide-by-two) to 700 mW (divide-by-four) for the 25 and 50 GHz products. Input sensitivity is typically less than –15 dBm for the 13 GHz versions, –12 dBm for the 25 GHz versions and –10 dBm for the 50 GHz versions, up to the maximum operating frequency. Output power is maintained at a level of > 0 dBm for single-ended operation and > 6 dBm for differential operation. Figure 1 shows a typical output voltage waveform of the 50668DV for an input frequency of 40 GHz. The peak-to-peak voltage (single-ended AC coupled) is 598 mV.
For optimum performance, the phase noise of a prescaler must be significantly lower than that of the oscillator. Figure 2 depicts the phase noise of a 50 GHz source and that of the 50668DV outputs at an input frequency of 50 GHz (the 50668DV supports both divide-by-two and divide-by-four operations). Test equipment limitations prevented the direct measurement of the added phase noise of the 50668DV. It is clear, however, that the added phase noise of the 50668DV is significantly below that of the 50 GHz source, as can be seen by comparing the two figures. The divide-by-four output of the 50668DV is approximately 14 dBc/Hz lower than that of the 50 GHz source, indicating that its added phase noise is indeed negligible relative to the 50 GHz source.
Inphi’s high frequency, low power prescalers enable system design engineers to perform the phase-locked loop function up to 50 GHz without having to down convert the RF signal to an intermediate frequency using a subharmonic mixer and separate LO. The new prescalers should significantly enhance the performance of broadband YIG oscillators, millimeter-wave VCOs and lidar synthesizers for advanced communications, electronic warfare, remote sensing and radar applications up to 50 GHz.
A new class of high frequency prescalers has been introduced for advanced synthesizer applications from DC to 50 GHz. These products are based on advanced InP HBT technology and offer significant improvements in operating frequency and power consumption over existing products.
1. Srivastava, et al., IEEE GaAs IC Symposium Late News Papers, October 2002.
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