Digital Microwave Radios have grown in sophisitication in order to support higher data rates. Gone are the simple modulation formats of years past, replaced by complex schemes such as 64-, 256-, and 512- state quadrature amplitude modulation (QAM).
Enabling these phase-sensitive modulation formats are stable local oscillators (LOs) such as the DFS series of frequency synthesizers from Elcom Technologies, Inc. (Closter, NJ).
Designed for C-, X-, Ku-, and Ka-band use, these stable sources operate in selected frequency bands from 1 to 23 GHz with the
low-noise performance neede to preserve high-speed data
transmissions in digital microwave radios as well as in very-small-aperture-terminal (VSAT) links.

The DFS synthesizers at a glance

The low-noise DFS synthesizers (see figure) measure only 3.75x4.0x1.0 in. (9.525x10.16x2.54 cm) yet incorporate an oven-controlled-crystal-oscillator (OCXO) reference source. They are available with standard step sizes of 125 KHz, 1MHz, 3 and 5 MHz, as well as 3 and 10 MHz for operating temperatures from -35 to +70 C. Typical DC power dissipation is 5W. This low DC power consumption maintains low operating temperatures for the DFS synthesizer active devices, enhacing long term reliability. The synthesizers achieve low spurious and harmonic noise in addition to low phase noise (see table). As users of digital microwave radios insist on more information carrying capability, the complexity of the modulation schemes in
these radios increases. Radios with 64 QAM and 4.55-b/s/Hz bandwidth efficiency are widely used for long-haul microwave links, and the systems up to 512QAM are under development. As the number of phase states increases, the amount of transmitted information increases as well, and the performance of the LOs in these radios becomes a creitical limiting factor in the overall performance on the radio. Conventional phase-noise measurement equipment tends to measure the long-termphase stabilities of a high-frequency source. It is the short-term instabilities, however, that wreak havoc on a digital microwave radio, causing large amounts of lost data. In a 135 Mb/s transmission system, for example, a phase transient of 100 us duration and sufficient amplitude results in a block of 13500 errors. In addition, if freaming bits are lost, then the necessary reframing process will cause even more erros. A phase transient will cause errrs when its peak value exceeds a certain threshold. The value of this threshold is only 7.69 deg. peak for 64QAM and 3.7 def. peak for systems using 256QAM (1). Phase transients may originate from within a synthesized LO (SLO) or from sources outside of it. Within the SLO, phase transients can be generated by mechanical contacts, unreliable solder joints, or defective components. Mechanical shoch and vibration can also trigger phase transiens from outside the SLO. Low microphonics or low gravitational (G)-force sensitivity are important in an SLO to minimize phase instabilities. With a special test system, the DFS series synthesizers were tested under shock and vibration as specified in the European Telecommunications Standards Institute (ETSI) 800019-14 standard for outdoor operation. The phase response to a 25-G shock is less than 8 deg. peak. The G-force sensitivity was measured by exposing the unit to sine-wave vibration from 2 to 200 Hz was 0.25x10^-10. To achieve this performance, the DFS's internal crystal reference source was moumted in a shochabsorvingstructure. Since low phase noise is critical for digital microwave radios using complex modulation, a special voltage controlled oscillator (VCO) was developed for the DFS synthesizers. The VCO provides low microphonics and achieves low phase noise of -107 dBc/Hz at an offset of 100KHz from an 11 GHz bandwidth. For narrower bandwidths, the pase noise is considerably lower.  P&A: 90days.

Reference: 1. M.E. Znujdewicz and B. Vassilukis, "Phase Transients in digital radio local osillators." IEEE Microwave Theory & Techniques Symposium Digest, 1987, pp. 475-476. 

MICROWAVES & RF * SEPTEMBER 1998