Frequency stability is Vital to modern communications systems.
Without it, bit-error-rate (BER) perfomance suffers in digitally-
modulated systems and signals may be lost in analog systems. For-
tunately, a line of phase-locked dielectric resonator oscillators
(DROs) from Elcom, Inc. (Closter, NJ) provides stable performance
at standard and customer-specified output frequencies. the sources
employ a unique multiplication scheme in order to take advantage of
the low phase noise of 5-and 10- MHz reference oscillators.

Crystal oscillators operating at 5 and 10 MHz tend to exhibit better frequency aging and temperature stability than their higher-frequency counterparts. These oscillators are commonly used as the reference sources in precision instruments, such as frequency synthesizers and frequency counters. Although these lower-frequency crystal oscillators require higher multiplication factors than 50- and 100- MHz reference sources, their superior phase-noise performance still holds up even with higher multiplication factors.

The phase-locked DRO (PDRO) sources (see figure) employ a unique technique in order to phase-lock microwave DROs to a crystal oscillator. The technique only requires a single loop for reliable phase-lock performance with 5-and 10- MHz crystal reference oscillators. By using just one loop, complexity, size, power consumption, and cost are minimized. Even with a low-frequency reference source, high microwave output frequencies are possible. For example, a model PDRO 601 source delivers delivers 6.1 Ghz phase- locked output signals from a 10 MHz reference oscillator.

By using fractional multiplication, the sources can operate at output frequencies that are a fractional multiplication of the input frequency. For example, model PDRO 1010 generates output signals at 1112.5 MHz, model PDRO 301 offers output signals at 3025 MHz, and model PDRO 1003 yields output signals at 10.025 GHz --all from a 10MHz crystal reference oscillator. The phase noise of this approach is very low: the phase noise of the PDRO 101 is -107dBc/Hz offset 1kHz from a 1112.5 -MHz carrier, dropping to -117 dBc/Hz offset 10 kHz from the carrier. The phase noise of the high-frequency model PDRO 201 is -100 dBc/Hz offset 1KHz from a 3025 -MHz carrier and -107 dBc/Hz offset 10 kHz from the same carrier frequency.

Low spurious-signal content is critical to many communications applications, particularly in frequency downconverters where high-level communications applications, particularly in frequency downconverters where high-level spurious signals can be confused with the signal(s) of interest. For example, if a 1.2-GHz signal is downconverted to a 140-MHz intermediate-frequency (IF) output signal, the local-oscillator (LO) signal leakage through the downconverter mixer is amplified and appears as a non-carrier-related signal. In many cases, the mixer isolation is 25 dB and the amplifier gain is 50 dB. A -90-dBm 200-MHz spurious signal at the LO output will generate -65-dBm spurious levels at the IF output. This level is too high in many demanding applications. The single-loop-locked DRO sources employ a multistage output filter to reduce the spurious levels to below -100 dBc. Spurious levels below 200 MHz are typically less than -110 dBc.

The rugged phase-locked sources in the PDRO line contain an internal voltage regulator and operate on supplies from +12 to +24 VDC. The current consumption is typically only 200 mA. The sources are designed for stable operation at temperatures from -25 to +75 degrees celsius.

MICROWAVES & RF * JANUARY 1997