Recent Advances in Phase Locked Dielectric Resonator Oscillators
By John Odell, Director of Engineering, Herley-CTI, Inc.

Phase Locked Dielectric Resonator Oscillators (PDROs) have been available for over twenty years. They are widely used in both commercial and military applications. PDROs offer low cost, small size, low noise, efficient power consumption and high reliability required for local oscillator generation in commercial applications such as modern radar, communication, and test instrumentation systems. In addition, because of the PDRO’s ability to perform in harsh environments, it is also an excellent choice for local oscillator generation in military electronic countermeasure, surveillance, radar, and missile systems. Recent advances at Herley-CTI have resulted in a new line of ruggedized PDROs that offers lower phase noise, lower spurious, smaller size, millimeter wave frequencies of operation, and higher output power options.

The harsh environments encountered in modern military systems require designs to meet the stringent specifications of military OEMs. To meet this need, Herley-CTI has developed ruggedized, hermetically sealed and painted phase locked dielectric resonator oscillators specifically designed for military applications. These units incorporate hybrid chip and wire MIC construction throughout and are rigorously tested and certified specifically for airborne military environments. By incorporating a hybrid chip and wire sampling phase lock board, these models avoid the use of injection molded semiconductors and comply with requirements banning plastic encapsulated parts. All assembly is done in compliance with an ISO-9001 certified quality system in an environmentally controlled class 100,000 clean room. These designs use no variable resistors or capacitors and incorporate established reliability passive components. Wire bond pulls are recorded on all assemblies and 100 percent visual pre-cap inspections are performed prior to electrical testing. All units then go through an extensive burn in and thermal shocking, plus full temperature testing. Following testing, the units are baked out, back filled with dry gas and then laser sealed and certified for fine leak rate of
10-6 atm cc/sec. These units have been qualified to operate under Over All Sound Pressure Levels (OASPL) of 130 dB, random vibration per MIL STD-202F Method 204A and Temperature Altitude Combinations of -1300 to +35,000ft at -45 to +85ºC. They are designed to withstand salt fog, sand and dust exposure, exposure to fluids, and MIL STD-202F method 213B condition J shock. Single and dual output models are available with references from 5 to 200 MHz, and with output frequencies from 800 MHz to 45 GHz. Spurious signals are below -80dBc, and standard output powers are 14 to 17 dbm, with optional power levels up to 30 dbm. Designed to operate in noisy power supply environments, these units operate off of a single 12V supply and incorporate passive input voltage filtering followed by a low noise internal regulator.

A block diagram of a standard PDRO is shown in Figure 1. The standard PDRO consists of a BJT or GaAs MESFET fundamental Voltage Tuned Dielectric Resonator Oscillator (VTDRO) which is phase locked via a sampling loop to a low noise crystal oscillator reference. The phase locked loop (PLL) acts as a low pass filter to the multiplied up reference phase noise and as a high pass filter to the VTDRO’s phase noise. Reference crystal oscillators available today exhibit noise floors lower than -165dBc/Hz at offsets 10 kHz and out from the carrier. In order to take advantage of these low noise references, the PLL noise floor of the PDRO needs to be equally low. If the noise floor of the PLL is not low enough, the noise floor of the PLL will limit the noise inside the loop. Digital loops with 100 MHz phase detector frequencies have a limited noise floor of around -150 dBc/Hz. In contrast, the sampling phase locked loop in the PDRO exhibits a typical phase noise floor of -162 dBc/Hz with a 100 MHz phase detector frequency. Equally important to the performance of a sampling PLL is the design of the reference amplifier. The reference amplifier on the input of the PLL must not degrade the reference phase noise. This amplifier must also maintain a constant signal level to the phase detector over variations in temperature from -45 to +85ºC along with reference level variations of +/- 5dB. Through the use of proprietary sampling phase locked loop circuitry, Herley-CTI’s single loop PDRO models provide exceptionally low phase noise, typically -120 dBc at 10 kHz offset from a 13 GHz carrier when phase locked to an external 100 MHz crystal reference. This proprietary low loop phase noise floor, along with the inherent low phase noise of the VTDRO, allows the use of wide loop bandwidths of approximately 200kHz to 300kHz. Wide loop bandwidths enable low microphonics and phase hit free operation, two critical parameters in both commercial and military systems.

Since fundamental oscillators are used at frequencies below 20 GHz , the far out noise floor of these oscillators is exceptionally low, generally below -160dBc/Hz at offsets greater than 10MHz. It is important to note that this far-from-carrier phase noise is significantly better than that of higher frequency models that use multiplied up L-band oscillators, where the noise floor is degraded by 20 log (N), where N is the multiplication factor. Figure 2 shows phase noise performance of a 13 GHz PDRO locked to a 100 MHz low noise crystal reference.

In addition to the standard external reference model, Herley-CTI offers two additional reference options. The first option provides an integrated high stability internal crystal oscillator in the 100 MHz range. In the past, these internal references have been add on modules to the standard external reference module and have only been capable of maintaining +/- 5 ppm stability over the -30 to +75ºC temperature range. By integrating a novel proportionately controlled heater circuit to the crystal oscillator, Herley-CTI is able to offer stabilities of +/-2 ppm over -45 to +80ºC with an AT cut crystal and +/-1 ppm over -45 to +80ºC with an SC cut crystal. By using a very efficient circuit layout, this model is available in the standard 2.25" x 2.25" x .62" slim line package.

For systems requiring an output that is not a multiple of the reference input, a second reference option is available. In this model, the external reference is locked to an internal crystal source in the 100 MHz range using digital synthesis techniques. Again, by using a very efficient surface mount circuit layout, Herley-CTI is able to offer this model in the standard 2.25" x 2.25" x .62" slim line package.

For local oscillator generation between 20 and 45 GHz, the PDRO line incorporates an integrated low noise frequency multiplier. Multiplier options of X2, X3 and X4 are available. These multipliers incorporate state of the art MMIC active multipliers, alumina thin film microstrip filters, and MMIC amplifiers. Sub harmonics of the output are typically less than -60dBc.

In addition to the fully militarized oscillators, Herley-CTI also offers a line of commercial off the shelf PDROs that are available with output frequencies from 800 MHz to 45 GHz locked to reference frequencies from 5 to 200 MHz. Standard output powers are 14 to 17 dbm, with optional power levels up to 30 dbm. These regulated units run off of voltages from 8 to 15 Vdc, have spurious content of -80dbc, and operate over the full temperature range of -40 to +75ºC.
Whether your application requires a standard COTS phase locked oscillator or a customized product to full military specifications specifically designed for your system, practical results have shown that the PDRO series developed and produced by Herley-CTI meets or exceeds the stringent requirements of today’s military and commercial electronic systems.
For further information, contact Herley-CTI at 973-884-2580 or contact us below.

HERLEY-CTI
www.herley.com
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