A Phase-coherent Frequency Synthesizer for Radar Applications

A Phase-coherent Frequency Synthesizer for Radar Applications

Programmed Test Sources Inc. (PTS)
Littleton, MA

The procurement policy of the defense establishment has recently undergone some significant changes. Traditionally, only equipment that met military specifications was available, often leading to very expensive solutions. Despite this restriction, maintenance requirements and reliability/longevity often were not commensurate with cost. This article describes a high performance, phase-coherent frequency synthesizer derived from established commercial building blocks for use in military radar system applications.

The model AN/MPS-39 multiple object tracking radar (MOTR) requires precise signals that are switched in microseconds to engage various targets in real time. High purity synthesized frequencies covering bandwidths of hundreds of megahertz are required. These signals must have low spurious and phase noise content and be capable of being switched rapidly and phase coherently. Synthesizers that employ dividers in their resolution sections do not produce phase-coherent signals when switched at arbitrary times.

Phase Continuous vs. Phase Coherent

Although the terms phase continuous and phase coherent sometimes are used interchangeably, they actually refer to two distinct properties. Phase-continuous frequency switching denotes that at the switching point the phases of the signal at both the old and new frequencies are equal and have no transients or discontinuities. Phase-continuous frequency switching is possible in direct digital synthesis (DDS) because of its ability to maintain an accumulated phase value during a frequency switch and after the next clock pulse begins generating the output signal at the new frequency from the phase value reached by the old frequency.

As applied to the switching behavior of a signal, the term phase coherent defines the signal’s steady-state phase after the switching process is completed. Beginning with two in-phase signals at frequency f₁ , one signal is assumed to undergo the switching sequence f₁ , f₂ , f₁ . If the two signals are again in phase after the switching sequence, phase-coherent switching has occurred. However, with arbitrary timing, the phase transients required for phase coherence preclude phase continuity.

Figure 1 shows a signal experiencing frequency switching at times t₁ and t₂ . With phase-continuous switching at time t₂ , the signal returns to frequency f₁ but now the phase is offset from that of the original unswitched signal. To obtain a phase-coherent switching sequence, some phase discontinuity generally must occur.

Phase Coherence in Synthesizers

Early frequency synthesizers were not necessarily controlled by a single crystal standard. Adequate frequency stability was obtained using several internal crystal oscillators that contributed to the overall frequency stability of the output. These devices were considered noncoherent. As applied to frequency synthesizers, phase coherence describes the relation of the frequency standard to the output frequency. If the output frequency accurately reproduces the relative frequency stability of the standard, the device is considered coherent.

Despite this definition, the assumption that all contemporary systems that use a single standard (external or internal) are coherent is incorrect. Many systems that utilize fractional-N or binary DDS fine-resolution subsynthesizers are not truly phase coherent; rather they have specified small but finite reference-to-output errors.

The Synthesizer DESIGN

Table 1 lists the specifications of the frequency source required for the MOTR system. The model PTS 250R4O1X-28 frequency synthesizer achieves these specifications. The synthesizer largely comprises standard modules and a minimum of custom design, resulting in a highly cost-effective product with predictable mean time between failures and minimum mean time to repair.

Figure 2 shows the synthesizer block diagram. Twenty-five 10 MHz steps are generated in the 10 MHz step section. In this portion, a VCO (stepped in 10 MHz increments) operates in a drift-canceling loop, which eliminates the free-running oscillator’s frequency drift. The VCO feeds both inputs to the final mixer –– one directly and one after an intermediate mix. Therefore, if the VCO frequency deviates from nominal, both mixer inputs move up or down in frequency together by the same frequency increment. As a result, the VCO deviation does not alter the difference of these two frequencies, which is the desired output frequency.

The diagram shows some of the VCO frequencies corresponding to certain 10 MHz steps as a means of clarifying the process. Essentially, this section selects and filters a 10 MHz line from a pulse generated from the frequency standard. Since all of the 10 MHz lines are generated continuously, frequency switching from one 10 MHz multiple to another occurs phase coherently. Careful temperature compensation of the oscillator and 505 MHz filter (which is as wide as practical from the standpoint of 10 MHz neighboring picket rejection) is essential to minimize phase drift during the observation interval.

Coherently switching 1 MHz and 200 kHz steps are produced in the DM/DMA section and via auxiliary standard frequencies that are derived in the SGA and SGB modules from the same 10 MHz pulse that is used for the 10 MHz steps. These frequencies are introduced to four DM modules, which each produce a 200 kHz line, and the DMA module, which generates the 1 MHz steps. The SGA and SGB standard generators, which operate together, are shown in Figure 3 .

A 5 or 10 MHz input is received from an external or internal frequency standard and fed to the input amplifier/multiplier. This block is followed by a narrow crystal filter. The pulse generator receives its input from this crystal filter and produces a spectrum of n x 10 MHz multiples, which is the basis for all fixed or standard frequencies in the synthesizer. The five low standard frequencies of 14, 16, 18, 20 and 22 MHz are produced by dividing a specified spectrum line through an amplifier filter circuit, digital divider and filter at the output frequency. Each of the two higher standard frequencies of 112 and 113 MHz also use one specific n x 10 spectrum line. Mixers add derivatives of certain low standard frequencies to obtain the final frequencies. Thus, by multiplication, division and addition (arithmetic operations) the SGA and SGB produce seven standard frequencies that are coherent with the 5 or 10 MHz input frequency. Three of the five lower frequencies are produced in the SGA.

A description of only the 20 MHz generator illustrates the process sufficiently since the sections are similar. Loosely coupled to the spectrum bus (n x 10) by a filter, a transistor amplifies the 100 MHz line; this signal reaches a divider after filtering. A divide-by-five process is used and the 20 MHz signal is connected to a tuned circuit. A low impedance output is fed to the 20 MHz bus in the deck for distribution.

A 33 MHz frequency is needed to produce 113 MHz in the SGB module. This signal is produced by dividing 22 MHz by two. After filtering, the 11 MHz third harmonic is fed to the SGB module and then added to 80 MHz. The SGB receives the 10 to 140 MHz spectrum from the SGA module. The low frequencies are produced by the same divide-by-five process used in the SGA module. The 70 MHz line is preselected, amplified and filtered before it enters the divider, and the resulting 14 MHz signal is fed to the output after filtering. The same IC divider also supplies a 14 MHz third harmonic. After amplification, this 42 MHz signal is injected into the final mixer, producing 112 MHz by adding a 70 MHz input.

Although dividers are used in various stages of the standard frequency process, it should be noted that they all operate in a CW fashion with no switched inputs, thus producing coherent signals once the unit is powered. These frequencies then are introduced to four DM modules (each producing a 200 kHz line) and the DMA module (which generates the 1 MHz steps). The DM modules, typically utilized in series and with a selectable output, are hardwired to produce only one frequency and are operated in parallel. Figure 4 shows the process inside each module. The 14 MHz input is upconverted to 126 MHz and, for each of the four modules, one of the four 16 to 22 MHz bus frequencies is added. After filtering, amplification and division by 10, output frequencies of 14.2, 14.4, 14.6 and 14.8 MHz are obtained (14.0 MHz is available directly from the bus). These 200 kHz raster signals (14.0 to 14.8 MHz) then are selected by PIN diode switches from the external remote frequency control for the 200 kHz steps.

The realization of the 1 MHz steps is similar except that no parallel module is required. The block diagram of the DMA module is identical to the DM diagram without the final division. Since all supply frequencies to this module (that is, the 14.2 and 14.8 input from the selected DM module, and the 112 and 113 MHz and 14 to 22 MHz bus frequencies) are coherent, frequencies generated in the 140 to 150 MHz band also are coherent. A biquinary selection process uses the appropriate supply frequencies to cover the 140 to 150 MHz band in 10 1 MHz steps with the actual frequency produced dependent on the remote frequency control input.

Conclusion

The model PTS 250R4O1X-28 frequency synthesizer is an example of a robust commercial product that has adapted to military applications successfully. The price of this complete synthesizer with a high stability 10–9/day frequency standard and special offset is less than $10,000 in small quantities.

Programmed Test Sources Inc. (PTS),
Littleton, MA
(978) 486-3008.