Functions of the Local Oscillator. The superheterodyne receiver utilizes one or more local oscillators and mixers to convert the echo to an intermediate frequency that is convenient for filtering and processing operations. The receiver can be tuned by changing the first LO frequency without disturbing the IF section of the receiver. Subsequent shifts in intermediate frequency are often accomplished within the receiver by additional LOs, generally of fixed frequency.
Pulse-amplifier transmitters also use these same LOs to generate the radar carrier with the required offset from the first local oscillator. Pulsed oscillator transmitters, with their independent "carrier" frequency, use automatic frequency control (AFC) to maintain the correct frequency separation between the carrier and first LO frequencies.
In many early radars, the only function of the local oscillators was conversion of the echo frequency to the correct intermediate frequency. The majority of modern radar systems, however, coherently process a series of echoes from a target. The local oscillators act essentially as a timing standard by which the echo delay is measured to extract range information, accurate to within a small fraction of a wavelength. The processing demands a high degree of phase stability throughout the radar. Although these processing techniques are described elsewhere (Chaps. 15 to 17 and 21), they determine the basic stability requirements of the receiver.
The first local oscillator, generally referred to as a stable local oscillator (stalo), has a greater effect on processing performance than the transmitter. The final local oscillator, generally referred to as a coherent local oscillator (coho), is often utilized for introducing phase corrections which compensate for radar platform motion or transmitter phase variations.
Stalo Instability. The stability requirements of the stalo are generally defined in terms of a tolerable phase-modulation spectrum. Sources of unwanted modulation are mechanical or acoustic vibration from fans and motors, power supply ripple, and spurious frequencies and noise generated in the stalo. In general, the tolerable phase deviation decreases with increasing modulation frequency because the doppler filter is less efficient in suppressing the effects. In a radar having two-pulse MTI, there is a linear relationship between the tolerable phase deviation and the period of the modulation. Their ratio is the allowable FM (frequency modulation) or short-term frequency stability sometimes encountered in the literature. This parameter does not adequately define the phase-stability requirements for pulse doppler or MTI radars where more than two pulses are coherently processed.
The phase-modulation spectrum of the stalo may be measured and converted into the MTI improvement factor limitation, which is dependent on range to the clutter and the characteristics of the two cascaded filters in the radar receiver. This conversion process involves three steps, described below.
It should be noted that some spectrum analyzers do not distinguish between frequencies below the desired stalo frequency and those above; their response is the sum of the power in the two sidebands at each designated modulation frequency. This is of no consequence in MTI radars which have equal response to positive and negative doppler frequencies. In radars using doppler filters unsymmetrical about zero doppler, it is necessary to assume that the stalo spectrum measured is symmetrical, generally a valid assumption. The examples shown subsequently employ measured data from this type of double-sideband (DSB) spectrum analyzer. If a single-sideband (SSB) spectrum analyzer is available, positive and negative modulation-frequency components can be measured separately and analyzed without any assumption of symmetry. It is essential that the measured data be defined as SSB or DSB, since there is a 3 dB difference in the two forms of data.
Range Dependence. Most modern radars use the stalo to generate the transmitted pulse as well as to shift the frequency of the received echoes. The transmitters are power amplifiers (traveling-wave tubes, klystrons, twystrons, crossed-field amplifiers, solid-state amplifiers, etc.) rather than oscillators (magnetron, etc.). It is this double use of the stalo that introduces a dependence on range of the clutter and exaggerates the effect of certain unintentional phase-modulation components by 6 dB, the critical frequencies being those which change phase by odd multiples of 180° during the time period between transmission and reception of the clutter echo from a specified range. At these critical frequencies, a maximum positive phase deviation on transmission changes to a maximum negative deviation at the time of reception, doubling the undesired phase modulation of the echo at IF.
Figure 3.4 shows this range-dependent filter characteristic, which may be expressed mathematically as dB = 10 log 4 sin2 (l-nfjllc) = 10 log 4 sin2 (ttfmt) (3.4)
where fm = modulation frequency, Hz
R = range, m c = propagation velocity, 3 x 108 m/s t = time delay = 2R/c
A short time delay can tolerate much higher disturbance at low modulation frequencies, as illustrated by the two cases in Fig. 3.4. Consequently, stalo stability needs to be computed for several time delays.
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