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RF Components: Equalizers

Understanding Fine Grain Equalizers

DEFINITION OF A FINE GRAIN EQUALIZERS FUNCTION: Multiple attenuation responses that can be adjusted in frequency and amplitude. (See Fig. #5) Fine Grain Equalizers are primarily used for minimizing gain ripple or gain spikes present in wide band traveling wave tubes, solid state amplifiers or systems where the phasing in and out of VSWR's cause excessive amplitude ringing. This type of equalizer can also be used for assembling attenuation curves of all shapes and amplitudes within its adjustment range. This is accomplished by setting each section to a different frequency and amplitude adjacent to one another. (See Fig. #6)

HOW TO SPECIFY: Parameters such as the number of sections needed, and the particular application must be defined prior to ordering these equalizers. The following is a partial list of those parameters along with a brief definition and a method for determining specifications.

ATTENUATION: The attenuation responses are adjustable to any frequency and specified amplitude within their operating band. This parameter can be specified in accordance with your particular applications. In wideband applications, various sections will be dedicated to a portion of the band.

INSERTION LOSS: The insertion loss of these devices is attributable to both absorptive and reflective losses which are present with all sections in a decoupled state. This parameter is specified as a maximum and is referred to as the relative zero attenuation point. Therefore, the specified attenuation level shall be relative to the insertion loss. A typical method for approximating insertion loss would be 0.10 dB for each specified section, and add 0.25 dB to that overall value. For example, if specifying insertion loss of a 10 section Fine Grain Equalizer, the approximate level of insertion loss would be 1.25 dB maximum throughout the frequency band. This parameter is measured with all sections decoupled. In most cases the insertion loss will be less than the approximated value.

VSWR: This parameter is dependent on many factors such as attenuation level, bandwidth, operating frequency range, number of sections, size, configuration, and adjustability requirements. Input and output VSWR's, under most circumstances, should not exceed 2.5:1 for accumulative attenuation levels below 5 dB. This can be further reduced by use of integrated isolators on the input and output.

ADJUSTABILITY: MICA can provide easy access adjustments which will allow you to vary the attenuation and frequency of each section independently. (See Fig. #5.) MICA's sales staff can help you determine what number of fine grain sections and what nominal bandwidths are needed to fulfill your particular requirements.

CONNECTORS: MICA provides SMA (F) as standard connectors. Other SMA series connectors can be provided on request. TNC and type "N" connectors are not recommended, but can be made available by special order.

WAVEGUIDE: Fine Grain Equalizers are available in all standard operating bands. (Specify flange type when ordering. )

SIZE: Design and size criteria can only be established after determination of all the applicable specifications. Package sizes can then be quoted and outline drawings provided for a detailed mechanical layout.

TUNING GUIDE AND OPERATIONAL CONCEPTS: The operation of MICA's Fine Grain adjustable Equalizers can be briefly described as follows. By forward or reverse movement of each resonator short, the electrical length can be varied. Therefore, the center frequency of these loss responses can be located anywhere within their operating frequency range. Adjustment of other parameters associated with each section allows the bandwidth, as well as the amplitude, to be set independently. Bandwidths of each section may vary as a function of frequency.

DESIGN AND CONSTRUCTION: The Fine Grain adjustable Equalizer consists of a main transmission line between the input and output ports, with multiple TEM mode resonant shunt branches coupled along its length. (Refer to Fig. #7)

The simplified equivalent circuit of the device is shown in Fig. #7. Each resonant section can be tuned by adjusting both the coupling (Cs) and the electrical length of the shunt stub (?s) associated with each section. By adjusting (Cs) and (?s) of the resonant shunt branch, center frequency, bandwidth, and amplitude can be adjusted within the operating frequency range. Although each section's bandwidth will be determined by the factory, it will vary linearily with frequency adjustment.

The electrical length of the shunt stub (?s) is varied by turning the frequency adjustment in each cavity. The (Cs) is varied by turning the amplitude adjustment of each cavity in such a way that the gap, with respect to the main transmission line, corresponds to the required capacitance value to achieve resonance. A standing wave is purposely introduced at these coupling points to minimize penetration and reduce the possibility of short circuit and interaction between sections. *

After the desired amplitude and center frequency of each cavity is properly adjusted to the required position, the amplitude adjustment may be slightly tuned in any sequence to obtain the required overall frequency characteristics. Minor changes of the frequency adjustment of each section may be necessary during final adjustment to obtain optimum response characteristics. After adjusting all the elements of tuning, each is locked. MICA's unique construction allows many readjustments without deterioration of the critical contact surfaces.

OPTIONS:

OPERATING BANDWIDTH: Each section's 3 dB bandwidth must be specified prior to ordering. Common bandwidths are 60 MHz to 3 GHz. The mixing of section bandwidths is common practice and an available option. The typical (Qs) are from 10 to 100.

BANDWIDTH: Bandwidth accuracy +/-15% is typical, when set at 3 dB above the insertion loss at a fixed freq.

FREQUENCY RANGE: 1.5 GHZ to 26.5 GHz.

ATTENUATION LEVEL AND AMPLITUDE ADJUSTABILITY: Continually adjustable from 0 dB to 5 dB per section. Extended attenuation levels are available by special order.

FREQUENCY ADJUSTABILITY: ±10% of center frequency as a minimum.

FIXED TUNE: These equalizers can be preset and sealed at the factory when field adjustment is not required. The customer must determine the pre-set value.

FIELD ADJUSTABLE: Fine grain equalizers are amplitude and frequency adjustable which will allow compensation for system level variation.

EXTENDED ADJUSTABILITY: In most cases standard adjustability can be further extended. Some trade-offs, such as degradation in VSWR and response symmetry may be necessary.

Note *: Gross mistuning of any section will damage the sensitive amplitude component of the resonator. When tuning the adjustment, screw clockwise, exercising care as it reaches its maximum travel.
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