A Review of Key Equalizer Specifications and What They Mean

By Richard M. Kurzrok Consultant

hether design- are characterized by Bode plots applicable to Here is a short tutorial that ing or buying an voltage transfer responses through frequency reminds engineers how Wequalizer, the selective amplifiers. This article will primari- to develop the proper key specifications must ly discuss frequency domain specifications for specifications for amplitude be understood in order to analog passive equalizers. and delay equalizers make the right design decisions or unit selec- Passive Equalizer Realization tions. This tutorial article offers a review of External passive equalizers often use equalizer technical specifications and what lumped or distributed circuit elements to cor- they mean, along with discussion of some of rect undesired transmission responses of other the applications that are most affected by par- circuits such as filters and transmission lines. ticular specifications. In commercial electron- In a usable frequency range, signal integrity ics, technical specifications are often subordi- requires carefully controlled transmission nate to unit cost and adequate performance is paths. Typical transmission responses with sometimes acceptable without optimization. linear, parabolic, or ripple behavior are correct- In government electronics, reliability consid- ed by equalizers normally operated between a erations can be paramount and high unit costs matched source (generator) and matched ter- will occur. Reliability is crucial in the commer- mination (load). To achieve desired correc- cial field of medical electronics tion(s), equalizers are designed for various Equalizers correct amplitude and/or group transmission response shapes. Good results delay aberrations in receivers and transmit- are obtained when applicable computer pro- ters for applications in communications, grams are used to predict equalizer perfor- radar, and electronic countermeasures. mance and determine necessary circuit ele- Equalizers are available as both passive and ment values. At microwave frequencies, elec- active units, with specifications that may be in tromagnetic simulation can be used for equal- either the frequency domain or the time izer design as an alternative to empirical tech- domain. Passive equalizers are characterized niques of the past. Finally, reflections at circuit per frequency responses between matched interfaces must be carefully controlled to avoid source and matched load. Active equalizers signal integrity degradation.

Figure 1 · L-C equalizer using two bridged-T sections.

50 High Frequency Electronics High Frequency Design EQUALIZER SPECS

Lumped-circuit equalizers have 140 MHz and other frequencies. been realized using cascaded Further front end cost reduction is bridged-T circuits which ideally are being achieved using direct conver- constant resistance circuits. A typical sion architecture which eliminates schematic of an L-C two section intermediate frequency circuits such equalizer is shown in Figure 1. Fixed as filters and equalizers [1]. pad attenuators are employed at Transmission-line equalizers equalizer input, output, and inter- have been employed at microwave stage to reduce interactions between frequencies using a circulator cou- source, load and adjacent sections. To pled unit with a reactive circuit con- meet Intelsat transmission require- nected to the second port. At ments for satellite communications microwave frequencies, the reactive Figure 2 · L-C equalizer using one links, 70 MHz elliptic function band circuit is usually realized using cavi- bridged-T highpass section pass filters of different carrier sizes ty resonators. The ferrite circulator, (bandwidths) were cascaded with configured as a four port unit, is a bridged-T group delay equalizers. reflective unit that provides useful circuit (ASIC). Alternate C and D sec- These L-C filter-equalizers circuits source/load isolation. Reactive cir- tions can be implemented using cou- were subsequently replaced by SAW cuits, using one and two resonators, pled transmission lines without fer- band pass filters that were inherent- have been popularly known as C and rite circulators. ly linear phase units requiring no D Sections. This reflective type of For equalization of coaxial trans- external equalization. These SAW fil- equalizer can also be realized using mission lines, the equalizer usually ters were physically smaller and pro- operational amplifiers [2]. Such an provides amplitude response correc- vided sharper selectivity at reduced equalizer could be ultimately realized tion that is inherently high pass. This unit cost. SAW units also took over at as an applications specific integrated can be achieved using one or more bridged-t sections. The high pass bridged-tee unit is a constant resistance circuit similar to the pre- emphasis circuit used with analog communications links. The schematic diagram of an L-C equalizer using one bridged-T high pass section is shown in Figure 2. The transversal equalizer has been implemented at microwave frequencies. It permits both manual and computer controlled switching of fixed increments of equalization. The transversal equalizer is compatible with digital electronics and can be used for adaptive equalization. Computed amplitude and group delay responses for some lossless and lossy C and D sections have been published in a prior article [3]. Also, the circulator coupled equalizer can employ reactive circuits with greater than two coupled resonators. Equalization fit is crucial to attainment of signal integrity objec- tives. Transmission performance specifications are highly dependent upon the specific type of applicable modulation. Equalizer realizations are often dependent upon performance and cost tradeoffs which can rather than external equalizers. entail several design iterations. Numerous structures are available for equalizers. Each equalizer Notes on Equalizer Specifications structure has its own problem areas 1. Transmission and Reflection relating to materials and manufac- Responses— turing processes. Special attention is Usable Transmission Responses: required when working at high sig- Maximum Insertion Loss in dB over nal levels. High average power levels specified frequency range(s), maxi- can result in overheating while high mum phase non-linearity in degrees peak power levels can result in volt- over specified frequency range (s), age breakdown. Some equalizer maximum differential group delay in structures are sensitive to shock and unit of time over specified frequency vibration. Circulator coupled equaliz- range. ers are sensitive to adjacent magnet- Usable Reflection Responses: ic fields; superconducting units Minimum Return Loss in dB over require control of temperature. In specified frequency range (s). some applications, intermodulation Source/Load Impedance: Ohms distortion in passive equalizers can (50 and 75 ohm impedances are typi- become a problem area. Switched fil- cal above audio) ter equalizers can vary from simple 2. Filter Interfaces— mechanical switching to local/remote Specify equalizer input/output computer controls. interfaces such as coaxial connectors, standard waveguide size, solder lugs, Acknowledgement surface mount tabs, etc. Equalizer design and develop- 3. Mechanical— ment can entail a wide of technology. Location of interfaces and The contributions of many other adjustable (if any) circuit elements. A engineers and technicians are grate- mechanical outline drawing showing fully acknowledged. For reasons of mounting information is usually nec- brevity, some useful equalization essary. techniques have not been discussed, 4. Environmental— and a bibliography of prior work was Specify both operating and stor- not included beyond the cited refer- age temperature, humidity, altitude, ences. shock and vibration, salt spray, etc. This can affect acceptable unit sur- References face finishes. Government specifica- 1. G. Breed, “Design Issues for tions should be included when appli- Direct Conversion Wireless Radios,” cable. High Frequency Electronics, pp. 48, 5. Reliability— 50, 51, June 2004. –This can be specified as avail- 2. C. Wenzel, “Low Frequency ability. Government specifications Circulator/Isolator Uses No Ferrite should be included when applicable. or Magnet,” RF Design, July 1991. Unit cost is directly related to the 3. R. Kurzrok, “Circulator- number of specifications imposed and Coupled Equalizers Applicable to their stringency. Imposition of source High-Speed Data Links,” Applied inspection and workmanship stan- Microwave & Wireless, pp. 86, 88, 90, dards can also affect the cost. Unit 92, June 2001. costs will go down as production vol- ume goes up. Author Information Richard M. Kurzrok is a consul- Miscellaneous Considerations tant, semi-retired after many years in NOTE: Specifications must be the RF/microwave industry. He can be altered for self-equalized filters reached at: [email protected]