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This article is about analog carrier telephony systems. For 12-channel implementation, see 12-channel carrier system. For digital carrier system, see T-carrier.
Early carrier telephony circuits

Carrier telephony is an analog multiplexing technique used to transmit multiple voice signals over a single pair of wires using frequency-division multiplexing (FDM). In early telephone systems, each conversation required its own physical circuit. In 1910, George Owen Squier demonstrated that multiple conversations could be transmitted on a single line by modulating one signal onto a higher-frequency carrier and combining it with a baseband signal, a method he described as "wired wireless".

After World War I, commercial carrier systems were introduced. In these systems, each voice channel (typically 300–3000 Hz) was translated to a distinct frequency band using modulation and filtering, allowing many channels to coexist on a single circuit. At the receiving end, filters and detectors separated and recovered the individual signals.

Carrier systems expanded through the 20th century as vacuum tubes, and later semiconductor devices, improved in performance and cost. Advances in circuit design and filtering enabled greater channel capacity and lower cost. By the 1970s, a single long-distance system could carry more than 100,000 simultaneous telephone conversations over thousands of miles on coaxial cables.

Analog carrier systems were gradually replaced by digital transmission, beginning with pulse-code modulation (PCM) systems and early implementations such as the T-1 system in 1962.

History

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Prior to carrier techniques, voice circuits were carried on opent wire pairs. Two pairs, or four wires, could also add a third circuit, a phantom pairs, using a pair of pairs. The east coast to Denver was the longest line used in this native mode, in 1915 amplification was added with repeaters to reach coast to coast.[1]: 4–5 

Early carrier telephony (1910–1920s)

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Carrier telephony developed from early demonstrations of frequency-division multiplexing. In 1910, George Owen Squier transmitted two simultaneous voice signals over a single circuit using a high-frequency carrier, describing the method as "wired wireless"[2]

Commercial systems were introduced by AT&T in 1918, building on this work.[3] Early implementations used open-wire lines and divided the available spectrum into bands carrying multiple voice and telegraph channels. A 1921 description by Colpitts and Blackwell outlined systems carrying several voice circuits alongside telegraph channels, using frequency bands extending from a few kilohertz to tens of kilohertz.

These early systems established the basic architecture of carrier telephony: frequency translation of baseband signals, bandpass filtering, and recombination on a shared transmission medium.

Expansion and standardization (1920s–1930s)

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Single balanced mixer used in 1921 carrier systems.

In 1921, E. H. Colpitts and O. B. Blackwell published a description of several frequency-division multiplexed (FDM) systems in use for telephony.[4]: 205–300  These systems were used on open-wire transmission circuits. The labels Type A and Type B were applied in later publications. In test systems put into use in 1914, satisfactory operation was described on communications between South Bend, Indiana and Toledo, Ohio, as well as between Chicago and Toledo. Demand for telephony during World War I resulted in installation of the test system between Pittsburgh and Baltimore.

In one carrier system[4]: 254–255  a ordinary telephony signal is carried in the 300 Hz - 3000 Hz band. 3 other 2-way circuits are assigned to carrier channels, with signals one way occupying the 10 kHz - 20 kHz band, and the other way the 20 kHz to 30 kHz band. Additionally, 8 two-way telegraph signals are multiplexed into the 3333 Hz - 10 kHz band.

Commercial systems were described, one of which transmitted the carrier with each modulated voice signal, used for the Detroit - Harrisburg circuit. It carried three carrier signals in addition to one normal telephony circuit over 596 miles, quadrupling the prior capacity.[4]: 280 

Another system suppressed the carrier, and tranmitted the modulated signal to the receiving end. This modulation is call single-sideband. Removal of the carrier made the transmission more efficient, as it used half as much bandwidth and less transmitter power. This was used in the Harrisburg - Chicago system. It carried one normal circuit and four carrier circuits on one line.[4]: 287 

Early systems also added multiple telegraph signals onto lines used for telephony.[5]

Carrier growth to 1928

In a 1928 paper, the four carrier channel system described in the 1921 paper was called Type A, and the three carrier channels without carrier suppression were called Type B. A further improvement in the system is described as Type C.[6]: 1360–1387  The type C system added three bi-directional carrier circuits to a bi-directional voice band circuit on an open-wire pair. This system was used for links from 150 miles to greater than 1000 miles. From 1926 to 1928, usage increased from 130,000 channel-miles to 230,000 channel-miles.

The Type C system was similar to the Type A system in that it used single-sideband with suppressed carrier. The system was designed for 200 miles between repeater. Two different frequency-planning strategies minimized crosstalk. Each modulator and demodulator used three vacuum tubes: one as an oscillator and two as a balanced mixer. The oscillator, an LC resonant type, maintained a 20 Hz maximum variation between modulation and demodulation oscillators. Amplifiers used in repeaters were balanced, with 2 push-pull tubes in the first stage, and 4 in the second stage, push-pull and parallel.

Two frequency plans were used, called the C-S and C-N. The highest frequency used was about 28 kHz. The C-S and C-N plans were design to minimize interfearemce with each other, and were used for different lines on the same poles.

Frequency allocations for the Type D system, showing coordination with the Type C carriers and with telegraphy.

The Type D system was for shorter circuits, 50–200 miles. It added one carrier circuit to a plain telephony circuit, doubling the capacity. The system was low cost. It used 10.3 kHz and 6.87 kHz as the carrier frequencies for the two directions, and single sideband modulation with suppressed carrier.

The system used two triodes to build both the local oscillator and the mixer. The two tubes operated in parallel for the oscilator, and differentially for the single-balanced mixer. This reduced cost and power dissipation. An terminal unit used 5 triodes, 2 for each frequency converter, and one for the ringer circuit.

Four crossarm telephone pole, using type C and type D carriers to carry 30 voilce band circuits, 6 type C circuits, 14 type D circuits, and 80 telegraph circuits

For the longer circuits, greater than 125 miles, an additional amplifier was added, making the D-A system. The type D-A circuits could be used for shorter runs on poles together with the type C circuits for the longer runs. A drawing showed usage with a 4-crossarm telephone pole, yielding 6 long-distance type C circuits as well as 14 shorter type D circuits together with 30 voice gand circuits.[7]

The last type-C circuit in the US was taken out of service around 1980.[1]: 6 

Morristown cable carrier trial (1933)

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In 1933, Bell Telephone Laboratories conducted an experimental cable carrier installation at Morristown, New Jersey. The trial used a 25-mile length of underground cable with repeaters at 25-mile intervals to simulate an 850-mile four-wire circuit.[8]

Nine carrier telephone channels were transmitted using frequencies from 4 kHz to 40 kHz, with 4 kHz spacing between channels. The trial demonstrated acceptable transmission quality and manageable crosstalk.[8]

The simulated circuit exhibited approximately 1,300 dB of attenuation at carrier frequencies, requiring very high overall gain. Stable operation was achieved using amplifiers based on the negative feedback principle developed by H. S. Black,[9] which allowed precise control of gain over long cascaded repeater chains. Engineers deemed negeative feedback to be key to developing long-distance systems.[1]: 73  For test purposes, carrier links were connected in tandem to form circuits up to 7,650 miles in length. Transmission quality remained satisfactory even at these extended equivalent distances.[8]

Although not adopted immediately for commercial service, largely due to economic conditions during the Great Depression, the Morristown trial established the practical basis for cable carrier systems. This work bridged earlier carrier systems and the standardized multi-channel group systems introduced later in the 1930s.

The 12-channel group systems (late 1930s–early 1940s): Types J, K, and L

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By the late 1930s, carrier telephony systems standardized on a 12-channel group as the basic multiplexing unit. Twelve voice channels were combined into a group occupying approximately 48 kHz. In the J, K, and L systems, channels were spaced at 4 kHz intervals using single-sideband (SSB) modulation, allowing one voice channel per frequency slot. Double-sideband amplitude modulation (AM) systems required wider spacing, typically 8 kHz, and were used primarily in shorter-haul carrier systems.

The principal implementations were the Type J system for open-wire lines, the Type K system for cable (bundles of twisted pairs), and the Type L system for coaxial transmission. These systems entered commercial service in the late 1930s and early 1940s and were used for progressively longer and higher-capacity routes.

The initial signal modulation to form the groups was identical in the J, K and L systems, and was based on double-balanced mixers made with copper oxide rectifiers[10] and crystal filters. The 12 signals are first translated to a band from 60 to 108 kHz, where as a group, they were then translated to the final frequency as needed.[11] The systems all operated from a master, 4 kHz clock, and a frequency multiplier system to synthesize all of the needed carriers. In the J and K systems, the oscillators were open loop, using tuning forks for frequency references. In the L system, given the high accuracy requirements for the greater frequencies involved (.25 ppm relative accuracy between the two terminals), crystal oscillators with a frequency locked loop was used. The variable frequency oscillator was implemented with a motor driving a variable capacitor.[11]

Type K cable systems

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In 1938, Bell introduced the Type K system for use on buried and aerial cable. It was designed for long-haul routes, typically from about 100 to 4,000 miles, with repeaters spaced approximately 17 miles apart. The system used four-wire operation, with separate wire pairs for each transmission direction. The same frequency band was used in both directions, with physical separation of the pairs preventing interference. Channel carriers occupied the range from 12 kHz to 56 kHz. Maximum frequency used was 60 kHz, double that of prior dable systems.[12]

Type J open-wire systems

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The Type J system, introduced in 1939, adapted the 12-channel group for open-wire lines.[13] It was used on long-distance open-wire routes, typically spanning hundreds to over a thousand miles. Both transmission directions shared the same wires but were separated by frequency. West-to-east transmission used frequencies from 36 kHz to 84 kHz, while east-to-west transmission used 92 kHz to 140 kHz.

Initial deployment included several long-distance routes, totaling approximately 55,000 channel-miles. Type J systems were often interconnected with Type K cable systems for extended routes.[13]

Type L coaxial systems

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Development of coaxial carrier systems began in the mid-1930s, extending the 12-channel group into higher-capacity structures. Five groups were combined to form a 60-channel supergroup occupying 240 kHz.[14]

The L1 system, introduced in 1941, was designed for very long-haul trunk routes, up to transcontinental distances. It used multiple supergroups on paired coaxial cables to provide up to 720 telephone circuits.[11][15]

By the late 1940s, coaxial carrier systems had expanded rapidly, with thousands of miles of L1 cable in service in the United States.[16]

Postwar refinement and solid-state transition (1950s–1960s)

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Type-42 48 channel system

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In 1950, the Lenkurt Electric Company announced a 48-channel carrier system. The paper's author described the history of carrier systems, including the J, K, and L systems. The life cycle of an invention was discussed: an initial phase demonstrating usefulness; a second phase of improvement, sometimes at high cost; a third phase focused on manufacturability; and a final phase realizing the full advantages of the method[17]

The Type-42 system first translated all 48 input signals, each occupying 200–3600 Hz, to 7800–4400 Hz by mixing with an 8 kHz oscillator and selecting the lower sideband. Each signal was then translated to its final frequency and combined with the other signals. This two-step method simplified filtering, requiring only LC filters. The hardware for each 12 channel group is separate, giving redundacy for reliability.[17]

Type N-1 systems

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The N-1 system, described in 1951 by the Bell System, was a 12-channel vacuum-tube-based system.[18][19] It used two frequency ranges, 164 kHz to 260 kHz and 44 kHz to 140 kHz. The system was designed for ranges between 15 and 200 miles. Unlike the J, K, and L systems, transmission used amplitude modulation transmitting both sidebands and the carrier. Carriers were spaced at 8 kHz intervals, using twice the bandwidth of a J, K, or L system.

Signals were companded, with amplitude compression before transmission and expansion at reception. This improved dynamic range, reducing noise for low-level signals.

Type O systems

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Type O was a 4-circuit system for 15 to 150 mile segments of open-wire lines, described in 1952.[20]

Type ON2

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24 single-sideband signals using the same fequency plan as the N-1 system, utilizing components from the N-1 and the O systems.[21]

Lenkurt 45BN system

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The 1955 Lenkurt system was coordinated with the N and ON carrier systems. It provided 24 voice channels using single-sideband in a 96 kHz band, either at 40–140 kHz or 164–264 kHz. The N allocations were 44–140 kHz and 164–260 kHz, and the O allocations were 40–136 kHz and 168–264 kHz. These systems could operate on the same cables.

A pilot tone corrected any frequency errors when operated with type N repeaters.[22]

In 1958, a repeater, compatible with the type N system, was described. It used transistors for low-level processing, and tubes as final amplifiers. Transistors were not considered ready for higher power applications.[23]

K24A system

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In 1960, ITT Kellogg described a solid-state carrier system, the K24 synchroplex.[24]

Type N2 system

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Direct distance dialing (DDD) drove the need for more circuits. The Bell System began using N2 systems for short-haul lines (up to 200 miles) in 1962. It was a solid-state, 12-channel double-sideband system. It was used to supplement and replace the N1 system. Western Electric produced over 9,600 N2 terminals in 1964.[25]

The input amplifiers were constructed with PNP germanium transistors, and silicon NPN transistors were used for the output stages. The silicon transistor was developed specifically for this design to permit high-power operation at elevated temperatures. The compressor and expander, used to increase dynamic range and reduce noise, required the development of a special diode[26] to be used as a variable resistance, termed a "variolosser".[27] They were designed to CCITT recommendations.[25]

The double-sideband frequency conversion was accomplished with a square-wave crystal oscillator driving an NPN switch-type unbalanced mixer.[27]

General Dynamics 12 channel system

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In 1963, General Dynamics reported a solid-state 12-channel carrier system for open-wire and cable applications. The architecture and frequencies were chosen to maintain line compatibility with Type J and Type O systems. It used amplitude modulation up to 350 kHz, with optional companding, primarily for higher-frequency carriers. Frequencies were staggered with 8, 12, and 16 kHz spacing to simplify filter design. Modulation mixers were simple four-diode single-balanced designs. Demodulation used a single-diode AM detector, with carrier level feedback to set gain using a diode attenuator. Filters included LC with ferrite, LC air-core, and crystal types.[28]

Type N3 system

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The Type N3 carrier system was reported in 1966. It was a 24-channel single-sideband system designed for distances of 35 to 200 miles. Single-sideband operation was reported to be more economical for distances greater than 35 miles.[21]

The 24 channels were modulated at 4 kHz intervals into a 96 kHz band from either 36–132 kHz or 172–268 kHz. A phase-locked loop corrected for any frequency drift between the modulation and demodulation oscillators.

The mixers were single-balanced transistor modulators, built with a germanium transistor designed specifically for this circuit, having high reverse gain. Crystal filters provided an audio bandwidth of 200–3450 Hz.[29]

High-capacity coaxial systems (1950s–1970s)

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Type L3 system

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The 1953 L3 system was designed to operate on the same coaxial cable as the L1 system so that the wiring could be reused. It provided 1,860 telephone circuits on each pair of coaxial cables, or 600 circuits and one television circuit.[30] Distances could be up to 4,000 miles. A total of 155 groups of 12 channels occupied frequencies up to 8.32 MHz. The design used vacuum tubes.

Type L4 system

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Placed in service in 1967, the L4 system provided 3,600 telephone circuits on each coaxial pair. It was a solid-state system, with repeaters spaced approximately 2 miles apart, and operated with signals up to 17.5 MHz.[31] Six hundred-channel mastergroups were defined as the standard within the Bell System, and L4 used six mastergroups.

Two families of transistors were developed for the L4 system: one for low-power, low-noise circuits, and another for medium-power applications. A new diode was also developed for use in fully balanced ring modulators up to 18 MHz.[32]

Lenkhurt 60 channel system

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In 1972, Lenkurt published a design in which 60-channel supergroups were modulated directly, rather than being formed from 12-channel groups.[33] The design used integrated circuits and custom crystal bandpass filters built with multiple quartz blanks for harmonic generation and bandpass filtering.[34]

Type L5 system

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Placed in service on January 3, 1974, the L5 system supported 10,800 telephone circuits on a coaxial pair. A cable with 10 coaxial pairs could support 108,000 simultaneous telephone conversations.[15] The channels were arranged in six jumbogroups, each containing six mastergroups, each mastergroup containing 600 channels. Repeaters were spaced at approximately 1 mile intervals. Frequencies above 60 MHz were used.[35]

Ultralinear transistors were developed for the L5 system.[36]

See also

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References

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  1. ^ a b c A History of Engineering and Science in the Bell System: Transmission technology (1925-1975). Internet Archive. [Short Hills, N.J.] : The Laboratories. 1985. ISBN 978-0-932764-08-9.{{cite book}}: CS1 maint: others (link)
  2. ^ Squier, George D. (May 1911). "Multiplex telephony and telegraphy by means of electric waves guided by wires". Proceedings of the American Institute of Electrical Engineers. 30 (5): 857–905. doi:10.1109/PAIEE.1911.6660586. ISSN 0097-2444.
  3. ^ Schwartz, M. (May 2008). "The origins of carrier multiplexing: Major George Owen Squier and AT&T". IEEE Communications Magazine. 46 (5): 20–24. doi:10.1109/MCOM.2008.4511639. ISSN 0163-6804.
  4. ^ a b c d Colpitts, E. H.; Blackwell, O. B. (January 1921). "Carrier Current Telephony and Telegraphy". Transactions of the American Institute of Electrical Engineers. XL: 205–300. doi:10.1109/T-AIEE.1921.5060708. ISSN 0096-3860.
  5. ^ Hamilton, B. P.; Nyquist, N.; Long, M. B.; Phelps, W. A. (January 1925). "Voice-Frequency Carrier Telegraph System for Cables". Transactions of the American Institute of Electrical Engineers. XLIV: 327–332. doi:10.1109/T-AIEE.1925.5061114. ISSN 0096-3860.
  6. ^ Affel, H. A.; Demarest, C. S.; Green, C. W. (October 1928). "Carrier Systems on Long Distance Telephone Lines". Transactions of the American Institute of Electrical Engineers. 47 (4). doi:10.1109/T-AIEE.1928.5055151. ISSN 0096-3860.
  7. ^ Black, H. S.; Almquist, M. L.; Ilgenfritz, L. M. (January 1929). "Carrier Telephone System for Short Toll Circuits". Transactions of the American Institute of Electrical Engineers. 48 (1): 117–139. doi:10.1109/T-AIEE.1929.5055185. ISSN 0096-3860.
  8. ^ a b c Clark, A. B.; Kendall, B. W. (December 1933). "Carrier in Cable". Transactions of the American Institute of Electrical Engineers. 52 (4): 1050–1053. doi:10.1109/T-AIEE.1933.5056444. ISSN 0096-3860.
  9. ^ US2102671A, Black, Harold S., "Wave translation system", issued 1937-12-21 
  10. ^ Caruthers, R. S. (April 1939). "Copper Oxide Modulators in Carrier Telephone Systems*". Bell System Technical Journal. 18 (2): 315–337. doi:10.1002/j.1538-7305.1939.tb03579.x. ISSN 0005-8580.
  11. ^ a b c Crane, R. E.; Dixon, J. T.; Huber, G. H. (January 1947). "Frequency Division Techniques for a Coaxial Cable Network". Transactions of the American Institute of Electrical Engineers. 66 (1): 1451–1459. doi:10.1109/T-AIEE.1947.5059597. ISSN 0096-3860.
  12. ^ Green, C. W.; Green, E. I. (May 1938). "A Carrier Telephone System for Toll Cables". Transactions of the American Institute of Electrical Engineers. 57 (5): 227–236. doi:10.1109/T-AIEE.1938.5057804. ISSN 0096-3860.
  13. ^ a b Kendall, B. W.; Affel, H. A. (July 1939). "A 12-Channel Carrier Telephone System for Open-Wire Lines". Transactions of the American Institute of Electrical Engineers. 58 (7): 351–360. doi:10.1109/T-AIEE.1939.5057972. ISSN 0096-3860.
  14. ^ Espenschied, L.; Strieby, M. E. (October 1934). "Systems for Wide-Band Transmission Over Coaxial Lines". Bell System Technical Journal. 13 (4): 654–679. doi:10.1002/j.1538-7305.1934.tb04442.x. ISSN 0005-8580.
  15. ^ a b "L5 Coaxial-Carrier Transmission System: Foreword". Bell System Technical Journal. 53 (10): 1897–1899. December 1974. doi:10.1002/j.1538-7305.1974.tb02724.x. ISSN 0005-8580.
  16. ^ Abraham, L. G. (January 1948). "Progress in Coaxial Telephone and Television Systems". Transactions of the American Institute of Electrical Engineers. 67 (2): 1520–1527. doi:10.1109/T-AIEE.1948.5059853. ISSN 0096-3860.
  17. ^ a b Erickson, L. G. (January 1950). "A Simplified 48-Channel Carrier Telephone System". Transactions of the American Institute of Electrical Engineers. 69 (2): 1493–1500. doi:10.1109/T-AIEE.1950.5060324. ISSN 0096-3860.
  18. ^ Caruthers, R. S. (January 1951). "The Type N-1 Carrier Telephone System: Objectives and Transmission Features". Bell System Technical Journal. 30 (1): 1–32. doi:10.1002/j.1538-7305.1951.tb01364.x. ISSN 0005-8580.
  19. ^ Kahl, W. E.; Pedersen, L. (April 1951). "Some Design Features of the N-1 Carrier Telephone System". Bell System Technical Journal. 30 (2): 418–446. doi:10.1002/j.1538-7305.1951.tb03664.x. ISSN 0005-8580.
  20. ^ Edwards, Paul G.; Montfort, L. R. (July 1952). "The Type-O Carrier System". Bell System Technical Journal. 31 (4): 688–723. doi:10.1002/j.1538-7305.1952.tb01402.x. ISSN 0005-8580.
  21. ^ a b Bleisch, G. W.; Irby, C. W. (1966-07-08). "The N3 Carrier System: Objectives and Transmission Features". Bell System Technical Journal. 45 (6): 767–799. doi:10.1002/j.1538-7305.1966.tb04221.x. ISSN 0005-8580.
  22. ^ "New type 45BN cable carrier system". The Lenkurt demodulator. 4 (6): 1–8. June 1955.
  23. ^ Babin, V.; Fish, R. (March 1958). "A transistorized repeater for use with the 45BN cable carrier system". Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics. 77 (1): 41–49. doi:10.1109/TCE.1958.6372756. ISSN 0097-2452.
  24. ^ Coetsee, B. G.; Curtis, G. L.; Halina, J. W. (January 1960). "The K24A syncroplex telephone carrier system". Transactions of the American Institute of Electrical Engineers, Part I: Communication and Electronics. 78 (6): 1044–1054. doi:10.1109/TCE.1960.6368515. ISSN 0097-2452.
  25. ^ a b Boyd, R. C.; Herr, F. J. (1965-05-06). "The N2 Carrier Terminal - Objectives and Analysis". Bell System Technical Journal. 44 (5): 731–759. doi:10.1002/j.1538-7305.1965.tb04157.x. ISSN 0005-8580.
  26. ^ Gardner, K. R.; Robillard, T. R. (September 1967). "Gold Doped Silicon Compandor Diodes For N2 and N3 Carrier Systems". Bell System Technical Journal. 46 (7): 1451–1477. doi:10.1002/j.1538-7305.1967.tb02470.x. ISSN 0005-8580.
  27. ^ a b Lundry, W. R.; Willey, L. F. (1965-05-06). "The N2 Carrier Terminal - Circuit Design". Bell System Technical Journal. 44 (5): 761–785. doi:10.1002/j.1538-7305.1965.tb04158.x. ISSN 0005-8580.
  28. ^ Jorgensen, O. A. (November 1963). "New 12-channel carrier system for extended-area trunk and subscriber service". IEEE Transactions on Communication and Electronics. 82 (5): 643–647. doi:10.1109/TCE.1963.6373278. ISSN 0536-1532.
  29. ^ Haner, R. L.; Wood, I. E. (1966-07-08). "Circuit Design of the N3 Carrier Terminal". Bell System Technical Journal. 45 (6): 801–844. doi:10.1002/j.1538-7305.1966.tb04222.x. ISSN 0005-8580.
  30. ^ Elmendorf, C. H.; Ehrbar, R. D.; Klie, R.H.; Grossman, A. J. (July 1953). "The L3 Coaxial System". Bell System Technical Journal. 32 (4): 781–832. doi:10.1002/j.1538-7305.1953.tb03713.x. ISSN 0005-8580.
  31. ^ Klie, R. H. (April 1969). "The L-4 Coaxial System". Bell System Technical Journal. 48 (4): 819–820. doi:10.1002/j.1538-7305.1969.tb04252.x. ISSN 0005-8580.
  32. ^ Chaplin, Norman J.; Dodson, Graydon A.; Jacobs, Richard M. (April 1969). "Solid State Devices". Bell System Technical Journal. 48 (4): 983–992. doi:10.1002/j.1538-7305.1969.tb04257.x. ISSN 0005-8580.
  33. ^ Reading, A.; Martin, R. (February 1972). "A New Modulation Concept in Channel Translating Equipment". IEEE Transactions on Communications. 20 (1): 67–72. doi:10.1109/TCOM.1972.1091100. ISSN 0096-2244.
  34. ^ Sheahan, D. F. (1975). "Polylithic Crystal Filters". 29th Annual Symposium on Frequency Control: 120–127. doi:10.1109/FREQ.1975.200072.
  35. ^ Kelcourse, F. C.; Herr, F. J. (December 1974). "L5 System: Overall Description and System Design". Bell System Technical Journal. 53 (10): 1901–1933. doi:10.1002/j.1538-7305.1974.tb02725.x. ISSN 0005-8580.
  36. ^ D'Altroy, F. A.; Jacobs, R. M.; Nacci, J. M.; Panner, E. J. (December 1974). "L5 System: Ultralinear Transistors". Bell System Technical Journal. 53 (10): 2195–2202. doi:10.1002/j.1538-7305.1974.tb02735.x. ISSN 0005-8580.
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