Grid-leak detector

Example of single tube triode grid leak receiver from 1920, the first type of amplifying radio receiver. In the left picture the grid leak resistor and capacitor are labeled.
A grid leak resistor and capacitor unit from 1926. The 2 megohm cartridge resistor is replaceable so the user can try different values. The parallel capacitor is built into the holder.

A grid leak detector is an electronic circuit that demodulates an amplitude modulated alternating current and amplifies the recovered modulating voltage. The circuit utilizes the non-linear cathode to control grid conduction characteristic and the amplification factor of a vacuum tube[1][2]. Invented by Lee De Forest around 1912, it was used as the detector (demodulator) in the first vacuum tube radio receivers until the 1930s.

History

Schematic diagram shows six vacuum tubes
A TRF receiver using a grid leak detector (V1).

Early applications of triode tubes (Audions) as detectors usually did not include a resistor in the grid circuit[3][4]. First use of a resistance to discharge the grid condenser in a vacuum tube detector circuit may have been by Sewall Cabot in 1906. Cabot wrote that he made a pencil mark to discharge the grid condenser, after finding that touching the grid terminal of the tube would cause the detector to resume operation after having stopped[5]. Edwin H. Armstrong, in 1915, describes the use of "a resistance of several hundred thousand ohms placed across the grid condenser" for the purpose of discharging the grid condenser[6]. The heyday for grid leak detectors was the 1920s, when battery operated, multiple dial tuned radio frequency receivers using low amplification factor triodes with directly heated cathodes were the contemporary technology. The Zenith Models 11, 12, and 14 are examples of these kinds of radios.[7] When screen-grid tubes became available for new designs in 1927, most manufacturers switched to plate detectors[8][2], and later to diode detectors. The grid leak detector has been popular for many years with amateur radio operators and shortwave listeners who construct their own receivers.

Functional overview

The stage performs two functions:

  • Detection: The control grid and cathode operate as a diode. At small radio frequency signal (carrier) amplitudes, square-law detection takes place due to non-linear curvature of the grid current versus grid voltage characteristic[9]. Detection transitions at larger carrier amplitudes to large-signal detection behavior due to unilateral conduction from the cathode to grid[10][11].
  • Amplification: The varying direct current (dc) voltage of the grid acts to control the plate current. The voltage of the recovered modulating signal is increased in the plate circuit, resulting in the grid leak detector producing greater audio frequency output than a diode detector, at small input signal levels[12]. The plate current includes the radio frequency component of the received signal, which is made use of in regenerative receiver designs.

Operation

The control grid and cathode are operated as a diode while at the same time the control grid voltage exerts its usual influence on the electron stream from cathode to plate.

In the circuit, a capacitor (the grid condenser) couples a radio frequency signal (the carrier) to the control grid of an electron tube[13]. The capacitor also facilitates development of dc voltage on the grid. The impedance of the capacitor is small at the carrier frequency and high at the modulating frequencies[14].

A resistor (the grid leak) is connected either in parallel with the capacitor or from the grid to the cathode. The resistor permits dc charge to "leak" from the capacitor[15] and is utilized in setting up the grid bias[16].

At small carrier signal levels, the grid to cathode space exhibits non-linear resistance. Grid current occurs during 360 degrees of the carrier frequency cycle. The grid current increases more during the positive excursions of the carrier voltage than it decreases during the negative excursions, due to the parabolic grid current versus grid voltage curve in this region[17]. This asymmetrical grid current develops a dc grid voltage that includes the modulation frequencies[18][19][20].

At carrier signal levels large enough to make conduction from cathode to grid cease during the negative excursions of the carrier, the detection action is that of a linear diode detector[21][22]. Grid current occurs only on the positive peaks of the carrier frequency cycle. The coupling capacitor will acquire a dc charge due to the rectifying action of the cathode to grid path[23]. The capacitor discharges through the resistor (thus grid leak) during the time that the carrier voltage is decreasing[24][25]. The dc grid voltage will vary with the modulation envelope of an amplitude modulated signal[26].

The plate current is passed through a load impedance chosen to produce the desired amplification in conjunction with the tube characteristics. In non-regenerative receivers, a capacitor of low impedance at the carrier frequency is connected from the plate to cathode to prevent amplification of the carrier frequency[27].

Design

The value of the grid condenser is generally chosen to be at least ten times the grid input capacitance and is typically 100 to 300 picofarads (pF)[2].

For minimum audio distortion, the time constant of the resistor and capacitor is chosen to be shorter than the period of the highest modulating frequency that is to be reproduced[28][29][30].

The resistance and electrical connection of the grid leak along with the grid current determine the grid bias [31]. For operation of the detector at maximum sensitivity, the bias is placed at the optimum point of curvature of the grid current versus grid voltage curve[19][32].

If a dc path is provided from the grid leak to an indirectly heated cathode or to the negative end of a directly heated cathode, negative initial velocity grid bias is produced relative to the cathode determined by the product of the grid leak resistance and the grid current [33] [34].

For certain directly heated cathode tubes, the optimum grid bias is at a positive voltage relative to the negative end of the cathode. For these tubes, a dc path is provided from the grid leak to the positive side of the cathode or the positive side of the "A" battery; providing a positive fixed bias voltage at the grid determined by the dc grid current and the resistance of the grid leak[35][19][36]

For maximum input signal capability rather than detection sensitivity, a grid leak of around 250,000 ohms is chosen and a tube requiring comparatively large grid voltage for plate current cutoff is used (usually a low amplification factor triode) [37][38]. Grid leak detectiion optimized for larger input signals is known as power grid detection [39]. The peak 100 percent modulated input signal voltage the grid leak detector can demodulate without excess distortion is about one half of the projected cutoff bias voltage [40], corresponding to a peak unmodulated carrier voltage of about one quarter of the projected cutoff bias[41][42].

Effect of tube type

Tetrode and pentode tubes provide significantly higher grid input impedance than triodes, resulting in less loading of the circuit providing the signal to the detector[43]. Tetrode and pentode tubes also produce significantly higher audio frequency output amplitude at small carrier input signal levels (around one volt or less) in grid leak detector applications than triodes[44][45].

Advantages

  • The grid leak detector potentially offers greater economy than use of separate diode and amplifier tubes.
  • At small input signal levels, the circuit produces higher output amplitude than a simple diode detector.

Disadvantages

One potential disadvantage of the grid leak detector, primarily in non-regenerative circuits, is that of the load it can present to the preceding circuit[27]. The radio frequency input impedance of the grid leak detector is dominated by the tube's grid input impedance, which can be on the order of 6000 ohms or less for triodes, depending on tube characteristics and signal frequency. Other disadvantages are that it can produce more distortion and is less suitable for input signal voltages over a volt or two than the plate detector or diode detector[46] [47].

See also

References

  1. ↑ Cruft Electronics Staff, Electronic Circuits and Tubes, New York: McGraw-Hill, 1947, p. 705
  2. 1 2 3 H. A. Robinson, "The Operating Characteristics of Vacuum Tube Detectors", Part I, QST, vol. XIV, no. 8, p. 23, Aug. 1930
  3. ↑ CDR S. S. Robison, Manual of Wireless Telegraphy for the use of Naval Electricians, Annapolis, MD: United States Naval Institute, 1911, p.125
  4. ↑ J. Scott-Taggart, Thermionic Tubes in Radio Telegraphy and Telephony, London, UK: The Wireless Press LTD, 1921, p. 118
  5. ↑ S. Cabot, "Detection - Grid or Plate", QST, vol. XI, no. 3, p. 30, Mar. 1927
  6. ↑ E. H. Armstrong, "Some Recent Developments in the Audion Receiver", Proceedings of the Institute of Radio Engineers, vol. 3, no. 3, pp. 215-247, Sept. 1915
  7. ↑ Schematics of Zenith models 11, 12 and 14. Three battery-operated Zenith grid leak models of the 1920s.
  8. ↑ E. P. Wenaas, Radiola: the Golden Age of RCA, 1919 - 1929, Chandler, AZ: Sonoran Publishing LLC, 2007, p. 336
  9. ↑ Cruft Electronics Staff, P. 705
  10. ↑ K. R. Sturley, Radio Receiver Design (Part I), New York: John Wiley and Sons, 1947, p. 377
  11. ↑ Cruft Electronics Staff, P. 706
  12. ↑ The Radio Amateur's Handbook (55 ed.). The American Radio Relay League. 1978. p. 241.
  13. ↑ J. H. Reyner, "Grid Rectification. A Critical Examination of the Method", Experimental Wireless, vol. 1, no. 9, pp. 512-520, Jun. 1924
  14. ↑ W. L. Everitt, Communication Engineering, 2nd ed. New York: McGraw-Hill, 1937, p. 418
  15. ↑ J. Scott-Taggart, p. 119
  16. ↑ J. Scott-Taggart, p. 125
  17. ↑ Signal Corps U.S. Army, The Principles Underlying Radio Communication, 2nd ed. Washington, DC: U.S.G.P.O., 1922, p. 478
  18. ↑ Landee, Davis, Albrecht, Electronic Designers' Handbook, New York: McGraw-Hill, 1957, p. 7-108
  19. 1 2 3 L.P. Smith, "Detector Action in High Vacuum Tubes", QST, vol. X, no. 12, pp. 14-17, Dec. 1926
  20. ↑ Cruft Electronics Staff, pp. 693 - 703
  21. ↑ Cruft Electronics Staff, p. 675
  22. ↑ Landee et al., p. 7-107
  23. ↑ W. L. Everitt, p. 421
  24. ↑ Signal Corps U.S. Army, p. 476
  25. ↑ Cruft Electronics Staff, p. 679
  26. ↑ Cruft Electronics Staff, p. 681
  27. 1 2 K. R. Sturley, pp. 379-380
  28. ↑ E. E. Zepler, The Technique of Radio Design, New York: John Wiley and Sons, 1943, p. 260
  29. ↑ J. H. Morecroft, p. 454
  30. ↑ Dr. H. Holden, "Grid Leak Detection in the 1920s", Old Timer's Bulletin, 2001
  31. ↑ J. Scott-Taggart, p. 125
  32. ↑ J. H. Morecroft, Principles of Radio Communication, New York: John Wiley & Sons, Inc., 1921, p. 451
  33. ↑ Giacoletto, Lawrence Joseph (1977). Electronics Designers' Handbook. New York: McGraw-Hill. p. 9-27.
  34. ↑ Tomer, Robert B. (1960). Getting the Most Out of Vacuum Tubes. Indianapolis: Howard W. Sams & Co., Inc. / The Bobbs-Merrill Company, Inc. p. 28. Archived from the original on 2009.
  35. ↑ RCA, The RCA Radiotron Manual, Technical Series R-10, Radio Corporation of America, p. 22
  36. ↑ Signal Corps U.S. Army, p. 477
  37. ↑ A. A. Ghirardi, Radio Physics Course, 2nd ed. New York: Rinehart Books, 1932, pp. 499
  38. ↑ E. E. Zepler, p. 104
  39. ↑ E. E. Zepler, p. 104
  40. ↑ K. R. Sturley, p. 23
  41. ↑ S. W. Amos, "The Mechanism of Leaky Grid Detection", Part II, Electronic Engineering, Sept. 1944, p. 158
  42. ↑ E.E. Zepler, p. 104
  43. ↑ K. R. Sturley, p. 381
  44. ↑ H. A. Robinson, "The Operating Characteristics of Vacuum Tube Detectors", Part II, QST, vol. XIV, no. 9, p. 42, Sept. 1930
  45. ↑ A. E. Rydberg, J. W. Doty, "The Superiority of Screen-Grid Detectors", QST, vol. XIV, no. 4, p. 43, Apr. 1930
  46. ↑ E. E. Zepler, p. 103
  47. ↑ H. A. Robinson, Part I, p. 25
  • Rutland, David (September 1994), Behind the Front Panel: The Design & Development of 1920's Radios, Wren, ISBN 978-1885391001
  • Schematic of Philco model 84 A superheterodyne cathedral radio from 1933 that uses a regenerative detector. (Note: The capacitor for the detector's control grid is the "tickler coil" winding on the IF transformer.)
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