Editing Dynatron oscillator

{{short description|Vacuum tube electronic oscillator circuit}}
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Dynatron signal generator 1931.jpg
| caption1 = Dynatron vacuum tube [[signal generator]], 1931.  It covered the range 1.8 to 15&nbsp;MHz.  The circuit was used in signal generators due to its frequency stability, which was compared to crystal oscillators
| width1   = 180
| image2   = Crosley 122 "Super Buddy Boy" radio receiver 1931.jpg
| caption2 = The dynatron oscillator circuit was also used as the [[local oscillator]] in early vacuum tube [[superheterodyne]] [[radio receiver]]s, such as this 1931 Crosley model 122 seven tube radio.
| width2   = 240
| footer   =
}}

In electronics, the '''dynatron oscillator''', invented in 1918 by [[Albert Hull]]<ref name="Kröncke">{{cite journal
  | last1  = Kröncke
  | first1 = H.
  | title = Oscillation without reaction
  | journal = Wireless World
  | volume = 18
  | issue = 12
  | pages = 467–468
  | location = London
  | date = March 24, 1926
  | url = http://www.americanradiohistory.com/Archive-Wireless-World/20s/Wireless-World-1926-03.pdf
  | access-date = March 20, 2015}}</ref><ref name="Hull">{{cite journal
  | last = Hull
  | first = Albert W.
  | title = The Dynatron - A vacuum tube possessing negative electric resistance
  | journal = Proceedings of the IRE
  | volume = 6
  | issue = 1
  | pages = 5–35
  | publisher = Institute of Radio Engineers
  | location = New York
  | date = February 1918
  | url = https://books.google.com/books?id=IUASAAAAIAAJ&q=hull+dynatron&pg=PA5
  | doi = 10.1109/jrproc.1918.217353
  | s2cid = 51656451
  | access-date = 2012-05-06| url-access = subscription
  }}</ref> at [[General Electric]], is an obsolete [[vacuum tube]] [[electronic oscillator]] circuit which uses a [[negative resistance]] characteristic in early [[tetrode]] vacuum tubes, caused by a process called [[secondary emission]].<ref name="Amos">{{cite book  
  | last = Amos
  | first = S. W. 
  |author2=Roger Amos
  | title = Newnes Dictionary of Electronics
  | publisher = Newnes
  | date = 2002
  | pages = 107
  | url = https://books.google.com/books?id=lROa-MpIrucC&pg=PA107
  | isbn = 978-0080524054 }}</ref><ref name="Gottlieb">{{cite book  
  | last = Gottlieb
  | first = Irving
  | title = Practical Oscillator Handbook
  | publisher = Elsevier
  | date = 1997
  | location = US
  | pages = 76–78
  | url = https://books.google.com/books?id=e_oZ69GAuxAC&pg=PA76
  | isbn = 978-0080539386}}</ref><ref name="Edson">{{cite book  
  | last = Edson
  | first = William A.
  | title = Vacuum Tube Oscillators
  | publisher = John Wiley and Sons
  | date = 1953
  | location = US
  | pages = 31–34
  | url = http://www.tubebooks.org/Books/vto.pdf
  }} on  Peter Millet's [http://www.tubebooks.org Tubebooks] website</ref><ref name="ArmyManual">{{cite book
 | title  = Technical Manual TM 11-665: C-W and A-M Radio Transmitters and Receivers
 | publisher = Dept. of the Army, US Government Printing Office
 |date=September 1952
 | pages  = 68–69
 | url    = https://books.google.com/books?id=f9QXAAAAYAAJ&q=dynatron+transitron&pg=PA68
 }}</ref>  It was the first negative resistance vacuum tube oscillator.<ref name="Kumar">{{cite journal
  | last = Kumar
  | first = Umesh
  | title = Design of an indiginized negative resistance characteristics curve tracer
  | journal = Active and Passive Electronic Components
  | volume = 23
  | pages = 13–23
  | date = April 2000
  | url = http://downloads.hindawi.com/journals/apec/2000/969073.pdf
  | doi = 10.1155/APEC.23.13
  | access-date = May 3, 2013| doi-access = free
  }}</ref>   The dynatron oscillator circuit was used to a limited extent as [[beat frequency oscillator]]s (BFOs), and [[local oscillator]]s in vacuum tube [[radio receiver]]s as well as in scientific and test equipment from the 1920s to the 1940s but became obsolete around World War 2 due to the variability of secondary emission in tubes.<ref name="Dietmar">{{cite web
  | last = Dietmar
  | first = Rudolph
  | title = Negative resistance oscillators
  | work = Principles of Schematics forum
  | publisher = Ernest Erb personal website
  | date = 17 December 2010
  | url = http://www.radiomuseum.org/forum/negative_resistance_oscillators.html
  | access-date = 29 November 2013}}</ref><ref name="Worthen">{{cite journal
  | last = Worthen
  | first = Charles E.
  | title = The Dynatron
  | journal = The General Radio Experimenter
  | volume = 4
  | issue = 12
  | pages = 1–4
  | publisher = General Radio Co.
  |date=May 1930
  | url = http://www.ietlabs.com/pdf/GR_Experimenters/1930/GenRad_Experimenter_May_1930.pdf
  | access-date = September 5, 2014}}</ref><ref name="Shunaman">{{cite journal
  | last = Shunaman
  | first = Fred
  | title = Transitron Oscillators
  | journal = Radio-Craft
  | volume = 16
  | issue = 7
  | pages = 419
  | publisher = Radcraft Publication Inc.
  | location = New York
  |date=April 1945
  | url = http://www.americanradiohistory.com/Archive-Radio-Craft/1940s/Radio-Craft-1945-04.pdf
  | access-date = September 6, 2014}}</ref><ref name="Palmer">{{cite journal
  | last = Palmer
  | first = C. W.
  | title = Recent advances in oscillator circuits
  | journal = Radio-Craft
  | volume = 11
  | issue = 9
  | pages = 534–535
  | publisher = Radcraft Publications, Inc.
  | location = New York
  |date=March 1940
  | url = http://www.americanradiohistory.com/Archive-Radio-Craft/1940s/Radio-Craft-1940-03.pdf
  | access-date = September 6, 2014}}</ref>

Negative transconductance oscillators,<ref name="Dietmar"/> such as the '''transitron oscillator''' invented by Cleto Brunetti in 1939,<ref name="Brunetti1">{{cite journal
  | last = Brunetti
  | first = C.
  |author2=E. Weiss
  | title = The Transitron Oscillator
  | journal =  Proceedings of the IRE
  | volume = 27
  | issue = 2
  | pages = 88–94
  | publisher = Institute of Radio Engineers
  | date = February 1939
  | issn = 0096-8390
  | doi = 10.1109/JRPROC.1939.229010
  | s2cid = 51644322
  }}</ref><ref name="Brunetti2">{{cite journal
  | last = Brunetti
  | first = Cleto
  | title = A Practical Negative Resistance Oscillator
  | journal = Review of Scientific Instruments
  | volume = 10
  | issue = 3
  | pages = 85–88
  | date = December 1939
  | doi = 10.1063/1.1751492
  | bibcode = 1939RScI...10...85B
 }}</ref> are similar negative resistance vacuum tube oscillator circuits which are based on negative [[transconductance]] (a fall in current through one grid electrode caused by an increase in voltage on a second grid) in a [[pentode]] or other multigrid vacuum tube.<ref name="Edson" /><ref name="Gottlieb2" >[https://books.google.com/books?id=e_oZ69GAuxAC&pg=PA76&lpg=PA78    Gottlieb, 1997, ''Practical Oscillator Handbook'', p. 78-81]</ref>    These replaced the dynatron circuit<ref name="Gottlieb2" /> and were employed in vacuum tube electronic equipment through the 1970s.<ref name="Dietmar" /><ref name="Shunaman" /><ref name="Palmer" />

==How they work==
[[Image:Dynatron tube.png|thumb|Dynatron tube, the first tube to produce dynatron oscillations, invented by Albert Hull in 1918.<ref name="Hull" />  It saw little use since the [[triode]] and the [[tetrode]], invented in 1926, proved capable of dynatron oscillations also.   ]]
The dynatron and transitron oscillators differ from many oscillator circuits in that they do not use [[feedback]] to generate oscillations, but [[negative resistance]].<ref name="Gottlieb" /><ref name="ArmyManual" />  A [[tuned circuit]] (resonant circuit), consisting of an [[inductor]] and [[capacitor]] connected together, is "almost" an oscillator: it can store electric energy in the form of oscillating currents, "ringing" analogously to a tuning fork.<ref name="Solymar">{{cite book  
  | last = Solymar
  | first = Laszlo
  |author2=Donald Walsh
   | title = Electrical Properties of Materials, 8th Ed.
  | publisher = Oxford University Press
  | date = 2009
  | location = UK
  | pages = 181–182
  | url = https://books.google.com/books?id=AiWyp0NQW6UC&q=%22negative+resistance%22+%22second+law%22+thermodynamics&pg=PA181
  | isbn = 978-0199565917}}</ref>   If a tuned circuit could have zero [[electrical resistance]], once oscillations were started it would function as an [[electronic oscillator|oscillator]], producing a continuous [[sine wave]].   But because of the inevitable resistance inherent in actual circuits, without an external source of power the energy in the oscillating current is dissipated as heat in the resistance, and any oscillations decay to zero.<ref name="Solymar" />

In the dynatron and transitron circuits, a vacuum tube is [[biasing|biased]] so that one of its electrodes has [[negative differential resistance]].<ref name="Gottlieb" /><ref name="ArmyManual" />  This means that when the voltage on the electrode with respect to the cathode is increased, the current through it decreases.<ref name="Gottlieb" />    A tuned circuit is connected between the electrode and the cathode.  The negative resistance of the tube cancels the positive resistance of the tuned circuit, creating in effect a tuned circuit with zero AC resistance.<ref name="ArmyManual" /><ref name="Solymar" />  A spontaneous continuous [[sinusoidal]] oscillating voltage at the [[resonant frequency]] of the tuned circuit is generated, started by [[electrical noise]] in the circuit when it is turned on.<ref name="Solymar" />

An advantage of these oscillators was that the negative resistance effect was largely independent of frequency, so by using suitable values of [[inductance]] and [[capacitance]] in the tuned circuit  they could operate over a wide frequency range, from a few hertz to around 20&nbsp;MHz.<ref name="ArmyManual" /><ref name="Dietmar" /><ref name="Worthen" />   Another advantage was that they used a simple single LC tuned circuit without the taps or "tickler" coils required by oscillators such as the [[Hartley oscillator|Hartley]] or [[Armstrong oscillator|Armstrong]] circuits.<ref name="Brunn" />

==Dynatron oscillator==
[[File:Dynatron oscillator circuit.svg|thumb|upright=1.3|Dynatron oscillator circuit]]

In the dynatron a [[tetrode]] tube is used.<ref name="Gottlieb" />  In some tetrodes the [[plate electrode|plate]] (anode) has negative differential resistance, due to electrons knocked out of the plate when electrons from the cathode hit it,  called ''[[secondary emission]]''.<ref name="Gottlieb" /><ref name="Edson" /> This causes a downward "kink" in the plate current vs. plate voltage curve ''(graph below, grey region)'' when the screen grid is biased at a higher voltage than the plate, as described below.   This negative resistance was mostly a feature of older tubes, of 1940s or earlier vintage.<ref name="Gottlieb" />   In most modern tetrodes, to prevent [[parasitic oscillation]]s the plate is given a coating which drastically reduces the unwanted secondary emission, so these tubes have virtually no negative resistance "kink" in their plate current characteristic, and cannot be used in dynatron oscillators.<ref name="Gottlieb" />

The tetrode wasn't the only tube which could generate dynatron oscillations.    Early [[triode]]s also had secondary emission and thus negative resistance, and before the tetrode was invented they were used in dynatron oscillators by biasing the [[control grid]] more positive than the plate.<ref name="Kröncke" /><ref name="Turner">{{cite book
 | last1  = Turner
 | first1 = L. B.
 | title  = Wireless
 | publisher = Cambridge University Press
 | date   = 1931
 | pages  = 297
 | url    = https://books.google.com/books?id=_bz1AAAAQBAJ&q=dynatron+triode&pg=PA297
 | isbn   = 9781107636187
 }}</ref>  Hull's first dynatron oscillator in 1918 used a special "dynatron" vacuum tube of his own design ''(shown above)'', a triode in which the grid was a heavy plate perforated with holes which was robust enough to carry high currents.<ref name="Hull" />   This tube saw little use as standard triode and tetrodes could function adequately as dynatrons.  The term "dynatron" came to be applied to all negative resistance oscillations in vacuum tubes; for example the [[magnetron|split-anode magnetron]] was said to work by "dynatron oscillation".

An advantage of the dynatron circuit was that it could oscillate over a very wide frequency range; from a few hertz to 20&nbsp;MHz.<ref name="ArmyManual" /><ref name="Dietmar" /><ref name="Worthen" />  It also had very good frequency stability compared to other LC oscillators of that time, and was even compared to [[crystal oscillators]].   The circuit became popular after the advent of cheap tetrode tubes such as the UY222 and UY224 around 1928.<ref name="Worthen" /><ref name="Brunn">{{cite journal
  | last = Brunn
  | first = Brunsten
  | title = Dynatron Oscillator Uses
  | journal = Radio World
  | volume = 19
  | issue = 22
  | pages = 15
  | date = August 15, 1931
  | url = http://www.americanradiohistory.com/Archive-Radio-World/30s/31/Radio-World-1931-Aug-15.pdf
  | access-date = September 5, 2014}}</ref>  It was used in [[beat frequency oscillator]]s (BFOs) for code reception and [[local oscillator]]s in [[superheterodyne]] receivers<ref name="Brunn" /> as well as in laboratory [[signal generator]]s and scientific research.  RCA's 1931 prototype [[television]] used two UY224 tubes as dynatron oscillators to generate the vertical deflection (28&nbsp;Hz) and horizontal deflection (2880&nbsp;Hz) signals for the CRT's deflection coils.

However the dynatron had some drawbacks.  It was found that the amount of secondary emission current from the plate varied unpredictably from tube to tube, and also within a single tube over its operating life;<ref name="Spangenberg">{{cite book
 | last1  = Spangenberg
 | first1 = Karl R.
 | title  = Vacuum Tubes
 | publisher = McGraw-Hill Book Co.
 | date   = 1948
 | location = New York
 | pages  = 718–719
 | url    = http://www.tubebooks.org/Books/Spangenberg_vacuum_tubes.pdf
 }}</ref><ref name="Ghirardi">{{cite journal
  | last = Ghirardi
  | first = Alfred A.
  | title = Practical Radio Course, Part 34
  | journal = Radio News
  | volume = 43
  | issue = 5
  | pages = 148–150
  |date=May 1945
  | url = http://www.americanradiohistory.com/Archive-Radio-News/40s/Radio-News-1945-05-R.pdf
  | access-date = September 5, 2014}}</ref> eventually it would stop oscillating.  When replacing the tube, several might have to be tried to find one that would oscillate in a circuit.   In addition, since dynatron oscillations were a source of instability in amplifiers, the tetrode's main application, tube manufacturers began applying a [[graphite]] coating to the plate which virtually eliminated secondary emission.<ref name="Gottlieb" />  By 1945 the use of the dynatron circuit was declining.<ref name="Shunaman" /><ref name="Palmer" /><ref name="Ghirardi" />

===Secondary emission===
[[Image:UY224 tetrode plate current negative resistance.svg|thumb|upright=1.1|Plate current ''I''<sub>P</sub> and screen grid current ''I''<sub>G2</sub> vs plate voltage ''V''<sub>P</sub> curves of RCA's UY224 tetrode which came out in 1929, showing negative resistance region ''(grey)''.<br />
&nbsp;Screen grid potential ''V''<sub>G2</sub> = 75 V<br />
&nbsp;Control grid potential ''V''<sub>G2</sub> = −1.5 V<br />
In this tube, secondary emission was sufficiently strong that it not only caused negative resistance (a declining slope) but reversed the plate current; more electrons left the plate than arrived at it.  ]]
{{multiple image
| align = right
| direction = horizontal
| header   = Plate current (''I''<sub>b</sub>) vs plate voltage curves for tetrodes:
| image1   = RCA 24-A screen grid tube anode characteristics.png
| caption1 = An early tetrode, the RCA 24-A from 1929, showing on the lefthand side the negative resistance "kink" in the curves due to secondary emission. At a screen voltage ''V''<sub>C2</sub> of 90 V it has negative resistance between about ''V''<sub>p</sub>&nbsp;=&nbsp;10 to 60 V.
| width1   = 265
| image2   = Tetrode.png
| caption2 = A modern tetrode, the 6P25. Due to a coating on the plate, there is very little secondary emission, so there is virtually no negative resistance region ("kink") in the curves, making this tube unusable for dynatron operation.
| width2   = 200
}}

In an electron tube, when [[electron]]s emitted by the [[cathode]] strike the [[plate electrode|plate]], they can knock other electrons out of the surface of the metal, an effect called ''[[secondary emission]]''.<ref name="Gottlieb" /><ref name="Edson" /><ref name="Spangenberg" />  In a normal tetrode amplifier this is an unwanted effect, and the [[screen grid]] next to the plate is biased at a lower potential than the plate, so these ''secondary electrons'' are repelled and return to the plate due to its positive charge.

However, if the screen grid is operated at a higher potential than the plate, the secondary electrons will be attracted to it, and return to ground through the screen grid supply.<ref name="Gottlieb" />  This represents a current of electrons ''I''<sub>G2</sub> ''away'' from the plate, which reduces the net plate current ''I''<sub>P</sub> below the cathode current ''I''<sub>C</sub>
:<math>I_P = I_C - I_{G2} \,</math>
Higher plate voltage causes the primary electrons to hit the plate with more energy, releasing more secondary electrons.  Therefore, starting at the voltage at which the primary electrons have enough energy to cause secondary emission, around ''V''<sub>P</sub>&nbsp;=&nbsp;10V, there is an operating region ''(grey)''  in which an increase in plate voltage causes more electrons to leave the plate than the additional electrons arriving at the plate, and therefore a net reduction in plate current.

===Negative resistance===
Since in this region an increase in plate voltage causes a decrease in plate current, the AC plate resistance, that is the differential output resistance of the tube, is negative:
:<math>r_P = {\Delta V_P \over \Delta I_P} < 0 \,</math>
As with other [[negative differential resistance]] devices like the [[tunnel diode]], this negative resistance can be used to create an oscillator.  A parallel tuned circuit is connected in the plate circuit of the tetrode.  The circuit will oscillate if the magnitude of the negative plate resistance is less than the parallel resistance '''''R''''' of the tuned circuit, including any load connected to the oscillator.
:<math>|r_P| < R \,</math>
The frequency of oscillation is close to the [[resonant frequency]] of the tuned circuit.
:<math>f = {1 \over 2\pi}\sqrt{1 \over LC} \,</math>

===Design===
As can be seen from the graphs, for dynatron operation the screen grid had to be biased at a considerably higher voltage than the plate; at least twice the plate voltage.  The plate voltage swing is limited to the negative resistance region of the curve, the downward "kink",  so to achieve the largest output voltage swing, the tube should be biased in the center of the negative resistance region.

The negative resistance of older tetrode tubes was around 10kΩ - 20kΩ, and can be controlled by varying the [[control grid]] bias.  If the magnitude of the negative resistance |'''''r''<sub>P</sub>'''| is just small enough to start oscillation, just a little smaller than the positive resistance '''''R''''' of the tuned circuit, the oscillation frequency will be very stable, and the output waveform will be almost sinusoidal.  If the negative resistance is made significantly smaller than the positive resistance, the voltage swing will extend into the nonlinear part of the curve, and the peaks of the sine wave output will be flattened ("clipped").

==Transitron oscillator==
[[Image:Transitron oscillator circuit.svg|thumb|upright=1.1|Transitron oscillator]]
[[Image:Transitron characteristic.svg|thumb|Screen current and voltage in transitron oscillator.  As the screen voltage ''V''<sub>G2</sub> becomes high enough that the suppressor grid voltage turns positive, electrons begin passing through the suppressor grid to reach the plate. Plate current increases and screen current decreases, giving the screen negative resistance ''(grey region)''.]]

The transitron oscillator, invented by Cledo Brunetti in 1939,<ref name="Brunetti1" /> (although a similar effect was observed in tetrodes by [[Balthasar van der Pol]] in 1926,<ref name="Drummer">{{cite book  
  | last = Drummer
  | first = G.W.A
  | title = Electronic Inventions and Discoveries: Electronics from its earliest beginnings to the present day, 4th Ed.
  | publisher = CRC Press
  | date = 1997
  | pages = 126
  | url = https://books.google.com/books?id=e35kJYAlyCgC&pg=PA126
  | isbn = 978-0750304931}}</ref> and Edward Herold described a similar oscillator in 1935<ref name="Herold">{{cite journal
  | last = Herold
  | first = Edward W.
  | title = Negative resistance and devices for obtaining it
  | journal =  Proceedings of the IRE
  | volume = 23
  | issue = 10
  | pages = 1201–1223
  | date = October 1935
  | issn = 0731-5996
  | doi = 10.1109/JRPROC.1935.227271
  | s2cid = 51656745
  }}</ref>) is a [[negative resistance]] oscillator circuit using a [[pentode]] vacuum tube, in which, instead of the plate, the [[screen grid]] has negative resistance due to being coupled to the [[suppressor grid]].<ref name="Edson" /><ref name="Gottlieb2" /><ref name="Spangenberg" />  See the circuit at right. In the transitron, the [[screen grid]] is biased at a positive voltage ''(battery B1)'' above the plate voltage while the [[suppressor grid]] is biased negatively ''(battery B2)'', at or below the cathode voltage.  Therefore, all the electrons will be reflected by the negative suppressor grid and none will get through to the plate.  The reflected electrons will instead be attracted to the screen grid, so the screen current will be high while the plate current will be zero.  However, if the suppressor grid voltage is increased, as it approaches zero (the cathode voltage) electrons will begin to pass through it and reach the plate, so the number diverted to the screen grid, and thus the screen current, will decrease. Since the other grids don't take significant current the cathode current <math>\scriptstyle I_\text{C}</math> is split between the plate <math>\scriptstyle I_\text{P}</math> and the screen grid <math>\scriptstyle I_\text{G2}</math>: 
:<math>I_\text{G2} = I_\text{C} - I_\text{P} \,</math>
The division of current between the screen grid and plate is controlled by the suppressor voltage.   This inverse relationship is indicated by saying the [[transconductance]] between the screen and suppressor grid (the change in screen current Δ''I''<sub>G2</sub> divided by the change in suppressor voltage Δ''V''<sub>G3</sub>)  is negative.

Since the suppressor grid voltage and not the screen grid voltage controls the screen current, if the suppressor and screen grid are coupled together with a capacitor (''C2'') so there is a constant potential difference between them, increasing the screen grid voltage will increase the suppressor voltage, resulting in a decrease in screen current.  This means the screen grid has [[negative differential resistance]] with respect to the cathode, and can be used to create oscillations.

In the transitron circuit, the screen and suppressor grids are coupled with a bypass capacitor (''C2'') which has a low impedance at the oscillation frequency, so they have a constant potential difference.  The parallel tuned circuit (''C1-L'') is connected between the screen grid and the cathode (through battery ''B1'').  The negative resistance of the screen grid cancels the positive resistance of the tuned circuit, causing oscillations.  As in the dynatron oscillator the control grid can be used to adjust the negative resistance.

Since the transitron oscillator didn't depend on secondary emission it was far more reliable than the dynatron. However, because the screen grid is not designed to handle high power, the oscillator's output power is limited.  Other tubes with multiple grids beside the pentode, such as the [[hexode]] and [[pentagrid converter]] tube, have been be used to make similar negative transconductance oscillators.  Pentode tubes used in this circuit have a negative transconductance of only around −250 microsiemens, giving a negative resistance of −4000Ω.   Tubes with more grids, such as the [[pentagrid converter]], can be used to make transitron oscillators with higher transconductance, resulting in smaller negative resistance.

==References==
{{reflist}}

{{DEFAULTSORT:Dynatron Oscillator}}
[[Category:Electronic oscillators]]