Dynatron oscillator Dynatron oscillator

1 dynatron oscillator

1.1 secondary emission
1.2 negative resistance
1.3 design

dynatron oscillator

dynatron oscillator circuit

in dynatron tetrode tube used. in tetrodes plate (anode) has negative differential resistance, due electrons knocked out of plate when electrons cathode hit it, called secondary emission. causes downward kink in plate current vs. plate voltage curve (graph below, grey region) when screen grid biased @ higher voltage plate, described below. negative resistance feature of older tubes, of 1940s or earlier vintage. in modern tetrodes, prevent parasitic oscillations plate given coating drastically reduces unwanted secondary emission, these tubes have virtually no negative resistance kink in plate current characteristic, , cannot used in dynatron oscillators.

the tetrode wasn t tube generate dynatron oscillations. triodes had secondary emission , negative resistance, , before tetrode invented used in dynatron oscillators biasing control grid more positive plate. hull s first dynatron oscillator in 1918 used special dynatron vacuum tube of own design (shown above), triode in grid heavy plate perforated holes robust enough carry high currents. tube saw little use standard triode , tetrodes function adequately dynatrons. term dynatron came applied negative resistance oscillations in vacuum tubes; example split-anode magnetron said work dynatron oscillation .

an advantage of dynatron circuit oscillate on wide frequency range; few hertz 20 mhz. had frequency stability compared other lc oscillators of time, , compared crystal oscillators. circuit became popular after advent of cheap tetrode tubes such uy222 , uy224 around 1928. used in beat frequency oscillators (bfos) code reception , local oscillators in superheterodyne receivers in laboratory signal generators , scientific research. rca s 1931 prototype television used 2 uy224 tubes dynatron oscillators generate vertical deflection (28 hz) , horizontal deflection (2880 hz) signals crt s deflection coils.

however dynatron had drawbacks. found amount of secondary emission current plate varied unpredictably tube tube, , within single tube on operating life; stop oscillating. when replacing tube, several might have tried find 1 oscillate in circuit. in addition, since dynatron oscillations source of instability in amplifiers, tetrode s main application, tube manufacturers began applying graphite coating plate virtually eliminated secondary emission. 1945 use of dynatron circuit declining.

secondary emission

plate current ip , screen grid current ig2 vs plate voltage vp curves of rca s uy224 tetrode came out in 1929, showing negative resistance region (grey).

 screen grid potential vg2 = 75 v

 control grid potential vg2 = −1.5 v

in tube, secondary emission sufficiently strong not caused negative resistance (a declining slope) reversed plate current; more electrons left plate arrived @ it.

in electron tube, when electrons emitted cathode strike plate, can knock other electrons out of surface of metal, effect called secondary emission. in normal tetrode amplifier unwanted effect, , screen grid next plate biased @ lower potential plate, these secondary electrons repelled , return plate due positive charge.

however, if screen grid operated @ higher potential plate, secondary electrons attracted it, , return ground through screen grid supply. represents current of electrons ig2 away plate, reduces net plate current ip below cathode current ic

i

p


=

i

c


−

i

g
2





{\displaystyle i_{p}=i_{c}-i_{g2}\,}

higher plate voltage causes primary electrons hit plate more energy, releasing more secondary electrons. therefore, starting @ voltage @ primary electrons have enough energy cause secondary emission, around vp = 10v, there operating region (grey) in increase in plate voltage causes more electrons leave plate additional electrons arriving @ plate, , therefore net reduction in plate current.

negative resistance

since in region increase in plate voltage causes decrease in plate current, ac plate resistance, differential output resistance of tube, negative:

r

p


=



Δ

v

p




Δ

i

p





<
0



{\displaystyle r_{p}={\delta v_{p} \over \delta i_{p}}<0\,}

as other negative differential resistance devices tunnel diode, negative resistance can used create oscillator. parallel tuned circuit connected in plate circuit of tetrode. circuit oscillate if magnitude of negative plate resistance less parallel resistance r of tuned circuit, including load connected oscillator.

|


r

p



|

<
r



{\displaystyle |r_{p}|<r\,}

the frequency of oscillation close resonant frequency of tuned circuit.

f
=


1

2
π






1

l
c







{\displaystyle f={1 \over 2\pi }{\sqrt {1 \over lc}}\,}



design

as can seen graphs, dynatron operation screen grid had biased @ considerably higher voltage plate; @ least twice plate voltage. plate voltage swing limited negative resistance region of curve, downward kink , achieve largest output voltage swing, tube should biased in center of negative resistance region.

the negative resistance of older tetrode tubes around 10kΩ - 20kΩ, , can controlled varying control grid bias. if magnitude of negative resistance |rp| small enough start oscillation, little smaller positive resistance r of tuned circuit, oscillation frequency stable, , output waveform sinusoidal. if negative resistance made smaller positive resistance, voltage swing extend nonlinear part of curve, , peaks of sine wave output flattened ( clipped ).