November
PROCEEDINGS OF THE I.R.E.- Waves and Electrons Sechton
1330
sulting at each level of modulation was determined. For
a typical adjustment and a frequency shift of ± 2 Mc,
second-harmonic amplitude was measured to be 32 db
below that of the fundamental- and third-harmonic
amplitude 37 db below fundamental. Somewhat better
linearity of modulation has been obtained by more careful adjustment of the circuit.
When operated from regulated power sources, the
6AK5 oscillator proved to be quite free of frequency
drifts. After a one-hour warm-up period, a drift of 25 kc
took place in the succeeding hour. Modulation at powerline frequency was measured and found to be ±8 kc.
AM effects have not as yet been evaluated but these effects are known to be small.
ACKNOWLEDGMENT
The writer wishes to acknowledge the helpfujl cooperation of W. M. Goodall and A. F. Dietrich of these
Laboratories in the design and testing of this oscillator.
The Reactance-Tube Oscillator *
HAN CHANGt
AND
V. C. RIDEOUTt,
Summary-The reactance-tube oscillator is a combination reactance-tube circuit and oscillator circuit which uses but a single tube.
It has two forms-one derived from the capacitive reactance-tube
circuit and one from the inductive reactance-tube circuit; with slight
variations the first may be made to resemble the Hartley oscillator
circuit, and the second the Colpitts oscillator circuit. Experiments
with this oscillator have shown that linear frequency variation versus
grid voltage change with constant output amplitude may be obtained
over a range of more than five per cent in the region of 1 to 4 Mc.
MEMBER, IRE
Then
YT
=
1
+
rp
-
gmnA
cos 0 +
igmA sin 0
1
j
+-. (3)
RT XT
= --
Here RT and XT are the effective parallel resistive and
reactive components of YT. It can be seen that RT may
be negative if rp is large and cos 0 is negative. The necessary condition for cos 6 to be negative is that the reacI. ANALYSIS OF OPERATION
tive parts of Z, and Z2 have opposite signs. If RT can be
C ONSIDER the schematic circuit of the reactance- made negative in this manner and if a parallel resonant
tube shown in Fig. 1. The admittance across ter- circuit is connected across terminals a-b, oscillations
may build up. Furthermore, since 1/rp and gm vary in
minals a-b is:
same way with grid bias voltage, RT might be exthe
1
1
gmZ2
to remain constant over a considerable range.
pected
(1)
+
+
Yab
Zl +Z2 rp Z1+ Z2
Fig. 2(a) shows such an oscillator circuit derived from
a capacitive reactance-tube circuit. In this case the freThe first term on the right side of equation (1) is the quency of and condition for oscillation are approxiadmittance of the phase-shifting network. The remain- mately given by
+gMCo(LpRv + LvRv)
1
g ..
CpEp(l + a) ]
2r/LpCp(l + -a) L +
RpCp(Lp- Lg) + CpLp2/Corp + Cp2LpRp/Co
-1/2
(4)
where
a =
ing terms represent the admittance added by the tube,
which will be called YT. Let
Z2
=
Aej0 = A cos 0 + jA sin 0.
(6)
Similar expressions for the oscillator circuit of Fig.
2(b), which is derived from the inductive reactance-tube
circuit, are
--I b
I
Fig. 1-Basic reactance-tube circuit.
ZI + Z2
(Co/Cp)(1 + Lg/Lp).
(2)
* Decimal classification: R355.911.1. Original manuscript received by the Institute, April 4, 1949; revised manuscript received,
August 1, 1949.
t University of Wisconsin, Madison, Wis.
f=
gm
>
b F
VF1+
1I
2 7r \LpCp
L_
+ b "]
gnLpRv
Lo(l + b)_
(7)
(Lo + Lp)(RpCO/Lp + C,,/Cprp)
(Cv + Cv) (Rg + Ro)
La,
(8)
Chang and Rideout: The Reactance- Tube Oscillator
1949
1331
II. NOTES ON DESIGN
In both types of circuits the frequency is approxib = (Lp/Lo)(1 + Cp/Cv).
(9)
mately 1/2rV/LPCP. The percentage frequency deviaIn equations (4) and (7) the circuit constants are so tion can be determined by considering the term gmRgLp/
chosen that the second term within the brackets is small. Lo in the inductive reactance-tube circuit and gmRgCo/
Both may then be expanded to show that a linear change Cp in the capacitive reactance-tube circuit. Note that
the latter expression is obtained from (4) by assuming
in frequency with gm is possible.
LpRp>>L,R,, a condition which is necessary in practice
for good operation.
The ratios Lp/Lo or Co/Cp should be very small, say
about 0.1. By fixing the gm variation range, R0 is determined. The impedance of Z2 should be about one-fifth
CoI
of the impedance of Z1. Under the above conditions a
tank circuit of effective Q of about 20 was found satisfactory in the case of a 6L6 tube. The exact value of Lp
or Cp is determined, with the help of the approximate
R
frequency, by the condition for oscillation. This condition should hold at the smallest value of gm in the range
L
of operation.
where
(a)
Cg
(b)
Fig. 2-(a) Capacitive reactance-tube oscillator circuit.
(b) Inductive reactance-tube oscillator circuit.
III. EXPERIMENTAL RESULTS
Experiments have been conducted chiefly in the 1- to
4-Mc range. Fig. 3 shows typical sets of curves of frequency variation and radio-frequency current in the
tank capacitor Cp as grid voltage is varied. Greater frequency variation is possible by increasing R0 and then
making other compensating adjustments, or by extending the grid voltage range to include positive voltages.
z
0
w
,4
z
From (6) it may be seen that the frequency of oscillation is lower than the natural frequency of the tank circuit, and that increase in gm causes the frequency to become still lower, with resultant operation still further
down on the resonance curve of the tank circuit. The
reduced output voltage which might be expected in this
case is counter-balanced by the increased plate current
which results when grid bias is decreased to increase
gm,,. Thus it is possible to see in a qualitative way that
reasonably constant output might be expected. By a
similar argument, constant output may be expected in
the inductive reactance-tube oscillator circuit. Here,
however, operation is on the high-frequency side of tankcircuit resonance, and increasing gm further increases
frequency.
The circuits of Fig. 2 may be simplified by making Cp
equal to zero in Fig. 2(a) or making Lp infinite in Fig.
2(b). It is interesting to note that the first reactancetube oscillator circuit then resembles the Hartley oscillator and the second the Colpitts oscillator.
0
a
4
C-
wa
a:
F
GRID BIAS (VOLTS)
Fig. 3-Typical curves of frequency and amplitude for a capacitive
reactance-tube oscillator (center frequency 1,322 kc), and an inductive reactance-tube oscillator (center frequency 1,553 kc).
A modified circuit resembling the Hartley oscillator
was tested and was found to give a 7 per cent linear
range of frequency variation with the constants used.
Tests made with a number of different types of tubes
show that beam power tubes are most satisfactory, although power triodes have been used successfully.