This document describes a patent for improvements to electronic time-base generators. It includes a circuit diagram and description of a generator that uses negative feedback to linearize the output waveform by differentiating the voltage across a scanning coil and using it to control a feedback valve. This counteracts nonlinearity from exponential charging/discharging of capacitors. It allows the scanning speed and sweep magnitude to be varied by controlling the bias on the feedback valve.

1. * GB780109 (A)
Description: GB780109 (A) ? 1957-07-31
Improvements in or relating to electronic time-base generators
Description of GB780109 (A)
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PATENT SPECIFICATION
7809109 Date of Application and filing Complete Specification: Aug.
10, 1955.
Application made in France on Aug. 19, 1954.
Complete Specification Published: July 31, 1957.
No. 23080/55.
Index at acceptance:-Class 40(6), P(1 Al:1 M5Al:1
M5B:2C2:2C4:3D:3E:3K:4T2).
International Classification:-HO3k.
COMPLETE SPECIFICATION
Improvements in or relating to Electronic Time-base Generators We,
METROPOLITAN-VICKERS ELECTRICAL COMPANY LIMITED, a British Company,
having its registered office at St. Paul's Corner, 1-3, St. Paul's
Churchyard, London, E.C.4, do hereby declare the invention, for which
we pray that a patent may be granted to us, and the method by which it
is to be performed, to be particularly described in and by the

2. following statement:-
This invention relates to electronic timebase or sweep generators as
used for energising a deflection coil (or coils) of a cathode ray tube
to produce a sweeping or scanning movement of the electron beam of
such tube, a deflection coil connected to be so energised being for
convenience termed a scanning coil hereinafter.
The invention is specially suitable for timebase generators intended
for use with oscil20loscopes employing cathode ray tubes of relatively
small diameter, but is also applicable in general to large screen
oscilloscopes and other cathode ray tube equipment.
The output from a time-base or sweep generator has usually a saw-tooth
or trapezoidal waveform the inclined portions of which correspond to
sweep periods of the electron beam. Especially for time-bases, it is
as a general rule desirable for these inclined portiofns of the
waveform to be as-nearly linear as possible in order that the time
scale shall also be substantially linear. The inclined portions of the
waveform are, however, often generated by the charging or discharging
of a capacitance and in the absence of correction tend to be
non-linear owing to the exponential charging or discharging
characteristic of the capacitance.
In accordance with the present invention, an electronic time-base or
sweep generator including means tending to correct non-linearity in
its output waveform, comprises a source of sweep voltage, a
cathode-follower valve connected effectively between said source and a
cathode ray tube scanning coil [Price 3/61 that is to be energised by
the generator, and negative feedback means including a differentiating
circuit connected to differentiate the voltage across the scanning
coil and during a sweep period to control accordingly 50 the extent of
conduction of a further, feedback valve effectively shunting the
source of sweep voltage, the arrangement being such as to tend to,
provide across the scanning coil a linearised voltage waveform
appropriate to producing a linearly varying current in the coil.
With such arrangement, the feedback means will have a correcting
effect tending to counteract non-linearity in the output 60 waveform
such as may be due for instance to the exponential charging or
discharging characteristic of a capacitance component of the generator
as previously indicated. It is also contemplated, as will appear
hereinafter, 65 that the scanning speed and/or the magnitude of the
sweep of the electron beam will depend on the control grid-cathode
bias of the feedback valve, and in order to provide a continuously
variable scanning speed or 70 sweep magnitude control means may
accordingly be provided for varying this bias.
In order to give a fuller understanding of the invention an embodiment
thereof suitable for use with an oscilloscope providing a panoramic

3. display in an electro-magnetic detection system will now be described
with reference to the accompanying drawings in which:Fig. 1 is a
circuit diagram of a time-base 80 generator in accordance with the
invention, and Fig. 2 gives various curves showing the variation with
time of the potentials at different points in the circuit of Fig. 1.
85 Referring to Fig. 1, a deflection coil 1 constituting a scanning
coil for the oscilloscope, not shown, is connected in series in the
cathode circuit- of a pentode valve 2 connected as a cathode-follower
and having its 90 j1 11.1cny780,109 screen grid maintained at a fixed
potential with respect to the cathode by any appropriate means
symbolised by the battery 3.
The anode of the valve 2 is connected to the positive terminal B+, of
a source of high tension through a resistance 4, while the control
grid of the valve 2 is connected to the terminal B'1 through a
resistance 6 of relatively high value and to earth through a
capacitance 7.
The anode of a triode valve 5 constituting the output valve of a
source of sweep voltage is connected to the control grid of the valve
2, while the control grid of the valve 5 is connected to the normally
non-conducting valve of a pair of valves (not shown) constituting a
monostable trigger circuit (flipflop) indicated by the rectangle 8.
This latter is synchronised by the tripping impulses of the
electromagnetic detection system.
When the trigger circuit 8 is in its stable state, the potential of
the grid of the valve is sufficiently high for this valve to become
conducting and grid current to flow. Under these conditions the
potential of the anode of the valve 5 (potential of the point A), and
consequently that of the control grid of the valve 2, becomes negative
with respect to the cathode potential of the latter valve; the anode
current of the valve 2 is therefore cut off at this time. When the
trigger circuit 8 is in its unstable state, the valve 5 is blocked and
the potential of its anode tends to increase according to an
exponential law de35termined by the time constant of the circuit
constituted by the resistance 6 and capacitance 7. The potential of
the control grid of the valve 2 rises above the cut-off value and the
valve 2 then becomes conducting.
A negative feedback circuit is provided including a triode valve 9 the
anode of which is connected to the -control grid of the valve 2 while
its cathode is connected to the terminal B-2 of the high tension
source through a resistance 11 and potentiometer 12 in series.
The control grid of the valve 9 is connected to the slider of the
potentiometer 12 through a resistance 10. Finally a capacitance 13 is
connected between the cathode of the valve 2 and the grid of the valve
9.

4. The curves VA, VB and Vc of Fig. 2 show the variation, as function of
the time t, of the potentials of the points A, B and C of the circuit
of Fig. 1. The upper (chain dotted) part of the curve VA (above the
point b) represents what would be the variation of the potential of
the point A in the absence of the feedback valve 9.
When, with the valve 5 blocked, the po60tential of the point A (curve
VA) increases and promotes unblocking of the valve 2 at point a, the
potential of the cathode of the valve 2 (curve Ve, point a) rises
abruptly.
This variation of potential is transmitted to the grid of the valve 9
(curve VB, point J) by the resistance-capacity circuit 14 constituted
by the resistance 10, a part of the potentiometer 12 and the
capacitance 13. As soon as the potential of the point B rises above
the cut-off potential UI, the valve 9 delivers 70 current and the
increase of the potential of the point A is strongly opposed. The
valve 9 thereafter conducts for the whole duration of the sweep
period.
The elements 10 and 13 are so chosen that 75 they constitute a
differentiating circuit, resulting in a trapezoidal voltage being
applied to the grid of the valve 9 at the point B (part fghi of the
curve Vn). This variation of the potential of the point B causes a
corresponding variation of the anode current of the valve 9 and an
inverse variation of the potential of the point A, thus tending to
linearise the resultant variation of the latter (part bc of the curves
VA and Vc, here being approximately coincident). A waveform is thus
obtained comprising a saw-tooth bed superimposed on a substantially
rectangular impulse a'bde, and it will be seen that the voltage
produced across the coil 1 has then the 90 necessary trapezoidal form
to obtain a correct scanning deflection of the oscilloscope electron
beam. The amplitude cd of this sawtooth is proportional to that (ed)
of the rectangular impulse and is determined by the 95 effective
cut-off voltage of the valve 9, which in turn is determined by the
effective bias on this valve as set by the position of the slider of
the potentiometer 12. The speed of displacement of the spot on the
sreen of 100 the oscilloscope (scanning speed) and the scale of the
image reproduced on the screen can thus be controlled by the
potentiometer 12.
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5. * 5.8.23.4; 93p