Tektronix TYPE 109, Инструкция по эксплуатации

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Applications— Type 109
F ig . 3 - 8 . D e ta ile d m ethod o f a c-c o u p lin g the s ig n a l d ire c tly to the v e rtic a l d e fle ctio n p la te s.
which will delay the pulse about 120 nsec to get the pulse
on the crt screen.
Only approximate values for two of the parts are given.
The values of these and all other parts depend upon the
crt, cable impedance, and lead lengths. Later on, some set­
ups employing a 540-Series Oscilloscope are illustrated and
described to show how optimum results were obtained by
modifying the circuit shown in Fig. 3-8 to suit the specific
application.
The coupling capacitors, C l and C2, provide the means
for ac-coupling to the plates. When selecting these parts
for use in the circuit, keep in mind that the physical size
should be small to reduce the lead-length inductance.
The cable which connects to the terminating resistor
should be long enough so that if a double-transit time re­
flection appears, it can be easily identified from the input
signal. Then, the undesired termination error causing the
reflection can often be corrected by physical or electrical
adjustments at the termination.
To find the resonant frequency (f0) of the lead induc­
tance and the deflection plate capacitance (C) for use in
the equation shown in Fig. 3-8, use the method that follows.
Turn off the oscilloscope power and disconnect the vertical
amplifier output leads to the crt. Cut a wire loop which
equals the total length of C l, C2, R l, R2, R3, R4. Substi­
tute the wire loop for these components between the verti­
cal deflection plate pins. Bring a grid-dip meter near the
loop and measure the resonant frequency.
A convenient method for making connections to the crt
deflection plates is to use clips removed from a standard
miniature tube socket.
After removing the wire lead, measure the total capaci­
tance between the plates with a Tektronix Type 130 LC
Meter, or equivalent, at the deflection plate pins. Capaci­
tance between the plates can also be found by referring
to a list of crt specifications.
The value of Rl is found by solving the equation in Fig.
3-8. Make R2 the same value as R l.
Since the deflection plates are placed close to the path
of the electron beam, a small amount of current will flow
in the deflection plate circuits. This current flow varies
nonlinearly with the beam position. The values of the re­
sistors R3 and R4 must be selected to keep the current
flow from producing' a large voltage drop at the deflection
plates. If the resistances are too great, the voltage drops
may become large enough to cause serious positioning
difficulties, defocusing, or distortion. These effects are most
noticeable when the beam is positioned near one side of the
crt. On the other hand, if the resistances are too small
the short r-c time constant of the coupling circuit may cause
the low-frequency response to be limited.
The risetime limitation is a combination of the limitation
imposed by the resonant frequency (fc) (which limits the
risetime arriving at the deflection plates), and the transit
time (tr) of the electron beam through the deflection plate
system (which limits the deflection plates' ability to change
the beam position rapidly).*
The deflection factor can be found from the reference
chart, or it can be measured as follows: Connect a dc volt­
meter between the vertical plates when the internal vertical
amplifier is connected to the deflection plate pins. Measure
the voltage change when the beam is positioned vertically
over the full height of the graticule. Divide this voltage
excursion by the graticule height in centimeters to obtain
the deflection factor in volts per centimeter.
If the output leads from the internal vertical amplifier
of the oscilloscope are disconnected and the power is on,
do not allow the leads to come in contact with the chassis
or tube shield. A short circuit of this type can damage the
amplifier circuits.
Typical risetime figures for Tektronix cathode-ray tubes
are listed in Table 3-1.
As a specific example, showing before and after results,
a Type 540-Series Oscilloscope with a Type K Plug-In Unit
was used in the setup shown in Fig. 3-9. In this example
* S e e I. A . D. Le w is an d F. H. W e ll, M illim ic ro se co n d Pulse Te ch ­
n iq u e s, Second e d itio n — 1 9 5 9 , C h a p te r 6 , Pergam on P re ss, Lo n ­
don an d N e w Y o rk .
3-6