Tektronix C-30B, Instruction Manual

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Operating Instructions—C-30B Camera
PHOTOGRAPHIC CONSIDERATIONS
WR1TING RATE
Writing Rate is a figure o f merit which roughly describes
the ability of a particular camera System mounted on a
particular oscilloscope to photograph fast-moving traces.
The w riting rate figure expresses the maximum spot rate
(usually in centimeters per microsecond) which can be
photographed satisfactorily.
The faster the oscilloscope spot moves, the dimmer the
trace becomes, since the electron beam strikes each point
on the phosphor coating fo r a shorter period of time. A
camera system and oscilloscope w ith a high w riting rate are
required fo r photographing low repetition rate displays at
fast oscilloscope sweep rates, or single sweep displays.
Figure 2-11 shows one method of calculating w riting
speed. A single trace of a damped sine wave is displayed.
The frequency of the damped waveform is such that the
rapidly rising and falling portions of the first two cycles
fail to photograph. The w riting rate of the system is found
as follows. Starting from the left, find the first rapidly
rising or falling portion of the damped sine wave that is
photographed in its entirety. Let D represent the vertical
distance (in centimeters) between the peaks connected by
this portion. If D is three or more times as great as the
horizontal distance occupied by one cycle (so that the
horizontal component of velocity is small compared to the
vertical component), the maximum w riting rate (in centi­
meters per microsecond) is given approximately by:
Maximum w riting rate = 3.14 Df
where f is the frequency of the damped wave in megahertz.
2 8 2 5 -1 3
Figure 2-11. Single trace of a damped waveform.
Absolute w riting speed of an oscilloscope and camera
system is d iffic u lt to calculate because so many variables
are involved. Among the variables are speed of the camera
lens, type of crt phosphor, type of film , crt accelerating
potential, camera lens, and development time of the film .
It is possible to compare the effectiveness of two films
by measuring their writing speeds under identical
conditions. In other words, the more effective of the two
films under those particular conditions can be chosen
w ith o u t assigning a specific value to either film .
The rated ASA speed of a film reveals little about its
effectiveness in recording single oscilloscope traces. This is
because ASA speed rating is measured for 1/50 second
exposure
to light of normal daylight and spectral
characteristics, while the very short exposures of fast crt
traces are several Orders of magnitude smaller and have
different spectral distributions. There is usually some
relationship between ASA rating and maximum writing
rate, however. Thus it would be safe to assume that a
film with a very high ASA speed rating would probably
have a higher maximum writing rate than a film with lower
ASA speed rating.
CRT SELECTION
There are a great number of crt phosphor types
presently available. Each of these phosphors has certain
advantages and disadvantages compared to the others. No
single phosphor is best for all applications. Of the many
types available, five are most commonly in use: P I, P2, P7,
P11, and P31. Other phosphor types are usually restricted
to special applications. Since the P1, P2, P7, P11, and P31
phosphors are most often used, the following discussion
is directed prim arily toward them.
The most important single characteristic of a phosphor
fo r photographic purposes is the color of its emitted light.
A blue or violet fluorescence is the most suitable for
photographic work. The shorter the wavelength of the
visible peak-emitted light, the better the phosphor for
photographic applications.
The P11 phosphor has the highest comparative writing
rate of any common phosphor and is thus the best for
photographic work. The medium-short persistence of this
phosphor is somewhat undesirable for general purpose
w ork, but the disadvantages are slight. Type P11 should be
chosen whenever the ultimate in photographic capability is
required. Type P11 emits a medium-short-duration,
purplish-blue light.
2-11