HP 212A - Page 9

50 ohm cable.
The output circuit of the Model 212A is terminated with ap-
proximately 50 ohms se termination of the output cable with
50 ohms is not absolutely necessary fox many measurements.
If the overall system is sensitive to frequencies on the order
of 20 to 30 megacycles, the output cable should be termin--
ated in a non-inductive 50 ohm load to prevent reflections
from the load which may cause ringing. This is especially
true when the ATTENUATION and OUTPUT controls are
set for full output.
Under these conditions the Model Z12A
OUTPUT source impedence is less than 50 ohms thus result-
ing in an imperfect output cable termination at the source end,
The SYNC OUT terminal may be connected to the synchronizing ter -
minals on an oscilloscope and the polarity of the SY NC OUT signal may
be switched to select the best performance. The SYNC SELECTOR
should be set to either the XI or the X10 position, and the PULSE RATE
dial set to the desired repetition rate,
The PULSE WIDTH dial may
then be set to select the pulse width desired and the PULSE POSITION
control is then adjusted until the pulse appears at the desired position
on the oscilloscope screen. The amplitude of the pulse can be adjusted
by means of the AMPLITUDE CONTROL arrd the OUTPUT ATTENUATOR
to give the desired output level.
CAUTION I
Do not set the PULSE POSITION delay to greater
than 50 micreseconds when a repetition rate of
5000 cps is used. Maximum delay of 1015 micro-
seconds may be used for repetition rates up to
2500 cps, Above 2500 cps the maximum delay
must be decreased as the repetition rate is in-
creased so as not to overload the tubes in the pulse
position circuit.
2-5 WAVESHAPE OF OUTPUT PULSE
The output pulse from the Model 2 1 ZA has extremely good wave shape
and, in general, the waveshape is quite independent of the load and the
operating conditions of the Pulse Generator. However, when the gen-
erator is operated at zero attenuation, with a length of cable between
the generator and an unterminated load, reflections may occur due to
the internal impedance of the generator being less than 50 ohms.
This
condition may be corrected by properly terminating the cable,
As the
attenuation is increased, the internal impedance approaches 50 ohms,