HP 214B - Page 112

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th e current switch (032 / 033) off. (034 is now conducting which turns 033 on and routes the positive
current to ground). The ramp capacitors now discharge into the negative current source with a current I.
When the ramp capacitor voltage drops to the lower Schmitt trigger threshold, the Schmitt trigger changes
state again and switches the
current switch on, thus repeating the whole cycle.
In External Trigger, Gate
or Burst modes, the VCO is disabled during re st periods by raising the Schmitt
trigger threshold - at the same time clamping the ramp side of the Schmitt trigger to 0 V via diode CR17.
The Schmitt trigger threshold is raised via the action of transistor switch 01 % 11, which switches
approximately +4 V through to the VR8 /CR 12 junction and thereby raises the 036 base potential
to +0.7 V.
A 214B
output pulse in External Trigger mode occurs for each pulse at the EXT TRIG input. This is
achieved by momenta ri ly switching 011 off, which allows the Schmitt trigger to toggle once (the
ramp capacitors therefore have no incluence on the output frequency).
In Gate mode, the internal VCO runs for the duration of the gate signal at the EXT input. This signal
switches
011 off, thus lowering the Schmitt trigger threshold and allowing it to toggle in response to
the ramp capacitor charge/discharge process.
In
Burst mode, the Schmitt trigger is enabled for the durat i on of the preselected number of pulses, a
'BURST COMPLETE' signal disabling the Schmitt trigger upon completion of the last pulse.
Delay Generator
The delay generator comprises the following functional blocks:
036 COLLECTOR
(REP . RATE )
047 COL LECTOR
044 BASE
( DELAY )
066
COLLECTOR
063 BASE
(WIDTH)
Figure 8- 3- 2. Timing diagram
8- 16
- adjustable
constant voltage source U 14a/ R45
- adjustable constant current source U14b / 049 /vern ier R 158
- ramp capacitors C47 - C52
-
current switch 050
- pre-amplifier / Schmitt trigger 044, 045 / 046, 047
Model 214B
Scans by ArtekMedia => 2009