Section I1 Paragraphs 2-22 to 2-29 figure 2-4. This additional resistance will reduce the source impedance reduce the voltage available from ohms and will also at the lower terminals to about 75 the lower terminals to approximately the same amount available a t the upperterminal. At the same time the additional resistance will form a more favorable time constant with the stray capacity Co at the lower terminals and thus speed up the normally slower rise time a t those terminals until it is comparable to that of the 75-ohm output. 2-27. Figure 2-4 suggests the use either of two 75ohm cables o r abalanced 150-ohm cable for connecting to the load. In.either of these cases it is normally unnecessary to terminate the cables, s o that they can be connected directly to the load. The arrangement has the advantage that it can be used with any load impedance. If cables of other impedances a r e used, they should be terminated inthe characteristic impedance of the cable. This will involve a consideration of the load impedance in some cases.
75n OUTPUT BALANCED 1 5 0 n CABLE 0R TWO 7 5 n COAXIAL cneLes
2-23. The clipper amplifier and output tubes in the Model 211A operate in a circuit designed for a 50% duty cycle. The balance of this circuit is maintained by the SYMMETRY control which balances the two outputs from the multivibrator. Any alteration of this circuit attempting to generate pulses, suchits padding as the SYMMETRY potentiometer to extend range of control, would overdrive one side of the clipper amplifier and output tubes beyond the 50% duty cycle factor to the eventual damage of the instrument.
2-24. B A L A N C E D O U T P U T .
2 -25. The 211A can be converted t o a balanced source without modifying the instrument in any way. Figure 2-3 shows the basic arrangement of the output circuit. The output tubesthemselves a r e in push-pull but have unequal loads a s shown A balanced voltage can thus be obtained by equalizing the tube loads. This can be done directly at the terminals on the panel.
7 5 n OUTPUT LEVEL CONTROL
nn
OUTPUT
'
T
ATTENuATOR
OUTPUT
OUTPUT
------SEE TEXT-
1
Figure 2-4.
Method of Obtaining a Balanced Output from the 2 11A
Figure 2-3.
Basic Output Circuit of 211A.
2-26. In figure 2-4 the upper terminal represents the 75-ohm output and the lower terminals represent the 600-ohm output. To equalize the source impedance a t the two outputs, a resistance of 86 ohms can be connected across the lower terminals as shown in
2-28. The output voltage can be selected most conveniently if the 75 S ATTEN. output is first set to 2 zero. The two OUTPUT AMPLITUDE controls will then have about the same voltage range and each should be set as necessary to obtain one-half of the desired output voltage. This will occur when both controls a r e a t about the same angular position. If desired, both line-to-ground voltages can be measured with a voltmeter o r an oscilloscope. 2 -29. Output voltage will be a s high a s 14 volts peakto-peak, open circuit, and 7 volts peak-to-peak terminated with 75 ohms.
