Agilent Technologies 2948, 2712A manual Simplified Diagram of the Sampler Drive

Model 8904A Service

R202	t15V								+15V	R204
4		0201							0202	I
			R206	c208	T I	c207	R205
		-					--
	4)			I I	- II				t I
						I		-

b- dl

61

FROM CLOCK SYNC U2A HI c221 - 0205 CR206+bVCR203 0206 (HIGH TRUE)

R226

-15V

Figure 8-9. Simplified Diagram of the Sampler Drive

Differential input transistors (Q205and Q206)are driven in complement by the outputs of D flip-flop U2A (the Clock Sync flip-flop). The emitter current for Q205 and Q206 is supplied by a common 10 mA current source. (The current source is Q207 and associated parts in the main schematic diagram. Diode CR207 thermally tracks the base-emitter junction of Q207 to thermally compensate the current source.) The current from the current source alternates between Q205 and Q206 each half clock period. Diodes CR202 and CR203 clamp the collectors of Q205 and Q206 to approximately +6V; CR204 and CR205 clamp the collectors to +lOV. Q204 and Q202 are emitter followers which drive one side of transformer T1; Q203 and Q201 similarly drive the opposite side of T1 but are 180” out of phase. Capacitor C201 forces the junction of R201 and R202 to follow the bases and emitters of Q201 and Q203 with a fixed dc offset. C205 performs a similar function to the other half of the circuit.

Sampler Switch

The Sampler Switch connects the DAC to the Sampler Amplifier during the “sample” half of the clock cycle; the Sampler Switch is open during “hold”. The switches are diodes U201A,B, C,and F.They are switched by the Sampler Drive through T1.

When the switch diodes are off, diodes CR212 and CR208 charge capacitor C214 to approximately 2.7V.The charge on C214 has no effect when the switch diodes are on, but provides a strong initial backbias to the switch diodes as they turn off.

Principles of Operation (A3) 8-25