Tektronix 1705A instruction manual LOW Load Average, High Load

Theory of Operation

+18 Volt Supply The +18 V supply is of the Buck-Regulator type. It uses a Switched Mode Power Supply Control Circuit, U583. Q783 is a buffer for slow startup and duty cycle limit. R681 and R680 are the limit resistors. The voltage on C684 ramps up to provide for slow startup when power is initially applied. The Internal Zener reference (VZ) for U583 is approximately 8 V and is present on pin 2. R586 and C583 are the frequency determining components for the IC’s internal sawtooth generator, which in this instance runs at approximately 50 kHz. R686 and R685 are the voltage setting components on the feedback input; C683, C684, and R682 form a frequency compensation network to prevent the IC from oscillating.

LOW LOAD

AVERAGE

LOAD

+40V

+18V

--0.7V

+40V

+18V

--0.7V

+40V

HIGH LOAD

+18V

--0.7V

Figure 4-2: Output duty cycle of the pulse width modulator used in the +18 V Power Supply

Q481 and Q482 drive the switching transistor. Q482 provides forward base current to the switching transistor, Q596. Q483 provides reverse base current to effectively turn off Q596. Q596 operates with voltage levels between -0.7 V and +40 V. CR594 is the commutating diode that sets the voltage level when Q596 is off (-.07 V) and current is still flowing in L591. The input filter circuit, composed of L591 and C690, is effectively driven with a square wave; however it is not a true square wave and is dependent on the loading of the +18 V supply. See Figure 4-2. The output voltage is fed back to the FB input of U583, which compares it to an internal reference voltage to determine the on time for the

1705A Spectrum Monitor 4- 13