MODEL 8559A

SERVICE

FIGURE8-52. LC POLEMODEL

A simplified schematic of the first LC pole is shown in Figure 8-53. The fundamental frequency-determining components are L6 and the center-tapped capacitance C16* and C20*. Positive feedback is applied to the center- tap at 21.4 MHz to compensate for losses in the tank circuit. The application of feedback makes it important that C16* and C20* be about the same value for proper pole operation. The level of the feedback is controlled by CR5, acting as a variable resistance. LC feedback control R26 establishes the current through CR5 and its resistance.

1 C 8 ' t 2 3+

/

 

ci6*

 

R25

 

 

 

 

 

 

 

(FEEDBACK)

 

R24

 

 

t V F

B I A S

FIGURE 8-53. FIRSTLC POLE, SIMPLIFIEDSCHEMATIC

When an LC filtered bandwidth is selected, BW5F is at + 15V; BW7F is at a voltage greater than or equal to

+6.8V and supplies bandwidth-determining bias current to CR3. Supply line +VF BIAS is always at +6.8V Control line BWSF reverse biases CR8 (block B), disabling the crystal pole, and forward biases CR1 (block B), opening the dc bias path to 4 2 (see Figure 8-47). During LC operation, CR6 is reverse biased, keeping C28 out of the circuit. When a crystal filtered bandwidth is selected, BWSF forward biases CR6 and allows C28 to ground the signal path.

Unity Gain Buffer Amplifier (E)

Operation of the Unity Gain Buffer Amplifier is similar to the 10 dB Input Buffer Amplifier, except that it has an FET input (Q5) and unity gain. The input signal path is activated by the BWSF line, which switches on CR9 (during LC mode) or CR8 (during crystal mode).

When the crystal mode is selected, the current through the input FET (Q5) is determined by 4 6 and constant current sink 4 7 (which sinks about 4 mA). During LC mode, current is supplied through R37 and CRlO from BWSE The input FET current is a good indication of the stage's operation and can be monitored by measuring the gate-to-source voltage. This voltage should be between +0.2V and + 1.W (an increase in current decreases the voltage).

Capacitor C68 and L19 form a feedback circuit that tunes Q7 to 21.4 MHz. Trimmer Resistor R31 (XTL FEEDBACK) adjusts the feedback and controls the stage gain as did R5 and R6 in block B.