MODEL 8559A

SERVICE

STEP GAIN ASSEMBLY A12, CIRCUIT DESCRIPTION

The Step Gain Assembly A12 provides from 0 to 50 dB amplification of the 21.4 MHz IF in 10 dB steps, as selected from the REFERENCE LEVEL control. A zero to - 12 dB REFERENCE LEVEL FINE attenuator control is also included on the front panel. Generated on the Step Gain Assembly A12 are the first mixer diode bias and a flatness control voltage proportional to the sweep plus tune (S + T) voltage.

Step Gain Amplifiers (E) (F) (G)

There are three step gain amplifiers, one 10 dB and two 20 dB, cascaded as shown in the schematic diagram. Full gain of any amplifier is selected by grounding the appropriate IFG line. The three step gain amplifiers can be considered as operational amplifiers. An equivalent circuit for the three stages is shown in Figure 8-57. The gain for each amplifier is: Gain = 1 + R,/R,. The feedback resistance, R,, for the 10 dB amplifier is R8, 562 ohms; for the 20 dB amplifiers it is R23 and R31, each 750 ohms. The input resistance, R,, is a combination of a fixed series resistance (56.2 ohms) and the controlled resistance of the PIN diodes. The resistance of the PIN diodes is approximately 10 to 1000 ohms and increases as the forward bias current is decreased from 100 milliamperesto 1 microampere. The input resistance, R,,for the 10 dB amplifier is approximately 260 ohms; for the 20 dB amplifiers, it is about 83 ohms.

FIGURE8-57.STEPGAIN AMPLIFIERS, SIMPLIFIEDDIAGRAM

Selection of the correct combination of step gain amplifiers is accomplished with the REFERENCE LEVEL switch. Rotating the switch grounds the emitter circuit of the selected amplifier (or amplifiers), allowing current to flow through the PIN di8de (or diodes). The possible switch combinations allow the gain to vary from unity (all switches open) to 50 dB maximum with all three emitter circuits grounded.

TestINorm Switch. In the emitter paths of the 20 dB step gain amplifiers are the TEST/NORM switches used to disable both 20 dB amplifiers during log amplifier adjustment.

0-12dBControl (H)

The REFERENCE LEVEL FINE control provides approximately 0.3 to 12.3 dB of attenuation at the base of 4 6 in the 0- 12 dB control circuit. By regulating the current flow through PIN diode CR7, the amount of signal attenuation is controlled. For example, if PIN diode current flow is increased, more RF signal is shunted or bypassed to ground. Capacitor C23 provides the RF ground path.

A minimum current flow through the PIN diode, which provides the maximum allowable diode resistance, is established by - 12 dB potentiometer R39 so that the diode is never completely cut off. Adjustment of R39 sets the 0.3 dB point and is adjusted with the REFERENCE LEVEL FINE control set fully clockwise (- 12 posi- tion).

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