MODEL8559A

SERVICE

Band Offset (D)

Operational amplifiers U4a, U4d, U4c, and their associated circuits provide offset and gain for the tilt voltage. Potentiometers R57, R58, R59, R60, R61, and R62 are used to adjust the offset of each band. A fixed negative offset is provided for all bands by operational amplifier U4c. The resulting flatness output voltage is applied to a voltage-controlled amplifier on Third Converter Assembly AlO.

Mixer Diode Bias (B)

Bias of the First Mixer Assembly A4 depends on the desired harmonic mixing number. Quad switch U1 and operational amplifiers U2a, U2b, and U2c with their associated components form the mixer diode bias sources. Varying power levels are coupled into the mixer diode due to irregularities in the YTO's swept power output, causing variations in the mixer diode bias conduction angle, or total bias power. The bias sources adjust to these instantaneous changes in the mixer bias conduction angle by increasing or decreasing bias in order to maintain a constant conduction angle. The circuit includes separate bias adjustments for bands 2-,2+, 3 - ,and 3 +. Bands 1 - and 1 + use a common bias adjustment potentiometer.

The four switches in U1 are normally closed, but the individual switches open when selected by a logic-high control voltage. Since the outputs from the band select decoder U3 are all high except one, the normal status of the switches in U1 is open until a low control input allows one to close. The switch then connects one of the three potentiometers (R70, R71, R72) through a factory selected fixed resistor to the positive input (pin 10) of operational amplifier U2c, forming a voltage source at that point. The table below shows which potentiometers and factory selected resistors apply to which band.

TABLE8-9. MIXERDIODE BIASADJUSTMENTS

Band

Control Name

Bias Adjust

Range Adjust Resistor

Resistor

(Factory-Select)

 

 

 

 

 

 

.01-3

V1

R72

R73

6-9

V1

R72

R73

3-9

V2-

R83

R84

9-15

V2+

R87

R88

6-15

V3-

R71

R74

12.1-21

V3+

R70

R75

 

 

 

 

Operational amplifier U2c forms a negative impedance converter that increases or decreases bias as needed to maintain a constant angle of conduction at the first mixer. This is necessary to maintain a constant insertion loss through the first mixer. Operational amplifier U2c is connected to the voltage source at the junction of R73, R74, R75, and Q1. This circuit multiplies its input source resistance by approximately - 1/110, thus converting the input voltage source and series resistance into an equivalent voltage source and negative impedance (here, approximately - 1000ohms).

Because of this conversion, as current increases in the circuit, the resultant output voltage decreases, just as it would if a negative resistance value (- R) were substituted for R in the familiar expression for Ohm's Law. The expression would then be rewritten as: E = I(- R). Notice now that an increase in current (I) results in a decrease in voltage (E). This is the equivalent action of this circuit. If all of Ul's switches are open (as in band 2- or 2+), transistor Q1 forces the junction positive, turning off CR15 and thereby removing the negative impedance converter from the bias output at P1-24. One of the other operational amplifiers in U2 is activated, providing voltage sources and positive resistances to the bias output (TPl or PI-24). When one of the operational amplifiers is selected, the diodes at the outputs of the other two are reverse biased, and disconnect the outputs from PI-24.

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