SERVICE

MODEL8559A

 

IF Preamplifier (F)

The IF Preamplifier voltage gain is provided by Q8 in a common-emitter amplifier configuration. Circuit gain is controlled with collector-to-base feedback through PIN diode CR4. The current through CR4 is adjusted from the front panel by the REF LEVEL CAL control and can vary the gain of the IF Preamplifier over a 10 dB range. Transistor 4 7 functions as an emitter follower buffer amplifier.

Flatness Compensation Amplifier (H)

Approximately 20 dB of compensation is available in the Flatness Compensating Amplifier to compensate for small changes in conversion efficiency that occur while sweeping through individual bands. Larger between- band changes in conversion efficiency are compensated for in the Band Conversion Loss Amplifier. The gain of the Flatness Compensation Amplifier is controlled by the Non-Linear Current Source, which draws current through PIN diode CR1. The more current it draws, the lower the gain.

Non-LinearCurrent Source (G)

The flatness voltage from the Step Gain Assembly A12 sets the base voltage of 43. Resistors R41, R42, R43, and diode CR5 establish the emitter current and cause it to vary non-linearly in response to changes in the base voltage. This non-linear current drives CRl and enables the gain of the Flatness Compensation Amplifier to be proportional to the base voltage (and flatness voltage) at about 0.4V per dB of gain.

Band Conversion Loss CompensatingAmplifier (I)

The Band Conversion Loss Compensating Amplifier changes gain in discrete steps to compensate for the changes in conversion Ioss associated with RF section harmonic band switching. In the fundamental mixing bands (Bands 1 and 2), the circuit has unity gain. During second harmonic mixing (Bands 3 and 4), CR2 is forward biased, allowing the gain to be set by R34 as shown in the following equation: Gain = 1 + R32/R34. In the third harmonic mixing mode (Bands 5 and 6), CR3 is forward biased and R36 establishes the gain as follows: Gain = 1 + R32/R36. See Figure 8-41 for a simplified schematic of the Band Conversion Loss Compensating Amplifier gain switching. A gain-versus-band table is shown on the Third Converter Assembly A10 schematic below function block I.

FIGUREMI. BANDCONVERSIONLOSSCOMPENSATIONAMPLIFIEDGAINSWITCHING,SIMPLIFIEDDIAGRAM