21.4 MHz Preamplifier 0

The 321.4 to 21.4 MHz converter drives the 21.4 MHz preamp which consists of Q6 and its associated components. Q5 provides bias for Q6 by setting the base current. LlO, C22, and C23 form a series resonant circuit producing a notch in the frequency response of this amplifier at 10.7 MHz. This is to prevent the second harmonic of 10.7 MHz from causing a spurious response.

Slope Attenuator @

The HP 8566 is a harmonic mixing spectrum analyzer, and as such has five bands of frequencies to cover the range of 0 to 22 GHz. When changing from band to band, the conversion loss of the input mixer changes. Also within each band there are variations of conversion loss with frequency.

The variations of conversion loss within each band are compensated by pin diodes CR1 to CR4 and associated circuitry. Pin diodes have a property of changing RF impedance as their dc current is varied. Thus CRl/CR2 and CR3/CR4 form the two elements of a voltage divider. The current through CRl/CR2 is fixed by RlO and Rll. The current through CR3/CR4 is determined by circuitry on the A6All Slope Generator. This forms a current controlled variable attenuator. Pin diodes CRl/CR2 are used in place of a fixed resistor to provide temperature tracking with CR3/CR4.

Step Gain Amplifier 1 @ and Step Gain Amplifier 2 @)

The variations of conversion loss from band to band are compensated for by two identical variable gain amplifiers made up of Q3 and QlO and associated circuitry (Q4 and Q9 are emitter follower buffers). A simplified circuit of one of these stages is shown in Figure 1. The gain of the amplifier is determined approximately by the following equation.

Impedance (CR8, CR9, and CRlO)

Gain M

Impedance (CR5, CR6, and CR7)

The impedance of CR8, CR9, and CR10 is determined by R23 and temperature compensation circuitry (Ql).

The impedance of CR5, CR6, and CR7 is determined by the current from the step gain circuit on the A6AlO Relay Driver.

2 A6A3