HP (Hewlett-Packard) HP 85660B ユーザーズマニュアル

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. 

 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.

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 

 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 

 and 

 form the two elements of a voltage

divider. The current through 

 is fixed by 

 and 

 The current through

 is determined by circuitry on the 

 Slope Generator. This forms a current

controlled variable attenuator. Pin diodes 

 are used in place of a fixed resistor to

provide temperature tracking with 

The variations of conversion loss from band to band are compensated for by two identical
variable gain amplifiers made up of Q3 and 

 and associated circuitry (Q4 and 

 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.

Gain 

Impedance (CR8, 

 and 

Impedance (CR5, CR6, and CR7)

The impedance of CR8, 

 and CR10 is determined by R23 and temperature compensation

circuitry 

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

 Relay Driver.

2