MODEL 8559A

SERVICE

BANDWIDTH FILTERS No. 1 and No. 2 ASSEMBLIES A l l and A13, CIRCUIT DESCRIPTION

Bandwidth Filters No. 1 and No. 2 Assemblies A l l and A13 are identical except for some off-board connec- tions. Bandwidth Filter No. 1 Assembly A11is described here. Bandwidth Filter No. 1Assembly A11operates at 21.4 MHz with a variable bandwidth of 3 MHz to 1 kHz. The RESOLUTION BW switch selects one of the following eight available bandwidths: 3 MHz, 1 MHz, 300 kHz, 100 kHz, 30 kHz, 10 kHz, 3 kHz, or 1 kHz.

Four stages of filtering are used for all eight bandwidths; each assembly contains two stages. The bandwidths from 30 kHz to 1 kHz are obtained from synchronously-tuned crystal filters. The remaining four bandwidths (100 kHz to 3 MHz) use synchronously-tuned LC tank circuits. The four crystal filter stages contain factory selected and matched crystals (AllYl, A11Y2, A13Y1, and A13Y2) that must be replaced as a set. If replacement of a bandwidth filter assembly is necessary, the new assembly is shipped with two crystals installed and two packaged separately to replace the crystals on the other assembly. In addition to the filter stages, each board contains a 10 dB Buffer Amplifier, a Unity Gain Buffer Amplifier, and an Output Buffer Amplifier.

10 dB Input Buffer Amplifier (B)

The 10 dB Input Buffer Amplifier is shown as a noninverting operational amplifier in Figure 8-45. Gain for the amplifier is expressed in the equation: Gain = 1 + RJR,,.The total resistance of R5, R6, and R7 forms the feedback path (RJ; R3 forms the input resistance (R,,).This ac model of the amplifier's operation is true for all but the narrowest bandwidths, as illustrated later.

- -

-

 

 

 

 

R 1 ~

 

 

dB -

R5tR6tR 7

GAIN I N

20 LOG li + -1

 

-

 

R3

GAIN

2 . 7 3

 

GAIN

I N

dB - = 8 . 7 5 4 8

FIGURE 6-45. 10dBINPUT BUFFER AMPLIFIER GAIN MODEL

nYo current paths are used for dc bias in the input buffer amplifier, one for crystal filter poles, another for LC filter poles. When a crystal filtered bandwidth (530 kHz) is selected, 4 3 (block D) and Q1 are the sources for the current through 4 2 (see Figure 8-46). The base voltage of 4 2 is fixed by the divider R9 and R10, while the

W 6 rnA DC BlAS

I N BLOCK D

-

~4 mA DC BlAS

FIGURE846. DC BlASPATH DURING CRYSTALPOLEOPERATION