HP 8559A Operation And Service Manual page 381

MODEL 8559A
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 A1 1 is described here. Bandwidth Filter No. 1 Assembly A1 1 operates
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
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, A1 1Y2, A13Y1, and A13Y2) that must be replaced as a set. If replace-
ment 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
feedback path (RJ; R3 forms the input resistance
but the narrowest bandwidths, as illustrated later.
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
a l l eight bandwidths; each assembly contains two stages. The bandwidths
+
1
=
This ac model of the amplifier's operation is true for all
FIGURE 6-45. 10dB INPUT BUFFER AMPLIFIER GAIN MODEL
DC BlAS
FIGURE DC BlAS PATH DURING CRYSTAL POLE OPERATION
The total resistance of R5, R6, and R7 forms the
GAIN I N dB 20 LOG
GAIN 2 . 7 3
GAIN I N dB
SERVICE
DC BlAS
I N BLOCK D