HP 8340A, Service Manual

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Model 8340A - Service
SMALL DELTA F SWITCH E
US functions as a single-pole double-throw switch, routing either
3 to 6 MHz from U2 or 0.15 to 0.30 MHz from Ul, to the A39 PLL3
Up Converter. The required range is selected by a TTL signal (SW2
from the A42 PLL2 Divider) on Pl-22. R33 and R34 shift the TTL
levels to ECL levels.
For 20-30 loop sweeps between 5 kHz and 100 kHz, SW2 is a TTL
HIGH and the 3 to 6 MHz divider output is routed to the A39 PLL3
Up Converter. For sweeps less than 5 kHz, SW2 is a TTL LOW,
selecting the 0.15 to 0.30 MHz output.
0.1-5 MHz DELTA F SWITCH F
For delta F or sweep widths of 0.1 to 5 MHz, the 15 to 30 MHz
output is used. The output of U3 in Frequency Dividers (Block
D)
is routed through U6D, U7D, and Tl. This combination serves as a
switch with 90 dB of isolation in the off state. The TTL signal
(SWl from the A42 PLL2 Divider) on Pl-24 controls this switch.
For maximum isolation, the two gates (U6D, U7D) are contained in
separate packages. Any stray signals that do reach the output
will be a common-mode input to transformer Tl. Since Tl will only
respond to differential mode signals from U7D, these common-mode
signals will be rejected.
C25 and Ll4 filter the output to reduce spurious responses.
8-231/8-232
Scans by HB9HCA and HB9FSX