Motorola R-20010 Maintenance Manual page 233

9 .1 DESCRIPTION
The primary func tion of t he Scope/DVM Cont rol
hoard is to route the required measureme nt and view-
ing signals to the digital voltmete r (DVM) and scope
circuitry. A large portion of the displayed data is
d etermined by DVM measurements on internal signal
points. Thus, for a rapid update of several data dis-
plays, several internal signal poin ts must be time-divi-
sion mul ti plexed to the DVM. The DVM Control
circuit ry and t he system processor provide this func-
tion.
With the Scope Control circuit ry, t he user can
choose to display eit her data information, or signals for
internal modulation or demodulation, or
ext~rnal
scope
inputs. The horizontal sweep provided for the spec-
t rum -analyzer display is coherent with the synthesizer
sweep generator. The coherent horizontal sweep and
sy nthesizer sweep can be used for an external filter-
alignment display. An external horizontal input is also
possible.
The Scope/DVM Control board also contains the
1-kHz notch fil ter and the rectifier circuit used for dis-
tortion measurements.
A block diagram of the Scope/ DVM Control b oard
is shown a t t he end of t his section in Figure 9-l , a
schematic in Figure 9-2, and the printed wiring board
assembly and pa rts list in Figure 9-3.
9.2 THEORY OF OPERATION
9.2.1 SCOPE VERTICAL CONTROL
Th E> input to t he scope vertical amplifier is switched
h. ' ·
1
he sc·opE> ve rtical-select swit ch (U6) to one of four
!'ou tTe:': the ra nge switch (V ERT FROM RN G SW),
the H' rt i<'al character-sweep generator. t he spectrum
anal~·zer
(SPECT ANA VERT), or t he 700-kHz IF.
Inputs to t he range switch, which a re routed through
1
he ra nge switch on the Front-P anel Interface board
(A 1:-i). rome from either 1) the scope's vertical -input
port on th e front panel or 2) t he internal modulation
s i ~twls.
as selected hy this board 's control circuitry for
modulatio n d is play. The vertical character sweep is a
>:a"·touth waveform generated by the vertical cha rac-
tE'r-sweE>p gene ra tor a nd synced hy the VERT CHAR
S Y: C signal fro m the cha racte r ge nerator on t he Pro-
('('s:'or hoa rd (A14). The detected and a mpl ified out -
put
'•I'
the Heceive r loga rit h mic IF is t he vertical inpu t
f, •r
t
lw !'pe('t rum analyzer. The remaining signal source
is the set'ond IF sig nal from
I
he Receiver fo r IF en ve-
lop(• ohsernlt ion.
9- 1
SECTION 9.
SCOPE/ DVM CONTROL BOARD (A7)
9.2.2 DUAL-DISPLAY CONTROL
The dual-display control circuitry produces a screen
wi t h a single row of cha racters at t he top of t he CRT
a nd a display for either t he spectrum analyzer or t he
scope. This fu nction is implemented with the vertical
sweep control by alternating the signal fo r the spec-
trum analyzer or t he range switch with the sign al for
the vertical characte r sweep.
The dual-display seque nce starts with t he synt he-
sizer sweep generator (Ul 2, U51 A, U59A), which is
common to the displays for both the spectrum a na-
lyzer and the scope. T h is sweep generator is started
with a pulse of the SYNTH 100 Hz input signal, which
fires t he one-shot U12B. This also enables the scope Z-
axis. Now the d isp lay for either t he spectrum analyzer
or the scope is being t raced on the CRT.
When t he synthesizer sweep reaches 2.5 Vdc (that
is, the scope's horizontal sweep is at the edge of the
screen), the dual-display cont rol circuitry (Ul O B, Ull,
Ul 2A, Q6) activates the CHAR GEN RST line. This
switches the scope's vertical and horizontal inputs to
the character-sweep generators. The vertical sweep
gene rator (U3, U4) determines the ·CRT frame rate of
61 H z. The horizontal character-sweep generator (U60)
determines the row rate of 7812.5 Hz; eight rows make
a cha racter height. T he vertical and h orizontal char-
acter generators a re synchronized by the character
generator on t he P rocessqr board (A14 ).
When th e first li ne of cha racte rs has been traced, a
1
rn nsit ion on the LI NE 1 input from the Al4 boa rd
t'E>set !' hot h character-sweep genera tors, and incre-
me nt ;:;
1
he cha racte r cou nter (Ul 3). This causes LINE
1
1
o he traced aga in. This process is repeated four
times. a t which point t he cou nter output resets t he
>:c·ope input!' to ";:;pectrum analyzer" or " range switch."
D urin~
the ret race time, the scope Z-axis is blan ked by
l
•:!OJ)
and is held off until the synthesizer sweep starts.
Th e timi ng of the p rocess allows the four character
t nlt't>S to he com pleted before the next synthesizer
s\\'ee p-s,,·nc.
9.2.3 SCOPE HORIZONTAL CONTROL
Switching for t he scope horizon tal input is d ivided
between t wo board s. The timebase gen erator a nd t he
ex ternal horizontal in put a re selected on the Scope
Amplifier board (A2). T he horizontal c ha racter-sweep
gene rator a nd the signals for t he synthesizer sweep
ge ne rator are selected on t he A7 boa rd by t he scope
ho rizontal switches (U52A and U52D ) and sent to the
I NT SCOPE HORIZ signal line.