Viking 242-2009-632 - Manual (Page 87)

CIRCUIT DESCRIPTION
6-15
March 1999
Part No. 001-2009-600
Figure 6-7 OCXO SWITCHING
U903D
U904A
U904D
U901C
U902B
U901B
U901D
U901A
U902A
U902C
U902D
1
2
3
4
5
6 11 10
12
13
11
10
9
8
4
5
6
74HC00
74HC08 1
2
3
12
13
11
12
13
11
1 2
10
9
8
74HC04
REMOTE CONTROL
(S904, U908)
OSCILLATOR SELECT
(S905, U909)
(Q906, DS902)
(Q909, DS903)
10 MHz
(OCXO 1, Q901)
10 MHz
(OCXO 2, Q902)
(U912, PIN 1)
OSC 1 STATUS
(U912, PIN 4)
OSC 2 STATUS
10 MHz SIGNAL OUTPUT
(U905, U906)
(X)
(S)
H
H
H
H
L
(OSC 1)
(OSC 2)
H
L
H
H
OSC 1
OSC 2
NOTE: The bottom circuit does not allow (S) to be high
and (OSC 1) to be low. If (OSC 1) is high and (S) is low,
(X) stays high, and OCXO 1 is selected as the main os-
cillator. Finally, if (OSC 1) is high and (S) is high, (X)
goes low and OCXO 2 is selected as the main oscilla-
tor.
6.3.4 OCXO PULSE DETECTION
The buffered OCXO 1 signal is sent to U907,
pins 9-10. U907 is a retriggerable one-shot multivi-
brator that is used for pulse detection. If four or more
10 MHz pulses from OCXO 1 are missing, the output
of one-shot multivibrator Q2 goes low. This turns off
LED driver Q905 and the green half of LED DS902
turns off. Simultaneously, Q2 goes high. This pulses
on/off Q907 and strobes the red fail LED for OCXO 1.
Relay driver Q908 can be driven by either Q2 or Q2
depending on the desired normal state of the OCXO 1
alarm relay. U911, pins 1-2 also serve as a pulse
detector for OCXO 2.
6.3.5 DELTA F DETECTION
The Delta F detection circuitry is made up of
U910, U911 and U913. The buffered output of OCXO
2 is applied to the D-input of flip-flop U910 and the
buffered output of OCXO 1 is applied to the CLK-
input. The Q-output is the frequency difference
between the two OCXOs.
The Q-output of U910, pin 5 is applied to the trig-
ger input (B2) of the one-shot multivibrator in U911,
pin 10. This device produces a 5 ms negative-going
pulse when it is triggered. It is triggered twice by
every output cycle from U910, pin 5. The second trig-
gering is caused by the undetermined states produced
when the signal goes low at the half-cycle point.
Because of this double triggering, the frequency on the
Q2 output of U911, pin 12 is twice the frequency dif-
ference of the two OCXOs.
The Q2 output of U911, pin 12 is applied to
U913, pin 14, which is one input of an internal phase
comparator. The other input of this phase comparator
is from a voltage controlled oscillator (VCO) internal