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10 MHz
(OCXO 1, Q901)
10 MHz
(OCXO 2, Q902)
REMOTE CONTROL
(S904, U908)
OSCILLATOR SELECT
(S905, U909)
H
OSC 1
(Q906, DS902)
H
OSC 2
(Q909, DS903)
The top section of Figure 6-7 determines whether or
not a switch is made to OCXO 2. In this section, an Ex-
clusive OR logic decision is made between inputs (S)
and (OSC 1). When U902B, pin 6 (X) is high, OCXO
1 is selected, when it is low, OCXO 2 is selected. If
(OSC 1) is low, (X) goes low and OCXO 2 is selected.
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
oscillator.
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.
U902B
4
6
5
U901A
74HC00
1
3
2
U902D
U902C
12
11
9
13
8
10
H
U902A
74HC08
1
3
2
Figure 6-7 OCXO SWITCHING
H
U901B
H
4
6
5
U901D
U904D
12
11
11
10
13
(X)
H
L
(S)
L
H
U904A
74HC04
U903D
1
2
12
11
13
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 (U910, U911, U913)
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.
6-15
CIRCUIT DESCRIPTION
U901C
9
8
10 MHz SIGNAL OUTPUT
10
(U905, U906)
(OSC 1)
(OSC 2)
OSC 2 STATUS
(U912, PIN 4)
OSC 1 STATUS
(U912, PIN 1)
October 1999
Part No. 001-2009-201
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