Fluke PM6690, Service Manual, Page: 57 / 200

X17. The oven oscillator should be powered also in standby
mode.
The oven oscillator outputs a 10 MHz signal if powered. It
should be 1.3 V
pp
measured at R282. If not selected, a gate
(U4) stops the signal, the control signal (U4:9) is then low.
The frequency is controlled by a DAC (U5). Its reference
voltage is derived from the oscillator, approxi5 V
(C174). The polarity of the reference voltage is reversed in an
op amp (U6), and the voltage at U5:1 should be -5 V. The out-
put voltage from the DAC should be between 0 and V
ref
, mea-
sured at R281. The DAC is controlled by the processor via the
SPI bus.
The frequency adjustment range should be wide enough to al-
low for more than 10 years of oscillator aging. The oscillator
must be replaced if the normal control voltage range cannot
make the oscillator output 10.000000 MHz.
As a last resort to exclude external causes of malfunction,
desolder the oven oscillator from the main circuit board.
Place it upside down and c12 V and ground accord-
ing to Figure x. A cold oven oscillator draws approximately
0.30 - 0.35 A. During heating the current consumption varies.
After 10 minutes it should stabilize on less than 0.1 A. The
output V
ref
should be approxi5 V and the 10 MHz
sinewave output signal should have an amplitude of more
than 2.5 V
pp
measured with a 1 M
W
, 10x probe. The control
input has an internal bias to keep the output frequency in the
middle of the range. Adjust the control voltage between 0 V
and +5 V and check the output frequency range with a fre-
quency counter. The minimum trimming range should be
±5 Hz. 10.000000 MHz must be reached somewhere between
0 V and +5 V.
If the oven oscillator circuitry is repaired, a new calibration
must be performed. See Chapter 7. A new factory calibration
by means of the utility program should also be performed.
External Reference Input
See Figure 6-14 and Figure 6-17.
The input signal is amplified in U31. The output signal from
the amplifier should be a square wave with logic levels, repro-
ducing the timing characteristics of the input signal. Check
the signal at U32:11. U32 generates a short pulse (approxi-
mately 40 ns) for each input cycle, check at U32:9. These
pulses generate a broad spectrum of harmonics, and the
following high-Q 10 MHz crystal filter allows only a 10 MHz
sinewave to pass. Measure at X19. Note that the trimmer
C442 is used for maximizing the amplitude at X19. Check that
the amplitude is not less than 1 V
pp
. If external reference is not
selected, the gate U33 stops the 10 MHz signal. The control
signal on U33:1 is then low.
100 MHz Multiplier
See Figure 6-14 and Figure 6-16.
100 MHz is used in the measuring logic, mainly as a reference
clock, but also for other purposes. A PLL is used for multi-
plying the 10 MHz reference to 100 MHz. On power-up the
processor sets up the PLL IC (U9) via the SPI bus. An output
signal, PLL LOCK, tells the processor if the loop is locked
(high level). A VCO, consisting of an inverter (U47) and an
LC circuit in the feedback loop, is controlled by the PLL IC.
The DC voltage from U9:2 is filtered and controls a capaci-
tance diode. The VCO frequency changes with the capaci-
tance. The loop can handle the switching of 10 MHz refer-
ence, from internal to external and vice versa. There is no
need for a new setup. If external reference is selected and no
such signal is connected to the instrument, the PLL will be un-
Troubleshooting 6-13
FPGA
100 MHz
PLL
EXT REF
IN
INT REF
OUT
STD
OSC
OVEN
OSC
U4 U4-
U7
10 MHz
U11
ON/OFF
100 MHz
PLL
LOCK
ON/OFF
m
P/SPI
m
P
m
P/SPI
U41, Q53, Q54
U31, U32,
U33, Q55
U9, U47, U48
Figure 6-14
Timebase reference system.
3
2
1 5
4
+12 V
Vref
Vcontrol
GND
10 MHz
OUT
Figure 6-15
Oven oscillator pinning (seen from bottom side).