Model 8340A - Service
voltage Reference
D
The voltage reference for the whole Pretune system is derived
from VRl, a low noise,
1%,
5 ppm reference zener diode. The
circuit provides a constant bias current of 7.5 mA through the
diode. RlO and Cll give extra filtering of diode noise.
Note that Cll is a polycarbonate capacitor which has low leakage,
noise, and current, and has good temperature stability. The
current from the op-amp is not sufficient to cover the worst case
needs of the board, so R49 provides the remainder of the current.
sweep Disable Switch
E
FET Q3 is a switch that grounds the junction of R25 and R26
(Block
C)
when LVSX (Low Voltage sweep disable) is LOW. This
prevents noise from the sweep generator from being added to the
PRETUNE voltage when the instrument is in the CW, MANUAL, or
narrow sweep modes. Narrow sweeps are sweep widths
�
500 kHz.
LVSX is HIGH any time that the 8340A is in the sweep mode and the
sweep width is >500 kHz. Therefore, any slight de offset that
might be at the output of the sweep generator when it is reset
(and supposed to be at zero) is added to the PRETUNE voltage
while the instrument is acquiring phase-lock. This offset will
not effect the sweep accuracy, however, as long as it is not
great enough to keep the YO from acquiring phase-lock.
YO
Delay Compensation
F
The frequency lag or delay in the YO during a sweep due to eddy
currents in the magnetic poles is described by the equation:
Delay(Freq)
=
[A+(B*F)]*dF/dt
where:
Delay(freq)
=
Frequency lag or delay
F
=
(measurement frequency) - (start frequency)
dF/dt
=
sweep rate
The YO DELAY COMPENSATION circuit generates a voltage, VCOMP·
(TP4), proportional to the delay shown above. This voltage is
sent to the A55 YO DRIVER board where it is used to speed up the
sweep to make the frequency more closely follow the PRETUNE
voltage and thus compensate for the YO frequency delay.
To generate this voltage, the circuit sums a de voltage (COFF
adjustment) and the VSWP voltage ramp (CGN adjustment) to
8-339
Scans by HB9HCA and HB9FSX
Содержание 8340A
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