HP 8340A Service Manual page 279

When· the sweep is reset, switch Sl (Q6) in Reset Amplifier 1
(Block R) is closed. Since the non-inverting input of U l4 is
grounded, it forces the inverting input to be at ground also. To
accomplish this, Ul4 will discharge C30 to its reset voltage and
divert the current from the current source away from C30 (once it
is reset) so that the voltage across C30 does not change. The
voltage across C30, after the ramp has been reset, is dependent
upon the gain from the non-inverting input to the output of U9,
and upon the gain from the VREF input to the output of U9.
Since the output of U9 is held at zero by the loop, it is
possible to write an equation which describes the voltage at the
output of U9 as a function of VREF and the voltage. across C30,
Vl. One can then solve for Vl .
0
V REF
approx. -7 volts;
=
•
This assumes that potentiometer R4 is set to zero Ohms. Vl will
vary with VREF.
A sweep is initiated by opening switch Sl (Q6) in the Reset
Amplifier 1 (Block R). The output of the current source is now
directed into C30. This constant current into a constant
capacitance gives rise to a voltage across C30 that increases
linearly. The output of the Ramp Generator (Block K), MKR
will increase linea�iy from
be stopped. At this point, the voltage across C30 will have
increased, become less negative, to a value that is dependent
upon the gain from C30 to the output of U9. This gain is
1+(4640/1620)
=
the voltage across C30 will increase by 10/3.864
Thus at the end of any sweep, the voltage across C30 will be
approximately -5.18 + 2.56
The foregoing discussion of Marker Ramp has been for the single
band case. During a multi-band sweep the ramp must pause at each
bandcrossing while the instrument phase-locks for the n�xt band.
To do this, the Current Shunt (Block Q)
diverts the output of the current source away from C30. CR2 keeps
the Current Shunt (Block Q)
is no longer going into C30, the voltage across it will simply
stay constant at the value that was present when the Current
Shunt was turned on. When the bandcrossing is finished, the
Current Shunt (Block Q)
8-366
Model 8340A - Service
Vl[l+(4640/1620)]
=
Vl VREF
=
therefore, Vl
to +10 volts where the sweep will
O
3.864. So when Marker Ramp increases by 10 volts,
-2.62 volts.
=
from discharging C30. Since current
is turned off and the Ramp Generator
Scans by HB9HCA and HB9FSX
- VREF(4640/1620)
0.74
X
approx. -5.18 volts
=
=
is turned on and U35B
•
RMP,
2.56 volts.