HP 8340B Manual page 175

Swept Mode
Single Band Sweeps >5
MHz.
In
swept mode,
the
instrument phase-locks the YO at the beginning of
each sweep, undergoing the
same sequence
of
events as
in the
CW mode. The sample
and hold on
the A49 YO loop phase detector assembly
holds the
YO
TUNE
error
signal constant at
its
phase-
locked value. In the hold
mode the YO phase-lock
loop
is
opened,
allowing
a ramp signal,
VSWP, to be
summed
into the
PRETUNE signal and subsequently applied to the YO main
coil to sweep the
YO.
This sequence
is called lock and
roll.
The processor writes the sweep time and sweep width numbers
to to the
ASS
sweep generator assembly, which generates the voltage
ramp,
VSWP, with the appropri¬
ate slope and duration. The
ASS
sweep generator assembly also generates
a
0
to 10V ramp
(MKR
RAMP),
that
has
the
same
duration as
VSWP.
Because of the inherent delay characteristics of the YO during sweeps,
a
ramping compensation
voltage,
VCOMP, is
summed with
PRETUNE
in the
ASS YO
driver.
VCOMP
is generated
by summing
correction
data from the processor with the
VSWP ramp. This
is done
in the YO Delay Compensation
portion of the AS4 YO pretune delay compensation assembly.
At each YO sweep retrace, the YO retrace
kick
pulse generator
(on the AS4 assembly) produces
a
pulse with
a
programmable width (via the microprocessor) that
is
sent to the ASS YO driver. This signal
causes
current drain from the YO tuning coil.
When activated,
this
momentarily tunes the
YO lower
in
frequency (by about
2.S
GHz)
to remove the effects of YO magnetic hysteresis.
Multiband Sweeps. The
operation of the sweep generator/YO loop circuitry in multiband
sweeps
is
essentially the same as in
single band
sweeps. The difference
is
that at each band crossing, the YO
is
retraced and the lock and roll sequence
is initiated again. This
produces a pause
in the instrument
sweep
at each bandcrossing while each loop is
being reset. VSWP
is
reset to zero volts between
bands, however the
MKR RAMP
signal remains fixed
at
its
end-of-band value
until the lock and
roll
sequence for the next band
begins.
The sum of the durations of the ramping portions of
MKR RAMP
will still equal the selected instrument
sweep time.
VSWP
has
a sensitivity
of
+2 V/GHz
of
YO
frequency.
The distinction
between YO frequency and
instrument frequency
is important because the instrument frequency is not necessarily the
same
as
the YO frequency.
In
the
10
MHz to 2.3 GHz
band (band 0). the YO output
is
heterodyned with
a 3.7
GHz
oscillator.
Consequently, the YO frequency
is 3.7 GHz higher than the instrument frequency.
In
the higher bands (SYTM multiply
bands
1
through
3
or,
in the
case
of the
HP 8340B,
band 4), the
instrument frequency
is derived from the
first, second, third, or fourth multiple
of the YO
frequency as
shown
in Table
D-1.
Table D-1.
RF
Output Frequency
vs.
YO
Frequency
Band
0
1
2
3
4*
*HP 8340B
only
Multiplication
Heterodyned (mixed)
YO
fundamental used
Multiplied by
2
Multiplied by 3
Multiplied by 4
RF Output Frequency
0 01
to 2.3 GHz
2.3 to 7.0 GHz
7.0to13.5GHz
13.5to20.0GHz
20
0
to 26.5 GHz
YO Frequency
3.71
to
6 2
GHz
2.3 to
70
GHz
3.5 to 6.75 GHz
4.5 to 6.67 GHz
5.0 to
6
625
GHz
HP 8340B/41B
Sweep Generator
-
YO
Loop Theory of Operation