HP 8340A Service Manual page 118

CONFIGURATION 1
This configuration is used in
kHz.
PLL2 phase-locks a 75 to 150 MHz
frequency derived from the Reference Loop. The PLL2
programmable in 5 kHz steps, and it can be swept a maximum of 25
MHz using a lock and roll analog sweep. To improve the linearity
of this sweep, a very linear discriminator is used in a feedback
loop around the
mainly a function of discriminator linearity, and removes the
effects of
vco
circuit for the PLL2 Loop has an extremely low amount of drift. A
capacitor cannot hold the error voltage constant during a slow
sweep, due to the amount of leakage that is present. Instead, the
majority of the error voltage is stored in digital form so that
it will remain stable during the sweep.
The output of the PLL2
of digital frequency dividers. By dividing the 75 to 150 MHz
by 5 an output is generated in the frequency range of 15 to 30
MHz. The resolution is 5 kHz, so the divided output will have
vco
1 kHz resolution. The
the divided output will sweep up to 5 MHz sweep widths. This path
is used directly as one of the three configurations of the 20-30
Loop. Notice that PLLl and PLL3 are not used at all in this
configuration.
CONFIGURATION 2 &
These configurations are used in sweep widths
sweep widths below .1 MHz, configuration 1 provides insufficient
resolution (1 kHz) to set the start frequency to closer than 0.5%
of the sweep width. To provide for finer resolution, the output
of the PLL2
vco
resultant signal has a 200 Hz resolution, is between 3 and
and can be swept with sweep widths up to 1 MHz. This PLL2 output
is used in configuration 2. For sweep widths less than 5 kHz,
configuration 2 will not provide sufficient resolution, so a
third output (configuration 3) is provided. The PLL2
divided by 500
output is used in configuration 3.
The two divided outputs of the PLL2
by 500) are used for configuration 2 and 3 to improve the
resolution of PLL2. The frequency, however, is also reduced such
that it is no longer in the 20 to 30 MHz range. PLLl and PLL3
8-162
Model 8340A - Service
YO
This results in sweep accuracies that are
vco.
tuning non-linearities. The sample-and-hold
is divided-down through several stages
VCO
can be swept a maximum of 25 MHz, so
VCO
3
is divided by 25
(divided by 5 twice, then divided by 20) and the
Scans by HB9HCA and HB9FSX
sweep widths from 5 MHz to >100
to a 10 MHz reference
vco
100 kHz. For
<
(divided by 5 twice). The
(divided by 25, divided
vco
is
vco
VCO
MHz,
6
is
VCO