A1 Reference Oscillator Board; A3 Source Board - Agilent Technologies E4991A Service Manual

A1 Reference Oscillator Board

A1 reference oscillator board provides reference frequencies of RF1, RF2, Int Ref Out, and
10 MHz reference for A3 and A4.
The RF1 is supplied to A3 source board for local signal that will be supplied to A4 finally,
and its frequency depends on measurement frequency as shown in Table 5-1. The signal
level of the RF1 is +6 dBm typically. The RF2 is supplied to A3 for producing the
measurement signal that is output from RF OUT port. The signal level of the RF2 is +6
dBm typically. The frequency difference between RF1 and RF2 is 12.5 kHz equal to the IF
signal.
Table 5-1 RF1 and RF2 frequencies for each band
Band1
Band2
Band3
Band4
A 150MHz oscillator(75MHz VCXO x 2) in A1 supplies a reference frequency signal for
the PLL-controlled oscillators that create RF1 and RF2 signals. If nothing is connected to
Ext Ref In connector on the rear panel, it is locked to an internal VCXO (Voltage
Controlled Crystal Oscillator). But if an external 10 MHz signal is supplied to the Ext Ref
In, it is locked to the external signal.
The RF2 is generated by a PLL circuit with the 6MHz reference signal (150MHz reference
signal / 25). The RF1 is generated by a PLL circuit with the reference signal that is the RF2
divided by the fractional N divider. If one of the VCO control voltages in the PLLs is out of
limits, "PLL Unlock" notification is displayed.
The created 150 MHz signal is also divided into a 10 MHz signal(150MHz/15), and it is
supplied to A3 and A4 for reference frequency through A10 analog motherboard. The
10MHz reference signal is output to Int Ref Out BNC connector on the rear panel directly.

A3 Source Board

A3 source board supplies a measurement signal through RF OUT port to A6 test head, and
a local signal to A4 receiver board. A3 source board also supplies a DC bias voltage which
is generated by A2 DC bias board(Opt.001), through RF OUT port to A6 test head.
VCO (Voltage Controlled Oscillator) frequency varies from 2305 MHz to 4152 MHz, and
it is mixed with RF2 signal to produce the measurement frequency(1MHz to 3GHz). The
relationship of the VCO frequency and measurement frequency is shown in Table 5-2.
After the mixing, the signal is filtered with a selectable BPF (Band Pass Filter) or a LPF
(Low Pass Filter) to eliminate the unwanted harmonics and spurious.
Chapter 5
Measurement Frequency
1 MHz  1739.9 MHz
1740 MHz  2259.9 MHz
2260 MHz  2679.9 MHz
2680 MHz  3000 MHz
Theory of Operation
RF1 RF2
2303.9875 MHz 2304 MHz
1151.9875 MHz 1152 MHz
1151.9875 MHz 1152 MHz
1151.9875 MHz 1152 MHz
Analog Section
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