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8712ET/ES and 8714ET/ES Theory of Operation

RF Network Analyzers System Theory

Service Guide 6-13

RF Path of the ET Analyzer.

The RF output signal from the source assembly passes through two directional couplers

before going to the RF OUT front panel connector. As the source signal passes through the

ﬁrst coupler, part of the signal is routed to the R input on the receiver board to be used as a

reference signal. The second coupler transmits the signal to the RF OUT front panel

connector and receives the reﬂected signal from this port. The reﬂected signal is routed to

the A input on the receiver board. The signal output from the RF OUT front panel

connector goes through the DUT and enters the instrument at the RF IN connector. This

signal is routed to the B input on the receiver board.

NOTE As seen in Figure 6-4, the ET Option 1E1 models have an RF step attenuator

(A5A1) in the signal path between the two couplers. The standard ET models

have one dual directional coupler in place of the two couplers pictured. The

signal path and theory are the same.

Narrowband and Broadband Processing.

The RF signals at the R, A, and B inputs on the receiver board can be processed as

narrowband measurements. Each RF signal is mixed with the LO signal from the A4

source assembly. The mixing produces a downconversion to an IF of 27.778 kHz. The

downconverted signal is ﬁltered, ampliﬁed, and sent to the ADCs on the receiver PC board.

The RF signals at the R and B inputs can also be processed as broadband signals. The RF

input is converted to a DC signal with a diode detector. The DC signal is chopped at a

27.778 kHz rate and then sampled at a rate of 55.5 kHz. The signal is then ﬁltered,

ampliﬁed and sent to the ADCs. These internal broadband inputs are referred to as B* and

R*. These inputs are used to make absolute power measurements.

One or two external broadband detectors can be connected to the rear panel. These

detectors convert the measured RF signal into DC signals, which are referred to as the X

and Y inputs. The DC inputs are multiplexed into the same choppers used for the B* and

R* detectors. The B* and Y inputs share a chopper, as do the R* and X inputs. The signal

then follows the same path as the one used for the internal broadband detection.

There is also a rear panel connector for an auxiliary input. This input allows the

measurement of DC or low frequency AC signals from −10 to +10 volts.

The analyzer uses adjustment routines to generate correction constant (CC) data for all of

the internal narrowband and broadband inputs. For the external detectors, the correction

constants are stored in an EEPROM in the detector. This data can be read by the receiver

to correct the data for the X and Y inputs. There are two 16-bit ADCs on the receiver

assembly. The two ADCs are multiplexed between the various narrowband/broadband,

internal/external signals. The ADCs require a 2.5 MHz clock signal from the CPU board.

They convert the 27.778 kHz signal into digital data, which is then sent to the digital

signal processor (DSP) on the A2 CPU board.

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Agilent Technologies 85032B/E 50 Specifiche Pagina 228