Broadband Reflectometry: Theoretical Background - Broadband Reflectometry for Enhanced Diagnostics and Monitoring Applications - page 34

Fig. 3.7 Generalized block diagram of a VNA [1]
The LO is either locked to the RF or the IF signal so that the receivers in the
network analyzer are always tuned to the RF signal present at the input. The IF
signal is bandpass filtered, which narrows the receiver bandwidth and greatly im-
proves sensitivity and dynamic range. This architecture is called superheterodyne
receiver , and it is used not only in VNAs, but, as well known, also in spectrum
analyzers.
4. processor/display for calculating and reviewing the results : this is where the re-
flection and transmission data are formatted in ways that make it easy to interpret
the measurement results.
The picture of a VNA (model Agilent E8361C) is shown in Fig. 3.8.
As will be detailed in the following sections, one of the advantages of FD mea-
surements is that there are well-established error correction models that, through
appropriate calibration procedures [such as the short-open-load (SOL) calibration],
can be used to reduce the influence of systematic errors [2].
Although VNAs are still costly instruments, in [9], a low-cost single-port VNA
for high frequency measurements was developed. The major feature of the VNA
proposed in [9] is that it eliminates the need for expensive heterodyne detection
schemes required for tuned receiver-based VNA designs; as a result, the realization
costs can be reduced dramatically.