Network Analyzer basics
We know that microwave measurement using slotted line has disadvantage that it can do measurement of both amplitude and phase but limited to single frequency. With this if one has to do broadband frequency measurement involving wide frequency range, it takes more time. To avoid this problem, network analyzer has been designed which measures both the amplitude and phase over the wide frequency range and within reasonable acceptable time. This page describes difference between SNA and VNA.
In the basic measurement setup accurate reference signal need to be generated, with this as standard reference signal, amplitude and phase of the signal emitted from DUT is measured.
Fig.1 Network Analyzer inside
As shown in the figure on left, typical network analyzer will have four parts which includes signal source, signal separation device, receiver or detector and signal processor/display section.
• Signal source provides incident signal.
• Signal separation device separates incident, reflected and transmitted signals.
• Receiver converts microwave frequency to lower IF frequency to make it easy for further processing.
• display or signal processor section which processes the IF signal and display the information on CRT screen.
Signal source: It produces the incident signal which stimulates the DUT. DUT or test device responds by reflecting part of the incident signal and transmitting the remaining part. By sweeping the frequency of the source frequency response of the DUT can be determined. There are two main type of sources viz. sweep oscillator and synthesized signal generator.
The second module is to separate incident ,reflected and transmitted signals. Once these signals are separated their amplitude and phase measurement can be carried out and their differences can be known. This task is accomplished with the use of directional couplers,power splitters,bridges or high impedance probes.
The third part is the receiver or detector, which basically converts RF voltage to lower IF or DC signal
to allow more accurate measurement. There are three main receiver techniques to accomplish this viz. diode,fundamental mixing and harmonic mixing.
• Diode is broadband detector which converts RF signal to the proportional DC voltage. For amplitude modulated signal, it strips the modulation. This technique is most commonly employed in scalar network analyzer.
• Other two are broadband tuned receiver techniques which converts RF signal to the low frequency IF signal. Both will have BPF at IF frequencies to reject the spurious ferquencies and extend the noise floor.
The display is the last part which produces the results as desired by the user. cartesian format indicate magnitude,phase or group delay as a function of frequency. Tha same can be displayed in polar format as well or in a impedance format in the form of smith chart. Modern network analyzer can also display in table format.
Network Analyzer types
There are two main types of network analyzer viz. SNA and VNA. SNA i.e. Scalar Network Analyzer helps measure magnitude related measurements. VNA i.e. Vector Network Analyzer helps measure both phase and magnitude related measurements. Following table summarizes difference between SNA and VNA network analyzer types.
|Perform sweeps faster than VNA.||Perform frequency sweep more slowly compare to SNA.|
|Measures only magnitude part.||Measures amplitude and phase of incident and reflected waves at ports of DUT.|
|Hardware required for downconversion and RF power detection is relatively simple and inexpensive.||VNAs are more complex compare to SNAs as they need full heterodyne architecture in receiver to measure both amplitude and phase. Hence they are expensive compare to SNA counterpart.|
Measurement using network analyzer
Transmission measurements: It includes transmission coefficient, insertion loss and gain measurements.
Reflection measurements: It includes reflection coefficient, VSWR, return loss and impedance measurements.
Scattering parameter measurements: S11,S12,S21,S222 can be measured.
Refer VNA tutorial➤ for more information.
SNA and VNA Manufacturers/Vendors
Scalar Network Analyzer 8757D from Agilent Technologies.
E5072A vector network analyzer from Agilent Technologies.
ANRITSU 37369C is the Vector Network Analyser, supports frequency range 40MHz to 40GHZ .
ANRITSU MS2026A is the handheld Vector Network Analyser supports upto 6GHz.
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