RF Network Analyzer Operation & Circuit
- notes and details of how an RF network analyser works and the main circuit blocks it contains..
An RF network analyzer contains a large amount of circuitry
In addition to this, an RF network analyser is a precision item of test equipment, many of which are able to operate over a very wide bandwidth, often extending from a few MHz or less right up to several GHz. As a result of this, RF network analyzers are not normally cheap items of test equipment.
In terms of the actual circuitry that an RF network analyser, the analyzer can be split into several sections:
- RF network analyzer stimulus / source: An RF network analyzer is an active test instrument. This means that it generates a signal that it applies to the device under test, and then it measures the response. The stimulus or RF signal source is essentially a form of signal generator. There are generally two modes. One is to sweep the power level, and the other is to sweep the frequency.
Typically the signal source for an RF network analyzer is separate to the main instrument, but this is not always the case. Also they may either be open loop voltage controlled oscillators, of they may be digitally synthesized. The open loop oscillators generally provide a good phase noise performance but their frequency accuracy and flexibility is relatively low. However they are much cheaper to design and build. Digitally synthesized oscillators are more expensive but they are able to provide an exact frequency signal which is essential for measuring narrow band frequency devices such as filters. However when measuring these devices, low levels of phase noise on the signal are essential otherwise the measurements are degraded and this considerably adds to the cost. Nevertheless most sources used with RF network analyzers are of the synthesized variety.
- Signal separation: The signal separation element within the RF network analyzer is often called the "test set". Often it may be a separate box, although in many instances it may be integrated within the main instrument. There are two functions that the signal separation hardware provides:
- Measure a portion of the incident signal to provide a reference for what is termed "ratioing". This may be accomplished using a splitter or directional coupler. Splitters are very broadband, but have the disadvantage that they introduce a loss of 6 dB. Directional couplers have a low loss through the main arm combined with good isolation and directivity. However they have an inherent high pass response usually giving them a low end frequency limit of around 30 - 40 MHz and as a result they are normally only used in microwave network analyzers.
- Separate the incident (forward) and reflected (reverse) travelling waves at the input of the DUT. Couplers are usually the preferred method because they are directional and they have low level of loss combined with a high level of reverse isolation. Their drawback is that they have frequency limitations and as a result bridges are often used instead. Although bridges work down to DC, they have a higher level of loss.
- Receiver and signal detection: Once the signal has been passed through the device under test and separated from the source signal, it is necessary to start to process it in the RF network analyzer so that the results can be gained. The first stage of this uses what is essentially a radio receiver with a demodulator or detector. The receiver can take one of a variety of forms. It can be a simple diode detector, but this only provides amplitude information, although it does provide a very wide bandwidth that may be needed in some instances. For example, one application where broadband diode detectors are very useful is measuring frequency-translating devices, particularly those with internal LOs.
Typically, though, a tuned radio receiver is used. This provides the best sensitivity, dynamic range as well as harmonic / spurious signal rejection. The narrow band filter within the receiver enables wide band noise to be limited and this provides a significant sensitivity improvement. Normally the radio receiver uses the superheterodyne principle.
Note on the superheterodyne receiver topolgy:
The superheterodyne radio receiver topology operates by changing the frequency of the incoming signal down to a fixed frequency intermediate stage where it can be amplified and filtered. A variable local oscillator signal is mixed with the incoming RF signal to achieve what is effectively a variable frequency filter.
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Once through the analogue sections of the receiver, today's RF network analyzers then apply the signal output from the receiver to an analogue-to-digital converter (ADC). This is done in such a way that both magnitude and phase information are extracted from the IF signal. Then digital signal processing, DSP, techniques can be used to process the signal. The tuned receiver approach is always used in vector network analyzers.
- Processor and display: With the processed RF signal available from the receiver and detector section it is necessary to display the signal in a format that can be interpreted. With the levels of processing that are available today, some very sophisticated solutions are available in RF network analyzers. Here the reflection and transmission data is formatted to enable the information to be interpreted as easily as possible. Most RF network analyzers incorporate features including linear and logarithmic sweeps, linear and log formats, polar plots, Smith charts, etc. Trace markers, limit lines and also pass / fail criteria may also be added in many instances.
These are the very basic blocks that can be found within RF network analyzers, whether a scalar network analyzer, SNA, or a vector network analyzer, VNA.
By Ian Poole
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