Network analyzers are invaluable instruments in the laboratory, used for a variety of tasks that include stability analysis, component characterization, and frequency response measurements. Many commercial models are available, with different frequency ranges and features, but all rely on the proper injection of a test signal to accurately perform tests. The quality of the test signal injector (or test adapter) and the injection method can directly impact test results. Too often, low-quality transformers are used to inject signals into the loops of power supplies during a measurement, and the results can be distorted due to the poor frequency response and impedance matching of the transformer.1

When selecting a test adapter, its bandwidth limitations and impedance should be well understood. In addition, the injection signal magnitude can have tremendous impact on the accuracy of network analyzer measurements. In order to avoid problems, several types of signal injectors should be used, each for a different type of test. These include injection transformers, solid state signal injectors (current and voltage), line injectors, and other coupling elements.

The injection transformer is by far the prevalent method for connecting a network analyzer to a circuit under test, and is primarily used for control loop stability measurements (see figure). The goal of the transformer is to inject a signal into the control loop being measured, without impacting the performance of the loop. To accomplish this to a reasonable degree, it is important to pick an injection point that is unaffected by the terminating impedance of the transformer, which is often in the range of 5 to 50 Ù.

The transformer itself is outside of the measurement, leading many engineers to falsely presume that is a noncritical element in the test setup. The frequency range of the injection signal is dependent on the circuit being measured. The measurement of a typical power factor correction (PFC) control loop generally requires a measurement frequency of 1 Hz or lower, as it is common for a PFC to have a control loop bandwidth of less than several hertz. The bandwidth of a high-performance linear regulator can be as high as several megahertz. While several different transformers can be used to address this range, it is beneficial to use a single transformer or two covering different frequency bands at most, due to the high cost of the transformers.

A transformer with significant permeability at 1 Hz and minimal attenuation at 10 MHz or more is difficult to design. The core materials are expensive and the transformers generally must be hand wound. This, combined with the small market size for such a product, dictate the high cost. Unfortunately, as a result, engineers sometimes use audio transformers or hum eliminators as signal injectors, which can yield poor results.

While high-quality injection transformers with bandwidths as wide as 1 Hz to 5 MHz or more are available, in some cases this is still insufficient for some tests. For example, a typical heater control loop might have a bandwidth of less than 1 Hz while some linear regulators and operational-amplifier (opamp) circuits can have bandwidths to 100 MHz or more. For these applications, a solid-state injector can provide the necessary bandwidth. A solid-state injector can perform at DC, while the upper frequency limit is dictated by the components selected and the printed-circuit-board (PCB) material and circuit layout. It is possible to obtain a solid-state injector with a working range of DC to 200 MHz, though above 50 MHz the interconnection between the injector and the circuit being tested can become quite critical. It is essential that ripple from the injector power supply does not dramatically degrade the dynamic range or the signal-to-noise ratio (SNR) of the measurement. Test results are often much cleaner when using a solid-state injector than with an injection transformer.

The selection of a valid injection point in the circuit is more critical when using a solid-state injector than with a transformer injector. A solid-state injector presents an infinite impedance between the injection points. To provide correct results, one side of the measurement must present a much higher impedance than the other side. In a typical power-supply control loop, the voltage sense divider is generally a good injection point, since the output impedance of the power supply is very low compared with the impedance of the voltage sense divider.

The solid-state injector has a limitation in operating voltage, with the majority limited to 10 or 12 V. This is not the amplitude of the injection signal, but the DC operating voltage.

While an injection transformer is a very wideband adapter, it is not useful for measuring power-supply ripple rejection (PSRR) or even an opamp. This is because the attributes that make the injection transformer perform so well also result in a transformer that is absolutely intolerant of DC current. Even very small DC currents (5 mA or less) can greatly reduce signal capacity or even saturate the transformer. For this reason, the line injector is an additional, essential test adapter.

The line injector allows the input DC supply voltage to be modulated by the analyzer source signal, as in the case of measuring PSRR. The line injector must be capable of a frequency range well below the AC line frequency and at least above the control loop bandwidth of the circuit being tested. The line injector, being in series with the input power to the circuit being tested, must be capable of operating at the input voltage and current levels of the circuit being tested, while minimizing the power dissipation within the injector.

A current injector is possibly the most versatile signal injector. While not designed to replace an electronic load, it is capable of performing a small signal step load at switching speeds and bandwidths beyond electronic loads. Also, the capacitance of an electronic load is generally too high not to impact a measurement. By incorporating a 40-MHz current monitor, a current injector can also measure output impedance as well as the stability of a filter, combined with the negative resistance of a switching converter or power supply. A current injector allows these measurements to be made noninvasively while connected to the system.

A current injector is a bilateral device, which works with positive or negative voltages and includes an internal bias for use with a network analyzer. The bias can be disconnected for use with an external waveform or arbitrary waveform generator such as the model G5100A from Picotest (www.picotest.com).

There are two common uses for attenuators when used in conjunction with the network analyzer. One is to attenuate the oscillator source signal. While this may seem odd, one of the most common errors in analyzer measurements is using a source signal that is too large. Even though the analyzer allows setting of the signal output amplitude, the lowest setting is often too high to allow an accurate small-signal measurement to be made. The correct amplitude is the smallest amplitude that exceeds the noise floor. 

Reference

1. Steve Sandler and Paul Ho, “Why Network Analyzer Signal Levels Affect Measurement Results,” Internal White Paper, Picotest, www.picotest.com.