Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 92

The VNA source provides the stimulus for the S‐parameter measurement. In the original VNAs these were open‐loop sweepers, but since about 1985 the use of frequency synthesizers has become the norm. Sweepers used open‐loop swept‐frequency oscillators to produce the stimulus signals; synthesizers replaced the open‐loop control with fractional‐N or multi‐loop signal generation where the output signal is digitally derived from a 10 MHz reference oscillator to a resolution of less than 1 Hz. Early sources were routed directly to the test sets of VNAs, which were also external, stand‐alone instruments, often with the first converter assembly inside.

More recently the source is provided internally to the VNA. In the first integrated VNAs, the signal quality of the internal source was less than that of external instrumentation sources but had the advantage of being much faster in sweeping frequency. For applications such as filter tuning and test, fast sweep times across wide frequency ranges were required. Common to VNA sources are the ability to vary power level over a prescribed range, usually called the automatic‐level‐control (ALC) range, which has values from 20 to over 40 dB. These ranges are often extended with integrated discrete step‐attenuators after the source. With new open‐loop control, the power range can be extended even farther, typically greater than 70 dB.

Most VNA sources also have a power flatness specification to provide some constant power to the DUT. In many cases, this level is digitally corrected by a factory calibration to be quite accurate at the test port. In addition, a user‐performed source‐power‐level correction is usually available to provide additional accuracy enhancement, as described later in Chapter 3.

A typical VNA source block diagram is shown in Figure 2.4.

Figure 2.4 Example of a VNA source block diagram.

Generally, there is a fundamental oscillator that provides a swept frequency response over one or more octaves. Previously, the output was often switched or split to use as an local oscillator (LO) to a low‐frequency heterodyne source stage; many modern VNAs use direct digital synthesis to get the low‐frequency response and divide the fundamental oscillator to get the higher range of the low‐frequency path.

The swept frequency oscillator is typically phase‐locked to a lower‐frequency fractional‐N (F.N) circuit or direct‐digital synthesizers (DDSs). In older VNAs, the phase locking was accomplished through the reference receiver, which reduced cost but required that the reference path signal remain present for all measurements. Modern analyzers separate the frequency synthesis for sources from receivers for greater flexibility and no longer require the any signal be present in the reference path. Because of this, the RF signal may be pulsed modulated without losing phase lock on the synthesizer.

The output may also go to multiple stages of division or multiplication, followed by amplification and filtering. The final output signal is combined from each of the input signals, resulting in broad frequency coverage. Typically, this common output has some RF detector on it to provide for ALC loop operation, maintaining constant source power over the different frequency bands and compensating for amplifier flatness. But often the ALC attenuator drive can be operated in an open‐loop mode to extend the power range beyond that of the detector diode.

The level control circuits often use an amplitude modulator before the amplification chain to complete the ALC loops. In some modern VNAs, a pulse modulator is added as well to provide for high‐speed pulsed RF measurements. When a pulse‐modulator is used inside the ALC loop, the ALC function must be disabled as it will attempt to respond to the pulsed signals. In this open‐loop mode, more sophisticated calibration or digital control must be used to control the source output power. Recently the use of a reference or test receiver as the ALC loop power control, rather than the internal diode detector, has become more common. This form of receiver leveling provides accurate results if the receivers are calibrated, a much wider range of leveling than the diode detector can support, and the ability to be programmatically controlled to correct for any external path loss or to provide a prescribed power profile.