Encyclopedia of radio electronics and electrical engineering / Civil radio communications

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MFJ antenna analyzers are widely used in amateur radio practice. Several of their modifications have been released (see, for example, "Radio", 2000, No. 5, pp. 78,79). One of the features of these devices is that they use RF generators with a very stable output voltage. This allows, with sufficient accuracy for amateur radio practice, without preliminary calibration of the device, to measure the SWR and other parameters of the antenna-feeder path. In this case, the RF generator of the analyzer covers (on several subbands) the frequency band from 1,8 to 170 MHz.

They have another interesting feature. This is the digital signal processing (in calculating SWR, feeder input impedance, etc.) used in the latest analyzers. It involves linear rectification of small RF voltages. Which circuit solutions provide this is illustrated by the figure, which shows part of the circuit diagram of the antenna analyzer model MFJ-259B. To stabilize the output voltage of the generator, the linearity of the rectifier is not fundamental, but they are all assembled in the device according to one scheme.

The RF generator is assembled on microwave field-effect transistors VT1 and VT2 according to a source-coupled circuit well-known to radio amateurs. The use of field rather than bipolar transistors (emitter-coupled circuit) provides a minimum load on the oscillatory circuit and a relatively high frequency stability of the generator. When changing ranges, inductors are switched (the diagram shows one - L1) and "stretching" capacitors to the KPI (C11). In the source circuit of transistors VT1, VT2, in addition to the resistor R5, a choke L2 is introduced, which increases the upper limit of stable generation to 170 MHz.

To ensure effective stabilization of the output voltage of the generator, a field-effect transistor with an insulated gate VT3 is also introduced into the source circuit of its transistors. The voltage supplied to the measuring bridge of the device is controlled by a rectifier assembled on a high-frequency Schottky diode (not shown in the figure). The rectified voltage of positive polarity is supplied to the linearizing stage on the operational amplifier DA1.

Linearization is carried out by a high-frequency Schottky diode VD1, included in the negative feedback circuit of the op-amp. There are two main requirements for this diode: it must be of the same type as the rectifier diode and, moreover, "paired" to this diode. In other words, have a close current-voltage characteristic. In this case, the drop in the efficiency of the rectification of the RF voltage will be compensated with sufficient accuracy by an increase in the transfer coefficient of the cascade at the op-amp DA1. From its output, the control voltage is supplied through the DA2 op-amp integrator to the gate of the regulating transistor VT3.

The efficiency of this automatic level control system is very high - the voltage on the measuring bridge practically does not change when the load in the measuring circuit changes from short circuit to idle. The word "practically" means that it lies at the limit of its registration by analog devices.

Good decoupling between the generator and the measuring bridge in the antenna analyzer is provided by two buffer stages: the source and emitter follower.

Author: E.Luksha (UA3AJV)

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