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DIRECTIONAL POWER HEADS

A-16

Technical description

Directional coupler assembly

The directional coupler is connected in series with the RF line under test and senses forward and

reverse RF power. The coupler has two outputs, one for forward power and the other for reverse

power. Because the coupler can be connected in either direction on the RF line, either output

could represent forward or reverse power. Each output from the coupler is routed to an RF

detector.

Channel circuit

There are two channel circuits, each comprising an integrator, AGC circuit and a peak buffer.

Both channel circuits are identical to each other, therefore only channel 1 is described below.

CW operation

With reference to Fig. A-6 the RF detector output is connected to one input of the integrator. The

output from an LF detector is connected to the other input of the integrator. The integrator output

controls the gain of the AGC circuit which controls the level of a 1 kHz signal to the LF detector.

When RF power is detected, an increased offset voltage exists at the integrator input which causes

the AGC to compensate by increasing the level of LF voltage to the LF detector. The increase in

LF level at the AGC circuit is routed to the Service Monitor, and is directly proportional to the

increase in RF sensed by the directional coupler.

The error correcting action of the AGC/LF detector loop circuit corrects for law variations in the

RF detector, thereby extending its linear output range.

The RF and LF detector diodes for each channel have matched characteristics and are mounted

close together to ensure thermal equilibrium. However, DC level differences due to temperature

are compensated by a unique auto-nulling scheme (patent applied for) which further extends the

detector dynamic range.

PEP operation

The integrator and AGC/LF loop circuit is not suitable for AM signals, therefore for PEP

operation it is switched out of circuit. The RF detector output is routed to a peak buffer which in

channel 1 outputs a positive voltage and in channel 2 outputs a negative voltage proportional to

the forward and reverse peak envelope voltage on the RF line under test.

Control circuit

Signal switching for CW/PEP signal paths is controlled by the Service Monitor through a control

circuit hybrid in the power head. The control circuit directly controls the operation of the zero-

loop circuit.

Power supply and oscillator

A power supply and oscillator circuit hybrid in the Power Head is powered by +12 V DC from the

Service Monitor. It provides the nominal 1 kHz signal to each channel AGC circuit, and also

±6.5 V DC supplies to the Power Head circuits.

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Aeroflex 2948B Betriebsanleitung Seite 244