Chapter 7

Preamplifier Operations

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7-2

INTRODUCTION

This chapter describes the use of external preamplifiers to boost signal

 

power at the input of an test amplifier.

USE OF

External preamplifiers can be used to boost signal level at the input of

EXTERNAL

the Amplifier Under Test (AUT). The maximum power per tone at the

PREAMPLIFIERS

AUT input without any external preamplifier is +5 dBm.Block diagrams

 

showing this feature are contained in Figures 7-1and 7-2.

 

As shown in Chapter 1, Figures 1-2and 1-3,a single preamplifier can be

 

used after the combiner and before the bi-directional input coupler. How-

 

ever the nonlinearity of this preamplifier may adversely affect the linear-

 

ity measurements of the AUT, unless it is far below the AUT.

 

If an individual preamplifier for each tone is used at the input to the

 

combiner, then this problem is avoided, but two other issues must be ad-

 

dressed:

 

￿ Driving the DRA Scorpion channels a1 and b1 into compression

 

￿ Possibility of damage to the combiner

 

The first issue is avoided by using an attenuator at the input of channels

 

a1 and b1. The value of this attenuator should be selected noting that the

 

maximum operating signal level at a1 and b1 is –5 dBm, and that the cou-

 

pling factor between AUT input and channel a1/ b1 input is ￿30 dB.

 

Thus, if measurements with an AUT input power of 1 watt (30 dBm) is

 

required, a 5 dB attenuator in each path is needed. In general,

 

attenuators are needed if Pin (max power at AUT input) is greater than

 

25 dBm and the value of attenuator is given by A (dB) = Pin (dBm) – 25.

 

Depending on the power output level of the preamplifier, two alternatives

 

exist for dealing withthe second issue.

Alternative 1

For moderate power levels (say up to 34 dBm at each preamplifier out-

 

put), the internal combiner can be used with a user-supplied circulator as

 

shown in the Figure 7-1. This will protect the combiner against a discon-

 

nected or mismatched AUT (e.g. power off) , but care must be exercised to

 

ensure that the source switch in the test set is correctly positioned before

 

power is applied to the preamplifier(s).

Alternative 2

For greater preamplifier power levels, an external combiner should be

 

used. Figure 7-2 is a suggested block diagram for this configuration.

 

PATS is designed to enable power calibrations in the presence of external

 

preamplifiers. For the flat power calibration, the user can specify his tar-

 

get power (at the AUT input which is the calibration reference plane) and

ME7840A OMM

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Anritsu ME7840A manual Chapter Preamplifier Operations, Use, External, Preamplifiers, Alternative
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ME7840A specifications

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