Anritsu ME7840A - Manual (Page 107)

Chapter 7
Preamplifier Operations
7-1
INTRODUCTION This chapter describes the use of external preamplifiers to boost signal
power at the input of an test amplifier.
7-2
USE OF
EXTERNAL
PREAMPLIFIERS External preamplifiers can be used to boost signal level at the input of
the Amplifier Under Test (AUT). The maximum power per tone at the
AUT input without any external preamplifier is +5 dBm.Block diagrams
showing this feature are contained in Figures 7-1 and 7-2.
As shown in Chapter 1, Figures 1-2 and 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:
q Driving the DRA Scorpion channels a
1
and b
1
into compression
q Possibility of damage to the combiner
The first issue is avoided by using an attenuator at the input of channels
a
1
and b
1
. The value of this attenuator should be selected noting that the
maximum operating signal level at a
1
and b
1
is –5 dBm, and that the cou-
pling factor between AUT input and channel a
1
/ b
1
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 P
in
(max power at AUT input) is greater than
25 dBm and the value of attenuator is given by A (dB) = P
in
(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 7-1