User's Manual

61-38-05 UserMan page 28 of 38
TXRX Systems Inc. Manual 7-9408-1.2 07/25/05 Page 28
Single Section Amplifier Subassemblies
Amplifiers with 400 Milliwatt RF power output rat-
ings are manufactured by TX RX Systems, Inc. for
use in the model 61-38-05 family of signal boost-
ers. These amplifiers utilize bi-polar transistors
operating as class-A linear amplifiers with varying
RF power output capability. They offer a good com-
promise between low noise figure and low levels of
intermodulation distortion. In addition, these ampli-
fiers use narrow band impedance matching cir-
cuitry which offers significant improvements in
noise figure compared to broadband designs. How-
ever, narrow band circuits necessitate having to
tune the matching networks to obtain best perfor-
mance. This tuning procedure needs to be done
when the RF transistor and/or matching network
components are replaced.
Each single amplifier section (see
Figure 20
) uses
a bias regulator circuit to keep the RF transistor
biased for constant collector current with changes
in temperature. The collector current remains con-
stant when these amplifiers are running properly.
The actual value of bias current will be different for
different types of amplifiers but can also vary
slightly if the power supply voltage varies. All ver-
sions of the current production bias regulators are
designed for fixed values of RF transistor collector
current.
The multi section amplifier assembly 3-11423 is a
combination of individual single section subassem-
blies (part # 3-8089) which have a 400 Milliwatt
output power rating per section. These single sec-
tion subassemblies are interconnected with short
lengths of double shielded coaxial cable for the RF
interconnection. The individual sections are
mounted on a common bracket with a DC distribu-
tion wire running internally between sections. A
multi section amplifier is tuned on a per section
basis. Never attempt to tune the interconnected
sections.
AMPLIFIER TUNING
Field repair and tuning of our amplifiers is sup-
ported by TX RX Systems Inc. and the following
procedure will allow satisfactory operation to be
obtained. At the factory, TX RX amplifiers are tuned
using two channel network analyzers that allow
adjustment of both gain, input/output return loss
and verification of reverse isolation. After this tun-
ing, the amplifiers are checked with an advanced
noise figure measurement system and are fine
tuned to obtain best noise figure. Because this
equipment is rarely available in even the most well
equipped service centers, we recommend return-
ing the amplifier to the factory for repair and retun-
ing if the specified noise figure has to be obtained.
In actual practice, most of the amplifiers retuned in
the field will exhibit noise figures that range from
being equal to the published specification or
exceed it by 0.5 to 2 dB. Low noise figure may be of
minimal importance in any system where very
weak signal sensitivity is not an issue.
REQUIRED EQUIPMENT
The following procedure was developed to be prac-
tical as a field bench service procedure. This pro-
cedure is intended for single sections only. All multi
section TX RX amplifiers are composed of cas-
caded single sections interconnected with short
lengths of double shielded coaxial cable. Individu-
ally tuned single sections do not require any further
adjustment when they are connected together
Analyzer
Input
Generate
Output
+30
+40
+20
+10
0
-10
-20
-30
-40
2" Square VHF
Notch Filter
Tracking Generator
Tune
Figure 19
: Tuning the Notch filter for max attenuation.