Communication Concepts AN762 Application Note - Manual (Page 3)

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AN762
3 RF Application Reports
The measured output voltage variations of the bias source
from zero to 1.0 A were ± 8 – 12 mV resulting in a source
impedance of
≈ 30 mΩ.
The Input Frequency Correction Network
The input correction network consists of R1, R2, C2 and
C3. With the combination of the negative feedback derived
from L5 through R3 and R4 (Figure 1), it forms an attenuator
with frequency selective characteristics. At 30 MHz the input
power loss is 1 – 2 dB, increasing to 10 – 12 dB at 1.6 MHz.
This compensates the gain variations of the RF transistors
over the 1.6 to 30 MHz band, resulting in an overall gain
flatness of approximately
± 1.0 to ± 1.5 dB.
Normally an input VSWR of 2.0:1 or lower (Figure 8) is
possible with this type of input network (considered sufficient
for most applications). More sophisticated LRC networks will
yield slightly better VSWR figures, but are more complex and
sometimes require individual adjustments.
Additional information on designing and optimizing these
networks can be found in reference (2) .
The Broadband Transformers
The input transformer T1 and the output transformer T3
are of the same basic type, with the low impedance winding
consisting of two pieces of metal tubing, electrically shorted
in one end and the opposite ends being the connections of
this winding (Figure 3A). The multi-turn high impedance
winding is threaded through the tubing so that the low and
high impedance winding connections are in opposite ends
of the transformer.
The physical configuration can be implemented in various
manners. A simplified design can be seen in Figure 3B. Here
the metal tubing is substituted with copper braid, obtained
from any co-axial cable of the proper diameter
(4) . The
coupling coefficient between the primary and secondary
windings is determined by the length-to-diameter ratio of the
metal tubing or braid, and the gauge and insulation thickness
of the wire used for the high impedance winding. For high
impedance ratios (36:1 and higher), miniature co-axial cable
where only the braid is used, leaving the inner conductor
disconnected gives the best results. The high coefficient of
coupling is important only at the high-frequency end of the
band, e.g. 20 to 30 MHz. Additional information on these
transformers can be found in reference
(5) .
Figure 2. Photograph of 180 W Version of the Linear Amplifier