Coupler CU-872/U (figure 3-37) is one example
of a typical HF electronic tube type
multicoupler. The CU-872/U is designed to
provide optimum coupling between a single
antenna, and as many as eight receivers.
Additional outputs from the same antenna are
possible by connecting an output from a
primary multicoupler to the input of a
secondary multicoupler in a cascade (cascading
is discussed below) arrangement as shown in
figure 3-38. A functional block diagram of the
CU-872/U multicoupler is shown in figure 3-38.
A 70-ohm input impedance is provided to match
the impedance from the antenna. From the
input connector, the signal is fed to
low-pass/high-pass filters. These filters pass only
the frequencies in the spectrum between 2.0
MHz and 32 MHz. Transformer T1 in the output
circuit of the low-pass/high-pass filters, provides
a transition between the low impedance
unbalanced input circuits and a relatively high
impedance balanced line. Each side of the
balanced line drives one section of the push-pull
distributed amplifier.
Tubes V1 through V5 and tubes V11 through
V15, and their associated circuitry, comprise
one-half of the push-pull distributed amplifier.
Tubes V6 through V10 and tubes V16 through
V20, and their associated circuitry, comprise the
other half of the push-pull distributed amplifier.
The distributed amplifier sections employ
cascade stages (discussed below) along artificial
transmission lines to obtain amplification over a
wide bandwidth. The cascade amplifiers aid in
reducing intermodulation by minimizing odd
harmonic distortion. Additionally, employment
of the distributed amplifier results in an
improved signal-to-noise ratio. The distributed
amplifier sections drive transformer T2 in a
push-pull manner, thereby reducing
intermodulation by minimizing even harmonic
distortion. The resulting signal, developed across
the secondary winding of transformer T2, is
applied to a cascaded hybrid network which
distributes the amplified signal to eight isolated
outputs.
