SP-210 RF Amplifier

(from TM 11-866)


a. The antenna is coupled to the grid of the 1st RF amplifier
(V1) through an input transformer having an untuned primary and
tuned secondary. There is an electrostatic shield or screen between
primary and secondary which prevents direct capacitive coupling
between the antenna circuit and any part of the tuned secondary
which is connected to the grid of tube V1. This arrangement results
in substantially pure inductive coupling only, permitting full advan-
tage to be taken of the noise-reducing properties of a balanced
transmission line lead-in. The impedance of the input circuit aver-
ages approximately 100 ohms throughout the tuning range of the

b. There are two stages of RF amplification preceding the 1st
detector or mixer. These stages are coupled by means of RF trans-
formers having tuned secondaries and low inductance untuned
primaries, except the 20-40 mc band of receiver BC-794-B which has
straight tuned impedance coupling (L49, L50). In receiver BC-1004-C
the RF transformers in the 540-1160 kc band (L42, L44) have 20-ohm
series damping resistors (R57, R58) inserted in their tuned secondaries.
This additional resistance materially reduces the "Q" of the RF
transformers and results in less side-band cutting, especially at
the low-frequency end of the band.