The converter section of the ARC-34 main receiver is shown in the illustration on page 97. This section consists of the first and second RF amplifiers, frequency multiplier (tripler), mixer, and first IF amplifier.
The RF signal is introduced into the first RF amplifier input cavity at J667, at the extreme left of the diagram. This amplifier is a grounded-grid amplifier, with the signal being applied to the tube through the cathode. The grid is connected to the AGC (automatic gain control) line, but is at RF ground potential due to the low impedance of grid capacitor C654.
The operation of the input cavity is similar to the operation of the resonant cavity discussed under Additional UHF Developments, on page 102. Inductive loop L651 induces a magnetic field in the cavity and since the dimensions of the cavity are correct for this frequency band, the cavity resonates, giving all the characteristics of an ordinary parallel-
tuned circuit. The cavity is tuned to a particular frequency in the band of 225 to 399.9mc by tuning capacitors C651A and C651B. These two capacitors effectively change the dimensions of the cavity to resonate at a particular frequency. C652 is a 5-8 mmf trimmer capacitor for aligning this amplifier with the second RF amplifier and the mixer. L652 is a small adjustable vane that makes it possible to vary the inductance of the cavity by a small amount when aligning.
The magnetic field in the cavity induces a voltage into cathode loop L653 and this causes the input signal to be applied to the cathode of the tube. C653 and R652 are conventional cathode bias components.
The output of the tube is applied to the common plate-grid cavity Z651B through C655, a 47-mmf capacitor. This signal sets up an electric field in this cavity. Tuning is accomplished in this cavity in the same manner that it was accomplished in the input cavity. The cathode loop L655 has the signal induced into it and thus the signal is applied to the cathode of the second RF amplifier. This is also a grounded grid amplifier stage, with the DC grid path connected to the AGC line. The output of this second RF amplifier is similar to that of the first.
The signal input to the mixer is applied by means of inductive loop L657 and C666 to the grid. The bottom end of grid resistor R664 is connected to receiver test jack so that the mixer grid current can be measured for alignment and troubleshooting purposes.
The HFO signal for the mixer originates at the master oscillator in the transmitter, as explained earlier. From there it is applied to two doublers before it is applied to the frequency multiplier (tripler) shown here. The range of this signal is from 70 to 128 mc, over the 225-399.9-mc range of the equipment. The plate circuit of the tripler and the cathode injection circuit of the mixer are the same. This cavity is tunable the same as the RF cavities.
Since the incoming signal is applied to the mixer grid and the HFO signal is applied to the cathode, there is a mixing action taking place in the mixer tube. The plate of the mixer tube is connected to ground through a parallel
resonant circuit tuned to 15.825 mc. This is the first intermediate frequency of the receiving equipment. From this stage on, the receiver is similar to the VHF receiver discussed earlier in the chapter.
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