Rfcs Receive System

Figure 3-24 shows the rfcs receive system used to receive the transmitted signal and translate it back to a usable output. You should look at this figure while studying the units in this section.

Figure 3-24. - Rfcs receive system.

ANTENNA FILTER ASSEMBLY. - The antenna filter assembly is connected to the antenna and receives the rf signal from the antenna. It filters out any unwanted rf signals and allows the desired band of frequencies to pass.

RADIO RECEIVER. - The radio receiver takes the rf signal passed on by the antenna filter and translates it to an audio signal.

RECEIVER TRANSFER SWITCHBOARD. - The receiver transfer switchboard is used to tie the receiver to any converter unit connected to it. This allows you a wide selection of equipment for connection to the same receiver.

CONVERTER-COMPARATOR GROUP. - The converter-comparator group is used with receivers in either space or frequency diversity operation. When diversity operation is not required, each converter can be used separately with a single receiver.

Each converter has its own COMPARATOR circuitry. This built-in design feature results in a considerable reduction in size from older units. The comparator was located in a separate chassis in the older units. Size has been further reduced through the use of microelectronics.

Figure 3-25 shows the basic method we use to convert a frequency-shift rf signal into a signal that controls the dc loop of a tty. The frequency shifts of the af output from the receiver are converted into dc pulses by the af discriminator. The dc pulses are then fed into the keyer. The keyer opens and closes the dc loop of the tty according to the mark and space characters received.

Figure 3-25. - Frequency shift receiving system simplified block diagram.

In diversity operation the comparator section of the converter-comparator group (shown in figure 3-24) compares the strength of the signals from two receivers. Signals from each converter are fed into a comparator circuit that compares the signals. This comparison is displayed on a crt on the front of the equipment. The comparison is in the form of LISSAJOUS PATTERNS. A lissajous pattern is a combined, simultaneous display of the amplitude and phase relationships of two input signals. One signal is applied to the vertical and the other to the horizontal deflection circuits. Lissajous patterns have many applications in electronics. They have operational uses as well as uses in corrective and preventive maintenance. Further coverage on lissajous patterns can be found in NEETS, Module 19, The Technician's Handbook. Figure 3-26 shows several typical lissajous monitoring patterns for the converter-comparator group. Once we have a correctly tuned signal, the comparator feeds it to the communication patching panel for patching to the tty. Now let's refer back to figure 3-24 while we discuss the rest of the units in the system.

Figure 3-26. - Typical lissajous monitoring patterns.

COMMUNICATION PATCH PANEL. - The communication patch panel serves the same functions on the receive side of the rfcs system as it did on the transmit side. It routes the dc signal to the proper cryptographic equipment. It also routes the decoded teletypewriter signal from the cryptographic equipment to the selected tty.

CRYPTOGRAPHIC EQUIPMENT. - The cryptographic equipment converts the transmitted coded signal to a decoded signal that can be printed out in its original state.

TELETYPEWRITER. - The tty equipment is used to convert the dc signal received from the communication patch panel to a printed copy of the original transmitted message. The tty shown is used only for receive and does not have the ability to transmit.

Q.27 What are the functions of a converter-comparator group?