Essential sections of a basic receiver and display
Antenna drive unit; rotates the antenna at constant speed
Heading marker switch; closes when main beam is in line with ships head and causes the PPI heading mark to appear
Bearing transmitter; driven by the antenna which transmits electrically the antenna beam bearing information
Electronic switch (T/R cell): or the transmit/receive cell (T/R), switches at high speed between transmit and receive modes: essentially a receiver protection device
The receiver circuit of the Radar is an extremely sensitive part of the receiver as it deals with signal voltages of an extremely low value.
The voltages of the received echo signal may be as low as a millionth part of a volt. The receiver must therefore amplify the signal by anything between 1 and 10 million times, so as to produce adequate voltages to be displayed onto the CRT.
The echo signals that are received have however the same frequency as the transmitted pulse as well as the same shape (envelope) as that which was imparted to the transmitted pulse by the modulator and pulse forming circuit.
The weak echo signals; with the high frequency have now to be amplified; however the amplification of signals with such high frequencies is extremely difficult. As such the frequency (I.F.) that is finally amplified is a much lower frequency (between 45-60MHz, depending on the manufacturer) but the shape of the pulse remains the same.
How do we get the lower frequency the Intermediate Frequency (I.F.)?
The process that is used is that, the incoming weak signal is mixed with another signal of nearly the same frequency. This signal is generated within the radar unit in the Local Oscillator.
Thus the local oscillator single cavity resonant oscillator generates a single frequency, which is mixed in the mixer circuit with the incoming weak echo signal. The frequency generated in the L.O. is very close to that generated by the magnetron.
The frequency obtained from the local oscillator can and is changed by two specific controls one a coarse control and the other a fine control.
The coarse control is control of the physical size of the cavity done by the manufacturer or at the time of installation by the technician.
And the fine control is exercised by small variations in the electronic conditions associated with the resonant cavity done by the radar operator mariner.
Mixer; an electronic circuit which converts the incoming echo signal at the transmission frequency to a much lower value known as the intermediate frequency (IF); since it is easier to process later.
9445 MHz from the echo, same as the transmit frequency
9505 MHz from the Local Oscillator
= 60 MHz Output of the mixer (IF) this is amplified.
Local oscillator; provides a frequency stable output signal having a value of frequency either higher or lower than the transmission frequency by a value equal to the IF.
Simultaneous application of the local oscillator output and the echo signal to the mixer will produce a difference frequency, which is the IF.
The IF signals contains the same information as the incoming echo signal
Demodulator; produces video pulses from the IF signal pulses
Video amplifier; amplifies and processes the video pulses to a level adequate to intensity modulate the PPI CRT beam current
Timebase waveform and control waveform generator; generates the timebase sawtooth sweep waveform and other rectangular waveform used to control the display of targets during the sweep time only: the circuits are synchronized to the transmitted pulse
Bearing receiver: a small machine, which receives antenna-bearing information and applies mechanical drive to rotating scan coils. The coils rotate in synchronism with the antenna. There are also other methods of producing a rotating scan at the PPI
Ranging circuits; two separate circuits one of that produces periodic short pulses to display accurately spaced concentric rings on the tube face (Range rings), the other circuit produces a variable radius ring (variable range marker) linked to an accurate range scale.
T/R cell (Transmit and Receive Cell)
The cell prevents magnetron high-level power from entering the receiver arm and protects the sensitive receiver crystals from damage when the transmitted pulse is present.
During periods of no transmission the cell allows the received signal to reach the receiver crystals. The cell is a chamber filled with inert gas. When the magnetron fires, the gas very rapidly ionizes producing a switching action, which directs the RF power to the antenna and away from the receiver.
At magnetron switch-off, the cell very rapidly de-ionizes to allow received signals at short ranges to be processed by the receiver.
Receiver crystals are in fact semiconductor diodes capable of rectifying radio frequency currents in the- Super Heterodyne Frequency (SHF) band and are arranged in a mixer circuit to operate in conjunction with the local oscillator.
The arrangement converts all incoming RF signals at super-high frequency down to a much lower radio frequency known as the intermediate frequency (IF) where the signal can be amplified and processed using conventional RF circuit design.
The conversion is achieved by superb heterodyning the received signals with a locally generated stable signal produced by a local oscillator circuit.
A Gunn diode is the preferred choice of local oscillator since it produces adequate output power, operates at low voltage levels and has lower noise output than its predecessor, the klystron.
The local oscillator is tuneable over a range of a few megahertz by means of an electronic tuning control.
The head amplifier amplifies the target signals now converted to the IF.
The effect of applying the target return signal is to produce sum and difference frequencies at the mixer output.
For example, if Echo frequency = 9400 MHz and
The local oscillator equals 9460 MHz or 9340 MHz, as the case may be.
Then the frequencies produced are 18 860 MHz or 18 740 MHz (being the sum values) together with 60 MHz, the difference frequency.
The sum and original frequency components are de-coupled at the mixer output and the difference frequency is the desired IF.
Raw echo signals converted to the IF will retain the same pulse length as that of the transmitted and received SHF signal.
The IF amplifier
The purpose of the IF amplifier is to amplify the IF signals produced by the mixer to a level sufficient to operate the video detector (usually a few volts).
A facility is provided to artificially lengthen the stored video signals by a fixed length of time. The effect at the display is to produce a more prominent paint of the digitized video.
Pulse stretching is usually made selective in that it is only applied to pulses above a fixed duration usually on the 6-96 nm ranges and only to displayed video beyond about 3 nm.
Echoes having duration less than that of the transmitted pulse are rejected and not elongated.
The Modulator unit includes a very fast operating switch (electronic) which allows the discharge of the energy stored in the Pulse Forming Network.
(Silicon controlled rectifier like a diode, allows current to flow only in one direction. The SCR however will allow current to flow only if a trigger pulse is applied to a control electrode. When the trigger pulse is present the opposition to current offered by the SCR drops instantly to virtually zero. This produces a pulse of current having an extremely fast rise time.)
The Pulse Forming Network stores the energy in capacitors. Capacitors can store energy and when triggered discharge this energy into a circuit but ordinary circuits have a long discharge time and consequently the charge falls to low levels thus this defeats the steady charge that is required in a Radar circuit. This is overcome by a series of capacitors and inductors.
Note that no signal is required to terminate the discharge, whenever the energy is drained out the pulse is terminated.
The duration of the discharge and thus the pulse length is therefore is a function of the amount of energy stored in the capacitors.
The amount of energy stored again is determined as to how many sections of the Network are put to use, this is determined by the selection of the Pulse Length.
Thus it is seen that selection of the Pulse length will determine the amount of energy released by the system and therefore the amount of energy contained in a pulse and this will affect the ranging capabilities of a Radar.