Controls Display Errors Antenna Plotting




Sections of a RADAR


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 ship’s 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.