Raymarine RAY 240 Manual page 12

MHz. Digital signals control the band of operation of he VCO, by switching in/out capacitors in the
VCO circuitry.
The synthesiser controls the frequency of the VCO by altering a voltage level on a varacter diode. To
reduce the phase noise in the adjacent channel, this tuning voltage is kept between 3 and 4 volts.
This tuning voltage is coarsely controlled by the switched capacitors mentioned above and another
varacter diode, which is controlled by a D/A converter.
The output of the VCO is switched between the transmit chain and the mixer in the Channel
Selectable receiver.
Transmitter
The transmit chain is comprised of two amplifiers. Firstly a buffer amplifier is used to increase the
power of the signal from 0 dBm to +18 dBm. This amplifier is heavily compressed to ensure the
input power to the main PA module remains constant. This signal is then amplified by the main PA
module to either 1Watt (+30 dBm) or 25 Watts (+44 dBm), with an RA35H1516M power amplifier
module from Mitsubishi. This device has a gain control pin, which in conjunction with an automatic
power control loop, controls the output power of the radio
Antenna switch/filter
A PIN diode switch connects the antenna connector of the radio to either the transmitter or the
receiver. To ensure the receiver is not damaged when the radio is transmitting at 25 Watts, the switch
has a high degree of isolation between the transmitter and the receiver. The antenna terminal of this
switch is connected to a low pass filter. This ensures that the harmonics of the output signal from the
Mitsubishi amplifier are attenuated to a level less than 0.25 µW (–36 dBm).
Receiver
The receiver port of the antenna switch is connected to a bandpass filter and a low noise amplifier.
The bandpass filter is tuned to 160.000 MHz and has a 3 dB bandwidth 10 MHz. This filter prevents
the LNA from being saturated by large out of band signals.
The output signal of the LNA is then split between the channel selectable receiver and the CH70
receiver. An attenuator can be switched into the channel selectable path allowing the user to
desensitise the radio when in the presence of high power signal levels
Both the receivers are comprised of the same blocks, and these are described below.
Bandpass filters and LNA
st
The main purpose of the bandpass filter is to attenuate the 1 image frequency of the receiver. This is
defined as Nominal frequency of the receiver – (2x 1 st
IF frequency).
As the nominal frequency of the channel selectable receiver can change, the bandpass filter has the
ability to be tuned accordingly. This is achieved by four varacter diodes, which are controlled by
voltages generated by the processor PCB.
The filters for the CH70 receiver are tuned using the method mentioned above, however these
voltages remain at a fixed level. The processor PCB also generates these voltages.
Page 12
Raymarine Limited. Registered in England. Company no. 1177969
Registered Office: Anchorage Park, Portsmouth P03 5TD