Raymarine Limited. Registered in England. Company no. 1177969
Page 13
Registered Office: Anchorage Park, Portsmouth P03 5TD
Mixer
The received signal is down converted to the 1
st
IF by mixing it with an LO frequency.
For the channel selectable receiver, the LO signal is generated by the VCO/Synthesiser. The nominal
frequency of the receiver is defined as
LO Fre 1
st
IF Frequency
For the CH70 receiver, the LO signal is generated by a crystal controlled oscillator at 46.524 MHz,
and then tripled to produce the 139.625 MHz signal.
Crystal bandpass filter
The crystal filters have a centre frequency equal to the 1
st
IF frequency of the receiver. These filters
are designed to attenuate two different frequencies. The 2
nd
image frequency of the receiver is
attenuated by this filter, which is defined as
Nominal frequency of the receiver – (2x 2
nd
IF
frequency)
.
The filters also attenuates signals >25 kHz away from the nominal frequency of the receiver (ie the
adjacent channel).
Detector IC
The outputs of the crystal filters are connected to a BA4116FV detector IC from Rohm. This device
down converts the 1
st
IF frequency to the 2
nd
IF frequency by using a crystal oscillator. This down
converted signal is then demodulated by a quadrature detector to produce the received audio signal
This device has an output pin whose voltage is relative to how noise the received signal is. This
voltage is used by the processor PCB to determine whether the squelch should be opened or closed.
