Codan Radio MT-3 - Manual (Page 34)

USER GUIDE | MT-3 ANALOG RADIO SYSTEMS
Chapter 4: Analog System Block Diagrams Page 26
AM BASE STATION (OR AM / FM CROSS-BAND) BLOCK
DIAGRAM
Figure 4-4 shows a block diagram of a Codan AM base station system (or AM/FM cross-band system)
using an AC-3E Audio Control Card. In an AM / FM cross-band system, either the receiver or transmitter
in the block diagram would be replaced with an FM receiver or transmitter.
The incoming AM transmission is routed through the preselector where the RF signal:
• is passed through a 20 MHz Elliptical Band Pass Filter
• is amplifi ed through a low noise amplifi er
• is fi ltered again through an image rejection fi lter
• is mixed with a local oscillator
• produces an IF frequency of 21.4 MHz.
The fi rst IF amplifi er provides linear IF amplifi cation and then crystal fi lters provide optimum bandpass
characteristics for good selectivity and low distortion. A second L.O. source of 32.15 MHz produces a
second IF of 10.75 MHz. Second IF fi ltering is achieved through the use of a 10.7 MHz ceramic fi lter.
The second IF is demodulated to recovered audio and an AGC output is applied to the RF amplifi er in
the preselector.
The recovered audio is sent to the squelch circuitry and the recovered (fl at) audio path. The squelch
circuitry will produce the COR output and control the audio squelch gate. The fl at audio will be routed
through the squelch gate and will then be split to the low level and high level audio adjust drivers. The
low level audio output will drive the speaker in the System Regulator. The balanced audio output (high
level), as well as the audio output (low level) to the system regulator speaker, are typically set for -8.0
dBm (308 mVrms) audio output, using a 1.0 KHz tone at 30% modulation at the RF input.
For an AM / FM Cross-band application, the COR signal and audio are routed through the AC-3E audio
control card and are sent to the transmitter as a PTT signal and audio. The AC-3E has audio routing
capability that can be confi gured to use the audio distribution amplifi er to send the audio from one
receiver to both transmitters. Audio level control variable resistors allow the user to adjust the audio
levels between the AM and FM receivers and transmitters using the front panel of the AC-3E.
For an AM Base Station application, the AC-3E audio control card has auxiliary audio inputs and outputs
for external control which can be used simultaneously with the internal audio routing, allowing a cross-
band-base confi guration or just a base station confi guration. For external control, the AC-3E also has
optically isolated COR and PTT inputs and outputs for connecting external equipment with high voltage
control signals such as E&M.
The transmitter will accept the incoming balanced audio and route it through the audio processor board
in the transmitter. The balanced level control and modulation control output level are set in the audio
processor. The balanced audio input level is typically set for a transmitter deviation of 30% modulation
at the RF output, using a 1.0 KHz tone at -25.0 dBm (44 mVrms) at the audio input. The incoming PTT
signal will activate all circuitry in the transmitter and will key the transmitter. The transmitter will then
modulate the audio signal to an AM carrier in the amplifi er.