Codan Radio MT-3 - Manual (Page 30)

USER GUIDE | MT-3 ANALOG RADIO SYSTEMS
Chapter 4: Analog System Block Diagrams Page 22
FM REPEATER / BASE STATION BLOCK DIAGRAM
Figure 4-2 shows a block diagram of a Codan FM analog repeater or base station system (or a repeater-
base) using an AC-3E Audio Control Card. A block diagram of a repeater system using an AC-3M-96
and AC-3S-96 Audio Control Card would be identical, with the exception of the auxiliary COR, PTT and
audio.
The incoming FM transmission is routed through the Front End sub-module where the RF signal:
• is fi ltered (VHF and UHF 400 – high selectivity multiple pole, helical resonator fi lter, 5 MHz pass band;
UHF 800/900 – dielectric bandpass fi lter)
• is amplifi ed through a low noise amplifi e
• is fi ltered again through an image rejection fi lter
• is mixed with a local oscillator
• produces an IF frequency of 21.4 MHz (VHF and UHF 400) or 45 MHz (UHF 800/900).
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 21.855 MHz (VHF and
UHF 400) or 44.545 MHz (UHF 800/900) produces a second IF of 455 KHz. Second IF fi ltering is
achieved through the use of a 455 KHz ceramic fi lter. The second IF is demodulated to recovered
audio which is then sent to the discriminator output (for CTCSS decoding), the squelch circuitry and the
fl at and de-emphasis audio paths.
The squelch circuitry will produce the COR output and control the audio squelch gates. The audio
will typically be routed through the de-emphasis circuitry and a jumper selectable voice band fi lter
(fl at audio is not commonly used). The de-emphasis audio output will drive the speaker in the System
Regulator. The balanced audio output level, as well as the other audio output levels, are typically set
for -8.0 dBm (308 mVrms) audio output, using a 1.0 KHz tone at 60% of maximum modulation (3.0 KHz
wideband; 1.5 KHz narrowband) at the RF input.
The COR signal and audio are then 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 (or the
AC-3M-96 and AC-3S-96 can send audio to all four transmitters). Audio level control variable resistors
allow the user to adjust the audio levels between the receivers and transmitters using the front panel of
the AC-3E. An additional benefi t of the audio routing circuitry is the capability of routing audio through
the CTCSS fi lter built into the CTCSS decoder board, allowing the voice band fi lter in the receiver to be
disabled to save current draw.
The AC-3E audio control card also has auxiliary audio inputs and outputs for external control which can
be used simultaneously with the internal audio routing, allowing a repeater-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 CTCSS decoder can also encode the same tone it is programmed to decode. If a separate tone is
required for encoding, an optional CTCSS encoder can be installed and for the separate tone.
The transmitter will accept the incoming balanced audio and route it through the audio processor board
in the transmitter. The balanced level control, compression control, and maximum deviation levels are
all set in the audio processor. The balanced audio input level is typically set for a transmitter deviation
of 60% of maximum modulation (3.0 KHz wideband; 1.5 KHz narrowband) at the RF output, using a 1.0
KHz tone at -8.0 dBm (308 mVrms) at the audio input.
The CTCSS tone is also added in the audio processor and set by the subtone 1 input level control.
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 FM carrier in the oscillator and then amplify the
output of the oscillator.