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MAXON
SP-300 HAND HELD
Page -9-
May 2001
THEORY OF OPERATION
The VHF and UHF scanning handheld series radios are com-
prised of one main PCB. The main PCB contains the trans-
mitter, receiver and control circuits. The control circuits
contain the micro controller and associated digital circuits.
DIGITAL CIRCUITS
IC 411 is a digitally-controlled analog switch which inter-
nally consists of three single pole, double throw switches.
By placing a high (5V) or low (0V) on the control lines
which consists of A, B and C. A controls the X ports, B con-
trols the Y ports and C controls the Z ports. Example: A high
on control A would connect X to X1. A low on control A
would connect X to X0.
CTCSS/DCS Decode Circuits
Discriminator audio from Pin 9 IC5 is fed into and associ-
ated parts, which are the first 2 poles of a 6th order 250 Hz
Chebeyshev low pass filter. The output from pin 1 (IC406A)
is fed into IC411 (Pin 2) and output to pin 15 (IC411). The
signal is then fed to Pin 8 (IC407) which is a 6th order low
pass Butterworth switched capacitor filter. The output from
the Butterworth filter (Pin 3 IC407) is then fed to the remain-
ing 4 poles of the 6th order Chebeyshev, which consist of
IC406C and one of the two operational amplifiers internal to
IC407 (MF6-100) along with associated components. Both
the Chebeyshev and the Butterworth combines for a 4dB rip-
ple low pass filter when programmed for 250 Hz. The output
of IC406C (Pin 8) is fed into the remaining operational
amplifier internal to IC407 (MF6-100) which forms the
squaring circuit for the Decode signal. The signal is output
from Pin 2 IC407 (MF6-100) and fed into IC409 (micro)
where it is matched with a preprogrammed frequency. If suc-
cessful, a Decode occurs, which is shown by a green L.E.D.
on the top panel of the VHF and UHF Scanning Handheld
and audio is heard. If valid Decode was not seen, the busy
L.E.D. (Yellow) would be shown.
CTCSS/DCS Encoder Circuit
During TX encode the tone squelch digital signal is pro-
duced as a 3-bit parallel word at Pins 15, 16, and 18 of the
micro controller (IC409). The 3-bit digital signal is con-
verted to an analog signal by resistors R478, 479 and 480.
The analog signal is fed into IC411 Pin 1. The signal is out-
put on Pin 15 (IC411) and fed into Pin 8 of IC407 (6th order
Butterworth clock tuned low pass filter). The filtered encode
output from Pin 3 (IC407) is fed into Pin 13 (IC411) and out-
put on Pin 14 (IC411). The filtered encode signal is fed to
IC406B and RV403 (sub-audible gain control), the output of
IC406B is then fed to the audio mixer circuit.
External Mic/PTT Control Circuit
The external microphone is connected via the CON701 con-
nector on the right side of the handheld. The internal mic and
speaker are disabled by grounding of pin 10 of CON701.
Pulling pin 10 of CON701 low will bring the base of Q702
low, switching the transistor off. This removes the ground
from the internal microphone, shutting it off. Pulling pin 10
of CON701 low will also pull the Gate of Q701 low, turning
it off. This will disconnect the speaker from the rest of the
circuit, shutting it off. To enable transmit the Q406 base
impedance is low (below 20k ohm), Q406 and Q407 turn on
and Q407 collector is low, which is connected to IC409
(micro) Pin-24.
Channel Select Circuit
One of 16 channels(SP-330 & 340) or one of 4 channels(SP-
310 & SP-320) may be selected, using the channel switch on
the top panel. The channel switch encodes the channel num-
ber selected into a 4-bit binary code. The binary code plus
one is equal to the channel number. The binary code is
decoded by the micro controller enabling the appropriate RX
or TX frequency and associated data to be selected from the
EEPROM.
z
NOTE: Any of the channel locations may be a scan
ning position. Refer to Operators Manual for fur-
ther instructions.
Low Battery Indicator Circuit
When the battery voltage drops below 5.4 VDC, D403 and
Q417 turn on. The micro controller disables the transmitter
and at the same time enables the red LED and sends an alert
tone to warn the user. The battery should be replaced or
recharged at this time (one tx allowed after low battery).
EEPROM
RX/TX channels, CTCSS/DCS as well as other data from
the programmer are stored in the EEPROM. The data stored
is retained without power supplied. This is a non-volatile
memory. The EEPROM may have information re-pro-
grammed or erased. IC408 is an EEPROM with 2048
(8x256) capacity and data is written and read serially.
High Pass Filter
The high pass filter is an 8-pole, 1 dB Chebeyshev active fil-
ter that comprises IC410 and associated components. The
de-emphasis is provided by resistor R451 and capacitor
C471. Receive audio is passed to IC410 by Pin 4 of IC411
where sub-audible tones below 300Hz are removed. Mic
audio is also fed into IC410 via IC 411 (Pin 4) where sub-
audible voice products below 300Hz are also removed.