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.
MAXON
SP-300 HAND HELD
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.
zNOTE: 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.
