63
amplifier, quadrature detector and active filter circuit. The second local oscillator at 20.945MHz with X202 and is
fed to the second mixer with the first IF signals to convert into 455kHz second IF signals..
The second IF signals leave through pin 3,and are fed to external ceramic filters XFL203, XFL204 which has
excellent selectivity, then fed to U201 (pin 5) again to be amplified and detected. Narrow /Wide band are switched
by software. The detected AF signals are output from pin 9.
* AUDIO AND SQUELCH CIRCUIT
The detected audio signals are put through a 6dB/oct de-emphasis circuit made up of R221 and C248. The signal
is then applied to audio power amplifier U3 through the volume control VR1 to obtain enough power to driver the
speaker.
Part of the recovered noise signal is fed to the integrated operational amplifier inside U201, which, with R232,
C256, C257 makes up an low pass filter . The DC signal detected by U201 (pin13) reaches the integrated DC
amplifier in U201 which has hysteresis to prevent jitter. The sensitivity of squelch is adjusted by VR201.
5. ALIGNMENT PROCEDURE
1) MEASUREMENT CONDITION
The following sections describes the alignment procedure for SPX-200 VHF transceiver under the following reference
environment conditions:
Temperature
:
25
C (77
F)
Relative Humidity
:
65%
Power Supply Voltage
:
7.5VDC +/- 5%
TEST EQUIPMENT / TOOLS REQUIRED
The following list of equipment is recommended for use in setting up the radio properly. Please ensure the test equipment
are calibrated according to the manufacturer’s instructions:
-
Frequency counter more than /-100Hz tolerance, high input impedance and high sensitivity
-
FM Signal generator, 1GHz with adjustable frequency, FM deviation, and RF output attenuators. 50
Ω
Output
impedance.
-
Oscilloscope, high input impedence.
-
16
Ω
1 Watt resistor as loudspeaker load
-
Audio Signal Generator, 10Hz to 20KHz, 600
Ω
impedance with attenuators.
-
RF Watt meter, with 50
Ω
10 Watt termination resistor (Or RF Voltmeter with 50
Ω
termination and external 50
Ω
attenuators)
-
Regulated Power Supply 7.5VDC 3A output
-
Digital A-V-O Multi-meter
-
SINAD Meter
-
External Speaker Mic. plug (or special audio test jig)
-
Interconnection test cable for RF and Control PCB
-
Circuit Diagram for SPX-200
-
PCB layout diagram for SPX-200
-
Tuning tools for RF/IF transformer and the VR potentiometers
2) DISASSEMBLING THE UNIT
The antenna
Disconnect the antenna
The Cover
-
Remove the battery.
-
Remove the 2 screws.
-
The case could then be opened for servicing.
-
Be careful NOT to disconnect the pin connector between RF board and Control board.
