Ritron RIT17-142R MODULAR VHF FM RECEIVER User Manual manual

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PRELIMINARY
DR-142
PROGRAMABLE VHF FM RECEIVER MODULE
OPERATING AND SERVICE MANUAL
RITRON INC.
TABLE OF CONTENTS
SECTION
TOPIC
PAGE
1.
DR-142 SPECIFICATIONS
SUB-D 15 PIN CONNECTOR
2.
INTRODUCTION
GENERAL
Model Identification
FCC ID
Unauthorized Modifications
MAINTENANCE INFORMATION
Surface Mount Repairs
Precautions for handling CMOS
Synthesizer shield
3.
DESCRIPTION OF INPUT/OUTPUT
4.
PROGRAMMING THE RADIO
PROGRAMMING THE RADIO USING A PC COMPUTER
Programmable Features
COMPUTER SOFTWARE COPYRIGHTS
5.
DR-142 THEORY OF OPERATION
INTRODUCTION
POWER SUPPLY AND VOLTAGE DISTRIBUTION
REFERENCE OSCILLATOR
SYNTHESIZER
RECEIVER
Receiver Front End
FM Receiver Subsystem
Voice and Tone Conditioning
MICROCONTROLLER
10
DR-142 ALIGNMENT PROCEDURE
RECOMMENDED TEST EQUIPMENT
RADIO PREPARATION
REFERENCE FREQUENCY
RECEIVER FRONT END TUNNING
RECEIVER SQUELCH
11
11
11
11
12
6.
DR-142 SPECIFICATIONS
FCC ID:
AIERIT17-142R
FCC Rule Parts:
15
Frequency Range:
136-156 MHz or 154-174 MHz
RF Channels:
8 Independent RX frequencies
Synthesizer Step Size:
5 kHz
Frequency Stability:
+/-2.5 PPM (-30 to +50 C)
Tone/Code Signaling:
CTCSS (Quiet Call) and DCS
Power Supply:
8 to 15 VDC
Battery Drain:
RX Standby:
RX Receive:
20 mA at 13.8 VDC
250 mA
Dimensions:
5”H x 3”W x 1.25”D
Weight:
3.8 oz.
Antenna Fitting:
None (customer supplied)
DR-142 SPECIFICATIONS
Wide band
25 kHz
Narrow band
12.5 kHz
Sensitivity (12 dB SINAD):
0.25 µV
0.25 µV
Adjacent channel:
-70 dB
-60 dB
Spurious rejection:
-60 dB
-60 dB
Image rejection:
-80 dB
-80 dB
Intermodulation:
-65 dB
-65 dB
FM hum and noise:
-45 dB
-40 dB
Conducted spurious:
-57 dBm
-57 dBm
Receiver attack time:
< 30 ms
< 30 ms
Noise squelch attack time:
< 15 ms
< 15 ms
RSSI squelch attack time:
< 5 ms
< 5 ms
RSSI squelch sensitivity:
Manually adjustable; factory set for –108 dBm
Noise squelch sensitivity:
Manually adjustable; factory set for 20 dB SINAD @ 1.5 kHz deviation
( 3.0 kHz used for wideband)
AUX OUT frequency response: 6 - 3000 Hz @ +1 / -3 dB
Audio Output
>700 mW into 8 Ω, with less than 5 % THD
Sub D 15 Pin Connector
The DR-142 is equiped with a 15 pin female sub D connector with the following functions:
Pin#
10
11
12
13
14
15
Function
least significant channel select bit (CS0)
channel select 1 (CS1)
Most significant channel select (CS2)
N/A
N/A
+ 8 to 15 VDC input
N/A
auxilary ouput
PC programing port
N/A
Auxilary monitor input (pull to ground to open squelch)
Speaker output
Carrier detect output (pulls to 5 VDC through 390 ohms)
N/A
ground
INTRODUCTION
GENERAL
RITRON's DR-142 receiver module is designed to operate in the VHF band. Two band splits are
available, 136-156 MHz (low) or 154-174 MHz (high). The unit receives voice through a nominal 1 watt
speaker output. It supports up to 9600 bps 4-level FSK through its auxiliary output port.
Each radio can be “dealer or factory” programmed to contain a unique set of eight operating frequencies
and options. When all three channel select inputs are tied to ground the radio operates on channel 1. The
channel selection is done in a binary manner on the CS2, CS1 and CS0 inputs. Quiet Call (CTCSS) or
Digital Quiet call (DCS) tones can be programmed per channel.
Model Identification
The DR-142 model, serial number and FCC Identification are displayed on a label located on the bottom
of the PCB.
FCC Label
An FCC label must be visible on the unit as installed in its final configuration. If the unit is to be used as
shipped from RITRON, this would be no problem since an FCC label is affixed to the bottom of the PC
board. If the DR-142 is to be installed in an enclosure, the installer must ensure that either the FCC label
on the unit is visible through a door, window or other opening, or add a label to the outside of the
enclosure. If a label is to be added to the outside of the enclosure, the label must be of a type which is
not easily removed or damaged and contains wording: Contains FCC ID: AIERIT17-142R.
Unauthorized Modifications
Note: The manufacturer is not responsible for any radio or TV interference caused by unauthorized
modifications to this equipment. Such modifications could void the user’s authority to operate the
equipment.
MAINTENANCE INFORMATION
Surface Mount Repair
RITRON surface mount products require special equipment and servicing techniques. Improper servicing
techniques can cause permanent damage to the printed circuit boards and/or components, which is not
covered by RITRON’s warranty. If you are not completely familiar with surface mounted component
repair techniques, RITRON recommends that you defer maintenance to qualified service personnel.
Precautions for Handling CMOS Devices
This radio contains complementary metal-oxide semiconductor (CMOS) devices, which require special
handling techniques. CMOS circuits are susceptible to damage by electrostatic or high voltage charges.
Damage can be latent, with no failure appearing until weeks or months later. For this reason, take
special precautions any time you disassemble the radio. Follow the precautions below, which are even
more critical in low humidity environments.
1) Storage/transport - CMOS devices that will be stored or transported must be placed in conductive
material so that all exposed leads are shorted together. CMOS devices must not be inserted into
conventional plastic “snow” or plastic trays of the type that are used for other semiconductors.
2) Grounding - All CMOS devices must be placed on a grounded bench surface. The technician that
will work on the radio/CMOS circuit must be grounded before handling the radio. Normally, the
technician wears a conductive wrist strap in series with a 100K Ohm resistor to ground.
3) Clothing - Do not wear nylon clothing while handling CMOS circuits.
4) Power off - Remove power before connecting, removing or soldering a PC board that contains
CMOS devices.
5) Power/voltage transients - Do not insert or remove CMOS devices with power applied. Check all
power supplies to be used for testing CMOS devices, making sure that no voltage transients are
present.
6) Soldering - Use a grounded soldering iron for soldering CMOS circuitry.
7) Lead-straightening tools - When straightening CMOS leads, provide ground straps for the tool used.
Synthesizer Shield
The synthesizer shield should not be removed, unless a component must be replaced. This shield is
soldered to the main PC board.
DESCRIPTION OF INPUT/OUTPUT
Channel Select (pins 3,2,1)
Three lines control the channel selection; CS2, CS1, CS0. The inputs have binary weighting of 4, 2 and 1
respectively. Tying an input to ground gives it a zero weighting. Thus, if no inputs are tied low channel
eight is selected.
Power Input (pin 6)
A power source of 8 to 15 VDC with 0.5 Amp capability should be connected here. Once power is applied
to pin 6 of the radio the microcontroller will start and load the receive frequency of the channel
designated by CS2, CS1 and CS0 (pins 3, 2 and 1). The radio will beep out the channel number on the
speaker line.
Auxiliary Output (pin 8)
An output loading of 600 ohms or greater should be applied to this output. With a 600 ohm load the
output exhibits a frequency response of 6 Hz to 3000 Hz. Higher load impedances will lower the low end
frequency response. The output is adjustable with an internal potentiometer. It is preset at the factory to
give 1 volt peak to peak output when receiving a +/- 1.5 kHz (+/- 3 kHz for WB) deviated signal. If the
correct CTCSS or DCS tone is present the radio will unsquelch the SPEAKER and AUX_OUT lines.
Programming Port (pin 9)
This line is a bi-directional programming port to be connected to a RITRON programming cable. The
other end of the programming cable connects to the PC’s serial port 25 pin D-sub connector. The
appropriate DR-142 programming software must be run for configuring the transceiver.
Auxiliary Output Monitor Input (pin 11)
Bringing this input to ground will switch the auxiliary and speaker outputs on. Normally the AUX_OUT
and SPEAKER_ OUT are switched on only when a carrier with required tone is detected. This pin can be
used as a tone enabled carrier detect output. It will be pulled to ground if a carrier with the proper tone is
detected. Otherwise, it will be pulled to Vcc via a 1.5 k ohm resistor
Speaker (pin 12)
A speaker with 8 ohms or higher should be connected to pin12. A nominal 1 watt of audio power at 10%
THD is expected. If the correct CTCSS or DCS tone is present the radio will unsquelch the SPEAKER
and AUX_OUT lines. The speaker volume is preset by an internal potentiometer (R350) to yield 2 volts
RMS into 8 ohms. If your application requires a different audio level two options exist. One is to open the
transceiver unit by removing the 4 corner screws on the connector end of the radio case, pull out the
PCB and reset the volume potentiometer (R350). The second is to place a power resistor or
potentiometer in series with the output and speaker to reduce or adjust volume.
Carrier Detect Output (pin 13)
This line will be pulled to 5 volts through a 390 ohm resistor when a carrier is present.
PROGRAMMING THE RADIO USING A PC COMPUTER
RITRON’s programming kit, DTXL-PCPK-2.0, allows programming of the DR-142 model radios using
a PC compatible computer.
The PC Programming Kit Includes:
1) Ritron Transceiver programming software, DTXL-PCPS-2.0, which is contained on 3.5" diskettes.
2) Ritron PC to radio adapter cable, 9/RTC-PAS, which is terminated at one end with a DB-25F
connector, at the other end with a 6 pin modular plug.
3) Another cable, DTXP-PAC, with a modular jack (which mates to the previous cable) on one end with
a DB-15M on the other end to mate to the radio. Power leads are also provided. Red is positive.
The PC Programming Kit Requires: A PC compatible computer with Windows 95 or later. The computer
must have an RS-232 serial port available. A hard disk drive is recommended.
Programming the DTX-142: To program the DR-142 do the following:
1) Connect the PC’s RS-232 port to 9/RTC-PAS and DTXP-PAC programming cable. Connect DTXPPAC to DTX-142
2) Apply 8 to 15 VDC power to pin 6 of the DB15 (Red wire)
3) Insert disk 1 in floppy disk drive. View the disk contents and double click on setup.exe. Follow the
instructions on screen. When finished the software will be resident on the PC’s hard disk.
Using the Programming Software
Upon starting the software a form appears identifying the radio frequency band and channel information.
The channel information will not be correct, however, until the radio is read. If no radio is connected
when the software is started the model displayed will be that of the last radio read by the software and
the default parameters for that radio. To read the radio contents click radio on the menu bar and then
read radio. Allow several seconds for the reading to take place. To change a parameter click a white field
or double click the channel field to access it. Once changes are made to program the unit click radio then
program radio. Again allow several seconds for programming. Files can be saved and read using the File
menu option.
Channel features can be programmed differently on each channel. Receive frequencies and QC or DQC
tones on a channel can be different. A channel can also be moved or deleted. An ID string of up to 72
characters can be entered to allow PC programmers to identify radios.
Computer Software Copyrights
The RITRON, Inc. products described in this manual include copyrighted RITRON, inc. computer
programs. Laws in the United States and other countries grant to RITRON, inc. certain exclusive rights
in its copyrighted computer programs, including the exclusive right to distribute copies of the programs,
make reproductions of the programs, and prepare derivative works based on the programs. Accordingly,
any computer programs contained in RITRON, inc. products may not be copied or reproduced in any
manner without the express written permission of RITRON. The purchase of RITRON, inc. products
does not grant any license or rights under the copyrights or other intellectual property of RITRON, inc.,
except for the non-exclusive, royalty fee license to use that arises in the sale of a product, or as
addressed in a written agreement between RITRON, inc. and the purchaser of RITRON, inc. products.
DR-142 THEORY OF OPERATION
INTRODUCTION
The DR-142 is an eight-channel synthesized receiver unit capable of rapid data reception at rates up to
9600 bps when using 4-level FSK. The unit maintains under 20 mA current consumption at 13.8 VDC
when in the receive standby mode. The current drain goes up as the voltage goes down.
POWER SUPPLY AND VOLTAGE DISTRIBUTION
The DR-142 is powered by an 8 to 15 VDC external power supply. F601 is a 3A fuse in series with
J301 for short circuit protection. Zener diode D601 prevents over voltage damage by blowing the F601
fuse when over 18 volts is applied. If reverse battery voltage is applied F601 will blow by conducting
current through D601.
The DR-142 is designed to consume low current by using a switching DC/DC converter called a buck
converter. Receiver standby current is less than 20 mA at 13.8 volts. The buck converter outputs about
40 mA at 5.4 volts. The conversion efficiency of the buck converter is about 90%. Lowering the supply
voltage will cause more current to be drawn from the supply. The buck converter’s output of 5.4 volts
feeds a 5 volt low drop out regulator U402. A voltage clamp consisting of Q601, 2 and 3 limits audio PA
voltages to about +14 volts. This circuitry is enabled by Q604 only when receiving a carrier or when
AUX_MON is enabled.
REFERENCE OSCILLATOR
Reference oscillator Y301 is a temperature controlled, voltage controlled oscillator (TCVCXO)
operating at 14.4 MHz. The Pin 4 output of the TCVCXO provides a reference for the frequency
synthesizer U401 at Pin 8. The reference oscillator also feed pin 7 of the U301 microcontroller through a
buffer amplifier Q310 and a Schmit inverter U303 to obtain a14.4 MHz clock. The Y301 pin 4 output is
also multiplied (tripled) up to 43.2 MHz by Q104 and its associated circuitry to provide a receiver second
local oscillator signal.
SYNTHESIZER
The DR-142 radio is built around a PLL synthesizer that consists of a receive voltage-controlled
oscillator (VCO). U401 contains both a prescaler and synthesizer controller. The prescaler squares and
divides the VCO output present at pin 5 by either 8 or 9, determined by a synthesizer controller logic
signal. The prescaler changes once every reference cycle at the appropriate time to yield the right
frequency. U401 contains a digital phase detector that works as follows. Microcontroller U301 clocks
new data into U401 internal buffer in synchronization with clock pulses. The channel information is
stored in EE memory of U301 and is loaded into RAM when the channel is selected.
The receive synthesizer works 43.65 MHz higher than the actual RF receiver band The loop filter
C403, C404,C405, R407 and R408 transform the pin 2 output signal to a DC voltage for application to
the VCO tuning varactors CR405 and CR406. The synthesizer system is “locked” when the phase and
frequency of both the reference and the divided VCO signal are the same. When the synthesizer is
locked pin 14 goes high. If the loop becomes unlocked pin 14 goes low.
RECEIVER
Receiver Front End
The signal from the antenna passes through the transmitter low pass filter and then goes to the
tunable bandpass filter L101 and L102. Q101 amplifies the signal about 15 dB before going through
another band pass filter and mixer matching stage. The receiver is turned on by Q107 and Q108
supplying RX_5v when RXEN of U301 goes high.
The amplified received input signal is applied to the base of mixer Q102. The 1st local oscillator
signal from the synthesizer module is buffered and filtered by band pass amplifier Q106 and then applied
to the source of Q102. L195, C114, C115 and C135 tune the drain output of Q102 to 43.65 MHz and
apply it to Y101 and Y103, a 43.65 MHz four-pole crystal filter. Q103 and associated components
amplify the 43.65 MHz IF signal and apply it to the input of the 2nd mixer at Pin 16 of U101.
FM Receiver Subsystem
A multi-function integrated circuit, U101 and associated components form the FM-receiver
subsystem. The subsystem performs the following functions as a 2nd mixer, IF amplifier and FM
detector. The second local oscillator at 43.2 MHz is applied to the 2nd local oscillator input at Pin 1 of
U101. The 43.65 MHz signal at Pin 16 and the 2nd local oscillator are mixed, with the resulting 450 KHz
output signal appearing at Pin 3. This signal is filtered by a 450 kHz 6-pole ceramic filter YF101 and
applied to the input of the limiting IF amplifier at Pin 5. IC101 pin 6 de-couples the IF amplifier. An
internal quadrature detector, whose center frequency is determined by the 450 kHz quadrature resonator
Y102, detects the FM IF signal. One input of the quadrature detector is connected internally to the IF
signal from pin11 while the other input is the phase-shifted signal from Y102 at Pin 10. Demodulated
audio appears at Pin 9, where a low-pass filter R323 and C309 removes spurious second IF output prior
to application to the voice, tone or squelch conditioning audio circuitry.
Two types of squelch circuits exist on the DR-142, an RSSI squelch and a noise squelch. Both
types can be used simultaneously or one or the other can be used. The RSSI (receive signal strength
indicator) squelch, which is typically set around –110 dBm, must be set to open at a higher level than the
noise squelch. This is necessary since the RSSI measures total power in the receiver IF band pass. All
background noise, which at VHF can be high, is seen as signal. The advantage of the RSSI squelch is
that it opens and closes the audio paths very quickly. The noise squelch has the advantage that it can be
set at a much lower level, typically –118 dBm for a 20 dB SINAD. It takes longer to open and close the
squelch. If both are used simultaneously, since they are wired ORed together at U301 pin 32, the squelch
will open quickly and close slowly for strong signals. If only RSSI is desired R145 can be removed.
Voice and Tone Conditioning in Receiver
Three post demodulation paths are provided in the DR-142. U304c provides DC level translation
to bias succeeding op amp stages at about 2.5 volts.
The audio path goes through a fourth order 300 Hz high pass filter U305c and d. C326, R353 and
R351 de-emphasis the audio. The 1 watt audio amp is turned on by Q311 and Q312. Error tones from
come through C325 from pin 13 of U301.
The data path goes through U304a and b. U305b is a second order 3000 Hz low pass filter.
U304a produces phase shift around 3000 Hz to equalize time delay. R337 and C316 provide another low
pass filter pole. U307a is an adjustable gain buffer stage. This stage is squelched by Q314.
Sub-audible signals go through a third order 250 Hz low pass filter U304d, R342, and C316. Pin
27 of U301 decodes the CTCSS or DCS signal. In the case of CTCSS an internal discrete Fourier
transform looks for the wanted tone. Decode bandwidth is about +/-2 Hz.
DR-142 THEORY OF OPERATION
MICROCONTROLLER
The DR-142 receiver is electronically controlled by U301, an 8-bit flash programmable microcontroller.
U301 has A/D inputs and PWM outputs for processing analog signals. Radio characteristics are stored in
internal EE memory. Its RS232 port is used in programming the radio’s personal characteristics such as
frequencies and tones.
PIN
10
11
12
13
14
DESCRIPTION
N/A
N/A
GROUND
+5 VDC VCC supply voltage.
GROUND
+5 VDC VCC supply voltage
XTAL1 is 14.4 MHz reference input from Y301.
Input is normally high when PLL is locked and low when unlocked.
Output goes high to produce high power RF output.
Output drops low momentarily to produce synthesizer latch enable (LE) pulses.
Output goes high to enable receiver (RXEN).
Output goes high to enable transmitter (TXEN).
N/A
SQUELCH output goes high to apply power to audio amp U306 for receiver speaker audio or
radio alert tones.
15
DATA output sends serial data to frequency synthesizer U401 to program frequency information.
Also used for flash programming (MOSI)
16
CLK output sends serial data clock pulses to frequency synthesizer. Also used in flash
programming (MISO).
17
output is set permanently low to allow CS0 to be a channel select input bit.
18
AVCC supplies +5VDC.
19
Input is used to measure receiver RSSI.
20
AREF supplies the reference level for the A/D and is connected to the regulated +5 VDC.
21
AGND supplies A/D ground.
22
Not used
23
Input is pulled low when the CS2 frequency bit input is tied to ground.
24
Input is pulled high when CS0 channel select bit is pulled low.
25
Output is set permanently low to allow CS1 to be a channel select input bit.
26
Input is pulled high when CS1 channel select bit is pulled low.
27
CTCSS IN is an A/D input sampling the CTCSS or DCS waveform.
28
Not used
29
RESET is held low to start the radio in a known state on power up.
30, 31 SERIAL DATA PORT links the microcontroller to communications from an external data terminal
via programming port pin 9 of J301. This allows programming of the DR-142 EE memory used
to store channel frequency and configuration information.
32
CARDET gets pulled low when a RF carrier is detected by the U101.
10
DR-142 ALIGNMENT PROCEDURE
An authorized RF service technician must perform test and alignment of the DR-142. Do not attempt
service of the DR-142 if not completely familiar with frequency synthesized radio operation.
RECOMMENDED TEST EQUIPMENT
1) 0 to +15 VDC, 0.5 A current-limited power supply
3) Oscilloscope
2) RF Communications Test with:
- Frequency Counter
- SINAD Measuring Device
4) VTVM or DMM
5) RITRON PC Programming Kit
RADIO PREPARATION
1) Solder a coaxial cable to the antenna port.
2) Connect the FM communications test set to the antenna connector.
3) Connect RITRON programming cable to radio and PC.
4) Apply 8 to 15 VDC to the red wire with the black to ground.
5) Run RITRON software and read the radio.
6) Program any two channels of your choice for band edge tune up; one to the low side of the operating
band and one to the high side ( Hint: use a bit of an offset from even frequencies to avoid receiver
interference. A good choice is 136.1 and 156.1 MHz for the low split or 154.1 and 174.1 MHz for the high
split.)
7) Type in these RX frequencies in both channels.
8) Program the radio with these parameters.
REFERENCE FREQUENCY
The DR-142 is preset at the factory to be set on frequency. If you need to readjust do the following:
1)
2)
3)
4)
Make sure that the unit is at room temperature (approximately +25° C)
Set the RF communications test set to the transmit frequency counter mode.
Select low edge channel on channel select inputs CS2, CS1 and CS0.
Insert a small insulated antenna loop probe into the access hole of the VCO shield. Make sure not to
short supply.
5) Read the frequency. Frequency read should be 43.65 MHz higher than desired receive frequency.
Error should be less than +/- 100 Hz.
6) Adjust the trimmer cap on the rectangular reference frequency oscillator Y301 if not within specs.
RECEIVER FRONT END TUNNING
The DR-142 receiver is factory tuned for a frequency range of 136 to 156 MHz (low split) or 154 to 174
MHz (high split). If receiver appears to be less sensitive on one of the band edges the front end tuning
coils may need readjustment. Connect a SINAD meter to the SPEAKER output (pin 12). Unsquelch the
audio amp by grounding pin 11. Set the generator to the upper band edge frequency programmed at –
121 dBm with +/- 1.5 kHz deviation for a narrow band and +/- 3.0 kHz for a wide band receiver. Adjust
coil cores L101 and L102 in until SINAD starts to drop. Back off slightly to maintain best SINAD. Now, set
generator to the lowest frequency programmed at –121 dBm. Similar SINAD readings as the high band
edge frequency should be obtained. If not, adjust coils slightly. Recheck high end and balance
performance.
11
RECEIVER SQUELCH
The DR-142 is capable of two squelch operating modes. Also, both can operate simultaneously.
See the theory above. The units can be configured as the user requires. When a low level signal appears
the noise squelch opens when the carrier level with 1.5 kHz (3.0 kHz for wideband) deviation is –118 or
greater. When the signal appearing is –108dBm or greater the RSSI squelch opens the audio with an
even faster attack time. The above levels are factory set levels. Ambient noise will affect the level at
which the RSSI should be set. The squelch level ideally should be set on the desired frequency, in the
actual location the radio will be used and connected to the desired antenna. The level should be set high
enough to reject most false signals.
If a very fast receiver decay time (or squelching after signal disappears) is needed the noise squelch
should be defeated by turning the R144 pot fully counterclockwise or removing Q110.
To adjust squelch levels for simultaneous use do the following:
Set RSSI squelch level first:
1) Temporarily defeat noise squelch pot by turning R 144 to counterclockwise minimum setting.
2) Adjust RSSI pot R123 fully counterclockwise to squelch audio. Be sure AUX_MON line is high to
allow squelching.
3) Set generator to –108dBm on channel desired with 1.5 kHz deviation ( 3.0 kHz for wideband) and 1
kHz tone. Add any CTCSS or DCS as needed.
4) Turn R123 clockwise until audio opens. Go no further if you want only RSSI squelch functionality.
Now set noise squelch:
1) Select a channel.
2) Adjust squelch pot R144 fully counter clockwise.
3) Adjust the communications test set generator on frequency with an output level of –118dBm. The
deviation should be +/- 1.5 kHz (3.0 kHz for wideband) with a 1 kHz audio signal.
4) Slowly adjust squelch pot R144 clockwise until squelch opens.
5) Reduce generator level until radio squelches.
6) Increase generator level until audio opens. Confirm squelch hysteresis is between 0.5 and 5 dB.
12

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