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Interface and Commands for the LM 10D and LM-16D Monitor Receivers The receiver requires an audio input for DTMF tones and provides tones and received audio output and a transmitter keying line. The receiver requires +5.5 to + 26V DC power at 25-35 mA. Outside case size is 4.7 x 3.7 x 2.2 inches. In normal operation, the LM-10D continuously compares the energy at 121.5 MHz with the energy at 121.46 MHz in the emergency frequency guard band. If a signal, modulated or carrier only, exists continuously on 121.5 MHz for the set delay time, usually 10 minutes, the transmitter is keyed and an alert signal, a CWID and a 6 second sample of the received audio is transmitted. The comparison integrator is then reset and a new comparison period is started. This process continues until the signal goes away or an operator issues a mute or an off command. The alert tone can contain one or more sequential tone or 5-6 tone paging sequences. In the LM-16D, two independent integrations, one for 121.5 and 121.46 and one for 243.0 and 243.2 MHz are simultaneously maintained. Either one can trip the alarm but only the audio from 121.5 MHz is transmitted in the alarm sequence. This frequency comparison or radiometer squelch permits reliable detection of signals of less than 0.07 u V at a quiet site while remaining insensitive to broadband noise like ignition noise and precipitation static. The LM-10D and LM-16D do not provide a DF function. They are designed to be used with an omnidirectional antenna which is not supplied. A broadband discone type antenna is good for the LM-16D. For best performance, the antenna should be located well away from transmitting antennas and should have a low loss feedline. The receiver could be tower mounted in a weatherproof enclosure to minimize feedline losses. The receiver case is not suitable for direct outdoor mounting as supplied. Commands. The LM-10D and LM-16D receivers can be interrogated at any time using DTMF tones. The monitor commands cause a reply consisting of the CWID and a 15 second sample of the received audio. During the first part of the audio sample, the receiver switches back and forth between the reference frequency and the requested frequency to assist detection of carrier only signals. Then the switching is turned off to give a clear sample of modulation. Switching does not occur if a reference frequency (command "1214" or "2432") is selected. On LM-10D receivers, the UHF commands will produce a reply and receiver noise but will not receive a signal. The unused crystal selection outputs, high during audio sample, tristate otherwise, could be used for other functions. Load should be limited to 4 mA. The unit will reject DTMF commands that are sent with less than 120 mS tone duration or more than 2 seconds between tones and will reset automatically if partial sequences or wrong sequences are received. Wait 3-4 seconds between commands if no reply is heard. If the alarm is muted, hold the first digit of a command at least 1/2 second if another request was made less than 2 minutes previously. Command List: "1210" Mutes the alarm. Reply is a single beep rather than CWID and audio. The alarm will stay muted as long as a signal on 121.5 MHz ( or 243 MHz for LM16D) is continuously present. If the signal goes away for 10 minutes or more and then comes back on, the alarm will again trip. "1211" Turns off the alarm circuitry. Reply is a single beep like "1210". Unlike mute, the alarm remains off unless it is manually turned on again with a "1214" control sequence. "1214" Removes the alarm mute, resets the off command and monitors the 121.46 reference frequency. Crystal position 1. This command should be sent after power is initially turned on. "1215" Monitors 121.5 MHz. Crystal position 2. "1216" Monitors 121.6 MHz. Crystal position 3. "1217" Monitors 121.775 MHz. Crystal position 4. For LM-16D only: "2430" Monitors 243.0 MHz. Crystal position 5. "2431" Monitors frequency in crystal position 7. "2432" Monitors 243.2 MHz UHF reference frequency. Crystal position 6. "2433" Monitors frequency in crystal position 8. Connections and adjustments. Audio input. Pin 5. This should be "speaker audio", that is, with deemphasis so that low and high tones are about equal amplitude. (no twist) Nominal level should be 0.5V peak to peak. An input level above 1.5 volt or below 0.1V peak to peak will cause the tone decoder to malfunction. Input impedance of the decoder is greater than 10K. Audio output. Pin 2. Up to 4 V peak to peak into 8 ohms or higher. No termination resistor required. Higher load impedance will reduce power used. Level is adjustable by a pot on the lower PC board to the left of the 9 pin connector. If millivolt microphone levels are needed, put an attenuator right at the modulator inside the transmitter to minimize noise pickup. Key output. Pin 7. FET open drain pulled to ground to key transmitter. Max. current 200 mA. Circuit has an RC timer to limit key down time to a maximum of 60-80 seconds independent of the control CPU. Threshold metering. Pin 3. (DFM) Test point to set receiver noise threshold. Requires a voltmeter connected between this point and ground. Voltage range is 0.5 to 4.3V. The threshold is at 2.5V. If this voltage is above 2.5V more than half the time, the alarm will eventually trip. Setting the threshold close to 2.5V with no signal will give best sensitivity but will produce some false alarms. We recommend a setting of 1.5 to 1.75V on site with the antenna connected. 2V is the maximum that should ever be considered. CW trip sensitivity at 2V is about-138 dBm while 1.5V gives about -134 dBm. Adjust using the OFFSET control on the receiver (top) board. Turning clockwise increases the meter reading. There is a significant lag in meter response and the voltage normally moves in steps and wanders about the set level because of the internal digital filtering of receiver noise. Have patience. Integrator metering. Pin 4. (STRM) Test point for the alarm integrator. Same range and connections as for threshold above. Should remain below 1V at 10 minute delay on site with proper threshold setting. Moves up scale if signal exceeds the reference, down scale if it does not. The alarm trips at about 4V. Either an alarm transmission or a manual interrogation will reset the integrator to about 0.5V. The integrator action is easiest to see during testing if the delay is adjusted to 30 sec (DELAY pot on the receiver board fully CCW) for testing. Don’t forget to reset it to 10 minutes (fully CW) for normal operation. The meter switches between VHF and UHF integrators about every 2 sec so the needle will swing back and forth if VHF and UHF have different signal levels in LM-16D receivers or if the signal comes on suddenly in LM-10D receivers. The first integrator to reach 4V trips the alarm. Both integrators must return to a level under 1 volt to reset the alarm. During the audio sample portion of an interrogation, the integrator meter indicates signal level for weak signals. 1.5V is about -132 dBm, 2.5V is about -124 dBm and 4V is about -112 dBm or higher. +12V Power. Pin 9. Ground. Pin 1. +12V is just a nominal voltage. DC power can be anywhere between +5.2 and +26V for normal operation. The regulator will shut down for overvoltage of up to 80V and reverse polarity of up to -40V continuous. If the voltage falls below +5V but not to zero, input current may rise as high as 200 mA. This is normal. Notes. Threshold should be checked one last time at the site with the receiver cover on tight. Sometimes a spur from the CPU will cause a small offset with the cover off. The circuit diagrams of the receiver board and the control board are included with the receiver. The DTMF decoder and control CPU are on a second board, mounted below the receiver board and held in by the connector. There are 5 selectable alarm delay steps: 30 seconds, fully counterclockwise or 7 o’clock; 1 minute at 10 o’clock; 2.5 minutes at 12 o’clock, 5 minutes at 2 o’clock and 10 minutes at 5 o’clock or full CW. The alarm cycle may be tested by momentarily grounding the ALARM point, the top of a resistor, as shown in the receiver board layout enclosed. The status of mute can be seen by connecting a voltmeter or scope to the MUTE test point shown on the same diagram. When muted, this point will pulse high and low at about a 2 sec rate. With no mute, it remains low. To remove the boards from the case, first unsolder the wire from the center of the RF jack. Remove the RF jack using an open end wrench to hold the nut. Remove the two side screws and lift the board up gently. Remove the two or three connectors from the bottom of the board. Putting a thin, flat screwdriver blade into the parting line of the connectors and prying will remove them in a controlled fashion. Please don’t just pull on the wires! Remove the two hex posts from the 9 pin connector. Mounting holes, as required, may now be put in the empty case.
Interface for the LM-10E and LM-16E Monitor Receivers. The LM-10E and LM-16E series monitor receivers serve the same purpose as the "D" versions above but have no controller built in. A common application uses power and auxiliary control signals from a radio repeater. The receiver provides an alarm indicator and audio outputs. The receiving frequency can be remotely selected. The repeater controller should provide an OFF or MUTE command, audio switching for the received signal, timing for interrogation and timing, ID and paging signals as needed for the alarm. The receiver uses unregulated DC power of 5.5 to 26V at about 30 mA max with audio output open. It provides continuous received audio output and an open collector alarm output. The audio output is capable of driving 4V p-p into a load of 8 ohms or more. It does not require a termination and the receiver will use less power if a high impedance load is used. An external antenna should be used. A ground plane works fine for 121.5 but a discone or other wideband antenna will give better results on 243 with an LM16E. The case size is 4.7 x 3.7 x 1.3 inches. The alarm output goes low if a signal above threshold is present on 121.5 MHz (or 243 MHz on the LM16E) for a time that is adjustable from 30 sec to 10 minutes. The alarm resets automatically 30 sec to 10 minutes after the signal goes away. The alarm is also reset when any single frequency is selected. Delay and other adjustments are the same as for the "D" models above. Connections are made through the attached cable as follows: Black Ground (power and audio) Brown Audio output. Use a resistive pad right at the transmitter input to reduce the level to minimize noise pickup if a microphone level input line is used. The output level is not adjustable. Red S meter output. See filter circuit on the bottom of the unit schematic. 0.5 to 4.3V output. Voltage rises with increasing strength. Range is about 0.05 to 1 uV. In alarm mode, shows integrator voltage. Alarm trips at about 4V. Needle swinging is from two independent integrations and is normal. Orange Threshold meter output. 0.5 to 4.3V range. Same filter as for S meter. Threshold is at 2.5V. If this voltage is above 2.5V more than half the time, the alarm will eventually trip. Set using the OFFSET pot. Setting the threshold close to 2.5V with no signal gives best sensitivity but will produce false alarms. We recommend a setting of 1.5 to 1.75V on site with the antenna connected. The reading has a long lag time. Be patient. Yellow Alarm output. Open collector with 22 ohm current limit resistor. 30 V max and 50 mA max. Output is low when alarm is tripped. Green +5V regulated source for metering networks. 10 mA maximum. Blue +5.5 to +26V unregulated power input. White Frequency and mode control line. The voltage on this line controls the receiver as listed below. The required voltages can be generated from open collector control outputs as shown on the schematic. One output is connected directly and four are connected through the resistors listed. The resistors are included. Any alternate scheme to generate the required voltages would also work. +5V or open circuit: Alarm mode. The receive frequency is switched for thresholding. The switching also makes carrier only signals audible. +4V or 39K to ground. 121.775 MHz receive or other frequency in position Y3. +3V or 15K to ground. 121.46 MHz reference frequency (243.2 reference in LM16E) +2V or 6.8K to ground. 121.46 MHz reference frequency.+1V or 2.4K to ground. 121.5 MHz (243 MHz in LM16E) 0V or ground. 121.5 MHz receive.
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