Midian Electronics MTSD-3, Manual

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SPECIFICATIONS
Operating Voltage 6-24 VDC
Operating Current 6 mA
Input Level 10 mV – 1 V RMS
15 mV – 1.4 V P-P
Input Impedance 100 K
Frequency Range
MTSD-3A
280 – 1600 Hz
MTSD-3B 500 – 2800 Hz
Ringing Output 2400 Hz
Momentary Output 200 mA
Latched Output 200 mA
PTT Transpond Current 100 mA
Min. Tone Time 150 – 2500 ms
Inter Tone Time 0 – 250 ms
Operating Temperature -30°C - +60°C
Dimensions .94”W x .32”H x 1.9”L
GENERAL INFORMATION
The MTSD-3 is a 2-Tone decoder that is available in 2 different frequency ranges. Two units can be combined
and have the outputs “anded” together to do two plus two type 2-Tone decoding (see Technical Notes section for
details).
CIRCUIT DESCRIPTION
The signal is fed into the MTSD-3 via the orange lead and is amplified by U1A. The value of “R Limit” may be
adjusted by the customer to adapt the audio signal available into the customer’s radio. This range, from 10 mV to
1 V RMS, is shown on the input level chart on the schematic.
Frequencies A and B are selected by the circuitry of U1B, U1C and U1D. The selected frequency from U1C Pin
14 band-pass output is fed to U2A, where it is detected, and the DC is then passed on to the Schmidt trigger U2B.
The output of the Schmidt comparator (pin 14) is fed to the timing circuitry of U3A through U3E. U3F is the 2400
Hz alert tone oscillator and is held off by a low from U3E until decoding. Upon decoding, the output of U3E is also
sent to U2C, and the resulting high output of U2C produces drive to transistor Q4. The output of Q4 is normally
high, and the Call (white) lead and the Positive Squelch (green/white) lead outputs will reflect this high until
decoding occurs. Upon decoding, the Positive Squelch floats, and the call output goes low. Inverter Q5 is driven
by Q4, and it’s output, Negative Squelch (yellow) lead, goes from a low to a float upon decoding.
Two monitor inputs that will open the radio squelch and turn on the call light are also available. Monitor 2, upon
going high, will activate the squelch outputs (outputs will not reset when the high is removed). When Monitor 1 is
used, a low input is normally placed on the input, and when it is removed (floats), the squelch outputs will be
activated. Returning Monitor 1 to a low will reset the squelch outputs. Note: Monitor 1 must be held low via JU2
when Monitor 1 is not used.
The Transpond Low (PTT) output is activated for approximately 1 second, when decode occurs, via the high
output of U3E, U2D and Q6.
U3E goes high for 3 seconds after decode and drives Q3’s output (gray) low assuming it is pulled up to 12 V via a
horn relay.
The MTSD-3 employs a 5V regulator to assure consistent operation over the wide range of input voltages.
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