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  Spectra TAC Receiver
Timing Notes

By Robert W. Meister WA1MIK
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There are some subtle things going on in the Spectra TAC receiver that are important for proper operation of the Spectra TAC comparator.


In the receiver, a 2175 Hz idle tone is generated whenever there is no signal present. The line card in the comparator detects this tone and uses it as a reference level to set the Automatic Gain Control (AGC) circuit to compensate for changes in gain over the phone line or RF link. When the receiver hears an input signal and un-squelches, the 2175 Hz idle tone is shut off and voice audio is passed across the line. The comparator detects this loss of tone and holds the AGC gain constant until the tone is detected again. In this way, the AGC circuit will not attempt to adjust the gain based on the voice audio coming down the line.

There MUST be a period of silence after the 2175 Hz idle tone shuts off, or before it turns on, when no other audio or tone is present at the input of the comparator's line card. This is necessary for the 2175 Hz tone detector and AGC circuit to function properly. Without these silent periods, things just don't work right. This is one reason why Motorola receivers and comparators sometimes don't play well with equipment from other manufacturers.

Based on some information I recently acquired, I investigated the timings involved in the Spectra TAC idle tone protocol. My findings are described below.

In the scope traces below, the top (red) trace is monitoring pin 11 (Un-squelched Indicator) at the 2175 Hz Encoder card. This 0-4.4V signal is also the trigger for the sweep, which occurs two boxes from the left side. The bottom (blue) trace is viewing the line output of the Spectra TAC receiver chassis. The horizontal time base is 10 msec per division. The narrow-band receiver is hearing a full-quieting signal with a 400 Hz tone modulated at 1 kHz of deviation.

No Signal Condition:

With no signal coming into the receiver, the squelch circuit on the Audio Control Module mutes the receiver audio (squelch noise) and allows the 2175 Hz idle tone to be present at the receiver's line output terminals.


When the carrier is detected and the squelch circuit opens, the red trace goes from 0V to 4.4V and triggers the sweep on the positive-going edge. You can see that the 2175 Hz tone in the blue trace goes away at exactly the same time the signal is received. The 400 Hz tone being fed into the receiver comes up slowly starting around 25 msec later, and is up to full level at about 40 msec after the carrier is detected. Note that there is a definite dead time of about 25 msec when no audio of any kind is present on the line terminals. The scope trace below shows this action:



When the carrier goes away, the squelch circuit takes a few milliseconds to close the squelch. When it does, the red trace goes from 4.4V to 0V and triggers the sweep on the negative-going edge. There is a brief noise burst of a few milliseconds between the time the input carrier goes away and the squelch circuit closes the squelch. About 20 msec later the 2175 idle tone slowly appears on the line and is up to full level by 25 msec. The scope trace below shows this action:


Delay Sources:

There are two delays present in the SpectraTac receiver; both serve to mute the received audio as well as the 2175 Hz idle tone when the received signal comes and goes.

When the receiver detects a signal and un-squelches, the Un-squelched Indicator signal immediately mutes the 2175 Hz tone in the Encoder card. The circuit that mutes the receiver audio is on the Audio Control Module. A partial schematic can be found here as a small PDF file. Q5 (AUDIO AMP) must now allow audio to pass. Q3 (INVERTER), C37, and Q4 (SHUNT SWITCH) on this module delay the un-muting of the audio at Q5, which is accomplished by shunting or shorting the audio signal to ground. When the squelch closes, the audio is muted immediately (or within a few milliseconds) by Q3, Q4, CR5, and Q5 with no delay. Audio is muted immediately whenever the receiver is squelched but it delays un-muting when the receiver un-squelches.

When the receiver loses its input signal and squelches up, the receiver audio in the Audio Control Module is muted immediately and the Un-squelched Indicator is sent to the 2175 Hz Encoder card. The full schematic can be found here as a small PDF file.There, Q1 (AND GATE), Q2 (INVERTER), C2, and Q5 (TONE SHUNT SWITCH) delay the un-muting of the idle tone, which is accomplished by shunting or shorting the tone signal to ground. This module immediately mutes the idle tone through CR3 when the receiver is un-squelched but delays the tone's return when the receiver squelches up.

Acknowledgements and Credits:

Information was obtained from the Instruction Manual for the Spectra TAC receiver 6881039E45 and the Reference Manual for the Spectra TAC comparator 6881039E50.

Spectra TAC is a registered trademark of Motorola, Inc.

Contact Information:

The author can be contacted at: (his-callsign) [ at ] comcast [ dot ] net.

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This page created on Saturday 05-Jul-2014.

Scope traces, article text, and hand-coded HTML © Copyright 2014 by Robert W. Meister WA1MIK.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.