phone line driver ic

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mikeu
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phone line driver ic

Post by mikeu » Tue Jun 28, 2005 4:53 pm

Hi,<p>Can anyone suggest an line driver ic for the cheap four conductor phone line. I want to send DTMF tones over a distance of at least 1,000 ft.
Could I use an RS-485 chip?

dyarker
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Re: phone line driver ic

Post by dyarker » Wed Jun 29, 2005 2:30 am

The standard level for DTMF from a telephone is -6dBm, and it should work at least 18,000Ft from the central office. A 1,000Ft should be no problem if the impedences are matched.<p>For example, if the unit creating the DTMF tones has an output impedence of 2K Ohms, use an audio transformer with 2K in and 600 Ohm out. At the receiver end if the amplifier input has a 10K Ohm input impedence, the use an audio transformer with 600 Ohm in and 10K Ohm out.<p>Since DTMF is two frequencies of analog audio, I don't see how an RS-485 (serial data) chip will do anything useful.<p>We'll need more info for more specific answers.<p>Cheers,
Dale Y

Gorgon
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Re: phone line driver ic

Post by Gorgon » Wed Jun 29, 2005 3:23 am

Hi Mikeu,
There are several questions in this:<p>Which type of equipment is generating and decoding the DTMF? Is it existing or something you want to build.<p>Do you want to transmit and receive in both directions? If yes, do you transmit/receive on the same pair of cable? and at the same time?<p>Do you use the same cable for speech? Bi-directional?<p>Give us your requirements, and we will confuse you in 'endless discussions'. :D <p>TOK ;)<p>[ June 29, 2005: Message edited by: Gorgon ]</p>
Gorgon the Caretaker - Character in a childrens TV-show from 1968. ;)

mikeu
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Re: phone line driver ic

Post by mikeu » Wed Jun 29, 2005 4:35 am

Hi,<p>I'm sending the tones in one direction only using a single pair. Currently there is a burst of several tones that make up a "command". I'm using a standard DTMF tone decoder chip at the receiving end with a processor to make the decisions.<p>Thanks,
Mike

dyarker
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Re: phone line driver ic

Post by dyarker » Wed Jun 29, 2005 6:09 am

"I'm using a standard DTMF tone decoder chip at the receiving end ..." No such animal, each manufacturer does their own thing as far as pin out. You need the spec sheet for the IC you have.<p>A Clare M-8870 has an on chip amplifier with input range -29dBm to +1dBm. Just DC block and the IC does the rest. On old decoder chips you have to add an external amplifier and filters to split the high group from the low group frequencies.<p>Good luck,
Dale Y

Engineer1138
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Re: phone line driver ic

Post by Engineer1138 » Wed Jun 29, 2005 8:56 am

It's pretty easy: you need to ensure that the signal you put on the line is no more than -6dBm like Dale said. That's about 300mV into a 600ohm load (quick back of envelope calculation: I could be wrong). So, use a resistive divider to drop your signal to that level and feed it into a 600:600 transformer for isolation. You'll find that kind of transformer much more easily than a 2k:600 one<p>Phone transformers are pretty cheap and I think Radio Shack still sells them. I have done it without the transformer, but I don't recommend it.<p>What are you using to decode the signal? I have used Silicon Systems SSI202 in a number of designs since it was pretty easy to find (and cheap) for years. Don't know if it's still in production, tho.

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Externet
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Re: phone line driver ic

Post by Externet » Wed Jun 29, 2005 9:06 am

Hello mikeu.<p>Just amplify the DTMF signal with a plain audio amplifier with balaced output to a level of a few volts p-p (which can be nothing else than a transformer).<p>The wires are just wires, and the signal will reach 1000 feet easily with plenty of level for decoding. If you prefer, the two pairs can be paralleled to a single pair.<p>There is no need for DC injection, no need to dramatic impedance matching, a standard DTMF decoder will manage the recovery.<p>Consider it as your stereo with a 1000 feet long wire to the speakers.
Just keep the noise out by using balanced signal with transformers as Dale says.
Miguel
- Abolish the deciBel ! -

mikeu
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Re: phone line driver ic

Post by mikeu » Thu Jun 30, 2005 4:30 am

Hi Dale and others,<p>I'm using a Holtek HT-9170B for the DTMF decoder. Very inexpensive and a very close replacement to the California Micro CM8870.
Thanks everyone for all the suggestions. Now, anyone have a favorite single supply op amp?<p>Mike

dyarker
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Re: phone line driver ic

Post by dyarker » Thu Jun 30, 2005 6:08 am

What's the op amp for? The HT-9170B has one inside, and detects down to -29dBm. Unless sending unit is very very weak, you only need to balance and match impedence. Since you are not connecting to the telephone system, expensive telephone transformers are not needed, just inexpensive audio transformers are okay.
Dale Y

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