What are Telephone circuit stages ?

[Alwaleed2]

mt8880 hybrid

I'm designing a telephone switch (Exchange) system an I want to know what are Telephone circuit stages ?

 

[xxargs]

I'm designing a telephone switch (Exchange) system an I want to know what are Telephone circuit stages ?

Is not easy to answer becuse little bit compex system...
(understatment)


I answer only for connection to analog standard phone unit.

You need know of design analog system and also know of transmissions teory to make right impedance, make working talking hybrid (also called 'fork' or 'side tone bridge/filters') for yours contrys phone system (different parameter on every contry depend of used cables in contry)

hybrid funktion is important to reducing 'hearing it self on talking' and good impedance matching important for reducing far end echo etc.

In most case on switch-side, using SLIC-circurits for interfacing to phone with embedded hybrid circurits. SLIC stand for 'Subscribe Line Interface Circurits' and makes from couple of manufactory, and one of then are is 'Mitel' (maker of more or less famous DTMF-reciver MT8870 and DTMF-receiver/tranciver MT8880)

I have only old book and thinking hard to design and make 'old' analog system and find circurits for this now, compare to days IP-based switch-phone system or PCM-based system...

but if still searcing document, try MH88500,MH88600, MH88628 etc. for different types of SLIC, and MH89625 is a PCM-interfaced SLIC on Mitel.

But this above is only interface to phone, you also need Power supply (good quality!!) and ringing power (AC 25 Hz or 50 Hz around 100 Volt), couple of analog switches, amplifiers, glue logic and CPU (for handle example ring, tip and analog switches), DTMF-reciver etc. etc. and know how build this in very low noise design.

Analog phone system is _not_ a 'low range' HiFi system in design and you have not margin to sloppy design and/or using cheap componets as LM324 etc. You need build with same eyes att good HiFi-system at least 80 dB spurrios free dynamic!.

And if any hearing noise must be a 'white noise' and not influese of techichal noise as ticking (watch on intrerrupts from CPU and noise from digital bus...) , whistle, ghost sound etc., and be aware from multiple not syncing switching power supply (can give 'ghost sound in swamp' in analog path depend of temperatures, peoples moving in room etc.) or different CPU-crystal in multiboard soloution (even few KHz differences between crystal can on many stage in digital world and in the end give wistle 10 dB over noise floor in analog path and women hear this and complain....)

People working in office switches and using headsets whole days, is very, very sensitive for unwanted colored and/or ticking technical noise, special women (hearing better compare to men) and most case need suppress unwanted non white or pulsed noise least 70 dB lower than normal speak level by design if not want complain or whine from users... ( I have experience of this...)

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In digital switches design, you need also good analog frontend to users with very high dynamic cabability and low noise and interferences. (compare earphone output on laptop to good quality soundcard on PC, listen in headset whole days - is same rule here)


/xxargs

 

[echo47]

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