Hi Roger & Treg I do not have a US spec for DTMF levels however if the -6 to -8 level is too low then there is a possible solution (although adding to component count). why not consider using a differential (or bridge) output stage on the transmit side amplifier? These circuits can double the available output voltage swing.
If you cannot get it to work using two op-amps on the 3.3v supply, maybe power the output op-amps from + & - 3.3 v using a MAXIM (or similar) dual supply rail generator chip. You can even control the output stage gain (using FET switches) so that DTMF level is different to Speech & data (if all originate from the same point). Just an idea, but I do appreciate that adding components is not always desirable! Regards: Bill Ellingford ---------- From: Roger Magnuson[SMTP:[email protected]] Sent: 27 September 1999 14:57 To: TREG Subject: DTMF in USA Hello TREGgers, According to FCC Part 68, 68.308(b)(2)(i)(A), the maximum limit for the DTMF signal power level is 0 dBm. We found that a majority of modems (at least those based on Conexant...) use a DTMF level of -4/-6 dBm for the US market. These sending levels are difficult to achieve these days (mainly "thanks to" 3.3V modem devices and high insertion loss of line transformers suitable for 56K). Now, to avoid the painful trial and error method I'd like to know if there is any published specification (Bellcore?) that defines a recommended power level for DTMF signals in the US? In your experience, would equipment with -8/-10 dBm power level operate correctly? Thanks for your help..... Roger Magnuson Manager, Marketing & Business Development TGC AB Dalvagen 28, 169 56 SOLNA, Sweden TEL: +46 856250050 (direct) FAX: +46 856250045 (NOTE our new fax number) mobile: +46 707770594 mailto:[email protected] internet: http://www.tgc.se
