Path: utzoo!attcan!uunet!mailrus!accuvax.nwu.edu!nucsrl!telecom-request From: mitel!spock!grayt@uunet.uu.net (Tom Gray) Newsgroups: comp.dcom.telecom Subject: Re: Data Access Lines Message-ID: <8312@accuvax.nwu.edu> Date: 26 May 90 15:34:12 GMT Sender: news@accuvax.nwu.edu Reply-To: Tom Gray Organization: Mitel. Kanata (Ontario). Canada. Lines: 42 Approved: Telecom@eecs.nwu.edu X-Submissions-To: telecom@eecs.nwu.edu X-Administrivia-To: telecom-request@eecs.nwu.edu X-Telecom-Digest: Volume 10, Issue 389, Message 2 of 10 In article <8293@accuvax.nwu.edu> John Higdon writes: X-Telecom-Digest: Volume 10, Issue 387, Message 3 of 12 >Chip Rosenthal writes: >> dBm is commonly used to specify a level referenced to a "digital ^^^^^^^ >> milliwatt" signal. This is a 1004Hz sine wave of 1mW power into >> 600ohms. >What was it before digital technology? I've always heard it referred >to as simply the "milliwatt". Also, to be technically pure, dBm can be >a reference to one milliwatt into any impedance, as long as it's a >milliwatt. The 600 ohms comes into play because everyone knows that >that when you measure 0.775 volts on across 600 ohms, you have a >milliwatt. If you measure 0.949 volts across 900 ohms, you still have >a milliwatt. And it is still 0 dBm. The digital milliwatt is defined in the CCITT standards. It is a sequence of eight PCM codes which when repeated in sequence produce a 1KHz tone. The digital miiliwatt is a means of defining the relationship between the analog and digital domains. Note that digital milliwatt or the digital test sequence produces a 1Khz tone when decoded not 1004Hz. 1004Hz digital tones are commonly used since it requires 2000 PCM samples to produce a single cycle of the tone. This produces a more exhaustive test of the decoders of PCM then the eight samples: of the strictly deefined 1Khz DTS. Because of roundoff error in the PCM sequences for the tones the 1Khz DTS will produce a level that is approximately .1db different than the 1004Hz tone. Thus for accurate level allignment of a PCM decoder the strict 1KHz DTS must be used. Thus the 1004Hz sequences are suitable for production or field testing of a PCM circuit but truly accurate allignments must use the DTS. I agree with Mr. Higdon in that dbm refers to one milliwatt in any impedance at any frequency. Indeed, even fibre optic transmitters and receivers are specified in dbm and their operating frequencies are rather higher than 1Khz.