Return-Path: Received: (qmail 8102 invoked from network); 8 May 2002 11:35:58 -0000 Content-Transfer-Encoding: 8bit Received: from unknown (HELO marstons.services.quay.plus.net) (212.159.14.223) by excalibur.plus.net with SMTP; 8 May 2002 11:35:58 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: (qmail 21222 invoked by uid 10001); 8 May 2002 11:39:48 -0000 Received: from unknown (HELO post.thorcom.com) (212.172.148.70) by marstons.services.quay.plus.net with SMTP; 8 May 2002 11:39:48 -0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 Received: from majordom by post.thorcom.com with local (Exim 3.33 #2) id 175R5f-0008WV-00 for rsgb_lf_group-outgoing@blacksheep.org; Wed, 08 May 2002 14:02:43 +0100 Received: from hestia.herts.ac.uk ([147.197.200.9]) by post.thorcom.com with esmtp (Exim 3.33 #2) id 175R5e-0008WQ-00 for rsgb_lf_group@blacksheep.org; Wed, 08 May 2002 14:02:42 +0100 Received: from gemini ([147.197.200.44] helo=gemini.herts.ac.uk) by hestia.herts.ac.uk with esmtp (Exim 3.22 #1) id 175PYj-0001y2-00 for rsgb_lf_group@blacksheep.org; Wed, 08 May 2002 12:24:37 +0100 Received: from [147.197.232.252] (helo=rsch-15.herts.ac.uk) by gemini.herts.ac.uk with esmtp (Exim 3.33 #1) id 175PYh-0001ID-00 for rsgb_lf_group@blacksheep.org; Wed, 08 May 2002 12:24:35 +0100 Message-ID: <5.1.0.14.0.20020508111932.00b04bc8@gemini.herts.ac.uk> X-Sender: mj9ar@gemini.herts.ac.uk X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Wed, 08 May 2002 12:18:04 +0100 To: rsgb_lf_group@blacksheep.org From: "James Moritz" Subject: Re: LF: RYCOM-meter In-reply-to: <173gH1-1aLBQGC@fwd08.sul.t-online.com> References: <5.1.0.14.0.20020502094245.00a23380@POP3.freeler.nl> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: At 18:51 03/05/2002 +0200, you wrote: >My slide rule says: 2 dBm with 600 ohms = 0.98 V Ueff >and RMS (roots mean square) nevertheless Ueff means or? I have a Rycom 3136 level meter, with similar scales. There are 3 input impedance settings - 600, 125, 75 ohms. The dBm scale measures the input power, with 0dBm = 1mW. It always reads correct, whatever impedance is selected - so with the same voltage across the input terminals, changing Rin from 600 to 75 ohms increases the reading by 10log(600/75) = 9dB, because more power is delivered to the low impedance load for the same input voltage. It does this trick by having an input transformer with different taps for the different impedances. However, this also means the voltage scales are only scaled correctly when 600 ohm Rin is selected - it reads about 7dB high on 125 ohms, and 9dB high on 75ohms. In addition, changing the input impedance will usually change the voltage delivered by the source, unless the source impedance is very low, or the "unterminated input" setting is used. The W&G meters I have allow you to select dBm or dBu. dBu is the voltage at the input with 0dBu defined as 0.775V (=1mW into 600 ohms), irrespective of the input impedance. So 0dBuV is -117.8dBu, or in 75ohms -108.8dBm. So I sometimes get totally confused! If in doubt, always use the dBm measurement, and work out from that and the selected input impedance what the voltage is. If you want to measure voltage directly, use the 600 ohm input, and connect a resistor in parallel to get the right input impedance if necessary. Cheers, Jim Moritz 73 de M0BMU