Return-Path: Received: from rly-mc09.mx.aol.com (rly-mc09.mail.aol.com [172.21.164.93]) by air-mc06.mail.aol.com (v126.13) with ESMTP id MAILINMC064-d984b28e18fbf; Wed, 16 Dec 2009 08:33:19 -0500 Received: from post.thorcom.com (post.thorcom.com [193.82.116.20]) by rly-mc09.mx.aol.com (v125.7) with ESMTP id MAILRELAYINMC098-d984b28e18fbf; Wed, 16 Dec 2009 08:33:05 -0500 Received: from majordom by post.thorcom.com with local (Exim 4.14) id 1NKtwL-00044l-RH for rs_out_1@blacksheep.org; Wed, 16 Dec 2009 13:29:17 +0000 Received: from [83.244.159.144] (helo=relay3.thorcom.net) by post.thorcom.com with esmtp (Exim 4.14) id 1NKtwL-00044c-Ar for rsgb_lf_group@blacksheep.org; Wed, 16 Dec 2009 13:29:17 +0000 Received: from smtp828.mail.ird.yahoo.com ([217.146.189.242]) by relay3.thorcom.net with smtp (Exim 4.63) (envelope-from ) id 1NKtyZ-0003ZL-H3 for rsgb_lf_group@blacksheep.org; Wed, 16 Dec 2009 13:31:37 +0000 Received: (qmail 51104 invoked from network); 16 Dec 2009 13:31:30 -0000 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=s1024; d=btopenworld.com; h=Received:X-Yahoo-SMTP:X-YMail-OSG:X-Yahoo-Newman-Property:Message-ID:From:To:References:In-Reply-To:Subject:Date:MIME-Version:Content-Type:Content-Transfer-Encoding:X-Priority:X-MSMail-Priority:X-Mailer:X-MIMEOLE; b=dtgUaZOdKrk9WgniTb6oESWs5H0h7ELEKkJsnmVOq36lc+VQG8mf3xsvWn0Yi32VFU+i3vty7s/A9oTVLPL6KwH+JaMVlzxYhkzCTXUYjTn9wwV0RXJRxdOd4nmbmUfLlwuDvx56MU1jz85U4AqPkX7vdTEv4TpxPyJ+sjwtt1A= ; Received: from unknown (HELO JimPC) (james.moritz@86.179.135.6 with login) by smtp828.mail.ird.yahoo.com with SMTP; 16 Dec 2009 13:31:29 -0000 X-Yahoo-SMTP: qKIhhNCswBB2TTHr2BORWcGGR2mpopxhCcunGIxpCKQYiG07Q7UOhNo- X-YMail-OSG: _xPjHMgVM1nHAmfhTxZNvXbOTWwnlTLGGfFn.XBhL3SvM71NoFPuLspELX.ahUc2vkyRVyDSkkj8V47CSJNwYN5A8ZtRRjfwnRDKG95G4I1wgpDsHQB4iE0fhltydq61jMs4dmbVFE2Lfe.AsvEk_Aj2I6JgQs5WxM4101tVzmVDiCWqrFHlg.M5lme4TTUBZDOL0RNwAtliTq2bgjvm2phV53c7UP4W8aa.T5Dw2134hQ-- X-Yahoo-Newman-Property: ymail-3 Message-ID: <88FB4290CF234F66933047AD790B3719@JimPC> From: "James Moritz" To: References: <38A51B74B884D74083D7950AD0DD85E828AC7C@File-Server-HST.hst.e-technik.tu-darmstadt.de> In-Reply-To: <38A51B74B884D74083D7950AD0DD85E828AC7C@File-Server-HST.hst.e-technik.tu-darmstadt.de> Date: Wed, 16 Dec 2009 13:31:27 -0000 MIME-Version: 1.0 X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Windows Mail 6.0.6002.18005 X-MIMEOLE: Produced By Microsoft MimeOLE V6.0.6002.18005 DomainKey-Status: good (testing) X-Spam-Score: 0.0 (/) X-Spam-Report: autolearn=disabled,none Subject: LF: "Gain" between qrss3 and qrss10 / PA design Content-Type: text/plain; format=flowed; charset="iso-8859-1"; reply-type=original Content-Transfer-Encoding: 7bit X-Spam-Checker-Version: SpamAssassin 2.63 (2004-01-11) on post.thorcom.com X-Spam-Level: X-Spam-Status: No, hits=0.1 required=5.0 tests=MISSING_OUTLOOK_NAME autolearn=no version=2.63 X-SA-Exim-Scanned: Yes Sender: owner-rsgb_lf_group@blacksheep.org Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rs_out_1@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false X-AOL-IP: 193.82.116.20 Dear Stefan, LF Group, DK7FC wrote: ...> Yes, on 137kHz a half bridge is also a good idea since the losses are quite small. But, for a half or full bridge like in these decca transmitters, one needs an output ferrite core that can handle the whole output power. I think these are expensive (?), not easy to get and could be driven into saturation when dimensioned well....> I have used ETD49-size SMPSU cores successfully for transformers at 136k at the 1kW level. These are readily available from big component distributors in this country such as RS Components and Farnell - the set of core halves, bobbin and clips costs about 5 - 10 GB Pounds. I suppose you should add some more for wire and insulation, but this is a lot cheaper than a suitably rated 50Hz transformer! In my experience, temperature rise due to hysteresis and other losses limits the power capability at these relatively high frequencies, core saturation is not the limiting factor. In the overall cost of a QRO TX, the most expensive part is usually the DC power supply. So, if you are trying to achieve a low-cost TX design, and you already have a high power DC supply, it is sensible to design the TX to make optimum use that supply. If you don't already have a PSU, economy is the big advantage of a 330V DC off-line rectified supply. The TX output power is a function of the DC supply V, the load resistance, and the circuit topology used. The DC supply is fixed by the mains voltage if we use a direct rectified supply. If, for further economy, we try to design an output stage with no output transformer, or other impedance matching, then the load impedance is also fixed. So the power output is then fixed for a particular circuit configuration. For a push-pull or full-bridge design we really need an output transformer to drive a single-ended load, but this leaves the Class D half-bridge and Class E configurations, which are single-ended. Using the text-book formulae for Pout of an ideal circuit with 330V DC input and 50R load gives about 1.2kW for an "optimum" Class E, and 440W for the half-bridge Class D. The class E circuit is perhaps simpler, at least on paper, and has attractive output power, but further calculations show that a switching device handling peak voltage 1200V and peak current about 10.5A is required, which would be a problem. We could re-design the class D stage to produce 1.2kW by reducing the load impedance to 18.4ohm, which could be achieved using an LC matching network, if transformers must be avoided. Then the peak current and voltage in the two switching devices would be 330V , 11.4A, which is a relatively easy requirement. So for this kind of power level, and off-line DC supply, the class D design is probably more practical . With current switching transistor technology, the 1200V peak voltage is the problem for any class E off-line design; but in the future it might not be, since there seems to be a lot of interest in developing high speed, high voltage switches based on GaN, SiC semiconductors at the moment. But, in the off-line TX, another important reason to have an output transformer is to isolate the output RF ground and antenna system from the mains supply, partly for safety - but with a full-wave rectifier, the "0V" DC terminal, and so the RF ground of the output stage, is not at ground potential anyway. The same also applies to the gate drive input side. So, if you don't have a suitable DC PSU, an off-line directly rectified supply is attractive, but it will probably need RF transformers! In any case, an output transformer does give a lot more flexibility in the design. Cheers, Jim Moritz 73 de M0BMU