Return-Path: Received: (qmail 17218 invoked from network); 15 Aug 2003 15:54:27 -0000 Content-Transfer-Encoding: 8bit Received: from netmail02.services.quay.plus.net (212.159.14.221) by mailstore with SMTP; 15 Aug 2003 15:54:27 -0000 Received: (qmail 19324 invoked by uid 10001); 15 Aug 2003 15:54:24 -0000 X-Priority: 3 X-MSMail-Priority: Normal Received: from post.thorcom.com (193.82.116.70) by netmail02.services.quay.plus.net with SMTP; 15 Aug 2003 15:54:23 -0000 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1106 X-SQ: A Received: from majordom by post.thorcom.com with local (Exim 4.14) id 19ngro-00061B-Gr for rsgb_lf_group-outgoing@blacksheep.org; Fri, 15 Aug 2003 16:51:52 +0100 Received: from [147.197.200.9] (helo=hestia.herts.ac.uk) by post.thorcom.com with esmtp (Exim 4.14) id 19ngrk-000612-FY for rsgb_lf_group@blacksheep.org; Fri, 15 Aug 2003 16:51:48 +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 19ngrg-000307-00 for rsgb_lf_group@blacksheep.org; Fri, 15 Aug 2003 16:51:44 +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 19ngre-0007ZZ-00 for rsgb_lf_group@blacksheep.org; Fri, 15 Aug 2003 16:51:43 +0100 Message-ID: <5.1.0.14.0.20030815154505.00b0a898@gemini.herts.ac.uk> X-Sender: mj9ar@gemini.herts.ac.uk X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Fri, 15 Aug 2003 16:48:13 +0100 To: rsgb_lf_group@blacksheep.org From: "James Moritz" In-reply-to: <000d01c362a3$216eb2c0$5967fea9@p4o2b7> MIME-Version: 1.0 X-MailScanner: No Virus detected Subject: Re: LF: Variegated witterings. Content-Type: text/plain; charset=us-ascii; format=flowed X-Spam-Status: No, hits=-1.0 required=5.0tests=EMAIL_ATTRIBUTION,IN_REP_TOversion=2.55 X-Spam-Checker-Version: SpamAssassin 2.55 (1.174.2.19-2003-05-19-exp) X-SA-Exim-Scanned: Yes Sender: Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group X-SA-Exim-Rcpt-To: rsgb_lf_group-outgoing@blacksheep.org X-SA-Exim-Scanned: No; SAEximRunCond expanded to false At 21:29 14/08/2003 +0100, you wrote: >Now the bitter bit. Things were now even worse, with it proving impossible >to get anything like the previous output figures or a decent waveform on >136 or even 73 at any over a couple of watts so I hastily put things back >the way they had been and opened the Pommeau. > >So todays puzzler is; > >How come? Anyone got any ideas? I've gone into sulk mode. > >Ian GI8AYZ/MI0AYZ Dear Ian, LF group, The main effect of the mods I did to the Maplin audio amp (very similar to the B&K modules) is to increase the slew rate by increasing the bias current in the input stage - this increases the output voltage swing at high frequencies that the amp can achieve and hence the output power for a given load. It is not easy to see how that would reduce the output. One thing to check is that the DC bias voltages in the circuit should be roughly the same after the mods as they were before. If there is a wrong component, it is most likely that the voltage at one or more nodes in the circuit will be stuck near one supply rail or the other. In what way is the output voltage waveform not "decent"? I found that my amp had to be overdriven to get reasonable output power and efficiency, so the output voltage waveform was not very sinusoidal, although with a high Q resonant load as most LF antennas are, the current waveform will always be a nice sine wave even with really horrible driving waveforms. One possibility that springs to mind is that speeding the amplifier up has resulted in instability problems - this type of circuit, with wide bandwidth (a few MHz), and an overall negative feedback loop, is sensitive to the load impedance over a wide frequency range, not just at the operating frequency. In particular, a load which is capacitive at HF will often cause the amplifier to be unstable at some frequency due to excess phase lag introduced into the feedback loop. A series tuned antenna by itself will be inductive at frequencies above resonance, but if there is a fairly long transmission line between loading coil and TX, the shunt capacitance of the line will cause the overall load impedance to become capacitive above a few 100kHz. If matching is done using a tapped loading coil with one end grounded, this will also introduce additional resonances at higher frequencies. If the amplifier output voltage waveform is OK with a resistive dummy load (or no load at all in the case of the audio amps) but not when the antenna is connected, this is probably why. With my audio amp, I found capacitive loads either resulted in severe "ringing" at the output, or caused a blurred waveform due to the thing taking off at some unrelated frequency. My cure was to add a low-pass filter between the matching transformer at the amp output and the coax feeder - the usual pi-section designs won't help due to the shunt capacitors at their input, so I used a tee configuration instead with a series inductor at the filter input. The low-pass filter effectively isolates the amplifier from the ipedance variations at the load. After this, I had no trouble. I have occasionally also had problems with class D amps and peculiar loads - these were also solved using a low-pass filter at the TX output. Cheers, Jim Moritz 73 de M0BMU