[From nobody Thu Jul 3 13:42:28 2003 Return-Path: <majordom@post.thorcom.com> Received: from post.thorcom.com ([212.172.148.70]) by immta3.bellatlantic.net (InterMail vM.4.01.02.27 201-229-119-110) with ESMTP id <20000712070454.LVOQ493.immta3.bellatlantic.net@post.thorcom.com> for <akestelo@bellatlantic.net>; Wed, 12 Jul 2000 03:04:54 -0400 Received: from majordom by post.thorcom.com with local (Exim 3.02 #1) id 13CGYh-0004X7-00 for rsgb_lf_group-outgoing@blacksheep.org; Wed, 12 Jul 2000 08:03:51 +0100 Received: from mail1.isys.net ([193.96.224.45]) by post.thorcom.com with smtp (Exim 3.02 #1) id 13CGYf-0004Wx-00 for rsgb_lf_group@blacksheep.org; Wed, 12 Jul 2000 08:03:49 +0100 Received: from k [195.64.97.36] by mail1.isys.net with esmtp (Exim 1.73 #1) id 13CGZV-0002lh-00; Wed, 12 Jul 2000 09:04:41 +0200 From: "Klaus von der Heide" <v.d.heide@on-line.de> To: rsgb_lf_group@blacksheep.org Date: Wed, 12 Jul 2000 09:05:19 +0200 MIME-Version: 1.0 Content-type: text/plain; charset=US-ASCII Content-transfer-encoding: 7BIT Subject: Re: LF: What is the best RX for 136 kHz? In-reply-to: <001201bfeae3$92861600$695b868b@zimslaptop> X-mailer: Pegasus Mail for Win32 (v3.12b) Message-Id: <E13CGZV-0002lh-00@mail1.isys.net> Precedence: bulk Reply-To: rsgb_lf_group@blacksheep.org X-Listname: rsgb_lf_group Sender: <majordom@post.thorcom.com> X-Mozilla-Status2: 00000000 From: "Graeme Zimmer" <gzimmer@bigpond.com> To: <rsgb_lf_group@blacksheep.org> Subject: Re: LF: What is the best RX for 136 kHz? Date sent: Tue, 11 Jul 2000 12:54:32 +1000 Send reply to: rsgb_lf_group@blacksheep.org Zim, VK3GJZ wrote: > The worst spurs are caused by the fact that the DDS is unable to change it's > output except at the clock edges. Definitely no! The sampling theorem in a DDS never is violated. Spurs only can be produced by non-linearity. One source is, as Johan wrote: > > In-band spurious signals are generated due to the limited DAC resolution. DDS means Direct Digital Synthesis. The main source of non-linearity is the generation of the sine function itself. Usually a lookup table is used plus a cubic interpolation or a Taylor expansion of third order. I use the latter in my DDS-implementation on the DSP. It is accurate in 23 bits causing no spurs above -135 dB. If the Taylor expansion is omitted the lookup table must be very very large to avoid spurs (note: it's totally digital, there is no ADC). So, if there are spurs I would guess the sine implementation is not as accurate as you want. This error vanishes when the interpolation is made for time points exactly lying on one of the points of the lookup table. That is just your observation: > At frequencies where the output is an exact sub-multiple of the clock, a > very clean signal is obtained. > But at all of the intermediate frequency steps, the DDS is actually > jittering between the two nearest sub-harmonics. I guess, then only the nearest table point is chosen for output instead of a correct interpolation. > The spurs caused by the DAC can be reduced by using more bits in the system, > but the FM spurs can only be reduced by using a (much) higher clock > frequency (there are various other schemes of course). No, the clock must be more than twice the output sine frequency. That's all. > > Another option is to make the receiver almost totally digital. > > The trouble here is that any digital RX will still need a digital VCO (NCO > or DDS) which will always have the above inherent FM spur problem. If the > DDS is in software, it will of course have a much lower clock frequency > (instruction step) than the hardware DDS chips. In a DSP-solution the samples are computed for virtual time steps. There is no relation between these time steps and instruction step or the processor clock. A conversion from virtual time steps (as on an audio CD) to real time is done by the A/D-converter. Therefore only the clock of the ADC determines the accuracy of frequencies in DSP-solutions, not the DSP-clock. As I said above, a DDS-routine for a DSP can be programmed to any precision. That is the reason why I prefer the almost totally digital receiver based on a programmable DSP. > I would love to be proven wrong in this rather pesimistic analysis... I hope I achieved that success. Thanks to Johan and Zim for their comments and to Petr for his root- question! Otherwise this clarification would not have been possible. 73 de Klaus, DJ5HG ]