Bruce,
[email protected] wrote on 12/16/2008 08:43:29 PM: > Joe > > Joseph M Gwinn wrote: > > Bruce, > > > > > > [email protected] wrote on 12/15/2008 06:42:59 PM: > > [snip] > >>> > >>> > >> However the proposed remedy has little or no effect on the errors caused > >> by such bias currents (eg transistor base currents). > >> The series resistor could be reduced to zero without effect on the mixer > >> offset due to the bias current. However the preamp offset due to the > >> source resistance would be reduced. > >> > > > > Hmm. It may be simpler than that. With the TDS3012B and 34410A connected > > in parallel across the IF output of a mixer, the bias currents from the > > TDS3012B developed a voltage across the mixer load resistor, and this > > voltage was sensed by the 34410A. All the phase detector hadto do was > > not short the bias current to ground. > > > > > > > AC coupling? At the expense of phase shifts and temperature sensitivity, but yes. And it makes it hard to sense a DC signal, if that's the intent. > >>>> > >>> I think that there are many top-end firewire soundcards. Whatever the > >>> music folk like the sound of would be a good place to start - musicians' > >>> well-trained hearing can be quite good. At least above 20 Hz. > >>> > >>> Actually, the people that make the AP192 do have firewire and usb > >>> offerings: > >>> > >>> <http://www.m-audio.com/index.php?do=products.family&ID=recording> > >>> > >>> > >>> > >> I've looked at all of the M-Audio offerings. > >> The more expensive ones have built in preamps plus 48V phantom supplies, > >> which can be switched off, however the presence of the switched +48V > >> supply is perhaps an invitation to disaster. > >> > > > > Given that capacitance to ground is more benefit than problem in this > > application, I would be tempted to use a pair of back-to-back rectifier > > diodes as a clamp to protect the mixer IF output et al. The 48 volt > > phantom supply will be short-circuit protected, so current will > > automatically limit. > > > > > A pair of coupling capacitors at the preamp output combined with clamp > diodes to the amplifier power supply rails would work well even if the > +48V can't be switched off. > The +48V appears between the balanced pair conductors and ground. > Unfortunately the power available from the phantom supply may not be > sufficient to power the mixer preamp. OK. The power limit does make protection easy. I gather that the limit is a few milliamps, so even a 1N4148 would work. > >> I've also looked at the specs for several other high end sound cards. > >> Quite a few only have single ended inputs. > >> Maybe, I should document the various cards and highlight their > >> shortcomings etc for this application. > >> > > > > That would be very useful. > > > I'll start on this shortly. Thanks. > >>>> The 4V rms input allows the mixer preamp to use devices like the THAT > >>>> 1646 to drive the balanced sound card inputs without degrading the > >>>> noise floor too much. > >>>> > >>>> > >>> Or build an isolation amp with some gain, and kill two birds with one > >>> stone? > >>> > >>> > >>> > >>> > >> A low noise isolation amplifier with a frequency response down to 1Hz or > >> so without using a transformer may be difficult to do. > >> > > > > People do make low noise common-base RF amplifiers, but 1 Hz would yield > > some pretty large bypass capacitors, even if the flicker noise can be > > controlled well enough. I would consider using ultracapacitors, which > > didn't exist until very recently, and of course have very large > > capacitance values. > > > > > A CB stage probably isn't optimum for the mixer preamp so that lower > value caps can be used provided that they effectively short the > amplifier input resistor Johnson noise at the frequencies of interest. But is a CB stage adequate? Elimination of hum pickup is worth a lot. > >>>> With a 1V rms full scale the noise floor degradation would be very > >>>> obvious when using a THAT 1646 (equivalent devices are even noisier). > >>>> It may be better to use a mixer preamp with a transformer coupled output > >>>> stage using hybrid feedback to achieve a low frequency cutoff below > >>>> 1Hz together with low noise. > >>>> > >>>> > >>> With a transformer, even if toroidal, keeping hum out may prove quite > >>> difficult. > >>> > >>> > >>> > >> High end (eg Lundahl LL1517) line output audio transformers come with mu > >> metal screens and metal foil interwinding shields. > >> > > > > They don't pass 1 Hz very well. I bet the rolloff is ~20 Hz. > > > > > > When driven conventionally the transformer cutoff is around 20Hz, > however if one uses the appropriate driver with a controlled negative > output R to cancel the transformer primary internal winding resistance, > the low frequency response can be extended significantly. This also > reduces the low frequency distortion. > However individual adjustment of driver to suit transformer is required > and tracking the winding resistance over temperature may be an issue. That would certainly work. See next. > > Certainly one can build a VLF transformer, but it will be a project for > > sure, and the transformer may be quite large. > > > > > The transformers only weigh about 65g. This weight estimate assumes the negative resistance circuit I assume, the intent being to allow use of the Lundahl LL1517 transformer. You might be interested in the following article: "A Broadband Active Antenna for ELF Magnetic Fields" by John F. Sutton and G. Craig Spaniol" in Physics Essays March 1993, Vol 6, #1, 1993. The negative-impedance trick is expanded upon. Sutton also has some US patents on this. US patent 5,296,866 covers the antenna, but is hard to understand without the article. > It may be simpler just to select a mixer for which the IF ground can be > isolated from the RF and IF grounds. > However a preamp with a transformer output may be useful if one uses a > mixer where all the grounds are connected together by the package. It has to be far easier to select the right mixer than to deal with a 1 Hz transformer. And cheaper. > >>>>> > >>>>> > >>>> No, I meant replace his 90 degree hybrids with a digital equivalent. > >>>> > >>>> > >>> I believe that his 90-degree hybrids are already digital. > >>> > >> I'm not convinced of that, if only because real time 10,000+ tap FIR > >> filters at 30+MSPS are probably still impractical. > >> > > > > I'm not convinced that one needs a 10,000-tap FIR to achieve this, and Sam > > Stein is one smart fellow. I recall some NASA patents from twenty years > > ago on how to get I+Q data from a single ADC, and while there was FIR > > processing of some kind, there were only maybe 8 or 16 taps. And Tayloe > > (US patent 6,230,000) gets much the same effect with one resistor, four > > capacitors, an analog mux, and two differential amplifiers. > > > > > I have read similar papers from that era on radar signal processing. > They either used a Hilbert transform or a pair of digital filters whose > outputs were in phase quadrature. > The quadrature accuracy for a given bandwidth depends on the the number > of taps. > The beat frequencies (in a dual mixer system) won't match exactly and > some correction for the resultant phase shift errors will need > to be made. > This may be less of a problem when the 2 beat frequency signals are > identical in frequency and just differ in phase. So long as we know the exact frequency, even if it isn't the exact desired frequency, all may be well. 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