Bruce,
time-nuts-boun...@febo.com wrote on 12/10/2008 08:38:13 PM: > Joe > Joseph M Gwinn wrote: > > Bruce, > > > > > >> Reflecting the sum frequency back into the mixer is actually necessary > >> to reduce the noise at the IF port. > >> I believe that one of Agilent's simulation application notes mentions > >> this effect but I don't recall the actual application note number. > >> This will affect the mixer RF and IF port impedance so adding a series > >> resistor may be required to improve the SWR. > >> > > > > How big an effect is this? Is the absolute noise decreased, or does it > > remain the same while the signal increase? > > > > > With the same difference frequency IF port termination impedance, noise > is actually decreased along with the mixer conversion loss. OK. Complicated beasts, those mixers. Do you know of a paper (or book) on the subject? > However if the sound card input noise dominates, reducing the mixer > effective output noise won't help. Yes. In the plots you posted in a different email, there was a big rise below 1 KHz (scan stopped at 1 KHz, so don't know the shape). Why is this? > > If I'm understanding Walls and Stein (paper 112) correctly, the advantage > > is because with the capacitor load the beatnote waveform approaches > > square, thus increasing the zero-crossing speed and therefor the phase > > sensitivity. This is no doubt true, but the question was if this also > > caused a small everything-dependent phase shift, something that would not > > have mattered in the measurement of phase noise. The object of paper 112 > > was to remedy a 10 to 20 dB error in phase noise measurements. The > > critical words are in the lower left column of page 337, in the paragraph > > beginning "If the mixer is terminated ...". > > > > > > > Saturating the RF port has a similar effect. Yes. But there are tradeoffs pushing the other way. > If one is time stamping the zero crossings an increased zero-crossing slope is an advantage. > For relative phase measurements a trapezoidal beat frequency waveform may be less useful. Fitting to the approximate waveshape, sine or trapezoidal, should yield a very robust estimate, due to the large data support, and zero-crossing slope won't much matter. Hmm. Actually, if the slopes of the trapezoid are too steep, we may not have all that many slope samples. [snip] > > Of course with a capacitive IF port termination, matching the RF and LO > ports becomes more critical as does the reverse isolation of the various > amplifiers driving the RF and LO ports. > It may be simpler in fact to use a level 17 mixer with high LO to RF and > LO to IF isolation with the RF port unsaturated as it relaxes the > reverse isolation specs for the isolation amplifiers. Another tradeoff. I'll have to think about it. I'm thinking of 6 db and 10 db attenuators on the LO and RF ports respectively, but no isolation amplifier. [snip] > > > > > The only configuration for which it makes any sense is an inverting > input amplifier with a finite input voltage offset. Why would non-inverting not work? Both inputs source or sink bias currents, and non-inverting presents a very high impedance. > > > >> It's hard to find such Firewire systems without such unnecessary frills > >> (for this application) as high gain preamps. > >> > > > > The AP192 has high-level inputs, but I don't know if this bypasses the > > preamps, or attenuates. Given their target market, I'd bet it bypasses. > > > > > There are no preamps other than an external differential input amplifier > that translates the 4 Vrms FS inputs at the input connector to a level > that the ADC can handle. > The ADC chip itself has no preamps built in. > There have been numerous complaint about this by some audio nuts, > however for this application not having such amplifiers is ideal. Bingo! Good to know. > >> The gain tempco and linearity of some variable gain audio preamps is > >> somewhat suspect. > >> > > > > I would think that none of these cards has a good tempco of anything, > > given the lack of necessity in their market. > > > > I would think that linearity would be quite good, given the horsepower > > competitions on linearity. > > > > > Since the 2 ADCs share the same reference their gain tracking tempco > should be quite good given that they use capacitors rather than > resistors within the ADCs. A happy accident, but we'll take it. We are converging on a soundcard wishlist: 1. True balanced inputs on XLR connectors. And good ground design, so we aren't bedeviled by ground loops. 2. 24-bit ADCs, and similar DACs. 3. Very good isolation all around. 4. Digital access via firewire (or USB3 I suppose), with the soundcard in its own box. 5. High-level input direct to the ADCs. While use of AKM ICs may be a very good idea, it is not a requirement per se. [snip] > >> Can alleviate [oddities at end of phase range} to some extent by driving > >> a pair of such phase detectors so that their outputs are in quadrature. > >> One just selects the phase detector output that is in the linear range. > >> The quadrature outputs also allow unambiguous assignment of the sign of > >> any phase change. > >> > > > > The Symmetricom 5120A does something very clever to alleviate this > > problem. Explained in US patent 7,227,346 and "Direct-Digital Phase-Noise > > Measurement"; J. Grove, J. Hein, J. Retta, P. Schweiger, W. Solbrig, and > > S.R. Stein; 2004 IEEE International Ultrasonics, Ferroelectrics, and > > Frequency Control Joint 50th Anniversary Conference, pages 287-291. > > > > Joe > > > > > I've read the patent. The paper is also worthwhile, and available on the web somewhere (don't recall where, but google found the pdf). I had to read the patent multiple times to figure out what's going on. The correlation approach is old as the hills, and only the digital phase detector was patentable. Joe _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
