Bruce,
[EMAIL PROTECTED] wrote on 12/09/2008 09:20:08 PM: > Joe > > Joseph M Gwinn wrote: > > Bruce, > > > >> A PCB mount mixer package is also preferable as its then much easier to > >> use a capacitive IF port termination (for lower noise) in conjunction > >> with series resistors at the RF and LO ports (for lower VSWR) than if a > >> mixer with SMA or other coax connectors were used. > >> > > > > I've been using 3 and 8 dB coaxial attenuators at the LO and RF inputs > > respectively, and it makes a big difference. > > > > But I don't understand the part about capacitive loading of the IF port. I > > would think that the low pass filter would need to present a matched > > impedance at the sum frequency, so the emerging high-level 20 MHz signal > > is not reflected back into the mixer. > > > 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? 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 ...". > > MiniCircuits AN-41-001 "FAQ about Phase Detectors" has on page 2 a 500 ohm > > resistor to ground and a 5000 ohm resistor to the first filter capacitor, > > so the capacitor is isolated from the IF port by the resistors. > > > > > I wouldn't take too much notice of that recommendation as I have little > confidence in the author's experience/knowledge. Well, OK, but: Stephen Kurtz says the same thing on the third column of the third page, a bit above Figure 6. Nelson and Walls (paper 971), Figure 4, also shows the low pass filter arranged to absorb the sum signal, not allowing it to be reflected back into the mixer. > >> Supposedly an SRA-1, but some caution is in order as some > statements as > >> to the effect of the input offset of an opamp based IF preamp in the > >> same application note were of dubious veracity unless one > were to use an > >> inverting opamp input stage. > >> > > > > This issue was mentioned in another app note, but their main issue > > appeared to be that the opamp bias currents could cause an offset. > > > > > > > But the circuit they suggest has no effect on bias current induced > offset, the same current flows into the mixer and termination impedance > independent of the series resistance. You're right that the proposed remedy didn't make sense. I don't know that this is a big problem with modern opamps, especially FET input ones (if needed). [snip] > > > > [Soundcards] > >>> > >>>> With identical beat frequency outputs, crosstalk between > >>>> channels within the sound card shouldn't be a great problem. > >>>> > >>>> > >>> I'm not sure I believe this, as there is likely ground coupling within the > >>> soundcard and the ear is famously insensitive to phase. Channel > >>> isolation of 60 dB isn't enough to prevent phase shifts. > >>> > >>> > >>> > >> It will be present but its effect in some cases (when the phase shift > >> between channels is such that the crosstalk phase is at 90 degrees to > >> the signal of interest) will be negligible, in other cases it is easily > >> measured and compensated for. > >> > > > > Isn't 90 degrees (quadrature) the worst case for causing phase shifts? > > > > > Yes, I should have said that when the 2 input signals are in quadrature, > any capacitive crosstalk will have little effect on the phase shift. Ah. Because the capacitor coupling adds a second 90 degree shift, bringing the total to 180 degrees. But crosstalk by ground coupling will be unaffected. As will crosstalk by transformer action. Those boards are pretty crowded. > > To get a one picosecond change at 10 MHz by injection of an attenuated > > quadrature copy of the main signal requires a relative voltage ratio of > > Tan[(10^-12)(10^7)(360)] = Tan[0.0036] = 0.0000628 of the main signal, or > > 20 Log[0.0000628]= -84 dBc. This is well exceeds the interchannel > > isolation of many sound cards. > > > > Cancellation by mathematical means could be possible, but will require a > > dynamic range well exceeding 84 dB. This ought to be easy to arrange. > > > > > The AP192 has a somewhat higher interchannel isolation than that, the > interchannel crosstalk spec is about -120dB. > With a sufficiently large number of samples the its easy to see > artifacts as low as -140dBFS. Yep. Seems like a very good card. > >>>> One concern particularly for low beat frequencies is the phase shift > >>>> in the sound card input coupling capacitors (usually electrolytics). > >>>> > >>>> It should be easy to test the sound card phase shift stability for > >>>> this application by driving both inputs from the same signal source. > >>>> > > > > Or terminate one channel input and drive the other, and measure the > > amplitude and phase of whatever comes out of the terminated channel, > > compared to the driven channel. Then swap channels and repeat. The phase > > and amplitude will depend on frequency, so a sweep will be required, and > > some frequencies may need to be avoided. > > > > > > > >>> > >> Joe > >> > >> I suspect that slow phase changes much less than 1ns or so are hard to > >> distinguish from gain drift given the gain tempco of the ECL > >> phase detector. > >> > >> A beat note near 1kHz appears to be even better if one is using > >> something like an enhanced Costas receiver or even using WKS > >> interpolation to locate and time stamp zero crossings. > >> > > > > But it limits the phase slope gain. I suppose there is an optimum > > somewhere. > > > > > > > >> So far only the M-Audio AP192 has been used. > >> Tests with an embedded motherboard 16 bit sound system show > >> significantly increased noise. > >> I've found that the noise level of motherboard sound systems varies > >> enormously from one motherboard model (sample of 2) to another. > >> > >> Any 24 bit sound card with a performance close to or better than that of > >> the AP192 should suffice. > >> > > > > In general, firewire connected sound cards should be better, because the > > soundcard maker has complete control of what's inside the box. Unlike > > inside a PC. > > > > > 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. > 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. > >> Other cards using AKM 24 bit ADCs should also be suitable. > >> > > > > Who is AKM? > > > > > Asahi Kasei EKM > > http://www.asahi-kasei.co.jp/akm/en/ > > http://www.asahi-kasei.co.jp/akm/en/product/proaudio.html > Thanks. I'll look into their data. > > 20 Log[ 2^24 ] = 144 dB, so something else will be the limit. > > > > > > > Actual ENOB ~ 19 to 20 bits. Makes sense. 20 Log [ 2^19 ] = 114 dB. Still plenty good enough. > >> Ideally an external sound card with balanced XLR inputs would be best. Yes. > >> HP produced a number of different phase comparators each with a > >> different type of phase detector. OK. And the PLL folk must have a million designs. > >> The K34-5991A design can't be older than the early 1970's because the > >> MECLIII devices used weren't available until then. > >> > > > > OK. I recall MECL. RIP. But we have PECL now. > > > > > Same thing, different supplies. > PECL when Vcc is +ve and Vee is GND. > NECL when Vcc is GND and Vee is -ve. > > > >> Warren built a similar phase detector (differential XOR or XOR + XNOR) > >> using CMOS ICs and for a common 10MHz input with a phase difference near > >> zero found short term output noise of of around 10uV or so (10V phase > >> detector FSR) using a passive low pass filter. > >> > > > > (10uV)/(10v)= 1 ppm. (100 nS)(10^-6)= 0.1 pS. > > > > > > > >> In principle an ADC like the LTC2484 could be used with a 2.5V CMOS > >> OR/XNOR phase detector and passive low pass filters. > >> The ratiometric conversion capability will significantly reduce the > >> sensitivity to the XOR gate supply if the XOR gate supply isalso used > >> for the ADC reference voltage. > >> If one used 5V logic a resistive output attenuator would be needed which > >> reduce the gain stability somewhat. > >> > >> All such phase detectors suffer from substantial nonlinearity near the > >> ends of the range due to gate output slew rate limitations. > >> > > > > If one is tracking through multiple phase cycles (as did the HP unit), > > this would matter. > > > > > Can alleviate it 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 _______________________________________________ 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.
