); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Bruce Griffiths wrote: > [EMAIL PROTECTED] wrote: > >>>> One trick I use to get better tracking is to build ratios out of multiples >>>> of a single resistor value. >>>> >>>> >> >> >> >>> Can you guarantee this works every time? >>> >>> >> Define "guarantee". For a reasonable risk ratio, yes. Resistor >> manufacturers don't go out of their way to package resistors from >> different batches on the same reel, so you if you use adjacent parts from >> the reel, the odds are overwhelming that you'll get a uniform material >> composition, which leads to a consistent tempco. >> >> That's certainly good enough for me to build a product out of >> the parts before doing the temperature testing. (It's not >> like parts always meet guaranteed specs, either. Remember the >> Signetics 25120 Sale price vs. AQL curve.) >> >> If you want it pre-tested, get a resistor network. Either custom, or >> again get multiple copies of the same resistance and connect them in >> the ratios you want. >> >> For more on the subjet of resistor tempco tracking, including all >> of the above suggestions echoed by a moderately experienced >> engineer, see "Debugging Analog Circuits" by Robert A. Pease. >> See chapter 2, page 28. >> >> >> > Even better use a device with built in trimmed resistors. > >>> LT1085 regulators are probably too noisy even when configured correctly. >>> The output voltage tempco is probably a little high also. >>> OCXO requires a supply voltage with better than 0.01% stability for time >>> intervals less than the discipling loop response time. >>> >>> >> Huh? Where do you get that from? Dont they internally subregulate? >> The raw power is used only for the oven... oh. Yes, and power is V^2/R, >> so epsilon change in voltage causes 2*epsilon change in oven power. >> >> >> > Try actually reading the OCXO datasheet rather than speculating. > The simple zener regulator built into the OCXO has finite rejection of > input supply fluctuations. > The output buffer stage is supplied directly from the voltage at the > OCXO pins, it has no internal voltage regulator. > >> Which boils down to a statement that the OCXO should internally >> subregulate. (With possibly a hack to bypass the regulator during >> warm-up.) Does anyone know how this works? >> >> >> > Stop speculating and look at the actual circuit schematics for the > 10811A and the 10544A. > neither the oven controller nor the oven heater are powered from an > internal regulator. > >>>> - Wouldn't it make more sense to configure U1A and U1B as simple >>>> non-inverting amplifiers, that will present a high-impedance load to >>>> EFC_input, thereby avoiding the need for all the J8-J10 and J21-J23 >>>> jumpers? You need a 3.3V reference for the U1A circuit rather than >>>> 1.5V, but that's easy enough to make out of a TL431. >>>> >>>> >> >> >> >>> Not really practical if one needs to compensate for small voltage >>> differences between the Fury EFC gnd and the OCXO EFC gnd, >>> However with a suitable star ground and independent supplies for the >>> Fury and the interface board this should not be an issue. >>> >>> >> Ah, I didn't realize that was an issue. That makes life trickier. >> >> >> > Theres also the voltage drop across the OCXO socket contacts with the > 10811A and the 10544A. > This will be modulated by the oven current variations. > >>> There is little point in using a low drift opamp with a TL431 reference. >>> >>> >> Good point; I just mentioned the TL431 because it was already there. >> FWIW, the LT1019 is excellent. >> >> >> > If you don't mind its noise. > >> And once you have one precision reference on board, it and a few pass >> transistors can give you a precision reference for your supplies. >> >> >> >> >>> Personally, I like the idea of feeding the EFC to a high impedance >>> amplifier. However, I am afraid that some would object to having the >>> EFC go through an op-amp. It will add noise and distortion. >>> >>> >> It's going through an op-amp on the Fury board anyway. If we have any >> specs on that, we can try to match it rather than overdesigning. >> >> >> > Yes and distortion is virtually irrelevant as the EFC frequency versus > voltage characteristic is unlikely to be that linear. > >>> I was thinking about a instrumentation amplifier, like the Burr >>> Brown INA103. It will provide complete isolation for the EFC signal. >>> >>> >> There's no need for a high-impedance input for the reference voltage or >> input ground, only the input EFC, but it would certainly work. >> >> However, the INA103 has, again, a rather high offset tempco >> (1 + 20/G uV/C) at low gains. This is because in-amps are generally >> designed for use at high gains, and so the second stage isn't optimized >> for low noise. >> >> I like the INA337, but it doesn't have the power supply range. >> The data sheet mentions some other parts... INA114? >> >> >> > The INA114 has much lower bias currents than the INA103 but again has > high noise and drift at low gains. > >> I'm a little worried about the gain resistor tempco for single-resistor >> solutions. (Which is why I'm fond of the INA337.) But how much of the >> EFC range is actually used during normal operation? Does it have to >> be 1e-13 stable at the extremes? >> >> >> > Typically a good 10811A will achieve an AVAR of a few parts in 1E12 for > tau> 1sec when being disciplined. > Its just necessary that frequency fluctuations due to power supply > fluctuations and other causes should be somewhat less than this for tau > < disciplining loop response time. > The disciplining loop cannot correct OCXO frequency fluctuations that > occur at rates beyond the loop bandwidth. > >>> As far as (J8-J10) only one would be connected at a time. I would not >>> be feeding all 3 at the same time. >>> >>> >> I was just thinking, if they were high impedance, there would be >> no need to ever disconnect them. >> >> (Another question is whether it's reasonable to use the same circuit >> for the 0-10V and +/-10V options, and just install different components.) >> >> >> >>> It has been made clear to me that the +24V(oven supply), +12V(oven >>> supply) and +/-12V(EFC interface) supplies must be isolated from each >>> other. >>> >>> The problem is that the oven is very noisy and will corrupt the ground >>> for everyone else. >>> >>> This needs some thinking. >>> >>> >> H'm... and if you really want the full +/-10V range, fitting a regulator >> and op-amp into the 2V of available headroom requires an LDO and a >> rail-to-rail output op-amp. >> >> >> > Most rail to rail output amplifiers tend to have poor PSRR when the > output approaches the rails. > However with an EFC voltage swing of 20V the absolute noise and > stability requirements may be relaxed somewhat but it depends on the OCXO. > Having specifications for candidate OCXOs having -10V to +10V EFC swing > would help in making a rational selection of components possible. > >>> The brute force approach is to get 3 separate supplies. However, I >>> don't think I need to go that far. >>> >>> Any ideas here are most welcome. >>> >>> >> Do you have any idea what the external power supply will be? >> It could be taken into account. >> >> It's noise coming OUT of the oven supply that matters, so subregulating >> it doesn't help; muting the current spikes requires energy storage >> (L and C). Outside the pi filters, take the EFC supply, drop it >> through an LDO, and watch the PSRR specs on the EFC components. >> >> >> > LDO's tend to be noisy and have poor high frequency line rejection. > >> (Remember, the OCXO is not *that* sensitive to its own oven hash, >> or it wouldn't work in the real world.) >> >> I often use the little things called "wound beads" or wideband >> chokes", e.g. http://www.ctparts.com/widebandchokes.asp, for generic >> supply filtering, but you may want a little more L in your pi filter. >> >> >> > A much higher inductance than can be achieved with a ferrite wideband > choke is required as the 10544A oven switching frequency is relatively > low being determined by a UJT oscillator. > The appropriate filters and supply regulators for the 10544A are > detailed on the datasheets which are available on Didier's site. > The recommended circuit has much lower noise than any 3 terminal LDO > regulator. > > Bruce > >
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