); SAEximRunCond expanded to false Errors-To: [EMAIL PROTECTED] RETRY Bruce Griffiths wrote: > [EMAIL PROTECTED] wrote: > >> >> In a message dated 11/2/2007 17:51:33 Pacific Daylight Time, >> [EMAIL PROTECTED] writes: >> >> >> >>>> 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. >>>> >>>> >> >> Hi guys, >> >> I've been following the discussion about increasing the Fury EFC range from >> 0-5V to -10V to +10V with great interest! >> >> I just had an idea on how to avoid all the issues potentially introduced by >> using an Opamp circuit. >> >> Let's take a step back and see how much EFC voltage deviation is really >> required: >> >> 1) let's assume we use an HP 10811, so temperature stability is very good >> and certainly requires less than +/-2.5V range to compensate for (on the >> MTI >> double oven units we typically see less than 100uV deviation on the EFC due >> to >> temperature!). >> >> 2) Now let's assume an aging of 5E-08 per year - certainly good OCXO's will >> be better than this. 5E-08 per year at 10MHz is about 0.00137Hz aging per >> day. >> >> 3) For 10811's I have measured a range of 4Hz for a 5V EFC change, so let's >> assume it's EFC gain is 0.8Hz/Volt. This into 0.00137Hz/day means a voltage >> change of 0.00171V per day. >> >> This means a -2.5V to +2.5V EFC range would be enough to compensate for >> about 8 years of aging on our well-aged theoretical OCXO, so going to +/-10V >> is >> probably much more than needed. >> >> 4) So why don't we just run the OCXO ground at +2.5V instead of 0V, and run >> the Fury ground at 0V? >> >> This means the Fury's EFC output (0V to 5V) looks like a -2.5V to +2.5V >> range to the OCXO due to the OCXO's ground being offset by 2.5V. >> >> The 10MHz output of the OCXO can be easily transformer-coupled into Fury as >> someone has said earlier, so no problem here. >> >> Offsetting the OCXO ground by 2.5V should be possible by adding a -2.5V low >> noise regulator to the system. EFC current is very low, so a low noise >> negative voltage reference may be used to generate the -2.5V. >> >> No need for opamps, complex bipolar voltage regulators, etc. Of course any >> noise or drift in the -2.5V regulator would show up in the EFC voltage as an >> >> error. >> >> What do you think? >> >> bye, >> Said >> >> >> >> > Why restrict operation to just the -2.5V to +2.5V portion of the > 10811A/10544A EFC range, isnt the 0 to +5V portion just as useful? > This requires no additional opamps or references or even a negative supply. > The only drawback is that one has to manually adjust the oscillator > somewhat more often than if the full EFC range were used. > > Another option is use a simple opamp reference inverter circuit using no > precision resistors to subtract 2.5V from the Fury RFC output. > The reference inverter can include a lowpass filter to reduce the > reference noise. > > The simpler we can make the interface with acceptable performance the > better. > Limiting the number of power supplies required is also useful, > particularly if one wishes to be able to run the system from batteries > in the event of the loss of a mains supply. > > Of course if one has an OCXO with an EFC range of 0 to -10V, or 0 to -5V > etc then using an opamp or 2 is unavoidable. > > If the interface is limited to OCXOs that have an EFC range that > includes 0 to +5V then additional (negative) supplies and opamps can be > avoided. > > Maybe we need to compile a list of suitable OCXOs complete with datasheets. > The list includes: > 10544A > 10544B > 10544C > 10811A > E1938A > Symmetricom 1000B > SRS SC10 > ............... > For which circuits and/or datasheets are available. > Tabulating their EFC ranges would also be useful. > > Bruce > >
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