Thanks for this. It is exactly the kind of response I was looking for. Based on what you say, it seems like a $10 eBay OCXO will always beat an temperature controlled TCXO.
The varactor tuning range is going to be a problem for which the only solution is a discrete crystal oscillator. I wonder if there are any non-custom sources for suitable crystals in a design like this. I assumed that they used AT cut crystals with a temperature curve of the same form if not the same uncompensated temperature range. Of course the temperature compensation circuit should completely dominate that anyway. I was hoping for something with a better risk for cost even if not as good as the typical Ebay used OCXO of questionable heritage. On Fri, 13 May 2016 17:57:58 -0400, you wrote: >Ok, with a TCXO you have a temperature sensor that tries to servo the crystal >on to frequency. >You also have a crystal with a temperature dependance. As you try to heat / >cool the TCXO >your thermal variation hits one before it hits the other. The net effect is >that the ADEV is actually >worse with a TCXO than with an un-compensated crystal. Consider that a good >oven has variation >in temperature on the order of micro C over a few seconds to tens of seconds. >Thats what you >are trying to emulate. > >The next issue is that the TCXO has a loop design oriented towards wide tune >range. Things like >varicap diodes have a much bigger impact on a TCXO loop than they do on a >narrow tune range >OCXO loop. That (and possibly a coil or two) are yet another source of >thermally induced variation >in the oscillator. Just like the temperature sensor, there is a delay in the >temperature hitting them >compared to other parts of the oscillator. Again, you get a degradation in >ADEV over the bare >TCXO crystal. > >Next up is the crystal its self. A TCXO crystal likely has a turn temperature >that is optimized to >match the compensation approach being used. In some cases there will not >actually be a turn >at all. Thus there is no temperature zero slope point to hit with your >temperature controller. This >means that rather than you now need much better temperature variation control >on your system. > >Bottom line: A $10 eBay OCXO is likely to beat an ovenized or cooled TCXO. > >Bob _______________________________________________ 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.