Use SMD PTC (resettable fuse) as simple "constant"temp source for OCXO? - Page 1
Forum section: electronics / rf-microwave

author: wb0gaz
author_rank: Frequent Contributor
timestamp: July 07, 2023, 07:02:37 pm
content: I am interested in adapting a small SMD TCXO to serve as a OCXO in a DIP-14 package using homebrew PCB.I do not need lab-grade stability - just looking for a way to keep temperature in the SMD TCXO well above ambient (thinking 60-70C range) +/- a few tens of degrees C during operation.The TCXO I have in mind is in a 2.0x2.5mm package. The PTC I have in mind is in a 1206 package. My power source is 5V DC. The whole thing would be on a small PCB fitting a 14-pin DIP form factor. The idea is to use the PTC (see link below, or same file attached) sitting across 5V rail to supply electrical energy the PTC would convert to thermal energy until a (device-specific) temperature max is achieved. At threshold where the PTC would go open circuit, it would stay open circuit until temperature drops to the point where the PTC would return to closed circuit state, and this cycle would continue indefinitely.https://www.mouser.com/datasheet/2/281/PRG21BBMB1RK-e-1485189.pdfWhy this simple-minded approach? No room on such a small PCB for a separate temperature sensor/controller, and no additional pins (beyond 4 - GND, VCC, VCont, OUT) to interface an external controller.)Is this approach too simple-minded?Any questions, comments, or requests for clarification greatly appreciated!

author: eb4fbz
author_rank: Regular Contributor
timestamp: July 08, 2023, 06:47:19 pm
content: It could make things worse, specially if it’s a TCXO and your oven is not perfectly stable. For a TCXO, both the crystal and the compensation loop are optimized to get the best performance at room temperature.OCXOs use special crystals with inflection point at the oven temperature.

author: iMo
author_rank: Super Contributor
timestamp: July 08, 2023, 06:59:33 pm
content: People use PTC "thermistors" for stabilizing the temp of oscillator's crystals since ever.Here the PTC thermistor is not the same as the PTC "fuse", afaik.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: July 08, 2023, 09:39:31 pm
content: OK! Both very helpful.I will investigate PTC (not as fuse) and understand caution advised by EB4FBZ!Dave

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 08, 2023, 09:45:17 pm
content: Look at the spec sheet for your TCXO, I do not think it is designed for running hot.Oven crystals that are designed to run at pretty hot temps also are controlled specifically to keep the temp as constant as possible. If the heating is turned on and off too abruptly you end up with what is called "band bang' control which makes your oscillator frequency unstable.

author: ogden
author_rank: Super Contributor
timestamp: July 08, 2023, 10:06:50 pm
content: Main idea of TCXO is to compensate frequency shift caused by crystal temperature change. So putting TCXO into constant temperature (oven) is useless overengineering which may lead to worse results compared to ovenizing plain crystal oscillator.

author: wb0gaz
author_rank: Frequent Contributor
timestamp: July 08, 2023, 10:31:17 pm
content: OK on all - I was looking for "is this too simpleminded", which I'm now convinced is the case.Thank you for steering me in better direction!

author: oilburner
author_rank: Contributor
timestamp: July 10, 2023, 05:16:56 pm
content: Roman Black had a great little circuit,https://www.romanblack.com/xoven.htmAnyone know what happen to him? Haven't seen anything new for a while...g

author: rhb
author_rank: Super Contributor
timestamp: July 13, 2023, 12:17:13 am
content: I respectfully suggest that instead of building your own oven for an OXCO you buy some of the CTI OSC5A02 OXCOs for $3 each while they are available on ebay. Spend your effort on an ovenized voltage reference, supply and a low tempco voltage divider. The highest spec trim pot I can find are Vishay nickel foil with a50 ppm/Ctempco.The OXCOs are pulls from old cell site GPSDOs. They are three orders of magnitude more stable than a new OXCO of any make. New these were spec'd at 0.5 ppb/day after 30 days, 100 ppb/yr aftere 1 year and 0.4 ppb/yr after 10 years. This is normal crystal aging behavior. The only way to get an OXCO crystal this stable is to run them for 10-15 years.I suggest that you design and build an oven for a CTI, used LM399 voltage reference and precision divider and supply voltage regulator in an outer oven. That alone should get you to roughly 0.005 ppb/day, 5e-12 drift, with most of the error being temperature and noise dominated.If you build the crystal OXCO yourself you will inevitably be frustrated by crystal aging. The shift from ovenizing the crystal to ovenizing the other parts will have a much bigger payoff for fundamentally doing the same thing..Quite simply put, for the same effort you can have a better frequency reference for under $100 than you can buy short of a rubidium clock. Pretty good for an OXCO without a GPS to discipline it.BTW voltage references exhibit the same aging characteristics, so used ebay LM399s are also better than new ones unless they are noisy.I do think your resettable fuse idea quite attractive for the suggested application. A large number heating an enclosure would have the statistical property of being very stable without any other control and would shift temperature very gradually by only turning a tiny portion of the heater system on or off.Have Fun!Reg