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Murata BG330N posistor HC18/49 crystal heater oven clip
Murata BG330N Posistor HC18/49 Crystal Heater Oven Clip
Simple cure for 'wandering' crystals Stabilise that Rx or counter!
On offer is one, new (old stock) Murata Oven Clip for HC18/49 case crystals.
As most readers will know, the resonant frequency of crystals changes with temperature and most crystals have their sensitivity listed in their datasheets in PPM (parts per million). At 10MHz each PPM corresponds to 10Hz, but when multiplied up to UHF that can be 500Hz/PPM and it's even worse in the microwaves. For test equipment this is quite unacceptable, and even regular radio applications can suffer from a particularly hot or cold day. The usual remedy is to raise the temperature of the crystal so it is always hotter than its surroundings, and maintain it at a constant temperature, ie. to "Oven" it.
The whole topic of ovening is very interesting, with a long history of R&D, but probably the simplest and most cost effective way to oven a crystal is to glue a PTC thermistor to the crystal's case and pass a fixed voltage through it. The current flowing causes the thermistor/crystal to heat up. However, due to the positive temperature coefficient (PTC) of the thermistor, its resistance rises as it heats up, reducing the current flow. At a certain temperature (with these clips, around 50C), the resistance has risen to the point where the temperature cannot rise further and the system stabilises. Stable temperature, stable frequency.
These clips, purpose made by Murata, are likely the most convenient way of applying this approach. Other vendors will sell you suitable thermistors (at several dollars apiece) and leave you the messy task of mounting them with a suitable glue (which will not fail at a constant 50C) or heatshrink. By contrast, these clips are designed to simply slip over an HC18 or HC49 can crystal or filter - and off again if you want to use them or the crystal somewhere else.
On these parts, one side of the heater is electrically common with the crystal case (usually ground).
* Reasonably insensitive to voltage changes
* General feeling is a 10x improvement in stability over a non-ovened unit.
* Easy to retrofit into existing applications - just need a regulated supply 5-12V.
* Used in Maxon radios to stabilize LO and Filter
An informal test in free air at 20C ambient gave the following temperatures at different voltages when the system had stabilised.
The BG330N are specifically mentioned in the article "Converting the NEC PASOLINK Transceiver to 47GHz" by David Robinson WW2R, G4FRE, online at http://f1chf.free.fr/hyper/pasolink2.pdf.
"All the Crystals were cut for operation at 50C, to cope with Texas summertime temperatures and are fitted with BG330N murata posistors run off a separate 7805 regulator to maintain 50C. For an excellent description of how to use these posistors effectively see the article by Doug Friend, VK4OE"
Unfortunately I do not have access to the article referenced, but you may ... it's "Crystal Heaters, some useful observations. Doug Friend VK4OE. RSGB Microwave newsletter Jan 2000"
Useful parts ... limited availability ...
With crystal packages moving to SMD, clips for these 'legacy' crystals seem no longer to be made. I have a handful of this type but supply is decidedly limited. No idea how much they 'normally' cost ...
