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1-8 of 8 messages
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Less-Expensive Alternative to Icom's High-Stabilit
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by GW0DIV on November 30, 2008
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I was browsing Mike Nadeau (N1EQ)'s site
http://www.n1eq.com/tech/page_05.html
and I came across his "Less-Expensive Alternative to Icom's High-Stability Crystal Unit". Does anyone know where I could get hold of some of these little "ovens"? Hopefully somewhere on-line?
Rhys
GWODIV
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RE: Less-Expensive Alternative to Icom's High-Stab
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by K7BAB on November 30, 2008
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Try here:
http://hamradio.cmcorp.com/inventory/CRYSTALS/Ovens.html
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RE: Less-Expensive Alternative to Icom's High-Stab
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by KA5N on November 30, 2008
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It is quite easy to make a crystal oven. One needs a source of heat, a method to set the desired temperature and a sensor.
A transistor (220 case)is a great heat generator and is easy to control. The sensor can be a thermister or some sort of bi-metal switch. Temperature selection is by means of a variable resistor in a bridge circuit with the thermister or just the bi-metal switch.
Of course I guess it is easier just to use a ready made one.
Allen
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RE: Less-Expensive Alternative to Icom's High-Stab
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by G4AON on November 30, 2008
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Rhys in order to fully stabilise a crystal with a heater, the temperature needs to exceed the normal maximum internal temperature of the radio... Typically heating the crystal to 50 degrees C or higher.
Unfortunately adding the heater to the normal crystal found in a radio will usually pull it too far off frequency to allow it to net back with the trimmer. You would need to use a crystal specifically cut to work at the higher temperature.
The bi-metal "thermostat" crystal ovens of yesteryear caused the frequency to drift about a centre point as the temperature rose and fell, that was before the contacts failed! They have long since been replaced with more reliable methods of temperature control.
73 Dave
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RE: Less-Expensive Alternative to Icom's High-Stab
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by N5EG on November 30, 2008
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Hi Rhys,
The article shown a good approach - the concept of a positive tempco thermistor to simultaneously dissipate power and regulate temperature has been used on a lot of two-way FM radios. The device in your referenced article is not an oven, but a positive-tempco thermistor bonded to a metal clip. these are also called PTC - for Positive Temperature Coefficient thermistor. Another article is: http://www.mlecmn.net/~lyle/all-in-one/all-in-1.htm which discusses making your own.
The key to stability is to prevent the temperature from cycling up and down. First step is to make sure that the temperature detector and heat generator are thermally coupled together very closely, otherwise there's a lag time between heating and heat-detection which causes thermal cycling. The PTC does that pretty well.
A worthwile addition to the approach shown in the article would be a small tight-fitting styrofoam cup over the crystal/heater combination to minimize the amount of heat leaking away from the crystal. That would slow down any thermal drift.
You can attach the positive-tempco thermistor directly to the crystal can with thermally-conductive epoxy, and save using a metal clip. The advantage of the clip is that it allows removal of the heater in case you want to change crystals. Soldering to a crystal can is possible if done carefully, but only if the crystal can was welded rather than soldered during manufacture (otherwise you may break a hermetic seal).
http://www.mouser.com has some of these devices. Base part numbers 527-3006 can be searched. That will get you to a range of units, temperatures, voltages, etc. so you can pick the appropriate one, and to the datasheets for the parts. They cost in the $0.50 to $1.00 area for small quantities.
-- Tom, N5EG
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RE: Less-Expensive Alternative to Icom's High-Stab
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by VK1OD on November 30, 2008
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Crystal ovens etc attempt to minimise the temperature induced frequency variation by raising the temperature of the crystal above the highest temperature that it would otherwise reach using a controlled heat source.
Nevertheless, the crystal is still subject to some temperature variation.
Crystals designed for use at elevated temperatures, such as in an oven or using a clip on ceramic heater have a different frequency / temperature characteristic. They are cut so that the turning point in that characteristic is at about the temperature of operation, some 50°C or so instead of around 25°C.
So, the little $1 PTC thermistor ceramic heater will likely improve things, but it is only half the answer.
Owen
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RE: Less-Expensive Alternative to Icom's High-Stab
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by TANAKASAN on December 2, 2008
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http://www.kuhne-electronic.de/en/shop/155_Crystal_Heater
They work great, performance is increased further if you build your oscillator in a screened can that has ceiling tile insulation all round.
Tanakasan
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RE: Less-Expensive Alternative to Icom's High-Stab
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by GW0DIV on December 4, 2008
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Thanks Tanakasan,
They look very interesting at only 13euro ($16.57, £11.32GBP) - definately a lot cheaper than Icom's units. They even have apage on fitting one to the 746! I wonder what they'd do in the 7400 & 706? They claim better than 0.2ppm. I shall have to experiment further I think.
Rhys
GW0DIV
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