EEVblog #236 - FE-5680A Rubidium Standard Teardown - Page 1
Forum section: electronics / general

author: BravoV
author_rank: Super Contributor
timestamp: January 13, 2012, 03:10:03 am
content: Really love this #236 vid, Dave.Btw, about that weird thing soldered above the crystal, just curios, does that crystal have padding below it ? Or its just hanging there ?

author: Rufus
author_rank: Super Contributor
timestamp: January 13, 2012, 03:32:22 am
content: Quote from: BravoV on January 13, 2012, 03:10:03 amThat thing is a bit of a mystery.Is it just a heater with a temperature sensor under the can? The lumps of foam in the case are for thermal insulation and draft prevention and it does sit under a lump.

author: EEVblog
author_rank: Unknown
timestamp: January 13, 2012, 07:26:38 am
content: From memory it was just glued down direct to the board.Dave.

author: Unknown
author_rank: Unknown
timestamp: January 13, 2012, 07:54:12 am
content: Can it be a Peltier heating/cooling element?

author: flolic
author_rank: Frequent Contributor
timestamp: January 13, 2012, 08:26:34 am
content: Quote from: HAL-42b on January 13, 2012, 07:54:12 amNo. Peltier element requires heat sinking of some sort.

author: amspire
author_rank: Super Contributor
timestamp: January 13, 2012, 02:15:43 pm
content: The part soldered to the crystal could be a thermistor. It could double at a heater for the crystal and temperature sensor. So it is heated till it reaches a preset resistance, that will correspond to a fixed temperature.

author: eliocor
author_rank: Unknown
timestamp: January 13, 2012, 02:36:52 pm
content: the part IS a thermistor, used to keep the quartz at a stable (higher than environment) temperature.

author: tnt
author_rank: Regular Contributor
timestamp: January 13, 2012, 06:34:59 pm
content: The whole assembly is something to keep the quartz at a given temperature. The foam also somewhat isolate it thermally.@Dave: It's is not frequency programmable like the previous models, it's only trimmable by the rs232.The 10 MHz output on the RS232 comes from the Xilinx CPLD and is a direct divided frequency from the onboard VCXO (60 MHz / 6). The square wave goes through a LC filter to that small RF connector and also to the DB9. The "odd looking" DB9 has actually a small filter in each pin that filters the signal even more.The Analog programmable chip is used in the loop that drives the cell for fine adjust. (and that loop is used to regulate the control voltage on the VCXO).Previous models had the DDS on the output, allowing for easy reprogramming, this is not the case on these new models.

author: kaz911
author_rank: Super Contributor
timestamp: January 13, 2012, 06:58:14 pm
content: according to the Spec sheet is is an option you can get when you order them.There are both versions available on eBay - but the 1-20 MHz versions are more expensive. :-)

author: JBeale
author_rank: Frequent Contributor
timestamp: February 01, 2012, 06:48:59 pm
content: On my unit, the 60 MHz crystal is glued down at the top edge to the PCB. The PTC soldered on top of it heats it to about 65 C according to my thermocouple.The digital tuning word on this model (60 MHz) has a resolution (tuning step size)of 6.8E-13. The older model 5680A with the 50.255 MHz crystal, has a tuning resolution of 1.78E-14. By the way, the 5680A can be modified to accept an analog voltage tuning signal to adjust the C-field, if you want.If you're interested in more info, check out the FE-5680A FAQ athttp://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:fe5680a_faq#adding_an_analog_adjustment_pot