7.750 MHz Crystals - Page 1
Forum section: electronics / projects

author: German_EE
author_rank: Super Contributor
timestamp: November 29, 2015, 09:27:03 am
content: I'm looking for a source of 7.750 MHz crystals (without having to get some custom made). Having tried Mouser, Digikey, Element 14, RS, Jameco and a few others I'm out of ideas. Can anybody suggest a company that has this frequency crystal as a stock item?

author: Zero999
author_rank: Super Contributor
timestamp: November 29, 2015, 10:39:07 am
content: I've just checked Digikey and Mouser. They sell 15.5MHz MEMS oscillators. Add a flip-flop to divide by 2 (there are many small, single flip-flop ICs available, if space is a problem) and you have 7.75MHz.

author: German_EE
author_rank: Super Contributor
timestamp: November 29, 2015, 01:12:44 pm
content: Sorry folks but I need 7.750 MHz crystals for a filter, not to build an oscillator.

author: ConKbot
author_rank: Super Contributor
timestamp: November 29, 2015, 07:22:50 pm
content: No ideas on the oddball crystal, bit I would like to point out for the mems enthusiasts, to remember that they have crap close in phase noise, and while the specs are selected carefully to make them look good, you can expect 20x or more long term jitter than a crystal. Set a scope to trigger on edge, scroll over 20ms and you'll have a good bit of jitter present in a mems osc, with a quartz osc, I had to scroll over 200ms, and there was about half as much jitter in the eye pattern.Not a problem in lots of applications, but keep it in mind. There's a reason normal MEMS oscillators spec the jitter integrating from 12KHz to 20MHz, the 10-100Hz phase noise jumps upto -30 or -40dbc.

author: ConKbot
author_rank: Super Contributor
timestamp: November 29, 2015, 11:53:42 pm
content: Quote from: blueskull on November 29, 2015, 09:11:29 pmUnfortunately I dont have the equipment to make an actual proper phase noise plot at work. We have a nice agilent VSA, but with next to no options, so I can only get a phase noise cursor, not a phase noise plot.But I was playing with a SiT9121 "ultra performance differential oscillator" which I suspect is the same core as the SiT8208, with a different output driver. Either way, I was using it with a PLL/VCO combo RF synthesizer. While a good bit better then the SiT8008, (the center peak was ~1 KHz wide and I couldnt even see the 'spurs' where the synthesizer output changes from reference phase noise to VCO phase noise, and I was running a ~200 Hz wide loop bandwidth, so it was bleeding over that badly) With the 9121, the central peak was as wide as the loop bandwidth (200 Hz) and with a crystal oscillator, it dropped down to ~ 30 Hz wide. The last 2 measurements were with a 3.1 Hz RBW iirc.Check outhttp://www.electronicproducts.com/Passive_Components/Oscillators_Crystals_Saw_Filters/MEMS_oscillators_vs_quartz_technology.aspxits from 2009, while figures have improved, I still wouldnt expect to be upto par with a quartz based unit.That being said, they are very well temperature compensated, they didnt drift at all after turn on. Vs the crystal where I'm chasing it all over when I'm running a 1 KHz span until the temperature of the board stabilized.If youre dividing the frequency down, or using it directly, it may be acceptable, but using it in a PLL where its getting multiplied up, the close in phase noise just gets stretched out wide. If I have a moment sometime soon, I'll take some measurements at standard offsets with the SiT9121, and PM you the results to keep from cluttering up the thread here with off-topic stuff too much more.

author: kony
author_rank: Regular Contributor
timestamp: November 30, 2015, 12:49:53 am
content: Try asking here:http://www.krystaly.cz/en/.

author: pmbrunelle
author_rank: Regular Contributor
timestamp: November 30, 2015, 05:22:37 am
content: Maybe you can multiply the signal with another frequency, and then do your filtering on the heterodyne?

author: babysitter
author_rank: Frequent Contributor
timestamp: November 30, 2015, 09:35:12 am
content: Why 7.750 MHz ?

author: German_EE
author_rank: Super Contributor
timestamp: November 30, 2015, 04:44:30 pm
content: DCF77 x 100

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: November 30, 2015, 04:54:45 pm
content: These people do custom crystals pretty cheaply - website looks quite old so probably worth checking they're still actually there :http://www.quartslab.com/