FE-5680A GPS discipline board - Page 1
Forum section: electronics / projects

author: nsayer
author_rank: Regular Contributor
timestamp: June 09, 2016, 07:44:49 pm
content: My latest fun projects have been GPS disciplined oscillators (I sell them on Tindie too). I'd send one in to Dave, but when I e-mailed him about that, he said that he doesn't get good GPS reception in the lab, so there wouldn't be a lot of point.My latest creation is a board that can power and GPS discipline an FE-5680A. You feed it 30W at 17-24 VDC (like from a surplus laptop power supply) and you get two independent 10 MHz TTL square wave outputs.FE-5680As are very, very stable, but when you get them from eBay, they may not necessarily be accurate. This board monitors the frequency and phase of the output and sends corrections as RS-232 serial commands.Turns out that it appears that this board can also discipline an FE-405B DCOCXO. I'm waiting for one to arrive from eBay to try it out. You just alter the jumpers on the board to turn the serial I/O from RS-232 to TTL and get the RF input from a u.FL jack instead of the DB9. The rest is firmware changes (and the 15 MHz output frequency instead of 10 MHz).I've got aHackaday.io projectthat covers all this. It's all open hardware / open source firmware.

author: Vgkid
author_rank: Super Contributor
timestamp: June 10, 2016, 12:23:41 am
content: That is pretty neat, Nick. I have been following along on the time-nuts list.

author: uncle_bob
author_rank: Unknown
timestamp: June 10, 2016, 01:14:01 am
content: HiBut does the Rb loose it's mind every so often when it is power cycled ...Bob

author: nsayer
author_rank: Regular Contributor
timestamp: June 10, 2016, 01:41:13 am
content: Quote from: uncle_bob on June 10, 2016, 01:14:01 amThe FE-5680A has two different commands you can use to set the frequency offset value, one saves to EEPROM, the other does not. I always use the second one in the current firmware. There is a third command that can read the current offset, but I don't do that either. So, long story short, the current firmware does start from a zero offset whenever it's started.That said, the behavior isn't as bad as it sounds. The initial FLL mode usually gets it quite close, and the PLL enters the second TC usually within an hour or two. It takes a while to get to the third TC, but by then the EFC value has already stabilized.EDIT: One thing you can do is watch the serial diagnostic log output. Once you see that the EFC has stabilized on a particular value, you can use some other software to permanently write that value into the 5680A. Having done that, your oscillator is now calibrated relative to GPS and can be used as an offline standard. That said, in my unconditioned workshop I see diurnal EFC shifts of ±20 units or so likely due to temperature, so "online" accuracy will still be higher.

author: Dubbie
author_rank: Unknown
timestamp: June 10, 2016, 07:43:13 pm
content: Have you considered doing a version with sine wave out instead of square?Sent from my phone using Tapatalk

author: nsayer
author_rank: Regular Contributor
timestamp: June 10, 2016, 07:53:32 pm
content: I'm not sure the best way to accomplish that without degrading the output.A simple LC pi network with a roll-off of just north of 10 MHz might do, but it's not something I've investigated, as all of my needs are met by the square wave output, and I don't have the equipment to measure phase noise.

author: uncle_bob
author_rank: Unknown
timestamp: June 10, 2016, 09:22:07 pm
content: Quote from: Dubbie on June 10, 2016, 07:43:13 pmHiThe various FE Rb's have a really dirty output in terms of phase noise and spurs. The sorts of things you might use a sine wave for probably are not going to be very happy with all the spurs and other "stuff" on the signal. That said, if you want to turn it into a sine wave it's pretty easy. Buffer the output with a single high speed '125 buffer. Run that buffer into 4 of the same in parallel. The paralleled outputs go to a 50 ohm to 50 ohm lowpass "Tee" matching network with a Q around 2. There are *lots* of calculators on the web that will give you the values. Play with the Q a bit to match up with the parts you have on hand. If the whole thing runs off of +5V, the result will be around +18 to +20 dbm into 50 ohms. It will be quite noisy, but not because of anything the gates did.Bob

author: nsayer
author_rank: Regular Contributor
timestamp: June 10, 2016, 09:27:16 pm
content: The rubidium is noisy as you say, but this board probably (not yet confirmed) can also discipline an FE-405B DCOCXO, which is much cleaner.The output chip presently is an NB3N551. It has a nominal 20 ohm output impedance, and I currently use a 33 ohm series resistor on each output. You think maybe adding the low-pass network onto that would work well enough?

author: uncle_bob
author_rank: Unknown
timestamp: June 10, 2016, 09:39:05 pm
content: Quote from: nsayer on June 10, 2016, 09:27:16 pmHiIf you take a look at the IBIS models for the chips and their dissipation ratings, you need multiple packages. The idea is to get them to as close to full saturation as you can. The power goes into the load and not into heating the driving chip(s). If you push it, you find that there are interesting "electro migration" rules that now limit your reliability.The 405 is cleaner than the Rb, but it's not clean when compared to a typical 10 MHz OCXO. There are still a lot of DDS spurs that come through inside their cleanup loop. The loop it's self is limited by the VCXO. Any sort of VHF to microwave radio (or other) application is going to need further cleanup rather than straight multiplication.Bob

author: nsayer
author_rank: Regular Contributor
timestamp: June 10, 2016, 09:44:46 pm
content: Well, there's always my OH300 GPSDO.I've been contemplating the design for an OH300 based clean-up oscillator, but I sort of doubt I'd sell even one.Still, I will try breadboarding the '125 + pi solution you suggest if for no other reason than to confirm a solution for those who really really need sine output.

author: uncle_bob
author_rank: Unknown
timestamp: June 10, 2016, 09:47:37 pm
content: Quote from: nsayer on June 10, 2016, 09:44:46 pmHiThere's several 100 million copies of that circuit out and running around. As long as you use a modern high speed gate, it works fine. You can play games with the number of gates in the output. They are cheap and even if 4 is overkill, you haven't spent much on them.Bob

author: Dubbie
author_rank: Unknown
timestamp: June 11, 2016, 09:40:33 am
content: I feel like a bit of a dunce in that I have never come across that solution before Bob.Nsayer, I'd be interested to see how you get on with the breadboard prototype.Sent from my phone using Tapatalk

author: uncle_bob
author_rank: Unknown
timestamp: June 11, 2016, 11:49:25 am
content: Quote from: Dubbie on June 11, 2016, 09:40:33 amHiHow much time do you spend tearing open sine wave 10 MHz signal sources and tracing out their internal workings?Bob

author: nsayer
author_rank: Regular Contributor
timestamp: June 14, 2016, 07:26:48 pm
content: I actually asked ON Semi about this directly. They said that there would be no reliability or lifespan problem adding a pi filter to the output of the NB3N551. The device has a characteristic internal impedance of 17-20 ohms, so adding a 33 ohm resistor in series before the pi filer (which my circuit already has) would make the output impedance close to 50 ohms for matching the pi network.This matches up with this page:http://www.wenzel.com/documents/waveform.html(scroll down to the beginning of part 2) - a resistor to raise the output impedance followed by a simple pi filter.The through-hole parts are still on order for testing, but assuming I don't see something horrible when I try this out, the next version of the board will have the footprints in place for the pi filter on one of the outputs. If it's desirable instead to have two square wave outputs, the inductor can be replaced with a 0 ohm "resistor," and the caps left off.

author: uncle_bob
author_rank: Unknown
timestamp: June 14, 2016, 10:44:37 pm
content: Quote from: nsayer on June 14, 2016, 07:26:48 pmHiYou really don't want a Pi, you want a Tee.Bob

author: nsayer
author_rank: Regular Contributor
timestamp: June 15, 2016, 12:15:37 am
content: Why?I'm not being an ass, I really don't know and want to hear the reasoning.

author: uncle_bob
author_rank: Unknown
timestamp: June 15, 2016, 12:36:55 am
content: Quote from: nsayer on June 15, 2016, 12:15:37 amHiIt's pretty simple:The PI has a capacitor in front of it. At high frequencies (harmonics) you are driving a short circuit.The Tee has an inductor in front of it. At high frequencies you are driving an open circuit.Since the short will pull current, and the harmonic energy does you no good, efficiency goes down. There's no reason to run an inefficient circuit. The higher circulating currents also degrade isolation.Yes, you *can* tweak the input inductor a bit for maximum efficiency. Spice models aren't much help. It is a try it and see sort of thing.Bob

author: DimitriP
author_rank: Super Contributor
timestamp: June 15, 2016, 08:49:34 am
content: Quote from: nsayer on June 15, 2016, 12:15:37 amHere is the TLDR version:http://www.ti.com/lit/an/swra064/swra064.pdfand the visual summary: