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| TruePosition GPSDO build question |
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| texaspyro:
--- Quote from: cdev on January 08, 2019, 04:28:11 am ---So the TP case I'm working on is ventilated and has a power supply. I'm just going to try to moderate the worst external extremes from getting to it. --- End quote --- You could try using Lady Heather's temperature control PID to PWM a fan and keep the temperature constant... |
| Squiddaddy:
I watched the YouTube video of CircuitValley's build, and he crammed so much into his little enclosure that he was running in the 50C+ range. I run no where near this, usually 35-38C. 40C when the sun warms the room up late in the day. He'll probably cook those boards in no time. Part of my heat is the 7805 regulator in there to power the AtMega328, the Max232, and the LCD. It's using a clip on heatsink, so I may bolt it to the enclosure. I have the 2011 board with 2 10MHz outputs, and a G revision of the Furuno GPS, which works out well, as I can feed the counter and the signal gen. No need for distribution yet. |
| cdev:
You wouldn't have any photos representative of what they do, do you? The one photo I have of the interior of the True Position LM-300 unit (from ebay) that actually shows the GPSDO part and its context) shows it right next to an (exhaust?) fan. --- Quote from: texaspyro on January 08, 2019, 04:08:14 am --- --- Quote from: edpalmer42 on January 07, 2019, 03:37:33 am ---Do NOT put a foam cap over the OCXO. It's designed to expect a certain amount of heat to escape. If you prevent that, the unit heats up to a point where the oven throttles back, but the temperature keeps going up! --- End quote --- That's not necessarily true. I have several GPSDOs where the manufacturer insulated the OCXO. Most used a thin flexible foam (often open at the top) but a couple have much more substantial insulation. But I would not recommend insulating a random OCXO... --- End quote --- |
| Squiddaddy:
Something else I noticed reading all of the TruePosition discussion on here is a command to set the cable delay of the antenna. I have 50ft of cable going to the outside timing antenna. Would it matter if the delay was added, or is it small enough to not bother? Does the cable delay affect the 10MHz output, or just the 1PPS signal? I'm not planning to use this as any time source, just a frequency reference. I don't know how to get the delay figure anyway. Also, the DAC value to the OCXO on it still changes. Once this thing locks to GPS, shouldn't the DAC stay put for long periods of time? Or is it always adjusting? These changes are in microvolts, so I believe it's normal. Currently at 2.071xxx volts, with the last 3 digits changing slightly every 15 seconds or so. And that's with the Atmega328 rounding it to 6 digits. Usually seeing 8 satellites in the sky. So far, I'm impressed with this thing. Been powered up a little over a week now. Unfortunately, I have to shut it down to put it on a UPS at some point. :( |
| edpalmer42:
--- Quote from: cdev on January 08, 2019, 10:19:53 pm ---You wouldn't have any photos representative of what they do, do you? The one photo I have of the interior of the True Position LM-300 unit (from ebay) that actually shows the GPSDO part and its context) shows it right next to an (exhaust?) fan. --- Quote from: texaspyro on January 08, 2019, 04:08:14 am --- --- Quote from: edpalmer42 on January 07, 2019, 03:37:33 am ---Do NOT put a foam cap over the OCXO. It's designed to expect a certain amount of heat to escape. If you prevent that, the unit heats up to a point where the oven throttles back, but the temperature keeps going up! --- End quote --- That's not necessarily true. I have several GPSDOs where the manufacturer insulated the OCXO. Most used a thin flexible foam (often open at the top) but a couple have much more substantial insulation. But I would not recommend insulating a random OCXO... --- End quote --- --- End quote --- Three points to remember: Under normal operating conditions, this equipment probably operates in an air-conditioned computer room. Once the unit's temperature stabilizes, it will never vary. So it doesn't matter if the OCXO is near a fan. We don't really know what the design criteria for this equipment was - accurate time of day, low phase noise, or rock stable frequency. The OCXO is operating inside a PLL so frequency changes will be slowly corrected. As amateurs trying to reuse this stuff, the best we can do is to try and keep the operating conditions as stable as we can. Ed |
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