[Mark] wanted an accurate frequency reference for his electronics lab. He specified some requirements for the project, including portability, ability to work inside a building, and low cost. That ruled out GPS, cesium standard clocks, rubidium standard clocks, and left him looking for a low cost Oven Controlled Crystal Oscillator (OCXO).
The Low Cost 10 MHz Frequency Reference is based around a Morion OCXO. These Russian oscillators are available from eBay second hand at about $40 a pop. With a stability well within the requirements, [Mark] order a few.
The next step was to stick all the components in a box. The two OCXOs in the box need about 3 amps to heat up, which is provided by a 12 V PSU. For portability, a sealed lead acid battery was added. The front panel shows the supply voltages, switches between mains and battery supplies, and provides connectivity to the OCXOs.
Since OCXOs work by heating a crystal to a specific temperature, they can use quite a bit of power in the heating element. To increase battery life, a neoprene foam insulator was wrapped around the OCXOs.
For less than $100, this portable tool will aid in calibrating equipment or creating very accurate clocks.
The battery is 1.3 Ah unit that is rated for 20 hour discharge, or 0.07 C discharge rate. It is woefully under-specced for a continuous load that can reach 3 Amps.
The runtime of such a battery will be measured in minutes because the proportionally high current draw reduces its coulombic efficiency. I would be surprised if can handle 3 amps for more than 15 minutes without the output voltage collapsing.
More precisely, the battery is rated for 1.3 Ah at 65 mA output current. while the oven needs 500 mA to stay warm, so he’s exceeding the battery’s nominal ratings by about 7.7 times. That means it will not deliver 1.3 Ah
These kinds of SLAs aren’t meant for high drain purposes, but more like memory backup batteries for automated systems in the field, and to work things like latching relays that only need short bursts of power.
The added insulation could decrease the load on the battery, although I don’t know if it’s a significant decrease.
datasheet of similar spec SLA: http://batteries.zeusbatteryproducts.com/Asset/PC1.3-12.pdf
From that datasheet, you have less than 6 minutes of backup for heating up the OCXO which is probably not designed to operated entirely on battery. On the other hand, the SLA can provide 2 hours of UPS backup once the OCXO hits steady state. (Assuming the discharge voltage of 10V and 500mA for steady state operation.)
3A is the warmup voltage it only pulls 476mA once warm. I only use the battery to carry the unit to my car and then plug it in to the car supply say 2 minutes then carry it inside when I need it and connect it back to mains, the foam reduced the load from nearly 1amp so it does make a difference
How long does he need for it to run on battery power? If he really only needs it to last for five or ten minutes, that could be fine. There are a lot of ways this project could have gone for superior battery performance, but if a couple of minutes of runtime are sufficient, this well have been the cheapest way to go.
Anybody ever considered using the reference off of GSM networks? They actually need the insane precision to work and because of the coverage availability should not be a problem…
GSM will typically use GPS disciplined OCXOs, or they will source timing from a T-1 (or other phone circuit). They several hundred dollars used, and many will need mods (Software and/or hardware) to output a standard 10mhz signal.
Its better to buy a regular used GPS OCXO.
Which is useless when you have to be in a building and can’t pull in antennas :P
I mentioned GSM because the signal can punch through several walls, whereas GPS will never make it even through a wooden roof.
There is an open-source GSM stack for a $20 phone, it can’t be that hard to pull out something that can be converted into a 10MHz signal…
Also, most OCXO are not meant to be moved around, shouldn’t acceleration and vibration cause deviations? (Coriolis forces and all that fun stuff…)
When you said Network, I thought you were talking about the tower side of things.
For the references in the phone, they use VCOs, which one might be able to access, though it might be running at a frequency annoying to divide down to the standard 10mhz. A custom receiver might be needed as the VCOs are increasingly integrated into the other chips in the GSM stack.
Modern GPS engines can pick up several satellites inside a room as long as it has a window.
People should take the time to proof-read the articles before publishing them, I know it can be difficult to spot mistakes but I find it helps if you have someone else proof-read it as well.
Quite! Caesium for example… There are other mistakes too… HaD is happy to tout their journalist credentials when it suits, but they don’t walk the walk – a great shame, they have the potential to have the same kind of authority and audience as say Elektor…
“Cesium” is the standard spelling in the US. Also, we call Natrium “Sodium” even though the symbol is still Na. There’s that whole Aluminum/Aluminium thing, too… don’t get your undies in a bunch!
The surplus FEI FE-5680A Rubidium frequency standards were really cheap a few years back when there was a mass upgrade of old 2G cell towers to 3G but they’re hard to find now. Any that do pop up on eBay tend to be overpriced as the surplus dealers have noticed that there is demand for them so they’ve ramped up the prices.
As noted in the article, if an OCXO is accurate enough then there is no need to mess about with more exotic frequency standards which come with their own problems.
I found a cheap 12$ glonass/gps receiver based on a ublox 7020, that can be reprogrammed to 10Mhz on the pps output by the u-center SW. config can be saved so it will start up at 10Mhz immediately. easiest to take the usb cable away after programming and hook up an rg316 coax for dc an 10Mhz combined. a dual UHS tiny logic inverter can drive the cable.
http://es.aliexpress.com/store/product/GPS-engine-board-Module-with-Antenna-USB-G-Mouse-LUY-VK-162/127766_1675335518.html?storeId=127766