Symmetricom TrueTime XL-DC changing antenna volatge - Page 1
Forum section: electronics / projects

author: zener
author_rank: Contributor
timestamp: May 02, 2019, 01:12:18 pm
content: I picked up one of these units and would like to change the 12 volt antenna power supply to 5 volt. Or even no powerto the antenna as my GPS antenna is powered by other means. I see mentions in other threads that this can be donebut no specific instructions. I have searched all over but no luck. My unit is not a down-converter type unit.How can I do this?Thanks,

author: DaJMasta
author_rank: Super Contributor
timestamp: May 02, 2019, 05:23:29 pm
content: You could open up the unit and try to replace the circuitry regulating antenna voltage (may not be easy, I've worked in a TimeSource 3500 with a similar antenna voltage issue and since it also monitored current consumption, the mod was more complicated), or you could develop an inline converter or injector for it.The basics of powering an antenna is adding a DC bias to the waveform, then in the active antenna you use an inductor to keep the RF out of the power supply section and a capacitor to keep the DC out of the RF section and have your circuit go about its amplification. You can do the same thing and throw in a linear regulator, so long as you don't need more current and can dissipate the power lost from the voltage drop - so you have a DC blocking capacitor in series with the antenna line, on the unit side you have an inductor that taps off the RF line to a linear regulator supply rail with decoupling caps, then the output of the linear regulator dumping back through another inductor into the antenna side of the DC blocking cap. Similarly, you could have a bias tee (DC blocking cap and single inductor on center tap) and use an external power supply to bias it with a different voltage, though if the unit is monitoring current consumption, this method wouldn't draw any power.As a caution, be sure the antenna line doesn't need an external LO downconverter. Units designed to be installed after long cable runs will sometimes feed an LO signal along with their DC bias for the antenna so that an inline frequency downconverter can be used near the antenna. This makes it so that the signal of interest going through the long cable is much lower frequency, but it also means that any antenna replacement or voltage mod still needs to mix the LO externally, since the unit itself does not accept the full frequency signal on its own and makes the whole process a lot more involved.

author: zener
author_rank: Contributor
timestamp: May 02, 2019, 09:10:52 pm
content: I poked around the circuit board with an ohmmeter and found 2 surface mount inductors that have DC connection to theBNC connector for the antenna. I was looking for a jumper to set the voltage but no luck. I have a document showingthe jumpers but the text describing the jumpers is not clear to me, no jumper mentions antenna voltage.The manual says that the units that require a LO converter have a sticker that says so. There is no such sticker on my unit.I have considered just getting a DC block to eliminate the issue but if the unit is measuring the current then that maycause a problem.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 02, 2019, 11:26:08 pm
content: In that case, if it's got a local regulator for the antenna rail you may be able to just swap out a part. Conversely, you could probably lift the leg/pad of the inductor connected to the DC rail and just route a clean 5V source to it, probably easiest to mod in a regulator from a different rail that seems to be overspecified, but just using the 5V on the board could be viable for lower antenna current requirements.

author: zener
author_rank: Contributor
timestamp: May 03, 2019, 01:12:45 am
content: Curiously there were 2 inductors side by side. I don't know if they are just wired in parallel or there is somethingmore complicated going on. It is hard to see the traces under the solder mask so it is hard to get a notion ofthe circuitry involved.

author: DaJMasta
author_rank: Super Contributor
timestamp: May 03, 2019, 02:36:13 am
content: Could be in parallel like with resistors for more power handling, but yes, probably worth buzzing out some traces and IDing any connecting parts to be sure.

author: ZigmundRat
author_rank: Regular Contributor
timestamp: May 03, 2019, 04:12:46 am
content: I have the info that shows the jumpers that need to be changed. ‘Jumpers’ in this case is zero ohm resistors etc that need to be moved. I’ve done this change myself and I don’t recall there being much more to it than that. I’ll post an update tomorrow night.

author: zener
author_rank: Contributor
timestamp: May 04, 2019, 05:28:13 pm
content: Thanks, that info would be really helpful.

author: ZigmundRat
author_rank: Regular Contributor
timestamp: May 06, 2019, 12:13:28 am
content: Sorry it took so long. Here's the jumper info (attached). In hindsight I should probably just have referred you to an earlier thread on the XL-DC. It's athttps://www.eevblog.com/forum/projects/symmetricom-truetime-xl-dc-gpsdo/. In that thread @pacAir included some useful scans of other manual pages which include the jumper locations. I also posted pics of the downconverter receiver board in that thread. There are distinct differences between it and the normal receiver - if you have any doubts you can compare them to what you have.As an aside, I did find that having the antenna voltage at 12V can be useful. My earlier GPS signal distribution method used a DBS splitter and a DBS inline preamp. But these don't work well at 5V, they work much better at 12V. That arrangement worked well for quite some time. If you've only got the one receiver then 5V will probably be the better choice.Have fun with the XL-DC. They still do a good job even though they are kind of vintage at this point.

author: zener
author_rank: Contributor
timestamp: May 07, 2019, 12:09:00 pm
content: I made the changes and it is working great. I just moved the zero ohm resistor from one set of pads to the other.