Avoiding Ground Loops Between PCB's (New Design) - Page 1
Forum section: electronics / beginners

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: July 09, 2014, 07:39:05 pm
content: Bear with me, I'm learning as I'm doing so I like to keep myself in just a bit over my head . . .I'm building a GPSDO as an educational project with a purpose. For several reasons, I've decided to go with separate boards for power supply, signal distribution, OCXO and control circuits (µC, GPS, PLL, display).The OCXO board will have a low noise buffer amp to send the oscillator's sine wave over to the distribution board via coax (board edge SMA connectors) as well as a squaring comparator to send a square wave over to the PLL board, also by coax. Then there are 12V and 3.3V supplies and the DC control line coming back from the PLL's phase detector.It occurred to me that I have a real ground loop problem here. Not only is there the supply ground, but 2 coax ground paths from the OCXO board to other boards which in turn will also be connected to each other.Should I connect the supply ground only to the digital board and let the various coax interconnects provide the remaining ground connections? That doesn't seem right so I'm a little baffled here. Thanks for your patience with me.

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2014, 01:22:49 am
content: Ground loops are important in high precision and low noise circuits but I suspect it will not be a problem in your case. High frequency signal components follow the path of least impedance so the coaxial shields still act as the ground returns where it matters. In test instrumentation, baluns or common mode chokes are often added between boards or modules to prevent ground loop issues.

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: July 10, 2014, 01:58:57 am
content: Oh, that makes sense! I could put a 1:1 transformer on each of the OCXO signal outputs so they are no longer current paths. Looks like CoilCraft has one with a 400 kHz - 600 MHz bandwidth and only 0.4 dB insertion loss (http://www.coilcraft.com/wbc.cfm).Thanks!

author: ve7xen
author_rank: Super Contributor
timestamp: July 10, 2014, 03:26:03 am
content: If you want to use transformers, Ethernet isolation transformers are designed for pulse use but should work fine for your purposes and are probably both cheaper and easier to get than an RF one. Make sure there are no additional internal components (e.g. termination).

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2014, 04:34:49 pm
content: Do not overlook using a current balun (common mode choke) which will subtract the difference in ground voltage from the signal. This has the advantage of passing DC. Implementation is as simple as a bifilar winding on a small toroid.Another thing worth doing is adding RLC power filters at various load blocks to decouple conducted noise both from getting out of noisy digital blocks and into quiet analog blocks. For the inductive element, I usually wind a single layer inductor directly around a low value resistor which serves to limit the Q.

author: Melt-O-Tronic
author_rank: Frequent Contributor
timestamp: July 10, 2014, 05:27:03 pm
content: If I understand correctly, you're recommending common mode chokes on the DC supply lines and the control signal from the PLL back to the OCXO. That sounds very wise. This is in addition to RLC filtering, right?On the 10 MHz signal lines, I updated my design to use CoilCraft WBC1-1LA 1:1 transformers to isolate DC so all the ground referencing is via the power connections.

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2014, 05:48:50 pm
content: Quote from: Melt-O-Tronic on July 10, 2014, 05:27:03 pmI am actually suggesting common mode chokes on the signal and ground pairs. If I wanted the lowest noise design possible, I would start off using the RLC power filtering at the separated digital and analog loads from the beginning because it is so easy to do. I learned the hard way about how powerful synchronous demodulation combined with phase locking can be in a design I did years ago.There is a clever trick you can use with the OCXO control or other sensitive analog signals though. If the control output is a current like from a transconductance amplifier, then common mode voltage changes will be suppressed. A load resistor at the OCXO converts the current to a voltage referenced to the OCXO ground. Bandwidth become easy to tailor by adding a shunt capacitor at the load.QuoteI have read discussions about doing this on the time-nuts email list (time-nuts@febo.com). I guess if you want very tight phase accuracy, the isolation method can become a source of error and high Q bandpass filters are particularly bad. I am inclined to think that the best way to go about this is to distribute a bandwidth limited square wave instead of a sine wave so the slope at the zero crossings is higher.I was working on my own GPSDO design for fun but had a setback when squirrels chewed through the cable at the antenna and receiver module and ran off with it.

author: AndyC_772
author_rank: Super Contributor
timestamp: July 10, 2014, 06:29:46 pm
content: Sounds like a problem that's easily overcome by applying a dc bias of 200V or more to your antenna.