RF design and ground loop - Page 1
Forum section: electronics / projects

author: RawCode
author_rank: Regular Contributor
timestamp: May 05, 2023, 08:09:26 am
content: Hello everyone.I work with devices that can work from 10 MHz up to 10GHz, and with measurement instruments like AWG, Digitizers, mixers, amplifiers and so on. The actual setup, in my opinion, is not great concerning ground isolation and looping.Consider also that some measurements were severely compromised, with the main suspect being ground looping.Now that I have to design some instrumentation that will interface with said setup, I'm starting wondering if there are some rules to rebuild the setup or at least to design a good instrument.For instance, considering the interconnections, should I use insulated BNC/SMA, coupling the signal ground to the chassis ground with a capacitor (or an inductor)? (if yes, which value is suggested?)Should I use RF transformers to galvanically insulate the RF signal that the DUT is generating in a big noisy environment, traveling through at least two meters of RF cables (that cannot be shortened), from the measurin setup?Which things should I care, which things I should not?

author: barshatriplee
author_rank: Regular Contributor
timestamp: May 05, 2023, 01:14:51 pm
content: Using insulated connectors like BNC or SMA with isolated signal and ground connections can help reduce ground loops. Make sure the connectors are rated for the frequency range you are working with.

author: profdc9
author_rank: Frequent Contributor
timestamp: May 05, 2023, 07:27:36 pm
content: Problems like ground loops are generally more of an issue with low frequencies, especially 50 or 60 Hz "hum" that occurs because of the magnetic fields of the mains power inducing a voltage into the loops. RF voltages tend to be relative to the chassis ground, which for unbalanced transmission line like coax, is joined between the chassis of two pieces of equipment. If the RF voltages are conducted on a transmission line with a low impedance, like 50 ohms, it is best to simply join all of the chassis together using the coaxial cable grounds as intended. Don't put an inductor in the ground, that is counterproductive. The grounds should be bonded together with as low impedance as possible. A transformer can be used to break a ground loop, but is generally not necessary for RF and can introduce distortion into the signal as a transformer has a lower and upper frequency at which it can produce a useful impedance transformation.However, if the RF source is a high impedance, such as a RF probe, this is a more difficult situation. In this case, it is best to place an amplifier with a low output impedance as close to the source as possible. A high impedance long cable is an electric dipole antenna and readily picks up noise.The transition between the frequencies that are considered RF for this case and those that are a problem for the ground loop is the frequencies at which the skin depth of the chassis or coax shield is no longer able to screen the stray magnetic fields. This is on the order of a few hundred kHz. For frequencies lower than this, the wavelength is typically very long compared to the components being connected, and a star grounding approach is used that avoids a ground loop.