Transient voltage on bias-tee? - Page 1
Forum section: electronics / rf-microwave

author: elroy
author_rank: Regular Contributor
timestamp: July 28, 2023, 11:54:08 pm
content: Today I was testing three bias-tee devices:-- A Mini-Circuits ZFBT-6GW+-- This bias-tee made by the company Ciglow:https://www.amazon.com/gp/product/B084MFKQ14-- A bias-tee custom-made by the designer of an active antennaWhen I connected a 9vdc source to the DC port of the custom-made bias-tee, the voltage appeared normally on the RF+DC port, with no sign at all of voltage on the RF port.When I did the same with either of the other two bias-tee's, the voltage also appeared normal on the RF+DC port. However, on the RF port, both of them showed a small and gradually declining voltage (as measured by my multimeter in DC volts mode).Is this normal? It was suggested to me that it may be, because of the load presented by the multimeter. Is there any chance that these bias-tee's could damage an SDR when connected to the SDR's antenna port? If not, why not?

author: Kim Christensen
author_rank: Super Contributor
timestamp: July 29, 2023, 03:50:19 am
content: Well, there's just a little SMD capacitor between your SDR and the bias supply ( C4 on the Amazon one you linked ) if that failed as shorted then the full DC supply would be applied to the SDR's antenna port.What you're seeing on your meter is that little capacitor being charged up via your meter's input impedance. There would be a little current spike when the SDR was attached or the DC supply was turned on if it was already connected. Damage potential depends on the design of the SDR. It's unlikely though.

author: elroy
author_rank: Regular Contributor
timestamp: July 29, 2023, 11:18:53 pm
content: Thanks. I haven't looked inside the Mini-Circuits bias-tee, but its small physical size suggests probably similar components. The custom bias-tee by contrast is larger, and the last time I opened it I remember seeing a larger electrolytic cap, among other components.I suppose I could put another capacitor in series between the bias-tee and the SDR as extra insurance. But would need to figure an appropriate value given the frequency range involved vs the existing cap inside the bias-tee.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 31, 2023, 05:22:05 pm
content: Depends on the bias tee and it's cutoff frequency. Common commercial bias tees could have anywhere between a 10 kHz and 1 MHz cutoff. those would have ~500 nF of capacitance or 5 nF on the RF port. This will form an RC circuit with your multimeter which most likely has a 10 megaohm input impedance. 500 nF + 10 megaohm gives a 5 second time constant which sounds similar to the behavior you observe.You can confirm this by measuring the bias tee with a capacitance meter. Measure between the RF and the RF+DC port with the DC port let open circuit.An SDR will have a 50 ohm input, so the pulse will decay much faster. It shouldn't cause any damage.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 31, 2023, 05:26:31 pm
content: I should add: transient voltages from bias tees definitely *can* destroy hardware. It depends on the magnitude of the bias voltage, the type of device on the RF port, the bias tee, and the rise time of the bias applied to the RF+DC port. However, in your specific case of a 9V bias voltage and a commercial SDR receiver there shouldn't be a problem.

author: coppercone2
author_rank: Super Contributor
timestamp: August 03, 2023, 04:41:47 am
content: Hmm I am pretty sure commerical bias T like to include a 10 ohm resistor or so. Not the biggest limiter but its sometimes? there. If not they seem to solder a resistor on the outside to feed it.At least I think its common practice. I am thinking a good bias T would have a 1 ohm ceramic composition resistor. I wonder how many circuits it would trouble if any.And they also have inductors. Actually not sure if the one i had was just a inductor but I think it had a carbon composition resistor and a inductor.But this is related to the power supply, its only a problem if the power supply does not soft start. AKA the bad habit of clipping leads on stuff

author: Hamelec
author_rank: Regular Contributor
timestamp: August 04, 2023, 01:04:33 pm
content: a bias T with a resistive component (> some ohm) is not of use.. supply voltage for your circuit is changing with supply current.

author: coppercone2
author_rank: Super Contributor
timestamp: August 04, 2023, 11:16:44 pm
content: not sure, I would have to find it, something broke off in there and I either put it in the junk bin or threw it out. Maybe dinged up connector.