50R Impedance matching - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: October 07, 2020, 11:47:13 pm
content: Hi,I am making a PCB coil for various general experiments. I am driving the coil from from a standard 50R signal generator. The frequency is not that crucial, it can be anything between 4MHz to 8MHz. It is just to play with some ideas. I want to make sure that maximum power is transmitted/emitted by the coil and that there is no signal distortion.1) I assume that, to achieve both, I "only" need to impedance match the coil to the signal generator. Or are there other requirements too?2) I am thinking of using a 50R coax cable on such PCB and between the 50R coax cable and the coil some sort of impedance matching network. Is that the correct way of doing it?3) How do I design such network and how do I tune it (in practical ways) to make absolutely sure there is no bounce back or signal distortion on the coil?Thank you

author: SilverSolder
author_rank: Super Contributor
timestamp: October 08, 2020, 03:35:29 am
content: One way of matching the impedance of a load to a source is to use a transformer - very old school, but it works! Or, you could use a buffer amplifier that converts from the 50 ohm source to the (presumably) low impedance of the coil. This has the benefit that you can design the amp to give as much power as you want, not limited by the signal source.

author: ricko_uk
author_rank: Super Contributor
timestamp: October 08, 2020, 04:22:08 am
content: Thank you SilverSolder,So if I use just a high-current op-amp that is enough to ensure there is no reflection or distortion?Thank you

author: bob91343
author_rank: Super Contributor
timestamp: October 08, 2020, 04:43:27 am
content: Not enough information. You are driving a coil? What is its impedance? A coil generally is almost purely reactive unless it radiates. If you want to match a pure reactance you need a pure reactance of opposite sign. A coil is inductive so you need a capacitor. Then it will be resistive at one frequency only. Its impedance depends on the Q of the circuit.So you have to know more about what you are trying to do.

author: hagster
author_rank: Frequent Contributor
timestamp: October 08, 2020, 05:35:41 am
content: As Bob says, you can't match 50R to a pure inductor. That is to say a purely reactive component(ideal C OR L) doesn't consume power.

author: SilverSolder
author_rank: Super Contributor
timestamp: October 08, 2020, 12:07:02 pm
content: It might be better to think of this project as a kind of transformer, rather than a coil. Presumably, the intention is to transfer power from the coil into something near it, right?

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 08, 2020, 01:29:29 pm
content: Alan has few videos on Smith chart and impedance tuning.. maybe relevant to you...

author: Marco
author_rank: Super Contributor
timestamp: October 08, 2020, 02:20:09 pm
content: Quote from: ricko_uk on October 07, 2020, 11:47:13 pmWhy do you care?Wasting a little energy in the signal generator 50 Ohm series termination isn't all that relevant. Everything is still linear, the coil is still seeing a sine.