# Superimpose a High Frequency AC Small Signal on a DC Power Signal

I am facing a challenging problem with a circuit that I am currently working on, I would like others input and thoughts.

I have a DC power signal that I am injecting a AC small signal this mixed signal is transferred over two conductors to a receiver circuit which then uses the DC power signal to power a load and also extracts the AC small signal for use.

Description of signals:

DC Signal: 30V generated by a boost converter that has noticeable amounts of noise under the working load of 500mA to 1A

AC Signal: ~2vpp @ 1MHz to 30MHz

simulate this circuit – Schematic created using CircuitLab

This is not an overly complex challenge given ideal components, however the real challenge I am seeing is the inductors sized to carry the currents required have a self resonant frequency typically <2MHz. From my understanding this means that they would not act as good filter component for the higher frequency AC small signal causing excess loading of the AC signal and a diminished signal at the receiver.

Very much looking forward to other's thoughts and opinions on the problem.

Thanks!

• Can you formulate that to a question? What inductors have you been looking at? COTS or wind yourself? Commented Feb 23, 2018 at 19:24
• There are hundreds of inductors in 1uH-10uH range and >1 A current with self-resonance above 100 MHz. Commented Feb 23, 2018 at 19:28
• What distance is this required to work over? Commented Feb 23, 2018 at 19:50
• is that into 50 ohms? with 1A and how much attenuation of noise and what frequency is the noise. Commented Feb 23, 2018 at 20:34
• @Ali Chen Based on the frequencies I'm looking to filter out I suspect that 10uH is on the low side (X (@1MHz) = 63ohm). Commented Feb 23, 2018 at 20:46

In the RF world, this circuit is called a bias tee.

You should realize that on the RF side, 1-30 MHz is a fairly wideband requirement, spanning nearly 5 octaves of frequency.

the real challenge I am seeing is the inductors sized to carry the currents required have a self resonant frequency typically <2MHz

You may find that a series combination of inductors gives a better result than a single inductor. One with high value and low SRF, one with lower value and higher SRF.

This is often done with coils or transformers to V+ with Common Emitter currents for AC to match load impedance and DC whatever coil and load requires.

Some CATV repeaters and many RF systems use AC or DC & RF on the same coax.

Start with specs for DC current, cable length and Signal impedance at source and destination.

Based on @Ali Chen's comment I was able determine that there is a limit on inductance values available that have suitable SRF once you get into the higher frequency filtering applications.

I will be conducting some tests with lower inductance inductors around 10uH and check performance.

Thank you all for your input!

• Please note that the answer feature is not supposed to be used for comments. Commented Dec 20, 2018 at 21:31