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

Question

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!

Answer 1

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.

Answer 2

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.

Answer 3

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!