DiSEqC circuit

I need a circuit to provide a 22 kHz signal coupled to a 13 to 18V DC line. I have a low frequency function generator for the 22 kHz square wave and a power supply for the DC. I've tried just using a cap to couple it on but as soon as I connect the DC my 22 kHz goes to almost nothing.

The DiSEqC standard is for controlling things like an LNB or a switch. In my case I have a Norsat Universal LNB that uses the voltage and signal to select 4 different LOs.

The 22 kHz level is supposed to be just a 0.6V square wave riding on the DC voltage.

I thought it would be fairly simple but it's doesn't seem to be. I'm hoping one of the circuit gurus here can help.

The signal is being attenuated by the filter capacitors in your power supply. You can use inductors in series with the DC power to minimize this attenuation. You need an inductor that can carry the DC current and also has enough inductive reactance (XL = 2 x Pi x f x L) at 22 kHz to not attenuate the signal significantly.

How much DC current is here? If it's 100mA or less, put a 27 ohm resistor in series with the DC power supply (between the power supply and the 22kHz).

But realize that putting an impedance in series with the supply's output will screw up the regulation.

I haven't hooked up the LNB yet but it's speced at 300 mA. I doubt it will pull much more than 170 if it's like their other LNBs.

I was wondering about using a 2 resistor summing op-amp circuit. If I just made a unity gain summer could I just put in the DC on one leg and the 22 kHz on the other?

Depending on the current being drawn off the power supply, that would take a "power" opamp (servo amplifier).

A summing op-amp would have to deliver the full 300 (170) mA.

I'm assuming that the LNB is OK with 13-18V and your existing power can tolerate some loss in an impedance. If the load is 170mA then 10 ohms in series will waste 1.7V. If your signal source could drive it, that is. Otherwise you'll need an inductor.

I was thinking it would take a higher voltage op amp since I need 19V and at least a few hundred mA.

The LNB needs either 13 or 18 depending on which band you want. The 22 kHz picks the other 2 bands.

So use an inductor that can carry at least 300mA. For about 1kΩ impedance at 22kHz, it should have a inductance of 7mH. Here's an example from Digikey.