Phantom power for TV antenna amplifier
I need to design a circuit that will provide power to a TV antenna amplifier.
My constraints are :
* Characteristic impedance : 75 Ohm,
* Power supply for antenna amplifier : 24V @ 100mA and
* Available power supply : 9V @ 1.6A
I think I will base the solution on a step-up converter.
But I have a concern about the harmonics of the switching frequency.
Moreover, I do not know how to calculate the inductor and the capacitor (not the one of the step-up convert but the capacitor which blocks DC to TV and the inductor which blocks HF to the step-up converter).
Thanks in advance
Thoma
Get a 24V @ 100mA supply. The switching noise will kill the performance of your amplifier.
Take apart the PSU and convert it to a Cockroft -Walton voltage multiplier. For the inductor, 10Turns air spaced around a 4mm former would do.The coupling capacitor 1nF.
Frank
Hi,
@ SLK001 :
It was my fear. But the original broken power supply was a switching mode power supply. Unfortunately, it is not longer available.
@ chuckey :
Does a 1nF ceramic capacitor do the job ?
Can I use a 150nH air inductor for this job ?
The 9V is a DC power supply so I can not use Cockroft-Walton voltage multiplier.
Thoma
The larger the Henry value, the more it chokes AC. The required inductor value is not easy to calculate. This might be called a rough estimate... You want to block frequencies above 30 MHz. Suppose your converter has an impedance in the output of 240 ohms (24V / 100mA). A reasonable inductive impedance could be 10x that, or 240 ohms.
Therefore find the inductance L that satisfies the formula XL = 2 * Pi * f * L.
240 = 2 * Pi * 30MHz * L.
As for the capacitor value, it should be large enough so that it charges to a stable DC level and doesn't change much. The less AC current you have going back and forth, the smaller can be the C value. Again this is not easy to calculate. You probably need to experiment.
Hi BradtheRad,
When you say 10x, I suppose you mean 2400 ohms. This gives a 12.7μH inductor.
Thoma
Xl = 2 X PI X 500 X 10^6 X 150 X 10^-9 ~ 450 ohms, this is OK because the coax will have a Zo of 75 ohms and 450 is higher then it, 750 ohms would be better, i.e. 150 X 750/450 or 250 nH. 24V seems a high value for the amp feed, in the UK it is standardised at 12V. Have a look inside it to see if there is some form of voltage stabiliser you can bypass. You are only dealing with small amplitude signals <100 mV.
Frank
Yes, you're correct. My mistake. Anyway it's a crude calculation, based on whatever parameters we can draw on. It could be off by an order of magnitude.