Best way to drive LED from 30kHz-500kHz without causing distortion?

[Mark]

I have a circuit that has a 30kHz - 500kHz sinewave at 1 - 20W into 50 ohms that requires an LED brightness to indicate the power level.  The only power in the circuit is from the input frequency of 30kHz - 500kHz. 

What is the best way to drive the LED without causing distortion to the signal? 
I'm seeing -50dBc on the 5th harmonic when driving it directly (just series R) and would like to improve on that. 
Thank you. 

[edavid]

1. put a second LED in anti-parallel to improve symmetry
2. use higher brightness LEDs, so you can increase the series resistance
3. add a low pass filter (or at least an inductor) between the circuit and the LEDs

[TimFox]

Instead of two LEDs back-to-back, you could also use a four-diode bridge with one LED across the "DC" diagonal.
The non-linear current through the series resistor into the LED load (even if balanced) will cause some distortion on the output from a generator with finite output resistance.
I have an old General Radio 1346 "Microvolter" that contains a simple AC/DC voltmeter before a wide-range attenuator.  Its specification is that it will induce less than 0.5% distortion when the meter rectifier is driven directly (higher-voltage ranges) or < 0.02% when driven through an internal transistorized amplifier (on lower-voltage ranges), on the waveform from a 600 ohm source. 
It can also be used to inject small DC voltages from an internal battery, or as a passive attenuator for AC or DC.
Of course, the load current in the panel meter is much, much less than the current required to light up an LED (perhaps 0.1 mA fs?).

[A.Z.]

I have a circuit that has a 30kHz - 500kHz sinewave at 1 - 20W into 50 ohms that requires an LED brightness to indicate the power level.  The only power in the circuit is from the input frequency of 30kHz - 500kHz. 

What is the best way to drive the LED without causing distortion to the signal? 
I'm seeing -50dBc on the 5th harmonic when driving it directly (just series R) and would like to improve on that. 
Thank you.

now... maybe I'm wrong, but what you are asking for is some kind of "simple, low components count, miracle device" and as far as I know, they are pretty out of stock nowadays, then,  again... maybe I'm wrong

[TimFox]

I have a circuit that has a 30kHz - 500kHz sinewave at 1 - 20W into 50 ohms that requires an LED brightness to indicate the power level.  The only power in the circuit is from the input frequency of 30kHz - 500kHz. 

What is the best way to drive the LED without causing distortion to the signal? 
I'm seeing -50dBc on the 5th harmonic when driving it directly (just series R) and would like to improve on that. 
Thank you.

now... maybe I'm wrong, but what you are asking for is some kind of "simple, low components count, miracle device" and as far as I know, they are pretty out of stock nowadays, then,  again... maybe I'm wrong

He's trying to obtain a specification of "0" for a non-linear circuit that must have positive values of distortion.  Done carefully, he can reduce the magnitude of the distortion, but zero will elude him.

[Mark]

Thank you for all the replies, the 5th harmonic distortion increased by 10dB when adding the LED. 

[rf-fil]

You might try putting a parallel resonant circuit tuned to the 5th harmonic, in series with your resistor and the LED. A parallel resonant circuit acts as "infinite" impedance at it's resonant frequency so it should not allow the 5th harmonic current to flow.


I mean something like this:





                   +------ CCCCCCCCC -------+
                   |                        |
[HOT SIDE] --------+                        +--[resistor]--LED---+
                   |                        |                    |
                   +------- LLLLLLLLL ------+                    |
                                                                 |
[GROUND]---------------------------------------------------------+                           



It won't prevent other harmonics though. If you wanted to reduce all the harmonics, you'd have insert a low pass filter between your circuit and the LED.

[radiolistener]

What is the best way to drive the LED without causing distortion to the signal? 

try to put diode rectifier loaded with capacitor and then put DC from the capacitor to LED.

[boB]

If LED modulation is not linear, you could try Pre-Distorting the sinewave drive so that it comes out less distorted as light

And/Or   drive the LED with a current source ?   LEDs brightness is more proportional to current as I remember.

Still won't eliminate the THD but could hopefully make it much lower. 

BTW, you're looking at the voltage across the LED or the light output ? Up to 500 kHz ?

boB  🌜🌛