How fast is a photoresistor when used in RF?

[neazoi]

Hello,
I am trying to build an RF attenuator for HF (1-30MHz).
My intention is to see how fast a photoresistor (LDR) is for this application.
I do not refer to the responce times of the LDR to the light intensity. I refer to the RF response of the LDR or otherwise the RF characteristics of the LDR for HF.
Would it be as good as a common resistor?

Take as an example the VTL5C1 vactrol

 

[davenn]

curious as to why you want to use this device ?

how much RF power were you planning to put through the LDR section ?

something like this is probably much better and it's likely to be impedance matched
https://www.hittite.com/products/index.html/category/250


Dave

 

[FvM]

Please review this paragraph from the PerkinElmer Vatrol databook:

Capacitance

The equivalent circuit for the output photocell is a resistor in parallel with the capacitance. The capacitance arises from the topside metallization of the electrodes which form a coplanar capacitor. The value of this capacitance is largely determined by the size of the ceramic base. For lower capacitance, a smaller cell is needed. The capacitance is so small (3.0 pF, typical on catalog AOIs) that it is negligible in most applications. However, there are applications such as wideband or high frequency amplifiers in which the capacitance needs to be considered. At 4.5 MHz, the video baseband frequency, the photocell capacitive reactance is only 12 kilohms. If the phase shift of the signal is to be kept below 10°, the highest useful cell resistance is only 2.0 kilohms. At high AOI input drive, where the cell is drive below 1.0 kilohm, the capacitance can increase additionally from 2 to 10 times, possibly due to distributed effects.

You can calculate that the LDR capacitance reduces the achievable control dynamic with increasing frequency.

 

[neazoi]

Please review this paragraph from the PerkinElmer Vatrol databook:


You can calculate that the LDR capacitance reduces the achievable control dynamic with increasing frequency.

Thanks very much! That completely answers the question