On 12/7/21 12:09 am, Alan G4ZFQ wrote:
I wonder why different frequencies? For higher values?
I ask because some criticise meters that measure at low frequencies
although for HF values I've not noticed any difference.
That one is complete, not a barebone assembly.
Your question prompted me to actually look this up. I own a GW Instek LCR-916 which operates between 100Hz and 100KHz. The manual contains a page-long list of accuracies for different L and C values at different frequencies, but I hadn't gotten around to working out why. Some quick searching turned up a worked example <> (I've not checked it):
At 100kHz the impedance of 1uF is only 1.6 ohms and you're trying to measure this in the presence of parasitic resistance, lead inductance and so on.
At 100Hz it is 1.6kohms which is much easier to measure.
For a 2pF capacitor at 100Hz its impedance is nearly 800Mohms so measuring it in the presence of leakage is going to be difficult.
(The applied voltage is typically only about 0.7V so your trying to measure a reactive current of less than a nA!)
At 100kHz it is a much more manageable 800kohms so it is reasonable to give more significant figures.
So for example, when measuring a 200mH inductor, the LCR-916 is accurate to 0.2%@100Hz/1kHz/10kHz, but only 5% at 100kHz. For contrast when measuring 200uH, accuracy is 0.2%@100kHz, 0.5%@10kHz, and unrated at the lower frequencies. Similar results apply for capacitors at various ranges.
This level of precision would appear unnecessary for building SoftRocks — as there's always a way to align when the components are within a few % of nominal — but it's conceivable that measuring very small inductors or capacitors at lower frequencies will yield much larger measurement errors. This is presumably a more serious concern for VHF and UHF circuits.
- Roland 9V1RT
