# How can I reduce cable capacitance for my Schering bridge?

• enamul101b
In summary, Kabir is seeking advice on how to minimize line capacitance for his research on building a Schering bridge. He has tried using two separate coaxial cables, but the capacitance has increased. The suggested solution is to "bootstrap" the shields by driving each side's shield with a buffered version of the test signal, which eliminates the capacitive current and reduces the line capacitance.
enamul101b

I am trying to build a Schering bridge to measure capacitance change for my research. I need to reduce the cable (line) capacitance to a minimum. I am using an ac 4V (rms), 250 KHz supply.

I tried to use two separate coaxial cables to connect the capacitor plates. The signal goes through the inner cable and the outer wrap wire is grounded in both cables (this should work as Faraday cage). But unfortunately I see the line capacitance increases for that.

Please suggest me what should I do to minimize the line capacitance.

Thanks in advance for helping me out.

Kabir

Use the highest impedance and the highest signal velocity coaxial cable you can get. This based on the following two equations for coax cables:

Cable impedance is Z = sqrt(L/C)
signal velocity = βc = 1/sqrt(LC)

where L = inductance per meter, and C = capacitance per meter.

enamul101b said:

I am trying to build a Schering bridge to measure capacitance change for my research. I need to reduce the cable (line) capacitance to a minimum. I am using an ac 4V (rms), 250 KHz supply.

I tried to use two separate coaxial cables to connect the capacitor plates. The signal goes through the inner cable and the outer wrap wire is grounded in both cables (this should work as Faraday cage). But unfortunately I see the line capacitance increases for that.

Please suggest me what should I do to minimize the line capacitance.

Thanks in advance for helping me out.

Kabir

To minimize the capacitance, you can "bootstrap" the shields. Drive each side's shield with a buffered version of the test signal. This basically eliminates the capacitive current to charge up the wire with respect to the shield in each coax.

## 1. How does cable capacitance affect signal quality?

Cable capacitance is the measure of a cable's ability to store electrical charge. High cable capacitance can lead to signal degradation, such as loss of high frequencies and distortion.

## 2. What are some common methods for reducing cable capacitance?

Some common methods for reducing cable capacitance include using cables with lower capacitance, using shielded cables, and keeping cables short and straight.

## 3. How does cable length affect cable capacitance?

Cable capacitance is directly proportional to cable length, meaning that longer cables will have higher capacitance. This is why it is important to keep cables as short as possible when trying to reduce capacitance.

## 4. Can using higher quality cables help reduce cable capacitance?

Yes, using higher quality cables with lower capacitance can help reduce cable capacitance. These cables are typically designed with better shielding and materials to minimize capacitance and improve signal quality.

## 5. Are there any disadvantages to reducing cable capacitance?

One potential disadvantage of reducing cable capacitance is that it can also reduce the amount of power that can be transmitted through the cable. This may not be an issue for low-power applications, but for high-power applications, it may limit the use of lower capacitance cables.

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