Coupling DC with RF signal

[toyonline]

Hi, I am working on generating a high voltage RF signal floated on a small DC voltage. The basic network I used is attached. RF signal was generated by resonance LC tank, and DC was from a DC power supply to generate ~10V.

Since the RF I used is 5kVpp @ ~1MHz, to protect DC power supply, the DC was connected into the network through a filter.

The expected output (J2) was a RF floated on a DC voltage. But the problem is, the network seems sometimes alter my DC voltage measured. For example, after a period of use, when I turn off RF signal (switch off linear amp), measured output (purely DC) was 0.2V, and the orignial set one was 5V.

This phenomenon doesn't regularly appear. I cannot control, but it does arise somehow.

Does this suggest I should replace my caps like F1 or C7? any other suggestions?

Thx.
----Toyonline

 

[chuckey]

you do not give a value for R1, it and the 150 ohms may be overheating. Also you do not say what the DC current load is. How do you monitor the DC level?
Frank

 

[toyonline]

Hi chuckey, although limit current of my DC power supply was set to be 1A, I don't think there is a DC current in the circuit, since the output is simply applied on a float electrode. I am using voltage and electric field within. R1 was removed, and let the two nodes across it to be open.

But your suggestion on R1 overheating gives me a hint. I have using a wire to connect DC power supply and DC input of the circuit. In order to further protect the DC supply, I fused a 100Mohm resistor at the end terminal of the banana pin. I am guessing a poor fusion of 100M with the pin, or worn out of the fusion. Is it possible the phenomenon was due to this reason?

I have used the circuit for quite long time (1year), this problem appears recently. The only thing I changed was replacing a C3 cap. But I don't think this will result in what I observed.

Thx.
----Toyonline

you do not give a value for R1, it and the 150 ohms may be overheating. Also you do not say what the DC current load is. How do you monitor the DC level?
Frank