I am trying to come up with a decent filter that will block HV DC and stop any AC signals above 10KHz with high gain as possible as per attached image, really I need an LC filter design. Any ideas/brainstorms welcome.
In post #1 i meant to say high attenuation not high gain by the way.
I have tried other designs with LC low pass but I has issues with using the T-shaped low pass filter that the VDSL signal would not work because there was a direct short to ground with the caps needed for the 1Khz signal.
Then you're going to need a very large capacitor for the DC blocking. Around 50uF to meet that 100Hz cutoff with a 50ohm load. Something like this. After that, a simple lowpass LC filter using lower voltage components will be fine. Keep in mind the filter response will depend greatly on the load impedance.
In post #1 i meant to say high attenuation not high gain by the way.
I have tried other designs with LC low pass but I has issues with using the T-shaped low pass filter that the VDSL signal would not work because there was a direct short to ground with the caps needed for the 1Khz signal.
Ok the next spec required is source , input, output and load impedance over entire range of audio and VSDL to avoid interference.
assume it is high impedance above 50MHz but need to know impedance to avoid resonant, same with Csource =? in audio at 1Khz
Ok the next spec required is source , input, output and load impedance over entire range of audio and VSDL to avoid interference.
assume it is high impedance above 50MHz but need to know impedance to avoid resonant, same with Csource =? in audio at 1Khz
Until you define source and load impedance, I assumed Cin, Rin and Cout and the rest was added to meet your requirements. The L, 100 mH is rather large, depends on the 100 Ohm source and has internal Rs of 10 Ohms to achieve the flat response. Lower Rs would sharpen the low end and much higher would degrade attenuation.
This plot is from 10 Hz to 25Mhz and is 20dB/decade at both ends.
Then again 100mH will have a lot of self capacitance and low SRF which improves high end attenuation and degrades low end.
I am trying to work this out, the signals will be coupled onto a 50ohm single coaxial cable over different lengths so its hard to initially see what these will be, i dont know how much the different lengths will effect this.
You ended up with a regular third order low-pass + coupling capacitor (= first order high pass), plausible so far. It can be tuned according to your specification which was yet clearly given.
Two comments:
- you may want to use a finer frequency increment than 1 point per octave in SPICE simulation
- I understand that it's telephone line problem. You should add realistic line impedances to your simulation, otherwise the results won't have much to do with real live. Why did you select 50 ohms load impedance?
With it connected to a 50 Ohm coaxial cable will these all ideally be 50ohm for impedance matching??
You should add realistic line impedances to your simulation, otherwise the results won't have much to do with real live. Why did you select 50 ohms load impedance?
Exactly this. Let's assume 10 kHz cut-off frequency is O.K., also third order slope (18 dB/octave), then acceptable passband ripple/gain deviation should be specified.
I agree that 50 ohm load impedance is a reasonable choice, but only if the source impedance isn't much greater than 50 ohms. This won't be the case for a telephone line. Load impedance must be known for an exact calculation of filter characteristic anyway.
I agree that 50 ohm load impedance is a reasonable choice, but only if the source impedance isn't much greater than 50 ohms. This won't be the case for a telephone line. Load impedance must be known for an exact calculation of filter characteristic anyway.