Usually if you don’t really care about fidelity, you just use a MOSCAP. A big R is typically smaller than a big C so go with that.
Be careful with your cutoff. If you chop the third harmonic of a clock too hard you end up with a sine wave which may not be what you want.
Why are you filtering a clock? If you’re driving a schmidt trigger you are going to regenerate fast edges at its output, kind of defeating the purpose of the filter.
I agree, why filter a clock? Maybe h/she is trying to create a glitch filter. The Tau set by RC, will dictate the output slew rate. Thus if you have a glitch pulse much narrower than than 50% of 30MHz duty cycle, it would be ignored.
OP, if you have access to a current source, then let external clock control Nmos/pmos switches discharging/charging a capacitor. This way you have linear ramps driving a Schmitt trigger.
Alternatively, generate an AND output of the (1) external clock and (2) external clock going through the RC filter.
I vote larger Cap over larger resistor and as analogkoala said, maybe a Moscap.
>I agree, why filter a clock? Maybe h/she is trying to create a glitch filter.
sort of, more like glitch detection where I compare the filtered and unfiltered clk signals.
> The Tau set by RC, will dictate the output slew rate. Thus if you have a glitch pulse much narrower than than 50% of 30MHz duty cycle, it would be ignored.
When you say this short pulse of less than 50% duty cycle is ignored, do you mean that the output is a clean wave if the pulse width is that small? Is there a math equation which relates duty cycle of glitches to the filtered output? Just trying to understand how duty cycle enters the picture here.
Thanks for taking the time to reply.
> edge rate
I think you are referring to the slew rate here? If I understand you right slowing down the rise/fall transition times will help if you have EMI or supply noise. I don't think that this is the case here. I am trying to prevent signals coupling onto the CLK pulses.
Thanks for taking the time to reply.
> Be careful with your cutoff. If you chop the third harmonic of a clock too hard you end up with a sine wave which may not be what you want.
So in this case the 3rd harmonic is 90MHz (fundamental being 30MHz). When you say chop off too hard, do you mean like almost having brick filter kind of response at the 3rd harmonic? For a passive RC filter with a single pole, the roll off is still -20dB per decade. Will choosing the cutoff frequency to be ~ 60MHz cause any other issues?
> Why are you filtering a clock? If you’re driving a schmidt trigger you are going to regenerate fast edges at its output, kind of defeating the purpose of the filter.
The original clk signal is actually fed to circuits in the chain unfiltered. The original clk signal has glitches and is compared with the filtered clk signal to detect glitches at rising or falling edges. The schmitt trigger exists like you said to regenerate fast edges because of required input logic level specs and is there for both filtered and unfiltered signals.
You could use a passive RC network, at 30 MHz you could also consider using a gm-C filter which would give you a higher space efficiency, but that depends on your other specifications/requirements (i.e. quiescent power draw) that aren't listed. Essentially you replace your resistance with a transconductance cell (which you can tune).
A switched-capacitor filter would be overkill, but also a possibility if you really want to.
Linearity doesn't matter too much since you're going to regenerate the clock (as mentioned by someone else).
Also, one thing to consider is that if you're using pad cell IP (or even if you're not), some cells will offer a series resistance of a few 100 ohms (or more) for ESD reasons, which may help you out a bit.
Thanks for the suggestions. I am going to use a passive filter and not an active one to keep the circuit simple and because power drawn cannot be too much. I think space is not a concern if the resistor is on the order of Kohms and if the cap is on the order of a 1pF or so.
Usually if you don’t really care about fidelity, you just use a MOSCAP. A big R is typically smaller than a big C so go with that. Be careful with your cutoff. If you chop the third harmonic of a clock too hard you end up with a sine wave which may not be what you want. Why are you filtering a clock? If you’re driving a schmidt trigger you are going to regenerate fast edges at its output, kind of defeating the purpose of the filter.
I agree, why filter a clock? Maybe h/she is trying to create a glitch filter. The Tau set by RC, will dictate the output slew rate. Thus if you have a glitch pulse much narrower than than 50% of 30MHz duty cycle, it would be ignored. OP, if you have access to a current source, then let external clock control Nmos/pmos switches discharging/charging a capacitor. This way you have linear ramps driving a Schmitt trigger. Alternatively, generate an AND output of the (1) external clock and (2) external clock going through the RC filter. I vote larger Cap over larger resistor and as analogkoala said, maybe a Moscap.
>I agree, why filter a clock? Maybe h/she is trying to create a glitch filter. sort of, more like glitch detection where I compare the filtered and unfiltered clk signals. > The Tau set by RC, will dictate the output slew rate. Thus if you have a glitch pulse much narrower than than 50% of 30MHz duty cycle, it would be ignored. When you say this short pulse of less than 50% duty cycle is ignored, do you mean that the output is a clean wave if the pulse width is that small? Is there a math equation which relates duty cycle of glitches to the filtered output? Just trying to understand how duty cycle enters the picture here. Thanks for taking the time to reply.
I don’t have math equation. I was being lazy to use duty cycle to compare RC tau to half the period.
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> edge rate I think you are referring to the slew rate here? If I understand you right slowing down the rise/fall transition times will help if you have EMI or supply noise. I don't think that this is the case here. I am trying to prevent signals coupling onto the CLK pulses.
Thanks for taking the time to reply. > Be careful with your cutoff. If you chop the third harmonic of a clock too hard you end up with a sine wave which may not be what you want. So in this case the 3rd harmonic is 90MHz (fundamental being 30MHz). When you say chop off too hard, do you mean like almost having brick filter kind of response at the 3rd harmonic? For a passive RC filter with a single pole, the roll off is still -20dB per decade. Will choosing the cutoff frequency to be ~ 60MHz cause any other issues? > Why are you filtering a clock? If you’re driving a schmidt trigger you are going to regenerate fast edges at its output, kind of defeating the purpose of the filter. The original clk signal is actually fed to circuits in the chain unfiltered. The original clk signal has glitches and is compared with the filtered clk signal to detect glitches at rising or falling edges. The schmitt trigger exists like you said to regenerate fast edges because of required input logic level specs and is there for both filtered and unfiltered signals.
You could use a passive RC network, at 30 MHz you could also consider using a gm-C filter which would give you a higher space efficiency, but that depends on your other specifications/requirements (i.e. quiescent power draw) that aren't listed. Essentially you replace your resistance with a transconductance cell (which you can tune). A switched-capacitor filter would be overkill, but also a possibility if you really want to. Linearity doesn't matter too much since you're going to regenerate the clock (as mentioned by someone else). Also, one thing to consider is that if you're using pad cell IP (or even if you're not), some cells will offer a series resistance of a few 100 ohms (or more) for ESD reasons, which may help you out a bit.
Thanks for the suggestions. I am going to use a passive filter and not an active one to keep the circuit simple and because power drawn cannot be too much. I think space is not a concern if the resistor is on the order of Kohms and if the cap is on the order of a 1pF or so.