# Very high frequency band pass filter

I designed a band-pass filter. I set the center frequency to 90 MHz. When I do an AC analysis, I see the filter graph I want, but the transient analysis results are not what I wanted.

I'm giving a single signal pulse to the input and I want to see that at the output, but the signal is distorted due to the filter.

I want to do good filtering without distorting the input signal.

What should I change in the circuit? What should I add?

I am sharing 2 filters that I have made with you.

The results of the filter circuit below are shown in the graphs. The frequency response of this filter is as I want it, but the results are very bad in the transient analysis. It distorts the input signal pulse.

Circuit:

Frequency response graph:

Filter input pulse:

Filter output pulse:

Now let's move on to the other filter circuit with a good transient result graph.

This filter circuit and its results are shown below.

Circuit:

Frequency response graph (this graph is not suitable.)

Filter input pulse:

The pulse I get at the comparator output (the results I want should be roughly like this.)

• I'm signal is linear at best with a 30 dB range and the Bode plot has a ridiculous >400 dB range so if you compare the plots just looking at the top 30 dB and that higher Q is what makes the difference, not the DC Jan 19, 2022 at 7:29
• But Dc analysis results do not occur as I want. I'm giving a single signal pulse to the input That's not DC analysis. In a DC analysis, the signal does not change over time, so simulationg a pulse response is impossible in a DC analysis. What you're doint is a time-analysis called transient analysis. Jan 19, 2022 at 7:50
• What bandwidth do you want, and why? Jan 19, 2022 at 9:30
• You can't eat your cake and have your cake. If you want a specific transient response then design your filter to suit that. Jan 19, 2022 at 10:19
• Are really needed R7, R14, R12, R16 ? In such filters ... are only needed the first and the last 50 Ohm. rf-tools.com/lc-filter Jan 19, 2022 at 13:04

The passive filter response looks normal for a highish-Q bandpass.

The active filter response is different because it doesn't work: the opamps don't have a negative power supply, yet the circuit would require them to output a negative voltage, so they're stuck in clipping when output voltage is near zero. In addition, the maximum input voltage spec of 4.5V is violated.

So you should use +/-2.5V power supplies for the opamps, and use a lower input amplitude.

Your analysis is far from being accurate. A BPF will slow down risetime ( due to -3dB BW) and cause ringing whenever Q>1. Its inverse is called Damping Factor. It seems you need a digital solution is a time delay, being a one-shot shorter than the input pulse.

To realize your transfer function, let's look at what you want.

## +ve Pulse narrower

$$\~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\$$Input: $$\~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\$$Output

$$\t_{PW50}\$$, width $$\~~~~~~~~~~~~~~~~~\$$~some pulse like 40 ns$$\~~~~~~~~\$$< 10 ns
$$\t_R\$$ , rise time $$\~~~~~~~~~~~~~~~~~\$$ ~1 V/ns$$\~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\$$same
V range$$\~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\$$5 V$$\~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~\$$ same
$$\t_{p}\$$ Propagation delay: $$\~~~~~~~~~~~~~~~~~\$$ TBD
Load Capacitance [pF]: $$\~~~~~~~~~~~~~~~\$$ TBD assume 30 pF max

• A typical trace over ground plane 3cm/ pF +IC load (let's assume 30 pF, a standard datasheet load).
• A ground plane is needed to lower the impedance if prop. delay approaches or exceeds rise time.

Classic One shots are found in logic '123 package.

74HC123: Transition time 15 ns max @ 4.5V , PWmin = 45 ns typ @ 5V (TI) This does not meet your requirements.

74ALC123: is lower output capacitance (4 pF) and half the driver resistance. ~ 25 ohms typ @ 5V but 45 ohms max @ 4.5V = Vol/Iol

Pulse Width (min) = 5ns . This looks similar to your requirements which you need to get into a habit of articulating in a list such as above.

For faster risetimes, use Current Mode Logic (CML), which has fewer choices and excludes one-shots. But you can make one with NAND gates and an RC delay as I have done here.

Then you may achieve sub-nanosecond pulse widths with a suitable controller impedance board design with test points, tip/ring probes and/or SMC connectors, cable with 50 ohm load.