comb generator design needed

Ac couple to the input of a cmos inverter gate.

can u provide this information in a detailed form

If this is not enough and you need a circuit to copy exactly then Google can help you; just search 'cmos gate comb generator'
Peter

with the repetition rate of 20 KHz, the output level of the pulses is varying a lot as the frequency is increased. how can i get a constant output level over the 100-180 MHz range and how can the output level be increased to about -20 dBm

That's easy. You can't.

The best you can do is use very fast logic with fast rise/fall times. It will not be all that flat.

As for -20 dbm.....which is 0.01 milliwatts. You have 80 MHz/20KHz=4000 comb teeth. 4000 x 0.01 milliwatt is 40 milliwatts total power. So if you had a 2 watt linear amp, you would probably get a relatively flat -20 dBm output height.

how far can an amplitude limiter be useful for the flat response. if so, what can be the specs

It won't work very well. You have a multitone signal. A limiter will simply clip the envelope in the time domain, which will give you a really screwed up un-flatness in the frequency domain.

What exactly is your application?

I suggest to calculate the basic spectral properties of the rectangular "needle" pulse of the suggested comb gen circuit from it's fourier series.

You can assume e.g. a 1 or 2 ns wide 5V pulse. Without considering the limited rise time, I got about -64 dBm @100 MHz for 5V/1ns/20 kHz
period into 50 ohm. For the 1 ns ideal square pulse, the amplitude is decreasing only slightly between 100 and 180 MHz, for 2 ns, the effect is
already considerable (due to sin(x)/x spectral shape). You may want to use an equalizer network for a systematic compensation.

The distance to intended -20 dBm is huge, however. To amplify the wide band comb generator output in a linear amplifier, it has to be band
filtered first.

I agree, you should filter before trying to amplify. I would go with a lowpass/highpass combo in series instead of a bandpass!

I agree, I didn't mean a particular filter implementation, just "cutting out" the band of interest.