You didn't particularly mention it, but I guess, the "220 MHz" output should have a constant pulse rate (no or low jitter)? In this case, a PLL is required. A special designed phase detector could trace the number of edges of both signals.
I imagine, they'll be counted separately and the results added. Possible in principle, but not necessarily serving a reasonable purpose.What happens when the edges of the two signals line up?
Nothing difficult. Square wave is just a combination of sinusoids (spectrum looks like sin(x)/x).If they were sinusoids, you could use a mixer and low pass filter, but because they are pulses, it's a LOT more difficult.
Yes, but the solution hasn't to do with digital design. It requires a high Q, accurately adjusted filter.After further review, I think you're correct, a mixer/bandpass just might work with square-wave inputs.
If he generates 200 MHz square wave in digital form, let him just generate 220 MHz instead and that is all, why doesnt he do it?
It is normal in a mixer to have one of the signals be essentially a square wave (switching the other signal). So maybe if one of the signals is bandpass filtered to make it mostly a sine wave then you could follow traditional heterodyne methods.Nothing difficult. Square wave is just a combination of sinusoids (spectrum looks like sin(x)/x).
Mixer, or multiplier produce a lot of different components with the frequencies +/-n*f1 +/- m*f2.
Anyway, if the inputs are sine or square.
Applying a Bandpass filter for 220 MHz you remove all other components and leave the single sine you need.
To convert it to square use amplifier + hard limiter.
Anyway the result of mixing will be a sum of lot of components, so it is unnessesary to convert both input signals to sine prior to mixing. Just a single good bandpass at output I think.It is normal in a mixer to have one of the signals be essentially a square wave (switching the other signal). So maybe if one of the signals is bandpass filtered to make it mostly a sine wave then you could follow traditional heterodyne methods.
Anyway the result of mixing will be a sum of lot of components, so it is unnessesary to convert both input signals to sine prior to mixing. Just a single good bandpass at output I think.
This is about the worst solution. It produces an irregular pulse train that could be anywhere between 200,000,000 and 220,000,000 pulses per second, depending on the relative phase of the two input signals and their precise frequencies.. The most likely count is 216,000,000. (I leave that as an exercise for the reader, assuming 1nS mono-shots.)The best option seems to be one which simply does edge detection of each signal, mono-shots this for about 1-2nS, and OR's the result. Along the lines of what Barry and FvM have been thinking.
as long as the signals are not precisely in sync, this will produce a better result than all the band-pass filtering, ...
This is about the worst solution. It produces an irregular pulse train that could be anywhere between 200,000,000 and 220,000,000 pulses per second
Even if the two signals are not in sync you will still get about 216,000,000 pulses per second out because about one in five of the 20 MHz pulses will just happen by random chance to fall close enough to a 200 MHz pulse so that it will not be counted separately. The only way your scheme could work is if the two signals were in fact in sync, but offset in phase just enough so that all the 20 MHz pulses missed all the 200 MHz pulses - a very unlikely scenario.who said anything about a requirement for a regular pulse train ? and i did say the 2 signals should NOT be in sync - implying non-aligned transitions.
thanks for the vote of confidence btw.
Even if the two signals are not in sync you will still get about 216,000,000 pulses per second out because about one in five of the 20 MHz pulses will just happen by random chance to fall close enough to a 200 MHz pulse so that it will not be counted separately. The only way your scheme could work is if the two signals were in fact in sync, but offset in phase just enough so that all the 20 MHz pulses missed all the 200 MHz pulses - a very unlikely scenario.
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