Bandwidth/Demodulation Bandwidth

hi,

i am interested in one of the following quadrature demodulators: ADL5387, AD8348

i am trying to build a laser rangefinder with an amplitude modulated laser at 300MHz. so the optical detector outputs a sine wave at 300MHz with random phase, depending on the distance of the object. the phase can change slowly, when the object comes closer or moves away from the rangefinder, or there can be quick jumps in the phase if the rangefinder moves along a wall and detects an edge. i need to determine the phase of this sine wave to get the current distance of the object.

since the phase is random and can make great jumps the bandwidth of my signal is basically infinite.

what happens if i use my signal with the above mentioned demodulators? the AD8348 has a demodulation bandwidth of 75MHz, the ADL5387 has a demodulation bandwidth of approx. 240MHz. does this mean that i have to make sure that the input signal bandwidth is not greater than 75MHz respectively 240MHz by filtering the signal with a bandpass at 300MHz+-37,5MHz (300MHz+-120MHz)? or does the demodulator simply clip off any signals beyond the 75MHz (240MHz) after the signal is in the baseband and the filtering in beforehand would be unnecessary effort?

thanks

jejo86

Parents
  • If your desired signal is at 300 MHz and you want to see phase changes, you need some sort of a phase detector, which also means you need a reference signal of constant phase. Here are some ideas, but the devil is in the details...

    1. AD8302 -- drive one input with your 300 MHz detector output and the other with a 300 MHz reference signal. It has a detected phase output. It's phase detector output may or may not be fast enough for you and there is a 180 degree phase ambiguity (it measures the magnitude of the difference but doesn't tell you the sign).

    2. Drive a mixer as a phase detector (likely suspects are the AD8342, AD8343, AD8344) with your detected signal driving the RF input and 300 MHz reference signal driving the LO input. The DC output term will be proportional to the phase difference. One note of warning: Mixers are generally designed to be AC coupled, so they do have offset and drift. These are single supply mixers so you will need level shift the output in order to drive an ADC. Or AC couple if you are looking at low frequency time-varying information.

    3. Use the PFD in a PLL... (look at the ADF4002 PLL) this gets a bit tricky but what you want to do in theory is drive the RF input (N divider) with your 300 MHz signal and the REF input with your 300 MHz reference signal. Set up the divider ratios so that the PFD sees the same input frequencies and then use the charge pump output as your phase detector output. You'll have to design a network that turns the up/down output pulses into a time-varying DC voltage that tracks the phase difference between the two input signals.

    One last thought: If your reference is one of our DDS ICs (the AD9912, for example) then you could use a nulling technique where you change the phase of the AD9912 until you measure a phase difference of zero; then the phase offset setting of the AD9912 will tell you the phase shift you wanted to measure.

    What you're trying to do is a bit nonstandard for these parts so this advice is on the theoretical side (i.e your application is new to us). Good luck!

Reply
  • If your desired signal is at 300 MHz and you want to see phase changes, you need some sort of a phase detector, which also means you need a reference signal of constant phase. Here are some ideas, but the devil is in the details...

    1. AD8302 -- drive one input with your 300 MHz detector output and the other with a 300 MHz reference signal. It has a detected phase output. It's phase detector output may or may not be fast enough for you and there is a 180 degree phase ambiguity (it measures the magnitude of the difference but doesn't tell you the sign).

    2. Drive a mixer as a phase detector (likely suspects are the AD8342, AD8343, AD8344) with your detected signal driving the RF input and 300 MHz reference signal driving the LO input. The DC output term will be proportional to the phase difference. One note of warning: Mixers are generally designed to be AC coupled, so they do have offset and drift. These are single supply mixers so you will need level shift the output in order to drive an ADC. Or AC couple if you are looking at low frequency time-varying information.

    3. Use the PFD in a PLL... (look at the ADF4002 PLL) this gets a bit tricky but what you want to do in theory is drive the RF input (N divider) with your 300 MHz signal and the REF input with your 300 MHz reference signal. Set up the divider ratios so that the PFD sees the same input frequencies and then use the charge pump output as your phase detector output. You'll have to design a network that turns the up/down output pulses into a time-varying DC voltage that tracks the phase difference between the two input signals.

    One last thought: If your reference is one of our DDS ICs (the AD9912, for example) then you could use a nulling technique where you change the phase of the AD9912 until you measure a phase difference of zero; then the phase offset setting of the AD9912 will tell you the phase shift you wanted to measure.

    What you're trying to do is a bit nonstandard for these parts so this advice is on the theoretical side (i.e your application is new to us). Good luck!

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