What I want to do is measure the bandwidth of a wireless video transmitter. I know for a fact that this is going to be at least 8 MHz if not much more as it gives a good video image with an OSD which has a pixel clock exceeding 4 MHz. (I'm trying to repurpose the transmitter to send other analog data.) Since I have no access to a high frequency signal generator (10 MHz+) to directly verify the -3dB point, is it possible to measure, say, the -1dB point and derive the frequency of the -3dB point from this?
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2 Answers
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Assuming your transmitter is NTSC, you have 4.2MHz to play with, according to Wikipedia.
To answer the original question, yes, it is possible to measure a signal at a lower frequency (using aliasing), but you must be able to sample at atleast twice the bandwidth of interest, and the signal band must start at a multiple of your sampling rate.
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\$\begingroup\$ I'm not trying to measure a signal using a lower sample rate, I'm trying to measure the bandwidth of a discrete analog transmitter (which AFAIK has no sample rate per se as it is a 2.4GHz tx.) \$\endgroup\$– Thomas OCommented Apr 12, 2011 at 18:21
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No you cannot, as it's all nonlinear and hardly predictable.
But you can build signal generator by yourself, 10Mhz or even 50Mhz is not deadly complex.
You can do rough estimations on square wave generator if you have osciloscope, which is deadly simple - just 1 digital invertor chip (F or LV/LVC series).
answered Apr 12, 2011 at 1:53
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\$\begingroup\$ Once generated, feed your signal into a flip-flop and divide it down to something you can measure. \$\endgroup\$ Commented Apr 12, 2011 at 3:32
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\$\begingroup\$ If you make a square wave, don't forget to filter off the harmonics before making response measurements. \$\endgroup\$ Commented Apr 12, 2011 at 3:33
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1\$\begingroup\$ I thought it would be exponential, like in this image: en.wikipedia.org/wiki/File:Butterworth_response.png and then that would be a simple equation to work out -3dB from -1dB... but perhaps it is not? \$\endgroup\$– Thomas OCommented Apr 12, 2011 at 7:44
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\$\begingroup\$ It depends on the scheme specifics. If might fall faster or slower, it might have input filter... \$\endgroup\$ Commented Apr 12, 2011 at 17:05
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1\$\begingroup\$ so could it be done from measuring many points on a curve? How do you build a reliable sine wave oscillator for a 10 - 50 MHz sweep? \$\endgroup\$– Thomas OCommented Apr 12, 2011 at 23:00
