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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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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).

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Once generated, feed your signal into a flip-flop and divide it down to something you can measure. – markrages♦ Apr 12 '11 at 3:32
If you make a square wave, don't forget to filter off the harmonics before making response measurements. – markrages♦ Apr 12 '11 at 3:33
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? – Thomas O Apr 12 '11 at 7:44
It depends on the scheme specifics. If might fall faster or slower, it might have input filter... – BarsMonster Apr 12 '11 at 17:05
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? – Thomas O Apr 12 '11 at 23:00
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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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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.) – Thomas O Apr 12 '11 at 18:21
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