IF to baseband conversion using MT downconvert passband VI

[SunJab]

Hi,

I want to frequency shift a signal from IF to 0Hz (baseband) using 'MT downconvert passband VI'.

I am reading the IF signal from a wave file and giving it to the MT downconvert passband VI....The VI help says it downconverts a real passband signal data of a user specified bandwidth and returns the complex IQ (baseband) data.

 I want to know what is the relationship between passband bandwidth , carrier frequency(i.e IF) and the sample rate ???

I know that the sample rate should be atleast twice the carrier frequency but what about the passband bandwidth???

Here is an example....i have tried to shift a PSK modulated signal (having carrier freq:10k Hz and Sample rate:32k) to baseband...but when i give a passband bandwidth value greater than 11k Hz, it gives me an error saying, 'Invalid sampling rate for requested carrier; Increase the sample rate or decrease the passband bandwidth'

Please help me out!

Thanks in advance 😃

 Sandy

[abhisheknag]

Hello Sandy,

According to Nyquists Theorem (http://en.wikipedia.org/wiki/Nyquist_theorem), sampling rate should be greater than twice the maximum frequency in a real signal. Let's look at your signal. If carrier frequency Fc = 10k and bandwidth BW = 10k, maximum frequency content of the signal is 10k + 5k = 15k. So, you can get away with a sampling rate of 32k. For BW = 12k, maximum frequency content of the signal would be at 16k. I would assume that MT does not allow sampling frequency to be exactly twice the maximum frequency. Which is why you get the error.

In general, for real signals, Fs should be freater than Fc + (BW/2).

Hope this helps. Please feel free to ask for further explanations, if required.

Abhishek