[hpsdr] spectrum increased frequency resolution?

[David McQuate]

I agree with most of what you wrote except the statement that frequency 
resolution is increased.
When you decimate, you reduce the effective sampling rate, but you also 
reduce the number of
samples.  The frequency resolution (SamplingFrequency / NumberOfSamples) 
is not changed.

Another expression for calculating the frequency resolution is ( 1 / 
TimeRecordLength ).
When we decimate, we eliminate all but every Nth data point, but the 
remaining points still span
the same length of time, leaving the frequency resolution unchanged.

Dave
wa8ywq

>  David McQuate wrote:
>> ***** High Performance Software Defined Radio Discussion List *****
>>
>> If I have a block of samples (say N = 4096, fs = 122.88 Msps),
>> a straightforward FFT produces data with frequency resolution of 30kHz.
>>
>> Is there a technique that will allow me to obtain information on a finer
>> frequency grid (say 1kHz or 300Hz or ...)--other than "simply" acquiring
>> additional data, extending the length of the time record?
>>
>> I've looked at Chirp-Z transforms and polyphase decimation, but neither
>> seems to produce higher frequency resolution.  I've also looked into DttSP,
>> but not found a solution there.
>>
>> Am I hoping for the impossible?
>>
>> Suggestions please.
> What if you take the time series data _before_ the 4096 points FFT, 
> and multiply it by a complex
> exponential so to bring the spot of interest at zero Hz, then you 
> low-pass filter and decimate it by
> a convenient factor. At this point you have a much reduced sampling 
> rate and doing now an FFT
> with the same number of points will give a much greater resolution.
>
> When you want to move up or down in frequency, you just change the 
> value of the complex exponential,
> which can be easily done in a semi-continuous way, given the high 
> sampling rate (122.88 MHz) you start with.
>
> 73  Alberto  I2PHD


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