On 4/12/2010 3:25 PM, David Barton wrote:
Hi,
I have questions about the parameters in the DBPSK and Low Pass
Filter blocks in GRC.
Low Pass Filter block:
1) If I set the lowpass filter block to interpolate by a factor
of 10 should the sampling rate parameter by set to the input sampling
rate or the output sampling rate since they will obviously differ by a
factor of 10?
2) If I set the lowpass filter block to interpolate by a factor
of 10 is the filtering operation performed before interpolation or
after interpolation? (Can the filter get rid of the spectral copies
centered at multiples of the input sample rate that are created during
interpolation or do I need another lowpass filter afterwards to remove
these spectral copies?)
One of my favorite things about GRC is that it allows you to quickly
create sample programs to test stuff just like this. You can lay down a
signal source, interpolating filter, and FFT and scope sinks to see
what happens when you adjust the parameters.
Specifically, 1) yes, the sample rate should be at the output rate of
the filter (it's generally true that you want to build your filter at
the highest sampling rate), and 2) yes, an interpolating filter will
interpolate before filtering to remove the spectral images (in actual
fact, the implementation details are more complicated, but the effect
is the same that the spectral images are removed).
DBPSK block:
1) What exactly is the Samples / Symbol parameter mean? I am
trying to figure out what the relationship is between the input sample
rate (bytes per second) to the output sample rate in (complex samples
per second).
2) Since the input is bytes is each bit of the byte converted to
a separate symbol meaning get 8 symbols for each imput sample(each
byte)?
Thanks,
Dave
With BPSK, yes, you get 8 bits out and each of those bits is converted
to 1 symbol. We then interpolate this by the samples_per_symbol >= 2
in order to satisfy Nyquist for the pulse shape filtering. So each
sample out of the shaping filter is represented by samples_per_symbol
number of samples. Play around with this value in GRC and see the
effects of using 2, 4 or 8 samples per symbol in both the frequency and
time domain.
Tom
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