Minor side point, if both the signal of interest and noise are
incoherent, then it makes sense to average spectral power density. But
if the signal is has long term phase coherency, complex FFT outputs can
be averaged. This is done with FMCW radar in Doppler processing. The
first FFT does range compression, and the second FFT puts the signal
into Doppler bins and dramatically increases SNR. Stationary objects are
lost in the zero Doppler clutter (which is why my car forward radar sees
moving cars fine but misses cars at a stoplight).
Best,
Karl
On 6/2/2026 1:33 PM, 'Dan Werthimer' via [email protected] wrote:
hi nitin,
since you don't want to add complex FFT outputs together,
and you instead want to gain access to the complex outputs of the FFT,
you could connect the FFT frequency domain complex output to a
snapshot block to capture the output from one or a few spectra,
and then after the data are captured, you can read the contents of the
snapshot block into a computer.
if you need to capture a continuous stream of complex data, then you
will run out of memory using a snapshot block,
so it's better to connect the complex data from the FFT output into a
high speed ethernet block,
and stream the high speed ethernet data to a computer.
best wishes,
dan
On Tue, Jun 2, 2026 at 6:35 AM Nitin Purohit
<[email protected]> wrote:
Dear Prof. Ross and Prof. David,
Thank you for your valuable insights.
As both of you correctly pointed out, I understand the bandwidth
implications associated with accumulating complex voltages rather
than power spectra. However, I am currently working on an
interesting project that requires access to the *complex FFT
outputs* from not one, but *three ADC channels*.
To begin with, I modified the existing single-channel spectrometer
design into a two-channel version, which appears to be functioning
as intended. During this process, I realized that the output data
being accumulated and transmitted is the *power spectrum* rather
than the underlying *complex FFT data*, which is what I ultimately
require for the application.
As suggested by Prof. David, I will explore theturn on the
Simulink option to show the signal data types to analyse the data
flow and will keep the group updated on the progress of the project.
In the meantime, any additional suggestions, references, or
example designs would be greatly appreciated, as I am still trying
to gain a deeper understanding of the CASPER blocks and their
implementation details.
Thanks you,
Nitin
On Tuesday, 2 June 2026 at 10:46:33 UTC+5:30 David Harold Edward
MacMahon wrote:
Hi, Nitin,
I think you have two questions. One about bit width
management and one about accumulating the FFT output.
For the bit width question, the first thing I recommend is to
turn on the Simulink option to show the signal data types.
Then when you do an “Update diagram” the data types of each
signal will be displayed. Some blocks will just output the
sensible output type (bit width/binary point, e.g. "Fix18_17")
given the input types, but many blocks let you specify the
output bit width and binary. Blocks of this type usually have
options to specify what to do when bits at the high end are
“lost”/“dropped” (aka overflow) and what to do when bits al
the low end are lost/dropped (aka quantization). The options
for overflow are “wrap” (i.e. just blindly drop the overflow
bits) or “saturate” (i.e. clamp at max/min value). The
options for quantization are “truncate” (i.e. just blindly
drop the extra bits at the low end) or one of several
different rounding modes. Wrap and truncate are “free”
because they don’t require any extra logic, whereas saturation
and rounding do require extra logic. You can check out the
distinction between the “reinterpret” block and the “cast”
block for more insights.
As for accumulating FFT output, Martin is right. If you
accumulate the complex voltages of the FFT you will
effectively be reducing the bandwidth of each FFT channel. If
you add two consecutive N channel spectra together
channel-by-channel then you will have essentially computed the
even channels of a 2N channel FFT.
Hope this helps,
Dave
On Jun 1, 2026, at 08:15, Nitin Purohit
<[email protected]> wrote:
Dear all,
I have a question regarding obtaining complex outputs from
the wideband spectrometer.
While going through the spectrometer design in detail, I
noticed that the power block appears to compute the
magnitude-squared of the complex FFT output by squaring the
real and imaginary components and then summing them.
<Screenshot from 2026-06-01 18-38-30.png>
In the complex spectrometer design, the FFT output consists of:
* First 24 bits: Real component (MSB first)
* Next 24 bits: Imaginary component (LSB side)
Since each component is multiplied by itself, the resulting
products are 48 bits wide. After the summation, the output
becomes approximately 49 bits (48 + 1 carry bit).
My difficulty is understanding how this output relates to the
subsequent *simple_bram_vacc* block, which is configured with:
* BitWidth = 64
* Binary Point = 34
How are these parameters derived from the incoming data stream?
A similar question arises in the real spectrometer design.
There, the real and imaginary components appear to be 18 bits
each, resulting in a power computation width of approximately
36 + 1 bits. However, the *simple_bram_vacc* parameters
appear to remain unchanged. I am therefore trying to
understand the rationale behind the BitWidth and Binary Point
settings of the accumulator.
<Screenshot from 2026-06-01 19-01-33.png>
From examining the *simple_bram_vacc (figure above the para)*
and *delay_bram* *(figure above the para) *block diagrams, my
current understanding is that:
* A pulse is generated every /vector_length/ samples (512
in this case).
* During the accumulation period, the delay BRAM stores
data at incrementing addresses.
* The accumulation continues until the count reaches
approximately /(DelayLen − bram_latency − 1)/.
However, I am unsure whether this interpretation is correct.
<Screenshot from 2026-06-01 19-11-38.png>
My current goal is to modify the spectrometer to preserve and
output the complex FFT values instead of computing power. If
I bypass the power calculation and directly pass the complex
FFT output into the accumulation stage:
1. How would *simple_bram_vacc* store the incoming complex
values?
2. Would separate accumulators be required for the real and
imaginary streams?
3. Is there an existing CASPER block or example design that
demonstrates accumulation of complex spectra rather than
power spectra?
I would greatly appreciate any explanation or pointers to
relevant documentation regarding this.
Hoping for a response soon,
Thank you,
Sincerely,
Nitin
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Department of Electrical and Computer Engineering
Brigham Young University
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(801) 422-1732
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