openlunar/chirpsim

Low fidelity time simulation of a chirp signal undergoing doppler shift

chirpsim

A super low fidelity time simulation of a chirp signal and doppler shifting. This tool was originally to help me (Aaron) gain a better understanding of signal mixing, by showing what a significant bandwidth signal (chirp) looks like:

• When upconverted...
  • aka mixed, and band-pass filtered for f_LO+f_IF
• When doppler shifted
• When downconverted...
  • aka mixed with an LO that's corrected and uncorrected for the doppler shift/distortion, and band-pass filtered for f_RF-f_LO

The following is a color-coded diagram of what the script simulates, corresponding to the outputs below.

Requirements

• Python 3.x
• Numpy
• Matplotlib
• Scipy

Installation

Clone this repository:

git clone [email protected]:openlunar/chirpsim.git
cd chirpsim/
pip install -r requirements.txt

Currently, this tool cannot be installed as a package. You must run it out of the repository directory.

Usage

Navigate to the repo's root folder, and run chirpsim.py with arguments:

cd chirpsim/
python chirpsim.py

Or with arguments as desired:

python chirpsim.py [-h] [-t T_MAX] [-s SIZE] [-d DOPPLER] [-bw BW]
                   [-fc F_CENTER] [-fr F_RATE] [-lf LO_F] [-lp PHASE]

Or for help, do the usual:

python chirpsim.py --help

Command Line Arguments

There are unfortunately quite a few args, but hey, what can you do:

optional arguments:
  -h, --help            show this help message and exit
  -t T_MAX, --t-maxx T_MAX
                        total simulation time, in seconds
  -s SIZE, --time-steps SIZE
                        number of timesteps
  -d DOPPLER, --doppler DOPPLER
                        amount of doppler shifting to apply to the chirp
                        signal, in percent

chirp_chars:
  chirp signal characteristics

  -bw BW, --bandwidth BW
                        bandwidth of the chirp signal, in hz
  -fc F_CENTER, --center-frequency F_CENTER
                        center frequency of the chirp signal, in hz
  -fr F_RATE, --frequency-rate F_RATE
                        rate that the chirp spans its bandwidth, in hz/s

lo_chars:
  lo signal characteristics

  -lf LO_F, --lo-frequency LO_F
                        frequency of the lo signal, in hz
  -lp PHASE, --lo-phase PHASE
                        phase shift of the demixing lo, from the mixing lo, in
                        deg

Output

The following plots should show up, which are both color coded to the overview diagram above.

A plot of the time domain:

A plot of the frequency domain: