¶ Signal Transmission — Visualizing AM and FM Waves (Grade 8)
Teach the properties of waves by visualizing amplitude-modulated (AM) and
frequency-modulated (FM) signals in the time and frequency domains using
software-defined radios on the COSMOS testbed.
¶ Overview
This tutorial is part of the COSMOS Education Toolkit series and targets
the MS-PS4-1 Next Generation Science Standard. Students use two SDR nodes on
the COSMOS testbed as a transmitter–receiver pair to observe FM signals in
real time. The experiment is driven entirely through a web interface served by
the toolkit software; no manual command-line radio configuration is required.
One node acts as the FM receiver: it listens on a selected center frequency
(400 MHz, 2.4 GHz, or 5 GHz), demodulates the incoming FM signal, and streams
the audio over TCP. The other node acts as the FM transmitter. Students switch
roles, observe the spectrum display, and record their findings before sharing
out as a class.
The tutorial assumes the COSMOS Education Toolkit setup steps (web server,
GNU Radio flowgraphs, etc.) have already been completed by following the
Education Toolkit parent tutorial.
¶ Learning outcomes
After completing this tutorial you will be able to:
- Explain the difference between amplitude modulation (AM) and frequency
modulation (FM) in terms of wave properties. - Observe the spectrum of an FM signal at the transmitter and compare it with
what is received. - Describe how the amplitude of a wave is related to its energy.
- Use the COSMOS Education Toolkit web interface to start and stop a remote
transmit/receive experiment. - Interpret time-domain and frequency-domain plots of a real-world RF signal.
¶ At a glance
| Difficulty | Beginner |
| Estimated time | 45 min |
| Domain / sandbox | BED (bed.cosmos-lab.org) — nodes sdr2-s1-lg1 and sdr2-md1 |
| Topic group | Education Toolkit |
| Last verified | Not re-tested (migrated 2026-06-20) |
¶ Prerequisites
Background knowledge
- Completion of the COSMOS Education Toolkit setup tutorial — the toolkit web server must be running before this experiment can be conducted.
- Basic concepts of waves: frequency, wavelength, amplitude.
- MS-PS4-1 background reading on wave properties (provided in the toolkit lesson plan).
Account & access
- A COSMOS account — register here.
- An approved reservation on the domain above — see Make a reservation.
- SSH access to the domain console — see SSH access.
¶ Resources required
Devices / nodes
| Resource | Role | Qty | Notes |
|---|---|---|---|
sdr2-s1-lg1 |
Node #1 (receiver or transmitter) | 1 | SDR-equipped node in BED |
sdr2-md1 |
Node #2 (transmitter or receiver) | 1 | SDR-equipped node in BED |
Disk images
| Image | Load onto | Provides |
|---|---|---|
| TODO: verify | sdr2-s1-lg1, sdr2-md1 |
COSMOS Education Toolkit web server + GNU Radio FM flowgraphs |
Software components
| Component | Version | Source |
|---|---|---|
| COSMOS Education Toolkit | TODO: verify | Preinstalled in toolkit image; web UI at http://localhost:8081 |
| GNU Radio | TODO: verify | Preinstalled in toolkit image |
| TCP audio client | TODO: verify | Attached to the toolkit — connects to port 8100 on the remote node |
Spectrum / RF
Center frequency options: 400 MHz, 2.4 GHz, or 5 GHz (selected in the web
interface). Experiment is conducted over the air between the two nodes.
Ensure the reservation covers the chosen frequency band.
¶ Topology
Two SDR nodes (sdr2-s1-lg1 and sdr2-md1) in the BED domain transmit and
receive FM signals over the air. Both nodes expose a local web interface at
http://localhost:8081 (tunnelled to the student's browser via the toolkit
setup). The receiver node additionally opens a TCP server on port 8100 to
stream demodulated audio back to the student's local machine.
¶ Setup
The toolkit web server and GNU Radio back-end must already be running as
described in the Education Toolkit parent tutorial.
Once that is complete:
- Reserve the BED domain and log into the console:
ssh <username>@console.bed.cosmos-lab.org - Confirm both nodes are powered on and reachable (the toolkit setup should
have handled imaging):omf stat -t sdr2-s1-lg1,sdr2-md1 - Open the toolkit web interface in a browser. The URL is served from one of
the nodes on port8081. If the setup was done correctly you will see the
toolkit home page.
¶ Instructions
¶ Step 1 — Navigate to the Grade 8 Signal Transmission lesson
-
In the toolkit web interface, press the Science radio button to open
the science lesson plan page. -
Select Grade 8.
Note: The toolkit contains 100+ lesson plans and labs. Only a subset
currently supports remote experimentation via the COSMOS website; the
selection grows over time. -
Select Signal Transmission from the Grade 8 lesson list.
The lesson page shows:
- A detailed description of the experiment.
- The MS-PS4-1 educational standard being addressed.
- Step-by-step experiment execution instructions.
-
Expand the Remote Experiment section on that page to reveal the
START/STOP controls for each node.
¶ Step 2 — Start the receiver
Press START under the receiver panel on either Node #1
(sdr2-s1-lg1) or Node #2 (sdr2-md1).
- The receiver begins listening on the selected center frequency (400 MHz,
2.4 GHz, or 5 GHz). - It demodulates any incoming FM signal.
- It opens a TCP server on port
8100to stream the audio back to the local
host.
In this tutorial Node #1 is
sdr2-s1-lg1and Node #2 issdr2-md1.
If Node #1 is the receiver, then Node #2 will be the transmitter, and
vice versa.
¶ Step 3 — Connect the local TCP audio client
Start the TCP client application on the student's local machine (distributed
as part of the toolkit) to connect to the remote receiver and play back the
incoming audio.
At this point students should hear noise only — the transmitter has not
yet been started.
¶ Step 4 — Start the transmitter
Press START under the transmitter panel on the other node (whichever
node is not the receiver in Step 2).
The transmitter begins sending an FM signal on the same center frequency.
Observe the spectrum display in the web interface and record observations.
Students should note changes in both the time-domain waveform and the
frequency-domain spectrum as the transmitter comes on.
 |
 |
|---|---|
| (a) FM signal at the transmitter | (b) FM signal at the receiver |
¶ Step 5 — Stop the experiment and share observations
When the experiment is complete, press STOP on both the transmitter and
the receiver nodes.
Students compare their observations of the FM signal and state their
interpretations. After all students complete their handouts, conduct a
class share-out of observations and comments.
¶ Verification
- After starting the receiver (Step 2), the audio client plays audible noise.
- After starting the transmitter (Step 4), the spectrum display shows a clear
FM signal peak at the chosen center frequency, and any modulated audio
becomes audible through the TCP client. - Stopping the transmitter causes the FM peak in the spectrum display to
disappear and the audio to return to noise.
¶ Cleanup
Press STOP on both nodes through the toolkit web interface, then power
off from the console:
omf tell -a offh -t system:topo:allres
(No omf save needed unless the toolkit image was modified.)
¶ Troubleshooting
| Symptom | Likely cause | Fix |
|---|---|---|
Web interface at http://localhost:8081 does not load |
Toolkit web server not started | Follow the Education Toolkit setup tutorial to start the server on the node |
Audio client cannot connect to port 8100 |
Receiver not yet started, or firewall/SSH tunnel misconfigured | Confirm the receiver node shows RUNNING in the web UI; verify the SSH tunnel forwarding port 8100 |
| No FM signal visible on the spectrum display | Transmitter not started, or center frequency mismatch between TX and RX | Confirm START was pressed on the transmitter panel and both nodes use the same frequency |
| Spectrum shows signal but audio is noise | Demodulation parameter mismatch | Stop both nodes and restart; verify toolkit software version |
¶ References
- COSMOS Education Toolkit parent tutorial
- NGSS MS-PS4-1 standard
- Available SDR hardware on COSMOS
- Make a reservation
- SSH access
Author(s): Panagiotis Skrimponis (NYU), Nikos Makris (NYU); COSMOS team · Last verified: Not re-tested (migrated 2026-06-20) · Tested image/release: TODO: verify · Tags: education, education-toolkit, fm, am, signal-transmission, grade-8, waves, sdr