EE 499 · Auxiliary material
dB Gym RF units practice
Welcome
One machine at a time.
Build the reflexes you need for RF engineering.
By the end, you should be able to look at dB, dBm, dBW, dBi and related units and immediately know what they mean and what you may do with them.
Practice, not an exam. Nothing is scored, nothing is locked, and you can revisit any machine. It supports Week 8 and Week 9 and belongs to no numbered week.
Machine 1 of 8
Ratio or Level?
Is this a ratio, gain or loss, or does it represent a level relative to a reference?
dBi is antenna gain relative to an isotropic radiator, not an absolute RF power level. dBu (0 dBu = 0.775 V RMS) is mainly audio and secondary in this RF course.
Machine 2 of 8
What Is the Reference?
Every referenced unit hides a reference. Name it.
What is the reference of ?
Reference
dBu is supplementary here: it comes from audio work and is not central to the satellite course.
Machine 3 of 8
10 log or 20 log?
Power quantity: \(10\log_{10}\). Voltage or field quantity: \(20\log_{10}\).
You are taking a decibel ratio of:
Caution (from the R&S note): the simplified 20 log voltage relation is valid only if both voltages are across the same resistance. With different resistances:
Try it: does the resistance matter?
Simplified \(20\log_{10}(U_1/U_2)\):
Correct power ratio:
There are not two different kinds of decibel. The factor of 20 appears because power is proportional to the square of a voltage or field quantity under the appropriate impedance conditions.
Machine 4 of 8
Signal Chain
Send a level through the blocks and watch it change.
The signal travels left to right. Edit any value, reorder with the arrows, remove blocks. Up to 8 blocks. At most one transmit antenna, and it is always the last block; this machine stops at the antenna.
Conducted power or EIRP? Before the antenna, the running level is the conducted RF power. An antenna gain is in dBi, a ratio against an isotropic radiator, and the level after it is the EIRP.
Machine 5 of 8
Can I Add These?
Can these numbers be combined directly?
Add gains and losses in dB.
Add physical powers in linear units.
Machine 6 of 8
Conversion Bench
Choose a conversion, then type in either box. The other updates.
dBu is mainly audio; it is here so you recognize it on instruments.
Power and voltage levels connect only through a resistance. Into 50 Ω, 0 dBm is 0.224 V, which is 107 dBµV. Without a stated resistance, dBm and dBµV cannot be converted into each other.
Machine 7 of 8
3–6–10–20 Reflex Trainer
Answer by reflex. No score and no timer.
Memory card (power):
| dB | +3 | +6 | +10 | +20 | −3 | −6 | −10 | −20 |
|---|---|---|---|---|---|---|---|---|
| power | ×2 | ×4 | ×10 | ×100 | ×½ | ×¼ | ×0.1 | ×0.01 |
Voltage: +6 dB ≈ ×2 and +20 dB = ×10 (power ×4 and ×100 respectively).
Machine 8 of 8
Spot the Mistake
Read the engineering statement. Is it correct or incorrect?
Final workout
Link Budget Machine
Now use everything you practiced on a real satellite link. Build it one step at a time and watch the waterfall grow.
This final workout uses the same link-budget reasoning developed in Week 9.
Step 1 of 8
Workout complete
If these calculations now feel mechanical rather than mysterious, the dB Gym has done its job.
Rohde & Schwarz — “dB or not dB?”, Application Note 1MA98_14e (2024).
Open reference (opens in a new tab)
Concepts and examples are informed by that note; it is optional reading.