Lab 10.2 — Simple attenuator pad

Lab 10.2 — Simple Attenuator Pad

Goal

Design a simple attenuation stage for safe SDR/RF experiments and document why attenuation is required before connecting transmitters and receivers.

Engineering question

How do we reduce signal level safely before an SDR receiver or measurement input?

Basic idea

An attenuator reduces signal amplitude and protects sensitive inputs from overload or damage. In SDR experiments, attenuation is mandatory for direct TX-to-RX cable loopback.

Simple voltage divider

Vout = Vin * R2 / (R1 + R2)

Attenuation in dB:

A_dB = 20*log10(Vout/Vin)

Example

Parameter	Value
R1	9 kOhm
R2	1 kOhm
Voltage ratio	0.1
Attenuation	-20 dB

Practical steps

1. Select desired attenuation.
2. Calculate voltage ratio.
3. Choose resistor values.
4. Check load impedance effect.
5. Draw schematic.
6. Measure input and output levels.
7. Document safe operating range.

Safety notes

RF safety

A simple voltage divider is educational. Real RF 50-ohm attenuators should be used for RF cable loopback measurements when power levels or impedance matching matter.

Report checklist

• [ ] State desired attenuation.
• [ ] Calculate resistor values.
• [ ] Calculate expected voltage ratio.
• [ ] Convert attenuation to dB.
• [ ] Explain load impedance limitations.
• [ ] Explain whether this circuit is safe for the intended SDR experiment.

Engineering conclusion template

Generate a matched 50-ohm symmetric π-pad reference and loaded-voltage budget:

python blocks/block_10_kicad_and_basic_electronics/python/lab_10_2_rf_passives_design.py \
  --attenuation-db 10 --impedance-ohm 50 --input-dbm -10

For the default 10 dB design the ideal values are approximately 71.151 Ω for the series resistor and 96.248 Ω for each shunt resistor. Use available resistor combinations, then measure the assembled network; the ideal calculation is not an RF safety certificate.

The attenuator uses R1 = ____ and R2 = ____, giving a voltage ratio of ____ and attenuation of ____ dB.
It is suitable / not suitable for the planned SDR experiment because ______.