Table of Contents
CHAPTER 1 — GENERAL INFORMATION
1-1. SCOPE
This manual covers design, construction, and calibration of fixed attenuator pads for use as calibration standards. Pi and T-topology pads can be constructed from precision resistors to provide attenuation values from 1 dB to 30 dB with ±0.1 dB accuracy and SWR <1.2:1 to 500 MHz.
CHAPTER 2 — THEORY OF OPERATION
2-1. PAD TOPOLOGIES
A matched attenuator pad maintains input and output impedance equal to Z0 (typically 50 Ω) at all attenuation settings. This ensures that the pad does not disturb the impedance environment of connected equipment.
Pi Pad (50 Ω)
T Pad (50 Ω)
| AdB | k | R1 = R3 (Ω) | R2 (Ω) | Nearest 1% std |
|---|---|---|---|---|
| 3 | 1.412 | 292.4 | 17.6 | 294 / 17.8 |
| 6 | 1.995 | 150.5 | 37.4 | 150 / 37.4 |
| 10 | 3.162 | 96.2 | 71.2 | 97.6 / 71.5 |
| 20 | 10.00 | 61.1 | 247.5 | 61.9 / 249 |
CHAPTER 3 — MATERIALS AND CONSTRUCTION
3-1. BILL OF MATERIALS (6 dB PAD EXAMPLE)
| Qty | Item | Specification |
|---|---|---|
| 2 | 150 Ω resistor | 0.1%, 50 mW, metal film (R1, R3 shunt) |
| 1 | 37.4 Ω resistor | 0.1%, 50 mW, metal film (R2 series) |
| 1 | SMA double-ended enclosure | Mini-circuits style, or BNC-BNC barrel with PCB |
| 1 | Small PCB | 50 Ω microstrip or air-wired |
3-2. CONSTRUCTION
Wire attenuator in pi configuration on small copper-clad PCB. Keep resistor leads <5 mm. Ground both shunt resistors directly to the case/shield. Use SMA or BNC connectors on both ends.
CHAPTER 4 — ASSEMBLY PROCEDURES
- Measure all resistors with calibrated ohmmeter. Select parts closest to calculated values.
- Solder shunt resistors (R1, R3) directly from center conductor pads to ground plane. Keep vertical.
- Solder series resistor (R2) between input and output center pads. Keep horizontal, minimize lead length.
- Install in SMA or BNC barrel enclosure. Verify no short circuits before applying signal.
- Label case: impedance, attenuation in dB, date.
CHAPTER 5 — CALIBRATION PROCEDURE
- Connect pad to TinySA: source on input, TinySA on output.
- Set TinySA to 0–200 MHz, measure insertion loss.
- Verify attenuation = Ameasured at multiple frequencies: 1, 10, 30, 100, 200 MHz.
- Attenuation should be flat within ±0.2 dB from DC to 100 MHz.
- Measure input reflection: |S11| should be <−20 dB (SWR <1.22) if resistor values are correct.
- Record actual attenuation vs. designed value. Correction = Adesigned − Ameasured.
CHAPTER 6 — TUNING AND ADJUSTMENT
Attenuator pads are not tunable after construction. If attenuation is off by >0.5 dB, check individual resistor values and replace as needed. A 1% error in R2 causes approximately 0.1 dB error in attenuation at high attenuation values.
CHAPTER 7 — VERIFICATION
- Cascade two 6 dB pads: total should measure 12.0 ±0.2 dB.
- Compare measured attenuation against NanoVNA S21 measurement for cross-check.
- Log: attenuation at 1 MHz, 10 MHz, 100 MHz; SWR at each end; date.
APPENDIX A — CALCULATIONS AND FORMULAS
APPENDIX B — EXAMPLE RESULTS
| Pad | Designed (dB) | 1 MHz (dB) | 10 MHz (dB) | 100 MHz (dB) | SWR in |
|---|---|---|---|---|---|
| 6 dB Pi pad | 6.00 | 6.02 | 6.01 | 6.05 | 1.02 |
| 10 dB Pi pad | 10.00 | 9.97 | 9.98 | 10.03 | 1.03 |
| 20 dB Pi pad | 20.00 | 20.1 | 20.1 | 20.2 | 1.04 |