Merv's Brain Dump

Gear Profile

• Example models: Generic bench or handheld units
• Frequency range: Varies by model; verify datasheet
• Connector type: BNC, SMA, or binding posts
• Typical use: General measurement and calibration checks

Purpose

This guide describes a homebrew, no-calibration-needed method to calibrate a capacitance meter using public reference signals and first-principles checks. It borrows techniques from:

• ../gps_calibration.md
• ../fm_broadcast_calibration.md
• ../radio_standard_calibration.md
• ../verification_procedures.md

What You Will Build

• A simple reference load (homebrew 50 ohm)
• A quarter-wave coax stub test fixture
• Optional GPS 1PPS counter (if you want tighter frequency checks)

Expected Accuracy

• Frequency axis: 0.1 to 1 ppm (with GPS or time standards)
• SWR/impedance: limited by resistor tolerance (typically 1-2%)

Calibration Philosophy

We use absolute references (GPS 1PPS, WWV/CHU, FM stations) and known physics (speed of light, coax velocity factor, resonance) to validate the analyzer without relying on any pre-calibrated lab equipment.

Key Principles

1. Frequency accuracy: Check displayed frequency against GPS/WWV/CHU/FM carriers.
2. Impedance accuracy: Validate using a known load and known reactance from a stub or LC.
3. Repeatability: Measurements should be stable over time and temperature.

Reference Sources

• GPS 1PPS: Atomic time tick (best accuracy).
• WWV/CHU: HF time standards (good accuracy).
• FM broadcast: Convenient local reference (moderate accuracy).

Build a 50 ohm Load

• Use four 200 ohm, 1% resistors in parallel.
• Solder directly inside a BNC/SMA connector shell if possible.
• Keep leads short to reduce inductance.

Build a Quarter-Wave Stub

• Choose coax with known velocity factor.
• Cut to calculated length (see Calculations).
• Short the far end (center to shield).
• Label the stub with its target frequency and VF.

Optional GPS 1PPS Interface

• GPS module with 1PPS output.
• LED + resistor for lock indication.
• Optional ESP32/CYD counter (see optional code example).

Assembly Steps

1. Build or verify the 50 ohm load.
2. Build one or more quarter-wave stubs (pick key bands).
3. Prepare short, known-good test leads.
4. Warm up the analyzer (10-15 minutes) before calibration checks.

Step-by-Step

1. Warm up the analyzer for 10-15 minutes.
2. Frequency check using WWV/CHU or FM station:
3. Measure known carrier.
4. Calculate ppm error.
5. Apply correction if supported.
6. Impedance check using the 50 ohm load:
7. Confirm 45-55 ohm range.
8. Record SWR.
9. Reactive check using the shorted stub:
10. Find resonance dip.
11. Compare with calculated frequency.
12. Document the offsets and repeatability.

Frequency Axis Check

• Measure WWV/CHU or FM carrier frequency.
• Compare to published frequency.
• If your analyzer supports frequency offset adjustment, apply correction.

Impedance/SWR Check

• Connect the 50 ohm load.
• The analyzer should read close to 50 ohm and low SWR.
• Connect the shorted stub and locate the resonance dip; compare to expected.

Trim and Iterate

• If stub resonance is off, trim length in small increments.
• Re-check until resonance aligns within expected tolerance.

Verification Checklist

• Re-measure the reference carrier after calibration.
• Measure a second independent reference (FM vs WWV/CHU).
• Confirm measurements are stable across 3-5 repetitions.

Acceptance Targets

• Frequency accuracy: within 0.1-1 ppm of reference.
• Load accuracy: within 1-2% of 50 ohm.

Quarter-Wave Coax Stub

Use a shorted coax stub to create a known resonance:

L = (c / (4 * f)) * VF
                

Where: - L = stub length (meters) - c = 299,792,458 m/s - f = frequency (Hz) - VF = velocity factor of coax (e.g., 0.66 solid PE, 0.78 foam)

Example

Target frequency: 14.200 MHz

L = (299,792,458 / (4 * 14,200,000)) * 0.66
                L = 3.49 m
                

Cut slightly long, then trim while watching the analyzer until resonance hits target.

50 ohm Load (Homebrew)

Parallel resistor network:

R_total = 1 / (1/R1 + 1/R2 + ... + 1/Rn)
                

Example

Four 200 ohm resistors in parallel:

R_total = 1 / (4/200) = 50 ohms
                

Use 1% or 0.1% resistors if possible.

Example Log

Date: 2026-01-15
                Gear: Example Analyzer
                
                Reference: WWV 15 MHz
                Measured: 15,000,012 Hz
                Error: +0.8 ppm
                
                Load Test: 50 ohm load
                Measured: 50.9 ohm
                SWR: 1.02
                
                Stub Test (14.200 MHz target)
                Measured dip: 14.198 MHz
                Error: -0.14%