Merv's Brain Dump

Chapter 1 — Introduction and Scope

This manual covers four audio filter designs for improving receiver selectivity and noise reduction: an LC notch filter (eliminating heterodyne interference), an active CW bandpass filter (300–500 Hz center, ±50 Hz bandwidth), an LC SSB bandpass filter (2.4 kHz, 300–2700 Hz passband), and a DSP audio processor (ESP32-based FIR/IIR filtering with 16-bit 44.1 kHz audio codec).

Chapter 2 — Theory of Operation

2-1 LC Notch Filter

A series LC circuit resonant at the interference frequency presents near-zero impedance in parallel with the audio path, pulling the interference signal to ground. At resonance f₀ = 1/(2π√LC), the notch depth depends on component Q. Practical notch depth: 30–50 dB. A variable capacitor makes the notch tunable across 200–5000 Hz.

2-2 Active CW Filter

A cascade of two Sallen-Key bandpass stages using LM833 (low-noise, 15 MHz GBW) op-amps provides approximately 36 dB/octave roll-off outside the passband. The center frequency Q is set by resistor ratios; Q = 10 for CW (narrow, 50 Hz −3 dB BW at 500 Hz center). This narrow bandwidth dramatically improves CW copy in high-noise conditions.

2-3 DSP Audio Processor

An ESP32-S3 with I2S audio codec (PCM5102A output, INMP441 microphone-grade input) implements digital filters in software. FIR filters achieve linear phase (no transient distortion); IIR filters achieve steep rolloff with fewer taps. Available modes: CW narrow (500 Hz), SSB bandpass, noise reduction (spectral subtraction), and audio peak filtering (sharp peak at detected CW frequency).

Chapter 3 — Equipment and Materials

Chapter 4 — Construction

4-1 LC Notch Filter

1. Wind 1 mH inductor on a ferrite pot-core for the highest achievable Q (target Q ≥50). Measure inductance with LCR meter before mounting.
2. Connect a variable capacitor (10–100 pF air-variable) in series with the inductor. This LC series combination connects in shunt (in parallel) with the audio signal path between the receiver audio output and the headphone or speaker.
3. Tune by listening: with an interfering tone present, adjust the variable cap until the tone disappears. Notch depth should be audibly dramatic (>30 dB).

4-2 Active CW Filter

1. Build two Sallen-Key stages. Stage 1: f₀ = 500 Hz, Q = 5. Stage 2: f₀ = 500 Hz, Q = 10. Series connection gives combined Q ∼50 (approximate).
2. Power: use a ±12V regulated supply. Use 100 nF NP0 + 10 µF electrolytic bypass on each supply pin within 5 mm of the op-amp.
3. Set gain <6 dB to avoid clipping. Check output at maximum received signal level with an oscilloscope — no flat-topping.

Chapter 5 — Operating Procedures

1. LC Notch: insert inline in the headphone lead. Tune the variable cap while a heterodyne tone is present until the tone nulls. Leave set; retune if the interferer changes frequency.
2. Active CW Filter: insert inline. Turn on with the band switch. The filter is narrowest in CW mode; switch to bypass or SSB mode if copying SSB. The filter does not switch fast enough for RTTY (use DSP mode).
3. DSP Filter: select mode on the CYD touchscreen. CW NARROW enables a 500 Hz FIR bandpass. NOISE REDUCE applies spectral subtraction; optimal for weak SSB signals with broadband noise.

Chapter 6 — Calibration

1. Inject a 1 kHz −30 dBV audio tone into the input. Measure output level with a true-RMS voltmeter. Passband insertion loss must be <3 dB for in-band signals.
2. Inject tones at 100, 200, 300, 3000, 4000 Hz. Verify attenuation ≥30 dB (LC notch) or ≥40 dB (active) for out-of-band signals.
3. DSP filter: use a sweep generator (PC software or TinySA audio output) to sweep 100–5000 Hz. Verify −3 dB points match the selected filter specification within 10%.

Chapter 7 — Verification and Acceptance

1. CW filter: passband 450–550 Hz, loss <3 dB; stopband outside 300–700 Hz, attenuation ≥30 dB.
2. SSB filter: passband 300–2700 Hz, loss <3 dB; >50 Hz and >3000 Hz at least 30 dB down.
3. LC notch: tunable to any frequency in 200–5000 Hz range; notch depth ≥30 dB at any tuned frequency.
4. No audible distortion on a steady 1 kHz tone at maximum receiver audio output (no clipping).
5. Log: date, filter type, passband −3 dB points, stopband attenuation at key test frequencies, operator.

Appendix A — Filter Design Formulas

LC series resonance (notch frequency):
  f0 = 1 / (2π × sqrt(L × C))

Sallen-Key BPF center frequency:
  f0 = 1 / (2π × R × C)    [equal-component design]

Q (quality factor):
  Q = f0 / BW_3dB

BW at -3 dB:
  BW = f0 / Q

Appendix B — Worked Example

CW filter, f₀ = 500 Hz, Q = 10:

BW = 500 / 10 = 50 Hz  (-3 dB bandwidth)
-3 dB points: 475 Hz and 525 Hz
Skirt at 300 Hz: well into stopband (200 Hz away from center)
Skirt attenuation (2-pole): ~12 dB/octave; at 300 Hz (~0.8 octave below)
  attenuation ≈ 2 × 12 × log2(500/300) = ~16 dB per stage × 2 stages = ~32 dB total