Chapter 1 — Introduction and Scope
This manual covers five SDR interface circuits that extend or improve the performance of RTL-SDR and similar low-cost SDR receivers: (1) HF upconverter (0–30 MHz to 125–155 MHz), (2) 16-band switchable bandpass preselector, (3) dual-band bias-T for remote LNA power, (4) PE4302 digital step attenuator with SPF5189Z LNA option, and (5) direct-sampling HF modification. All circuits interface via SMA connectors; the preselector and attenuator are ESP32-controlled.
Chapter 2 — Theory of Operation
2-1 HF Upconverter
An SA612A double-balanced mixer converts HF input (0.1–30 MHz) to IF (125.1–155 MHz) by mixing with a 125.000 MHz TCXO local oscillator. The RTL-SDR is then tuned to 125–155 MHz to receive HF signals without needing direct-sampling mode. An SPDT relay bypasses the upconverter for 6m and above (where RTL-SDR tunes directly). Optional SPF5189Z LNA (+19 dB, NF = 0.6 dB) precedes the mixer.
| Parameter | Value |
|---|---|
| Input frequency | 0.1–30 MHz |
| LO frequency | 125.000 MHz (±2.5 ppm TCXO) |
| IF output | 125.1–155 MHz |
| Conversion gain (passive) | −2 dB |
| Conversion gain (with LNA) | +17 dB |
| Image frequency | >155 MHz (suppressed >45 dB by IF BPF) |
| LO leakage to RF port | <−40 dBm |
2-2 16-Band Switchable Preselector
Sixteen bandpass filters (one per amateur band, 160m through 23cm plus one bypass) select with an SP4T×4 relay tree driven by a 4-to-16 decoder (74HC154) from ESP32 GPIO[3:0]. Filter topology: 3-pole Chebyshev BPF for 160m–10m; interdigital for 2m/1.25m; stripline/SAW for 70cm and above. Rejection of out-of-band signals: ≥50 dB adjacent band, ≥60 dB for FM broadcast (88–108 MHz) when tuned to HF.
2-3 Bias-T
A bias-T injects DC power onto coax to supply a remote LNA or active antenna. An RF choke (L, >10µH SRF) passes DC while blocking RF; a DC-blocking capacitor (C) passes RF while blocking DC. The HF version (Version A) covers 0.1–50 MHz; the VHF/UHF version (Version B) covers 10 MHz–3 GHz.
2-4 PE4302 Digital Step Attenuator
The PE4302 provides 0–31.5 dB attenuation in 0.5 dB steps via 6-bit SPI control. Frequency range DC–3 GHz, IP3 = +40 dBm, NF = attenuation setting + 0.5 dB. Combined with an SPF5189Z LNA (+19 dB gain, NF = 0.6 dB), the combination provides adjustable gain from −31.5 dB (full atten) to +18.5 dB (no atten, LNA on).
Chapter 3 — Equipment and Materials
| Module | Key ICs | Connectors | Supply |
|---|---|---|---|
| HF Upconverter | SA612A, 125 MHz TCXO, K1 relay | SMA in/out | 5V USB, 80 mA |
| Preselector | 74HC154, 4× SP4T relays, 16 BPF modules | SMA in/out | 5V, 200 mA |
| Bias-T (HF) | L = 10µH RFC, C = 100 nF | SMA in/out/DC | 5V or 12V from SDR |
| Bias-T (VHF) | L = 100 nH, C = 10 nF NP0 | SMA | 3.3V/5V/12V |
| Attenuator/LNA | PE4302, SPF5189Z | SMA in/out | 3.3V |
| Controller | ESP32 WROOM-32 | USB-C | 5V |
Chapter 4 — Installation and Setup
4-1 Stacking Order
Connect SDR interface modules in this order from antenna to SDR:
Antenna → Bias-T (if using remote LNA) → Preselector → Attenuator/LNA (optional) → HF Upconverter (HF only) → RTL-SDR
4-2 Preselector Band Selection
The ESP32 selects the correct filter automatically if configured with
the current receive frequency. Manual override: set GPIO[3:0] to the
filter index (0–15) using the CYD touchscreen or serial command
BAND <n>.
4-3 Upconverter LO Alignment
The RTL-SDR must be told to offset the displayed frequency by the LO value. In SDR# or GQRX: set “LO offset” or “frequency correction” to +125.000 MHz. In GNU Radio: subtract 125e6 from the reported frequency to get the true HF frequency.
Chapter 5 — Operating Procedures
5-1 HF Reception with Upconverter
- Enable bypass relay for frequencies ≥60 MHz (GPIO or front-panel switch). For 0.1–30 MHz: use upconverter path.
- In SDR software: tune to 125 + f_HF (MHz). E.g., 40m = 7.15 MHz → tune SDR to 132.15 MHz.
- Enable preselector for the band in use to reduce out-of-band interference. FM broadcast (88–108 MHz) causes severe IMD in RTL-SDR at HF; preselector eliminates this.
5-2 Adjusting Gain
- Start with maximum attenuation (31.5 dB) on PE4302. Reduce in 6 dB steps until the noise floor drops and desired signals become visible.
- Enable SPF5189Z LNA only for weak-signal work (<−100 dBm); for strong-signal environments (near broadcast transmitters), keep LNA off to avoid IMD.
Chapter 6 — Calibration
- Inject a known signal (−50 dBm at 14.175 MHz) into the upconverter input. Verify the SDR displays the signal at the correct displayed frequency (offset by LO).
- Preselector: verify each filter passes its intended band within 1 dB of bypass, and rejects adjacent bands by ≥40 dB.
- Attenuator: apply 0 dBm CW; step through 0, 10, 20, 30 dB settings. Verify SDR S-meter drops by 10 dB per step (±1.5 dB).
Chapter 7 — Verification and Acceptance
- WWV reception check: with upconverter active and 40m preselector selected, verify WWV at 5.000 MHz or 10.000 MHz is audible at noise floor SNR ≥15 dB.
- FM rejection: verify FM broadcast stations at 88–108 MHz are not visible while preselector is set to any HF band (≥60 dB suppression relative to bypass mode).
- Log: date, upconverter LO error (measured vs. GPS reference), preselector insertion loss at band centers, attenuator step accuracy, operator.
Appendix A — SDR Software LO Offset Settings
| Software | Setting | Value for 125 MHz LO |
|---|---|---|
| SDR# | Shift frequency | +125000000 Hz |
| GQRX | LNB LO (Tools → Offset) | 125.000 MHz |
| GNU Radio | Frequency subtract block | 125e6 Hz offset in flowgraph |
| CubicSDR | Not supported natively; use IF math | — |
Appendix B — Image Frequency Calculator
For any LO: Image freq = LO + f_if_desired = LO + (LO + f_hf) = 2×LO + f_hf With LO = 125 MHz, f_hf = 7.15 MHz: f_image = 2×125 + 7.15 = 257.15 MHz This is above the RTL-SDR's tuning range → image is rejected by the IF BPF at 125–155 MHz (>45 dB suppression).