================================================================================ direct_sampling_hf_ascii.txt — TM-SDR-SCH-002 Rev A Direct Sampling HF Interface for RTL-SDR v3 / RSP1A / Airspy HF+ Anti-Aliasing Filters, Impedance Matching, Direct Q-Branch Input Document: TM-SDR-SCH-002 Rev A ================================================================================ OVERVIEW -------- The RTL-SDR v3 and similar dongles support "direct sampling" mode: HF signal is fed directly to the RTL2832U ADC, bypassing the R820T2 tuner. The ADC runs at 28.8 Msps (nominally), giving a Nyquist limit of 14.4 MHz. With proper anti-aliasing filters, this covers 0.1–14.4 MHz (160m–20m+30m). Two input paths on RTL-SDR v3: Q-branch direct: 0–14.4 MHz (HF direct sampling) I-branch direct: same, used for some dongles Practical usable range: 0.5–28.8 MHz with careful aliasing management. KEY SPECIFICATIONS ------------------ ADC: RTL2832U, 8-bit, 28.8 Msps nominal Nyquist BW: 0–14.4 MHz (clean), alias risk 14.4–28.8 MHz Dynamic range: 48 dB (8-bit ADC), improved by bandpass filtering MDS: −110 dBm typical (without LNA, 500 Hz BW) MDS (with LNA): −125 dBm typical Input impedance: 50Ω (with matching network) RTL-SDR v3 Q input: ~2 kΩ || 3 pF (requires impedance transformation) BLOCK DIAGRAM ------------- [SMA IN] ─── [LPF 30 MHz] ─── [BPF band] ─── [50:2k transformer] ─── [Q-branch] 50Ω HF optional impedance match RTL-SDR v3 OR for better performance with Airspy HF+: [SMA IN] ─── [LPF 32 MHz] ─── [LNA SPF5189Z] ─── [50Ω coax] ─── [Airspy HF+] (optional) 50Ω input SCHEMATIC — ANTI-ALIASING LOW-PASS FILTER (7-pole Elliptic, 30 MHz) ---------------------------------------------------------------------- Purpose: Attenuate signals above 30 MHz before ADC. Also provides harmonic/image suppression. Topology: 7-pole elliptic (Cauer) LPF, −60 dB at 45 MHz Characteristic: equiripple passband 0.1 dB, 60 dB stopband above 45 MHz L1 L2 L3 IN ──┬── ... ──┤─────────┤─────────┤── ... ──┬── OUT │ │ │ │ │ │ │ │ C1 C2 Ct1 C3 Ct2 C4 Ct3 C5 │ │ │ │ │ │ │ │ GND GND GND GND GND GND GND GND Simplified: L-C-L ladder with shunt traps at stopband frequencies. NODE LAYOUT: SMA IN ──+── L1 ──+── L2 ──+── L3 ──+── L4 ──+── SMA OUT │ │ │ │ │ C1 C2 C3 C4 C5 │ ─┤─ │ ─┤─ │ GND Ct1/Lt1 GND Ct2/Lt2 GND (trap) (trap) Values (50Ω system, 30 MHz cutoff, 7-pole elliptic): L1 = L4 = 100 nH L2 = L3 = 390 nH C1 = C5 = 56 pF NP0 C2 = C4 = 150 pF NP0 C3 = 180 pF NP0 Trap 1 (Lt1 = 68 nH, Ct1 = 15 pF): resonates at ~50 MHz Trap 2 (Lt2 = 39 nH, Ct2 = 10 pF): resonates at ~80 MHz All inductors: Coilcraft 0805CS series or wound toroid (T25-12) All capacitors: NP0/C0G 50V 0805 Performance: Insertion loss: < 0.3 dB from 0.5–28 MHz Attenuation at 30 MHz: 3 dB (by design, cutoff) Attenuation at 45 MHz: > 60 dB Attenuation at 88 MHz: > 80 dB (FM broadcast rejection) Return loss: > 20 dB from 1–28 MHz FM BROADCAST NOTE: FM broadcast at 88–108 MHz produces intermodulation in the ADC. This LPF provides >80 dB FM rejection, critical for urban use. SCHEMATIC — IMPEDANCE MATCHING (50Ω TO RTL-SDR Q INPUT) ---------------------------------------------------------- RTL-SDR v3 Q-branch input impedance: ~2 kΩ at HF frequencies (Measured: the Q-branch input pin is essentially ADC input through ESD diodes) Option A — Transformer matching (recommended): Broadband toroidal transformer, 50Ω : 2000Ω (ratio √(2000/50) = 6.32:1 turns) Use: 1:6 turns ratio on FT-37-43 ferrite toroid Primary: 4 turns Secondary: 25 turns Frequency range: 1–30 MHz (FT-37-43 good below 30 MHz) Wind: bifilar style, equal coverage around core LPF OUT ── L_primary (4T) ──── GND │ L_secondary (25T) ──── Q-input pin │ GND (center tap if desired) Option B — Resistive termination (simpler, more loss): LPF OUT ──┬── R_term (1.8 kΩ) ──── GND └── C_couple (10 nF) ──── Q-input pin This presents ~1.8 kΩ to the ADC but wastes signal (10 dB loss). Useful for strong-signal environments; simple and flat response. Option C — JFET follower (best for high-Z, low capacitance): LPF OUT ── J310 JFET source follower ── Q-input J310 gate impedance: >1 GΩ; source output: ~50Ω to ~500Ω Provides isolation; JFET source sets working impedance. Drain: +5V via 1kΩ Gate: input (via 100Ω protection) Source: output (10 µF blocking cap to Q-input) JFET sets source-follower gain ~0.9 (−0.9 dB) RECOMMENDED: Option A (transformer) for lowest noise; Option C for portability. SCHEMATIC — OPTIONAL PRESELECTOR BPF BYPASS --------------------------------------------- A relay (K2) allows switching between wideband (anti-alias LPF only) and narrow preselector BPF (from preselector board, detailed separately). Signal path: LPF OUT ── K2 (COM) ──> K2 (NO) ── directly to impedance match ──> Q-input └─> K2 (NC) ── to/from preselector board ──> back here K2: 5V SPDT relay, Axicom V23105 or equivalent. GPIO_BPFSEL (from ESP32) controls K2 via ULN2003A. SCHEMATIC — OPTIONAL LNA FOR DIRECT SAMPLING --------------------------------------------- For Airspy HF+ or RSP1A (which have 50Ω input), add LNA before ADC: SMA IN ── LPF ── [SPF5189Z] ── 50Ω coax ── SDR dongle SMA +19 dB NF 0.6 dB SPF5189Z wiring (same as upconverter section): Pin 1 (IN): via C_in (100 pF) from LPF output Pin 2 (GND): ground Pin 3 (OUT): via C_out (100 pF) to SDR SMA Pin 3 (Vcc): 3.3V via 100 nH choke, 100 pF bypass cap to GND CAUTION: With +19 dB gain, strong HF broadcast can overdrive 8-bit ADC. Use inline step attenuator (0/10/20 dB) to set proper ADC level. Target: ADC input −20 to −30 dBFS (not clipping; not too quiet). SCHEMATIC — POWER AND CONTROL ------------------------------- USB Micro-B J1: VBUS ─── F1 (500 mA PTC) ─── +5V rail GND ─── GND +5V → AMS1117-3.3 → +3.3V rail: 3.3V → SPF5189Z Vcc 3.3V → logic ICs Control header (6-pin, 2.54mm): Pin 1: +3.3V Pin 2: GND Pin 3: GPIO_LNA (HIGH = LNA active) Pin 4: GPIO_BPFSEL (HIGH = preselector engaged) Pin 5: GPIO_ATT10 (HIGH = 10 dB pad inserted) Pin 6: GPIO_ATT20 (HIGH = 20 dB additional pad) ALIASING MANAGEMENT FOR RTL-SDR DIRECT SAMPLING -------------------------------------------------- RTL2832U ADC: 28.8 Msps (3.2 MHz × 9 = 28.8 MHz) Nyquist: f_N = 14.4 MHz Aliases fold as: f_alias = |f_signal − n × 28.8 MHz| for integer n With 30 MHz LPF: Signals 0–14.4 MHz: received directly (no alias) Signals 14.4–28.8 MHz: alias to 28.8−f (mirror image) Example: 21.225 MHz → alias at 28.8−21.225 = 7.575 MHz (overlaps 40m!) Signals 28.8–30 MHz: very weak due to LPF rolloff SOLUTION for 17m/15m/12m/10m (14.4–29.7 MHz): Option 1: Use UPCONVERTER mode (125 MHz LO → RTL-SDR tunes normally) Option 2: Use SDR with Nyquist > 30 MHz (Airspy HF+, RSP1A) Option 3: Accept aliased display and use software correction (impractical) BANDS SUITABLE FOR DIRECT SAMPLING (with 30 MHz LPF): 160m (1.8–2.0 MHz): YES, clean 80m (3.5–4.0 MHz): YES, clean 40m (7.0–7.3 MHz): YES, clean 30m (10.1–10.15 MHz): YES, clean 20m (14.0–14.35 MHz): YES (below 14.4 MHz Nyquist — barely fits) BANDS REQUIRING UPCONVERTER OR HIGHER-SAMPLE-RATE SDR: 17m (18.068 MHz): exceeds Nyquist → use upconverter 15m (21.0 MHz): exceeds Nyquist → use upconverter 12m (24.89 MHz): exceeds Nyquist → use upconverter 10m (28.0 MHz): exceeds Nyquist → use upconverter ALIGNMENT PROCEDURE -------------------- Equipment: NanoVNA, 50Ω SMA cables. 1. Set NanoVNA: Port 1 to LPF input SMA. Port 2 to LPF output SMA. Sweep: 0.1 to 100 MHz. 2. Check S21 (insertion loss): Target: < 0.3 dB from 1–28 MHz. If high: check solder joints on inductors, verify capacitor values. 3. Check S21 at cutoff: 30 MHz: should be −3 dB ± 1 dB. 45 MHz: should be < −60 dB. 88 MHz: should be < −70 dB. 4. Check S11 (return loss): Target: > 20 dB (< −20 dB S11) from 1–28 MHz. If poor: adjust L1 or L4 value (trim by bending turns on toroid). 5. With RTL-SDR connected: Tune to known signal (e.g., WWV 10.000 MHz, 5.000 MHz, 15.000 MHz). Verify signal readable on GQRX/SDR++. Compare to directly connected antenna (should be similar or better). BILL OF MATERIALS — DIRECT SAMPLING INTERFACE ----------------------------------------------- Qty Part Value/Type Package ───────────────────────────────────────────────────────── 1 U1 AMS1117-3.3 LDO regulator SOT-223 1 U2 SPF5189Z LNA (optional) SOT-89-3 1 T1 FT-37-43 4:25T transformer toroid 2 K2 relay SPDT 5V coil SIL-4 2 SMA SMA edge 50Ω edge mount PCB 1 USB Micro-B USB power PCB 1 F1 500 mA PTC polyfuse 1812 2 L1,L4 100 nH 0805CS 0805 2 L2,L3 390 nH 0805CS 0805 2 Lt1 68 nH trap inductor 0805 2 Lt2 39 nH trap inductor 0805 5 C1–C5 56–180 pF NP0/C0G 50V 0805 2 Ct1 15 pF NP0/C0G 0402 2 Ct2 10 pF NP0/C0G 0402 various bypass caps 100 nF/10 µF 0805 ================================================================================