The Boatanchor SDR Integration Series: Index and IF Buffer Amplifier Reference

A consolidated index to the five SDR integration articles, plus a shared technical appendix on the buffered IF tap that most of them depend on.

1. Series Index

The series was written to be read in order for anyone starting from scratch, but each article also stands alone for its specific receiver. Articles are listed here in publication order.

2. Quick Cross-Reference

For readers who know what equipment they own and want to jump to the right tap point:

3. Why a Buffer Amplifier Is Necessary

The appendix begins here. Everything from this point forward is a shared technical reference that applies across the series.

Four things go wrong if you connect an SDR directly to any of the tap points listed above:

1. Loading of the original circuit. Tube-era IF stages are high-impedance — plate impedances of 50–500 kΩ are normal. An SDR’s 50 Ω input presents an enormous mismatch that will load the circuit heavily, detune the IF transformer, and reduce the receiver’s own performance. The original signal path must be preserved.
2. DC exposure. Plate circuits sit at +150 to +250 VDC with respect to chassis ground. Even with a coupling capacitor in place, capacitor failure or a momentary short could apply that voltage to the SDR’s input, destroying it instantly.
3. Signal level mismatch. An SDR’s IF input typically wants signals in the −20 to −10 dBm range (around 20–70 mV into 50 Ω). A tube-era IF plate signal can swing 1–10 V peak-to-peak. Direct connection overloads the SDR’s front end and produces spurious products that make the waterfall useless.
4. Ground-loop and RFI coupling. A long coaxial cable between receiver chassis and SDR chassis without proper buffering becomes an RF antenna in both directions, picking up ambient noise and radiating switching-mode-supply harmonics from the SDR back into the receiver’s sensitive IF stages.

A proper buffer amplifier solves all four: it presents a high-impedance input to the receiver (no loading), blocks DC with margin (no exposure), attenuates the signal to a level the SDR expects (no overload), and provides an isolated, shielded 50 Ω output (no RFI).

4. Reference Buffer Amplifier Design

The design that follows is a JFET source follower with optional tuned input bandpass. Single 12 V supply; single-ended signal path; small enough to fit inside a 1×2 inch shielded enclosure that mounts inside the receiver cabinet or on an external bracket. It is intentionally simple — the philosophy throughout this series has been that modifications to vintage equipment should be minimal, reversible, and conservative.

Topology

J310 N-channel JFET in source-follower configuration, DC-biased via a 1 MΩ gate resistor to ground, with a tuned or broadband input network depending on the variant. Output is taken from the source through a 51 Ω series resistor to a BNC connector. Nominal gain approximately −6 dB (attenuation) to ensure signal levels stay below the SDR’s overload point.

The schematic above is intentionally ASCII-rendered so it displays correctly across WordPress themes and mobile devices without requiring an image asset. A proper schematic capture with component references for KiCad or Eagle will be included in a follow-up post.

Operation

C2 is the DC-blocking coupling capacitor from the receiver’s IF point to the buffer’s input network. The tuned (Variant 1 or 2) or broadband (Variant 3) network provides the bandpass shaping. R2 sets the gate DC reference to ground, presenting approximately 1 MΩ of input impedance that has negligible loading effect on the tube IF stage. The J310 operates at its zero-bias drain current (typically 8–10 mA, well within its 20 mA maximum); the 220 Ω source resistor R3 sets the output impedance and DC operating point. R4 terminates the output in 51 Ω looking back, matching the coax line.

Overall small-signal gain from input node to 50 Ω load is approximately 0.5, i.e. −6 dB — the source follower itself has gain near unity, and the output network divides by roughly two. This deliberate attenuation gives headroom: IF stages can produce large transient signals during strong-signal conditions, and it’s safer to start with 6 dB of loss than to discover you’re overdriving the SDR.

5. Variant 1 — 455 kHz Tuned Buffer

Use this variant for: R-390A (V506), SP-600 (V7), S-Line receivers, KWM-2/2A (455 kHz tap).

The L1/C3 tank resonates at approximately 455 kHz with Q around 40, giving a −3 dB bandwidth of approximately 11 kHz around the centre frequency. That’s wider than any mechanical filter passband in the receivers listed, so the tank doesn’t constrain the panadapter view — it just keeps out-of-band signals and broadcast-band breakthrough from reaching the buffer’s active device.

6. Variant 2 — 500 kHz Tuned Buffer

Use this variant for: Collins 51S-1 (V4A mixer plate tap).

Identical topology to Variant 1, with tank components adjusted for 500 kHz resonance:

All other components are unchanged. L1 resonance with C3 at 500 kHz is approximately 560 µH; as with Variant 1, a slug-tuned adjustable coil is preferable for alignment.

7. Variant 3 — Broadband 400–4,000 kHz Buffer

Use this variant for: SP-600 upper-HF first-IF tap at 3.955 MHz; KWM-2/2A first-IF tap at 2.955–3.155 MHz; any application where a wider window is desired and the tap point itself provides bandpass shaping.

The tank is replaced by a simple 100 µH series choke that presents high impedance to RF while passing DC bias to the gate bias resistor R2. The resulting bandpass is very broad — essentially limited by C2 on the low end and by parasitic capacitances on the high end — covering approximately 400 kHz to 4 MHz with minimal roll-off. This is deliberate: when you’re tapping a first-IF stage that already has transformer-shaped bandpass of its own, adding another tuned stage just narrows the window for no benefit.

8. Construction Notes: Shielding, Grounding, RFI Mitigation

A correctly-designed buffer amplifier can still perform badly if the construction is sloppy. The following practices matter:

Physical enclosure

Build the buffer inside a small shielded metal box — Hammond 1590A, 1590B, or a homemade aluminium can. Single-sided PCB with continuous ground plane on the back, if you’re making a board. Dead-bug (ugly-construction) on a piece of scrap PCB with a continuous ground plane also works. What matters is that the active device and the tuned circuit sit in a metal-enclosed space with input and output coaxial connectors mounted directly through the enclosure wall.

Grounding

One point ground at the buffer enclosure. The coax shield from the tap point connects to this ground at the buffer end. The coax shield going out to the SDR connects to this ground at the buffer end. Do not ground the receiver-side coax shield to the receiver chassis at both ends — you’ll create a ground loop that picks up mains-frequency hum and radiates it into the SDR’s low-frequency input.

Ferrite chokes

A clamp-on 31-mix ferrite bead at each end of each coaxial cable run reduces common-mode current by 10–15 dB at HF. Not optional if you have any switching-mode power supplies in the shack (which means: you do). Type 31 is preferred over 43 at the 455–500 kHz frequencies in most of this series; type 43 is acceptable above 2 MHz.

Coax choice

For short runs (<1 m) inside the receiver chassis or from receiver to adjacent SDR, RG-174 or RG-316 is fine and physically convenient. For longer runs to a remote SDR, use RG-58 or better. Do not use audio-grade shielded cable — its shield coverage is inadequate.

Supply wiring

The +12 V supply to the buffer should come from a linear regulated source, not a switching-mode wall-wart. A LM7812 on a small heat sink fed from a 15–18 V wall-wart or a small dedicated 12 V bench supply is the right approach. 100 mA is more than adequate for any single buffer.

9. Testing and Alignment

Before connecting to any receiver:

1. DC tests. Apply +12 V. Measure drain voltage (should be ~10–11 V, close to supply minus choke drop). Measure source voltage (should be ~1.5–2.5 V depending on device). If drain voltage = +12 V exactly, the JFET is open or not conducting; if it’s near zero, the JFET is shorted or mis-biased.
2. Signal injection test. Feed a signal generator into the input at the tap-point frequency (455, 500 kHz, or upper-HF first IF). Use −40 dBm input level. Measure output into 50 Ω termination with a scope or SDR. You should see clean signal at −46 dBm ±3 dB at the design frequency.
3. Tank alignment (Variants 1 and 2 only). Sweep the input across ±100 kHz around the design centre frequency. Peak the slug-tuned coil (or verify the fixed tank) for maximum output at centre. The −3 dB bandwidth should be 10–15 kHz.

Only after these tests pass should you connect the buffer to the actual receiver tap point.

After installation in the receiver:

1. With the receiver tuned to a known signal (use a calibration crystal or WWV), verify the SDR’s waterfall shows that signal at the expected frequency offset from the buffer’s output centre frequency.
2. Listen to the receiver’s own audio output. If the audio now has a tone, whine, or buzz that wasn’t there before, the buffer is radiating back into the receiver — check shielding and ground bonding.
3. Tune the receiver across one full band. The SDR waterfall should remain centred with the full IF envelope visible; the receiver’s own sensitivity should be unchanged from before installation.

10. Common Problems and Fixes

Frequent issues encountered by builders working through this series:

Symptom: Waterfall is weak and noisy

Most likely cause: tank misaligned or coupling capacitor C2 wrong value. Check tank resonance with a GDO or signal generator sweep. Verify C2 is NP0/C0G ceramic, not Y5V or X7R (the latter two change value dramatically with voltage and temperature).

Symptom: Receiver loses sensitivity after buffer installation

Most likely cause: C2 value too high, loading the tap point. Try 470 pF or 220 pF instead of 1 nF. The buffer should be near-invisible to the receiver — if the receiver notices the buffer, the coupling is too tight.

Symptom: Tuning whine or buzz in receiver audio that wasn’t there before

Most likely cause: Switching-mode supply (SDR’s, PC’s, or LED lamp’s) injecting RFI into the receiver through the coax shield. Add clamp-on ferrites to both ends of all coax runs; verify linear regulated supply to the buffer itself; if present, relocate the SDR farther from the receiver.

Symptom: SDR shows an extremely strong signal at 455 kHz (or 500 kHz) even with no antenna on the receiver

Most likely cause: Broadcast-band signal leakage directly into the buffer, bypassing the tank. Improve shielding of the buffer enclosure; shorten the lead from tap point to C2; verify the tap-point lead is not running parallel to any high-level signal path inside the receiver.

Symptom: Buffer output drifts in level over warm-up

Most likely cause: JFET drain current drifting with temperature. Normal J310s exhibit ~20% I_DSS variation over 25–50 °C temperature rise. If this is problematic, a 2N5486 has slightly better temperature coefficient; or add a small degenerating resistor (10 Ω) in series with the source. Most panadapter applications don’t care about 1–2 dB drift.

11. Parts Sourcing

All parts for the buffer are inexpensive and widely available. Approximate 2026 pricing for a single build from common parts distributors:

• J310 JFET: $1–3 from DigiKey, Mouser, eBay. 2N5486 is a drop-in alternative at similar cost.
• Moulded RF chokes (680 µH, 560 µH, 100 µH): $0.50 each. Mouser, DigiKey, or eBay bulk packs.
• NP0/C0G capacitors (180 pF, 1 nF, 10 nF): $0.10 each.
• Metal film resistors: $0.05 each.
• Hammond 1590A enclosure: $5–8.
• BNC connectors: $2–4 each.
• RG-316 coax: $1–2 per metre.
• Type 31 clamp-on ferrites: $3–5 each from Fair-Rite distributors.
• LM7812 regulator: $0.50.

Total cost per buffer, including enclosure and connectors, runs $25–35 in small quantities. Building two or three at once (for a station with multiple receivers) reduces per-unit cost to around $20.

Closing Thoughts

The buffer amplifier described here is deliberately unremarkable engineering. A JFET source follower with a tuned input is a circuit that every RF engineer built at some point in their career, and that’s the point — there’s no need to invent anything new to integrate modern SDRs with vintage tube receivers. The design solves a narrow, well-defined problem with known-good techniques, and does so with parts that cost less than a meal out.

Where the engineering effort belongs is in the integration thinking covered across the five series articles: which tap point, which SDR for which role, which architectural choice preserves the vintage receiver’s character while adding useful modern capability. A buffer amplifier is a means; the thoughtful pairing of boatanchor and SDR is the end.

If there are common problems, variations, or receiver-specific adaptations that this appendix doesn’t cover, let me know — this is a living reference that will expand over time based on what builders actually encounter.