DENKOThe Signal chain page
The path from antenna to converter, drawn as a live diagram at /chain. Select a block to read what it is; select a branch or a control to change the path. This is also where inputs are enabled, disabled and inverted.
1. Reading the diagram
The diagram input buttons 1–4 choose which of the four amplifier channels is drawn. Inputs 1–3 are the H-field (magnetic) channels on the RJ45 connector; input 4 is the E-field (electric) channel on the coax connector, and its chain is simpler — its amplifiers are unity-gain followers.
| Drawn as | Meaning |
|---|---|
| Cyan outline | In the signal path right now. |
| Dim | Not selected, or not fitted on this board. |
| Dotted | Unknown — the gain chain has not been programmed yet (for example while a survey or sweep is running), so there is nothing to show. |
| Amber dashed | A change is being saved. |
The line under the diagram states the whole path in words: channel, input path, both gain stages, filter, and the total gain — including automatic when the station is steering the gain itself (gain_auto).
2. The blocks, left to right
| Block | What it is |
|---|---|
| Input | The antenna connection: an RJ45 pair for the H inputs, coax for the E input. The preamplifier gain of the head is declared, not measured — nothing in the hardware reports which head is on the far end of the cable. Declare it on the block: ×10 for the stock PCB 16.2 head, ×45 for PCB 23.1. An H input with no preamp declared is flagged, because without a head there is no signal at all. |
| Transformer | 1:1 pulse transformer giving 2 kV isolation on the H inputs. The E input uses a coupling network instead. |
| LC ladder | A passive filter shared by both input branches, −3 dB near 43 kHz. It shapes the signal before the first fork — and on a board without the digital filter it is the only real low-pass in the chain. |
| First fork | Amplified path (×4 with a ~1.6 kHz high-pass) or direct tap (×1, ~3.3 kHz high-pass). A station on the direct tap has four times less total gain. |
| PGA A | First programmable gain stage: ×1, ×2, ×4, ×5, ×8, ×10, ×16 or ×32. |
| Second fork | Through the LTC1569-7 — a clock-controlled 10th-order low-pass whose cutoff the station sets (the filter cutoff selector) — or around it on the bypass. The filter is not fitted on every board (Blue Basic omits it); a board without it is detected and forced onto the bypass. |
| PGA B | Second programmable gain stage, same ladder. The total gain is preamp × path × PGA A × PGA B. |
| Anti-alias, ADC | A fixed ~720 kHz section in front of the converter, then the STM32's 12-bit ADC at 500,000 samples per second. One count is about 0.79 mV at the converter; divide by the total gain to refer a reading to the antenna. |
| DAC self-test | Drawn below the chain: a signal the station injects ahead of the whole chain, used by the Tests page. It is what makes the sweep and gain check possible with no external equipment. |
3. Changing the path
Selecting a branch switches its multiplexer; the selectors on the diagram set the declared preamp gain, PGA A, PGA B and the filter cutoff. Changes apply automatically, a moment after you make them — the note under the diagram says Saved; checking hardware and then confirms the chain is on the new path. The Front end table below the diagram lists every resulting value, including the total gain calculation.
These changes are configuration changes: they apply now, and the next remote configuration fetch can replace them (the default fetch interval is 5 minutes). If the station reports back Saved, but hardware differs, the station itself overrode the request — typically a filter that is not fitted, or automatic gain control moving the gain. What lasts and what does not: chapter 6.
4. The software chain
Under the hardware diagram, a second, read-only strip follows the digitised signal: measured baseline and noise, the detection threshold (and whether the noise floor is allowed to raise it — trig_auto), the tone track single-frequency reject, the trigger rate with any rate-cap skips, and packets held back by send_gap. These are live measurements, refreshed as the page polls; if the strip reports Status unavailable; values may be stale, the station stopped answering — check the network, not the antenna.
5. Enabling inputs, and polarity
The STM32 inputs buttons switch each of the four inputs on or off:
- A disabled input is excluded from detection, surveys and the Live view. At least one input must stay enabled, and you cannot disable the input you are currently viewing — switch the viewed/diagnostic input first.
- The choice is stored on the board and survives restarts and remote configuration refreshes — unlike the path changes above, this one is yours, not the fleet's.
- Use fleet channels clears your local choice and follows the remote configuration's mask again.
The input polarity buttons invert individual inputs — the same control as on the Live page — and are likewise stored on the board. A note under each group reports the save (channels saving… / polarity saved, or a failure to retry).
6. What this page does not know
The diagram is computed from the PCB netlist, and the page says so itself: nothing on it is measured. The gain stages are write-only chips — the page shows what the firmware believes it sent (the “programming shadow”), never a readback; the preamp gain is the value someone declared; and the component values are family references from the netlist, not verified for every board revision.
The measurements that close those gaps live one page over: the response sweep measures this board's actual frequency response, and the gain check measures whether the programmed gains actually follow the ladder.