Mixer plot

The Mixer plot (web /plots/mixer, or /mixer) shows the power spectrum of the channel baseband two ways — GopherTrunk’s take on OP25’s Raw Mixer and Tuned Mixer tabs. It answers “where is the carrier sitting, and is the receiver pulling it onto centre?”

The daemon runs a parallel P25 receiver on the selected channel (the same engine the other Plots use) and FFTs its channelized baseband, so the plot reflects exactly what the production demod is working with — not a re-implementation.

What you see

Two stacked spectra, both Power (dBFS) vs Frequency, spanning the channel baseband (±½ the DDC output rate, ~±24 kHz on P25):

• Raw mixer — the channelized baseband as the receiver first sees it. The carrier sits wherever its residual frequency offset leaves it; the amber marker flags the loop’s carrier-offset estimate.
• Tuned mixer — the same window re-mixed by that carrier-offset estimate, so a locked loop pulls the carrier onto the centre (0 Hz) line.

Comparing the two is the whole point: the shift from the raw peak to the centred tuned peak is the carrier-recovery correction.

Symptom	Reading
Raw peak off-centre, tuned peak centred	Healthy lock — the loop is correcting the offset
Raw peak near centre already	Little tuner error on this channel
Tuned peak still off-centre	Loop hasn’t acquired — weak signal or wrong Mode
No peak, just noise floor	No active signal at this Offset, or wrong channel

A large, steady raw offset is your tuner’s PPM error (a 420 MHz / 50 ppm RTL-SDR can sit ~20 kHz off); the tuned view should still recentre it once the loop locks.

Controls

Pick the receiver with Mode (C4FM or CQPSK — match the site), and use the Offset / Hold / Centre controls (shared with the other scopes) to point the receiver at a locked control or voice channel. With Hold off, the Offset follows the newest active call on the SDR. See the Constellation panel for the DC-spike explanation of why off-centre channels are mixed down before channelizing.

How it works

• The panel opens WS /api/v1/diag/mixer?device=...&proto=<…>&offset=<hz>. The daemon runs a symbolscope engine with its FFT taps enabled.
• Each frame carries two FFT-shifted dBFS arrays: raw_bins (the DDC output) and tuned_bins (that window multiplied by exp(-j·2π·f_off·n/Fs), where f_off is the receiver’s own carrier_offset_hz). Reconstructing the tuned view from the loop’s estimate needs no extra receiver tap and works identically for C4FM and CQPSK.
• The FFT math is the same spectrum.PowerDB helper the wideband Spectrum waterfall uses, so the two views are normalized consistently.

The numeric side of the same loop state — the carrier-error trend, AGC and clock meters — lives on the Tuning panel; the Mixer plot is its graphical FFT counterpart.

Implementation

Path	Role
internal/dsp/spectrum/spectrum.go	PowerDB — shared windowed-FFT dBFS helper
internal/scanner/symbolscope/mixer.go	Baseband accumulator → raw/tuned FFT frames
internal/scanner/symbolscope/scope.go	Feeds the accumulator with the channelized baseband + carrier estimate
internal/api/mixer.go	MixerProvider interface + WS /api/v1/diag/mixer handler
cmd/gophertrunk/mixer_provider.go	Daemon provider over the iqtap broker
web/src/panels/Mixer.tsx	Two stacked FFT line plots (raw + tuned)