User Manual for SCARP

Firmware B178 featuring 8 channel scanning, 4 mhz analysis and single channel zoom  

SCARP

Aviation Band Scanning Receiver

User Manual

Manual Revision 7 • Covers Firmware B178

Read this manual before first use.

Revision history

Rev 5 — Appendix C added  •  Rev 6 — Appendix C expanded  •  Rev 7 — audio output clarified: external amplification required, low-level signal, no direct headphone drive

Table of Contents

1. Welcome................................................................................................................. 4

2. Safety Information.................................................................................................. 4

3. Getting Started........................................................................................................ 4

3.1 Charging......................................................................................................... 4

3.2 Powering On — Important First Step............................................................ 4

4. Setting Up SCARP.................................................................................................. 4

4.1 Antenna.......................................................................................................... 5

4.2 External Filter................................................................................................ 5

4.3 Audio Output................................................................................................. 5

4.4 Setting the Volume........................................................................................ 5

5. Controls at a Glance................................................................................................ 5

6. Understanding the Display...................................................................................... 6

6.1 Main Screen Layout....................................................................................... 6

6.2 Battery Indicator............................................................................................ 6

6.3 Display Update Cadence................................................................................ 6

7. Scanning Basics...................................................................................................... 6

7.1 Channel Status Colors.................................................................................... 6

7.2 Enabling Channels......................................................................................... 6

7.3 Monitor Mode (Yellow)................................................................................ 7

7.4 What Is Saved and What Is Not..................................................................... 7

8. Squelch.................................................................................................................... 7

8.1 AGC Squelch Mode....................................................................................... 7

8.2 AI Squelch Mode........................................................................................... 7

8.3 Choosing Between AGC and AI.................................................................... 7

9. Programming a Channel......................................................................................... 8

10. Waterfall Mode..................................................................................................... 8

10.1 Entering Waterfall Mode............................................................................. 8

10.2 Reading the Waterfall Screen...................................................................... 8

10.3 AGC Waterfall — What the Colors Mean.................................................. 8

10.4 AI Waterfall — What the Colors Mean....................................................... 9

10.5 Waterfall Controls....................................................................................... 9

10.6 The WF SCALE Overlay............................................................................. 9

10.7 Saving a Frequency from the Waterfall..................................................... 10

10.8 Getting the Best Results............................................................................. 10

11. ZOOM (Service Mode)....................................................................................... 10

12. Troubleshooting & Common Issues................................................................... 10

13. First Flight Setup Checklist................................................................................ 12

Appendix A: Quick Reference.................................................................................. 12

Appendix B: Service Notes....................................................................................... 12

Appendix C: Standalone RF Board / Arduino Integration........................................ 14

1. Welcome

This manual covers setup, controls, and everyday operation of your SCARP receiver running firmware B178. SCARP scans aviation-band frequencies across up to eight programmable channels (CH0–CH7), offers two squelch systems (signal-strength AGC and audio-based AI), and includes a live spectrum “waterfall” view for visualizing RF activity.

Read this manual fully before your first flight or field session. SCARP's controls are gesture-based — the same button does different things depending on a short tap versus a long press — so Section 5 (Controls at a Glance) is worth keeping handy until the gestures become familiar.

Note: This manual matches firmware B178. Earlier or later firmware may behave differently. If your unit's behavior does not match this manual, check which firmware it is running.

2. Safety Information

●      SCARP is a receive-only monitoring device. It does not transmit.

●      Use only the included charger to charge the unit.

●      Do not operate SCARP as a primary or sole means of navigation or situational awareness. It is a supplemental monitoring tool.

●      Do not expose the unit to moisture, extreme heat, or physical shock.

●      If used in a cockpit or vehicle, secure the unit and its cabling so it cannot interfere with controls.

Caution: SCARP is a listening aid, not a certified avionics instrument. Always cross-check critical information against certified equipment and official sources.

3. Getting Started

3.1 Charging

●      Charge SCARP for at least 4 hours using the included charger before first use.

●      A full charge provides approximately 4 hours of operating life. Actual battery life varies with temperature, volume level, and usage pattern.

Caution: Do not expect good reception or accurate measurements while the charger is attached. The charging/USB power environment is electrically noisy and can degrade reception. Charge the unit, then unplug the charger before use.

3.2 Powering On — Important First Step

Power on the unit using its power control. After boot, SCARP displays all eight channels, but every channel starts in the Red (disabled) state. This is normal — SCARP does not scan anything until you enable channels.

To start scanning:

●      Short-tap a channel's right-side key to enable that individual channel, or

●      Long-press the bottom-left button to enable all channels at once.

Note: If the unit appears “dead” after power-up — all channels red, nothing scanning — nothing is wrong. Enable one or more channels and scanning begins.

4. Setting Up SCARP

Before scanning, confirm the following. Skipping any of these is the most common cause of “no reception” or “weak audio” complaints.

4.1 Antenna

●      SCARP requires an antenna to receive anything meaningful. Without one, expect little or no usable signal.

●      An antenna is included with certain configurations; confirm yours is attached before use.

4.2 External Filter

●      For best performance, use SCARP with its external filter. An external filter is included with certain configurations.

●      The filter reduces overload from strong, nearby out-of-band signals, which makes squelch behavior far more predictable (see Section 8).

4.3 Audio Output

SCARP's audio output is a low-level, high-impedance audio signal. It is similar to a microphone-level or detector-level signal, not a speaker-level or headphone-amplifier output.

For listening, connect SCARP to a powered external speaker, an amplified headset, or another external audio amplifier with a suitable high-impedance input.

Caution: SCARP is not designed to directly drive earbuds, passive headphones, low-impedance headphones, or an unpowered speaker. Connecting those loads will produce little or no usable audio and may make the unit appear to be malfunctioning.

4.4 Setting the Volume

Volume is adjusted through an on-screen overlay, not a physical knob:

1.      Long-press the bottom-center button (hold about 1.5 seconds). The volume overlay appears.

2.      Bottom-right = volume up. Bottom-left = volume down. There are 8 volume steps.

3.      Bottom-center = OK (confirms and exits).

●      Your volume setting is saved and restored at the next power-up.

●      If you do nothing for 10 seconds, the overlay exits automatically.

5. Controls at a Glance

SCARP's buttons are gesture-based: a short tap and a long press on the same button do different things. This table covers the main screen. Waterfall mode and the volume overlay temporarily remap some buttons, as described in their own sections.

Button	Short tap	Long press
Bottom-left	—	Enable all channels for scanning
Bottom-center	Enter PROG (frequency programming) mode	Open the volume overlay (hold ~1.5 s)
Bottom-right	Step the AGC trigger up by 25 (wraps to 0 above 400)	Toggle between AGC and AI squelch modes
Right-side channel keys (CH0–CH7)	Change that channel's status (enable / monitor / disable)	Open the Waterfall mode chooser
CLR (in PROG mode)	Backspace one digit	Exit PROG mode

Note: Holding the bottom-center button while powering on performs a factory reset of the channel frequencies — see Appendix B: Service Notes.

6. Understanding the Display

6.1 Main Screen Layout

The main screen shows all eight channels at once, plus a status row at the bottom:

●      Eight channel rows (CH0–CH7), each showing the channel's frequency, its status color bar, and a per-channel signal number (AGC level / squelch floor). In AI squelch mode the per-channel numbers are intentionally blank — see Section 8.2.

●      The bottom row shows a timer / PROG label, the current global AGC trigger number, the battery icon, and a mode tag showing whether AGC or AI squelch is active.

6.2 Battery Indicator

The battery icon is a segmented bar with up to four segments. The segments have fixed colors — from left to right: green, green, yellow, red — and the bar fills from right to left.

Judge remaining charge by how many segments are lit, not by their color. Seeing a red segment as part of a mostly-full bar is normal. Fewer lit segments means less charge remaining; when only one or two segments remain, plan to recharge before your next session.

6.3 Display Update Cadence

While SCARP is scanning, the left-side numeric values beside the channel rows may appear to update in a “ripple” pattern from top to bottom. This is normal.

The display is intentionally updated more slowly than the receiver scan loop. SCARP may visit and evaluate channels faster than the screen is repainted. To keep the display readable and avoid unnecessary screen flicker, each channel's displayed number is refreshed only periodically.

As a result, you may see CH0 update, then CH1, then CH2, and so on down the screen. After the last active row updates, there may be a short pause before the update pattern begins again at the top. This pause is normal and does not mean scanning has stopped.

Important: the visual update cadence is not the same thing as the RF/audio scan cadence. The screen is showing a readable status update, not a real-time animation of every scan decision.

7. Scanning Basics

SCARP scans up to 8 channels, labeled CH0 through CH7. Each channel has a stored frequency and a runtime status color that controls how the scanner treats it.

7.1 Channel Status Colors

Color	Meaning
Green	Enabled — the channel is included in the scan.
Yellow	Monitor — the scanner parks on this channel and stays there. Only one channel may be yellow at a time.
Red	Disabled — the channel is skipped entirely. All channels start red at power-up.

7.2 Enabling Channels

●      Short-tap a channel's right-side key to change its status.

●      Long-press the bottom-left button to enable all channels at once.

●      When you enable or monitor a channel, SCARP copies the current global AGC trigger value into that channel as its squelch level (see Section 8.1).

7.3 Monitor Mode (Yellow)

Setting a channel yellow tells SCARP to stop scanning and park on that channel — useful when you want to sit on one frequency (for example, a tower or ATIS) without hearing anything else.

●      Only one channel can be the yellow monitor at a time. Setting a second channel yellow automatically demotes the previous yellow channel to red (disabled).

●      To resume normal scanning, change the yellow channel back to green or red.

Note: If SCARP seems “stuck” on one channel and never scans, check whether that channel is yellow. That is monitor mode doing its job.

7.4 What Is Saved and What Is Not

●      Saved across power cycles: channel frequencies, volume setting.

●      Not saved: channel status colors (all channels reset to red at boot) and per-channel squelch levels (these are runtime copies of the global AGC trigger, refreshed when you enable a channel).

This means your normal power-up routine is: turn on, enable the channels you want, and go. Frequencies will be exactly as you left them.

8. Squelch

Squelch determines when SCARP “opens” a channel — stops scanning and passes audio. SCARP B178 has two squelch systems. Long-press the bottom-right button to switch between them; the active mode is shown by the AGC/AI tag on the main screen.

8.1 AGC Squelch Mode

AGC mode opens a channel based on signal strength. It is controlled by a single global number, the AGC trigger:

●      Short-tap the bottom-right button to step the trigger up by 25. Above 400 it wraps back to 0.

●      The current trigger value is shown on the bottom status row.

●      The trigger is copied into a channel as that channel's squelch level at the moment you enable or monitor it. To apply a new trigger value to an already-enabled channel, adjust the trigger and then re-enable the channel.

Practical guidance:

●      If SCARP stops too often on noise or weak junk, raise the trigger.

●      If SCARP misses aircraft you can hear on other radios, lower the trigger (remember it wraps past 400 back to 0).

●      Using the external filter (recommended) makes AGC squelch behavior much more predictable.

8.2 AI Squelch Mode

AI mode does not use signal-strength thresholds at all. Instead, SCARP listens to the audio itself and opens a channel only when the audio classifier judges the content to be voice-like, muting hiss and dead carriers.

●      In AI mode the per-channel squelch numbers are intentionally blank on the main screen. This is normal — they are not used in AI mode.

●      The AGC trigger number and the trigger-adjust tap have no effect on channel opening while AI mode is active.

8.3 Choosing Between AGC and AI

●      AGC mode is simple and predictable: one number decides everything. Best when you know your RF environment.

●      AI mode is best in noisy RF environments where signal strength alone cannot distinguish a real transmission from noise — it opens on voices, not on energy.

●      You can switch modes at any time with a long press of the bottom-right button.

9. Programming a Channel

Use this procedure to set any channel (CH0–CH7) to a new frequency. Valid frequencies are 108.000 to 136.975 MHz.

1.      Short-tap the bottom-center button to enter PROG mode.

2.      Type the frequency as exactly six digits, in kHz (example: 134.025 MHz → 1 3 4 0 2 5).

3.      Press the right-side key of the channel you want to store it into (CH0–CH7).

4.      SCARP stores the frequency and returns immediately to the main screen. There is no confirmation screen — the redraw to the main screen is the confirmation.

Corrections and exit:

●      Short-tap CLR to backspace one digit.

●      Hold CLR to exit PROG mode without storing.

●      You can reprogram any channel at any time — there is no separate “unlock” step.

10. Waterfall Mode

Waterfall is SCARP's RF visualization mode. It shows activity across a 4 MHz slice of spectrum as a live, rolling display — useful for seeing what's actually on the air rather than guessing from audio alone.

10.1 Entering Waterfall Mode

1.      Long-press any right-side channel key. The Waterfall mode chooser appears.

2.      Choose with the bottom buttons: bottom-left = AGC waterfall, bottom-center = EXIT (cancel), bottom-right = AI waterfall.

●      If you do nothing for 5 seconds, the chooser cancels itself and returns to the main screen.

10.2 Reading the Waterfall Screen

Span. The waterfall always displays a 4 MHz slice of spectrum, centered near the channel you long-pressed to enter. The mode label on screen — 4MAGC or 4MAI — means “4 MHz span, AGC mode” or “4 MHz span, AI mode.”

Frequency axis (left to right). The left edge of the display is the start frequency of the slice. A frequency reference runs along the top: major tick marks are 1 MHz apart, minor tick marks are 250 kHz apart. Each vertical column of the display therefore corresponds to one small step in frequency. The cursor marks your current focus point, and the cursor's exact frequency is shown below the display.

Time axis (top to bottom). Each horizontal row of the waterfall is one complete sweep across the 4 MHz slice. New sweeps are drawn one row at a time; when the drawing reaches the bottom of the screen it wraps back to the top and overwrites the oldest rows. A vertical stripe therefore means a signal that is present sweep after sweep; an isolated dot or short dash means a signal that appeared briefly and went away.

●      Persistent bright vertical line = a continuous transmitter (a carrier that is always on).

●      Bursts or dotted vertical patterns = an intermittent transmitter, such as two-way voice traffic.

10.3 AGC Waterfall — What the Colors Mean

The AGC waterfall paints signal strength (RF energy). The color ramp, from weakest to strongest:

Color	Meaning (AGC mode)
Black	Little or no energy — at or below the display floor.
Green	Weak but real energy above the floor.
Yellow	Moderate energy.
Red	Strong energy.
Magenta	Very strong energy at the top of the display range — the signal is at or beyond full scale (possible overload/clipping). If large areas turn magenta, the display range needs adjusting (see 10.6) or the front end is being overloaded (see 10.8).

Where the color boundaries fall depends on the display floor and range settings — see the WF SCALE overlay (10.6).

10.4 AI Waterfall — What the Colors Mean

The AI waterfall does not paint signal strength. It paints voice-likeness: how strongly SCARP's audio classifier judges each frequency to contain voice activity. A frequency can be strong in the AGC waterfall and black in the AI waterfall — that simply means “energy is present, but nobody is talking.”

Color	Meaning (AI mode)
Black	No voice activity detected.
Green	Some voice-like activity detected.
Yellow	Repeated voice-like activity.
Red	Strong, repeated voice activity — someone is talking here regularly.
White	Smear marker — see below. Not a voice hit.

White smear marks. A very strong nearby transmitter can overload the receiver front end and “bleed” across many adjacent frequencies at once, which would otherwise paint a false horizontal scar of voice hits. SCARP detects this condition and suppresses those false hits, painting the affected columns white instead. White therefore means: “activity was seen here, but SCARP judged it to be overload bleed from a strong transmitter, not a genuine signal on this frequency.” The real transmitter causing the smear is usually the strongest signal nearby.

10.5 Waterfall Controls

●      Left / Right: move the cursor across the displayed spectrum slice. Hold a direction key for auto-repeat.

●      EXIT (short tap): returns to the main screen.

●      EXIT (hold): opens the WF SCALE overlay (see 10.6).

●      Right-side keys CH0–CH6: save the cursor frequency into that channel (see 10.7).

●      Right-side key CH7: enters ZOOM, a service/calibration mode — see Section 11 before pressing it.

10.6 The WF SCALE Overlay

The WF SCALE overlay (opened by holding EXIT from the Waterfall) adjusts how energy levels are mapped onto the display colors. It shows the following values:

●      FLOOR — the level treated as the bottom of the display. Anything at or below the floor paints black. Raising the floor hides weak noise; lowering it reveals weaker signals.

●      RNG (range) — how much signal above the floor is spread across the color ramp (green through red, with magenta at the top). A small range makes colors change quickly with small signal differences; a large range compresses them.

●      SUG (suggested) — the floor and range values SCARP suggests based on what it is currently receiving.

●      AUTO — applies the suggested values in one step.

Use the bottom buttons, as labeled on the overlay, to select and adjust the values or apply AUTO.

When to adjust the scale:

●      Display is almost entirely black, even on frequencies you know are active → the floor is too high. Lower it, or use AUTO.

●      Display is washed out — large areas of yellow/red/magenta everywhere → the floor is too low or the range too small. Raise the floor or use AUTO.

●      After moving to a very different RF environment (for example, from home to an airfield), AUTO is usually the fastest way to a readable display.

10.7 Saving a Frequency from the Waterfall

When the cursor is on a signal of interest, press one of the right-side keys CH0 through CH6. SCARP stores the cursor frequency into that channel and exits to the main screen.

Note: CH7 cannot be saved from the Waterfall, because the CH7 key is used to enter ZOOM. To program CH7, use PROG mode from the main screen (Section 9).

10.8 Getting the Best Results

●      Use a proper antenna and, ideally, the external filter.

●      Avoid operating with the charger attached.

●      In a high-RF environment, reduce overload sources (or add filtering) before trusting fine detail in the display. Widespread magenta in AGC mode or white smear marks in AI mode are both signs of front-end overload.

11. ZOOM (Service Mode)

ZOOM is a service and calibration mode, not a normal operating mode. It is entered from Waterfall mode by pressing the right-side CH7 key, and it contains the unit's PPM and IF LO calibration controls. Do not use ZOOM in normal operation.

Caution: Exiting ZOOM with BACK automatically saves the current PPM and IF LO calibration values to permanent memory. There is no separate “save” step — exiting is saving. Calibration affects the tuning accuracy of every channel on the unit.

If you enter ZOOM by accident:

●      If you have not touched any calibration control (LO+ / LO− / PPM), press BACK to exit. The unchanged values are re-saved, and nothing is affected.

●      If you may have changed a calibration control — or you are not sure — do not press BACK. Power the unit off instead, then power it back on. Calibration is only written when ZOOM exits, so powering off without exiting restores the previously saved values.

Symptoms of disturbed calibration: all channels off-frequency by a consistent amount, stations sounding distorted or “off-tune” across the whole band. If this happens after ZOOM was used, the unit needs recalibration — see Appendix B.

Note: Deliberate calibration using ZOOM is a service procedure and is outside the scope of this manual.

12. Troubleshooting & Common Issues

Symptom	Likely Cause / Fix
All channels red and nothing scans after power-up	Normal — all channels boot disabled. Short-tap channel keys to enable individual channels, or long-press bottom-left to enable all. (Section 3.2)
SCARP sits on one channel and never scans	That channel is set yellow (Monitor mode). Change it back to green or red to resume scanning. (Section 7.3)
Per-channel squelch numbers disappeared from the display	AI squelch mode is active — the numbers are intentionally blank in AI mode. Long-press bottom-right to switch back to AGC mode if desired. (Section 8.2)
The left-column numbers update one row at a time, then pause briefly before starting again	Normal operation. SCARP scans independently of the screen refresh. The display is rate-limited for readability, so the numbers may appear to ripple down the channel list with a short pause before restarting at the top. (Section 6.3)
AGC trigger adjustment seems to do nothing	Either AI mode is active (trigger is not used), or the channel was enabled before you changed the trigger — re-enable the channel to copy in the new value. (Section 8.1)
Cannot save CH7 from the Waterfall	By design — the CH7 key enters ZOOM from the Waterfall. Program CH7 via PROG mode from the main screen. (Section 10.7)
Entered ZOOM by accident	If no calibration control was touched, press BACK to exit — nothing changes. If a calibration control may have been touched, power off WITHOUT pressing BACK; exiting ZOOM auto-saves calibration, powering off does not. (Section 11)
Waterfall chooser vanished before I picked a mode	The chooser cancels after 5 seconds of no input. Long-press a channel key to bring it back. (Section 10.1)
Waterfall is almost all black, or washed out in red/magenta	Display floor/range needs adjusting for your RF environment. Hold EXIT in the Waterfall to open WF SCALE and use AUTO. (Section 10.6)
White marks in the AI waterfall	Normal — smear suppression. A strong nearby transmitter overloaded the front end and SCARP marked the false hits white instead of counting them as voice. (Section 10.4)
All channels off-frequency by a consistent amount	Calibration (PPM / IF LO) may have been changed and saved on ZOOM exit. See Section 11 and Appendix B.
Weak, noisy, or degraded reception	Charger is still attached. Charge fully, then unplug before use.
No reception / dead air on all channels	No antenna connected, or antenna not seated. SCARP needs a real antenna to receive anything meaningful. Also confirm channels are enabled (green).
Signals sound overloaded, “weird,” or inconsistent	Missing or bypassed external filter. Reconnect the external filter, especially in high-RF environments.
No audio, or audio very faint	The audio output is being treated like a headphone/speaker output. SCARP provides a low-level, high-impedance audio signal and requires a powered external speaker, amplified headset, or external audio amplifier. It will not directly drive earbuds, passive headphones, low-impedance headphones, or an unpowered speaker. Also check the volume overlay (Section 4.4).
Stops too often on noise (AGC mode)	AGC trigger set too low. Short-tap bottom-right to raise it, then re-enable the affected channels. (Section 8.1)
Misses traffic you can hear elsewhere (AGC mode)	AGC trigger set too high — or it wrapped past 400 back to 0 and was then raised too far. Lower it and re-enable channels. Consider AI mode in noisy environments.
Battery drains faster than expected	Expected life is approximately 4 hours and varies with temperature, volume, and usage. Judge remaining charge by the number of lit battery segments, not their color. (Section 6.2)

13. First Flight Setup Checklist

A quick reference for getting SCARP ready to go:

1.      Charge 4+ hours, then unplug the charger.

2.      Connect the antenna (and external filter if you have it).

3.      Connect a powered external speaker, amplified headset, or external audio amplifier. Do not use passive earbuds, passive headphones, or an unpowered speaker.

4.      Set the volume: long-press bottom-center, adjust, press bottom-center to confirm.

5.      Choose your squelch system: long-press bottom-right to toggle AGC / AI.

6.      If using AGC mode: set the AGC trigger with short taps of bottom-right.

7.      Program your channels: short-tap bottom-center for PROG, six digits, then a channel key.

8.      Enable channels: short-tap individual channel keys, or long-press bottom-left for all.

9.      Scan.

10.   Long-press a channel key any time you want the Waterfall view instead of guessing.

Appendix A: Quick Reference

Channel Status Colors

●      Green — enabled / scanned

●      Yellow — monitor / parked (one channel maximum; a new yellow demotes the old one to red)

●      Red — disabled / skipped (all channels start red at boot)

Gestures

●      Bottom-center: tap = PROG, hold = volume overlay

●      Bottom-right: tap = AGC trigger +25 (wraps past 400 to 0), hold = toggle AGC/AI

●      Bottom-left: hold = enable all channels

●      Channel keys: tap = change channel status, hold = Waterfall chooser

●      In Waterfall: CH0–CH6 = save cursor frequency, hold EXIT = WF SCALE, CH7 = ZOOM (service mode — see Section 11 first)

Common Gotchas

●      All channels boot red — enable them before expecting anything to scan.

●      Charger attached = degraded RF performance. Charge it, then unplug for real use.

●      No antenna = nothing meaningful. SCARP needs a real antenna.

●      External filter matters. If signals seem “weird,” overloaded, or inconsistent, the filter is often the fix.

●      Audio output requires external amplification. Use a powered speaker, amplified headset, or audio amplifier. Passive headphones, earbuds, and unpowered speakers will not work properly.

●      Blank squelch numbers = AI mode is active, not a fault.

●      Exiting ZOOM auto-saves calibration. If you touched calibration controls by accident, power off instead of pressing BACK.

Appendix B: Service Notes

These functions are intended for setup and service rather than everyday use.

Factory Reset of Channel Frequencies

Holding the bottom-center button while powering on resets CH0–CH7 to the factory default frequencies:

Channel	Frequency (MHz)	Channel	Frequency (MHz)	Channel	Freq (MHz)
CH0	118.200	CH3	126.700	CH6	134.800
CH1	122.700	CH4	133.800	CH7	134.850
CH2	123.000	CH5	134.025

Caution: A factory reset overwrites all eight stored frequencies. Note your programmed frequencies before performing one.

Calibration (ZOOM Service Mode)

PPM and IF LO calibration values are accessible from ZOOM (Section 11). Exiting ZOOM with BACK automatically saves both calibration values to permanent memory — there is no separate save step. These values affect the tuning accuracy of the entire unit and should only be changed as part of a deliberate calibration procedure. If calibration is disturbed accidentally and saved, the unit must be recalibrated against a known-accurate reference signal.

Appendix C: Standalone RF Board / Arduino Integration

This appendix is for customers using the SCARP RF board on its own, connected to their own Arduino or other controller. It does not apply to the complete SCARP receiver, where all of the interface circuitry described below is already provided on the SCARP backbone board.

Standalone board warning: the 7-pin connector is not a complete plug-and-play Arduino shield interface. The complete SCARP receiver includes additional interface circuitry on the backbone board, including I²C pullups and AGC series resistance. A standalone RF-board user must provide those parts externally. Wiring the board directly to an Arduino without these interface details may cause communication failure, unstable AGC behavior, poor reception, or misleading test results.

C.1 The H1 Connector

All connections to the standalone RF board are made through the 7-pin H1 connector. The schematic below shows the RF-board side of H1:

Figure C-1: H1 connector, RF-board side.

C.2 Pinout and Interface Requirements

H1 pin	Signal	Function / requirement
7	5.0V	Main regulated 5V input to the RF board. Use only clean regulated 5.0 V DC. In the complete SCARP receiver this rail is filtered and locally decoupled.
6	GND	Ground return. Use a short, low-impedance ground connection. Poor grounding will directly degrade RF performance.
5	AUDIO_HP	Detected audio path used by the SCARP audio/AI analysis. AUDIO_HP is not a speaker or headphone driver. It is a low-level, high-impedance audio/detector signal intended for AI analysis or for feeding a suitable high-impedance external amplifier/input. This signal is highly layout-sensitive. For any AI use, the required anti-alias / 455 kHz shunt capacitor (approximately 2.2 nF to ground) must be physically at the controller ADC pin — see C.4. Do not expect AI mode to work over jumper wires.
4	AGC_PROXY	Analog AGC/signal-strength output. Must be connected to the user's ADC input through a 270 Ω series resistor. This resistor is provided on the SCARP backbone board in the complete receiver. Keep wiring short and filtered.
3	SDA	I²C data line. User must provide I²C pullup on the controller side. Keep wiring short.
2	SCL	I²C clock line. User must provide I²C pullup on the controller side. Keep wiring short.
1	MUTE	Audio mute control. In SCARP firmware, mute is controlled by driving the line low to mute and releasing/high-Z to unmute. Do not blindly drive this pin high unless the interface design specifically supports it.

C.3 Mandatory User-Side Interface Requirements

Two parts of the interface exist on the SCARP backbone board in the complete receiver and are NOT provided by the standalone RF board. If you buy only the RF board, you must provide both yourself. Copying the pinout alone is not enough.

Important — I²C pullups required: the SDA and SCL pins on the RF board are I²C control lines. For standalone use, the user must provide I²C pullup resistors on their controller board. In the complete SCARP receiver, these pullups are provided on the SCARP backbone board. If the standalone RF board is wired directly to an Arduino without proper pullups, I²C communication may fail or behave intermittently.

Important — AGC_PROXY series resistor required: the AGC_PROXY signal must be connected to the user's ADC input through a 270 Ω series resistor. Do not connect AGC_PROXY directly to an Arduino analog input. In the complete SCARP receiver, this resistor is located on the SCARP backbone board. The resistor is required for the intended AGC measurement/discharge behavior and helps protect the controller pin and analog node during operation.

C.4 Audio Filtering — Required for Any AI Use

There are two different filters in SCARP's AI audio path, and they must not be confused:

●      A hardware anti-alias / 455 kHz shunt filter at the controller's audio ADC pin. In the SCARP implementation this is approximately 2.2 nF to ground at the A7 input. Its job is to kill 455 kHz IF leakage and prevent aliasing before the ADC samples the audio.

●      A software high-pass filter inside the AI classifier. Its job is to remove low-frequency drift, hum, and DC-like behavior before extracting audio features. It lives in firmware and requires nothing from you.

Caution: The hardware shunt capacitor must be placed physically at the controller ADC pin. Placing it at the RF board, on a breadboard, or at the far end of a jumper wire is not equivalent — the wire between the capacitor and the ADC pin can pick up 455 kHz energy before the signal reaches the filter, defeating its purpose.

C.5 Power and Grounding

●      Use clean regulated 5.0 V DC only. For standalone RF-board work, a regulated lithium-battery-derived supply is strongly preferred.

●      Do not assume a laptop USB port, USB wall charger, USB power bank, Arduino 5V pin, or breadboard power rail is quiet enough for meaningful RF evaluation. These supplies commonly inject noise that shows up as false RF, unstable AGC behavior, noisy audio, or misleading Waterfall patterns.

●      Keep the 5V and ground path short and low impedance. Treat ground routing as part of the RF design, not just a DC return.

●      Keep analog signal wires short and away from digital lines.

C.6 Why Jumper Wires Are a Problem

At aviation-band RF frequencies and at the 455 kHz IF, jumper wires are not harmless conductors. They act as antennas, ground inductors, coupling paths, and noise injectors:

●      Long 5V and GND leads inject supply ripple and digital current pulses into the receiver.

●      I²C activity can couple into the analog AGC or audio wiring.

●      Breadboard grounds have too much impedance and parasitic coupling for reliable RF evaluation.

●      AUDIO_HP and AGC_PROXY wires can pick up internal 455 kHz energy or external noise before they reach the controller.

●      Moving a wire or touching a cable can change the RF behavior, making the setup appear unstable when nothing is actually failing.

A custom controller PCB with short, disciplined wiring and analog/digital separation is strongly preferred over a jumper-wire breadboard.

C.7 What to Expect from a Standalone Setup

●      Reasonable goals: basic frequency control, AGC_PROXY measurement, AGC-based squelch experiments, and simple reception testing.

●      Not reasonable: complete SCARP receiver performance from a generic Arduino, USB power, and jumper wires.

Caution: AI mode should be considered unavailable for normal homebrew Arduino use. It depends on filter placement, shielding, grounding, ADC behavior, and firmware timing that the complete SCARP hardware controls as a system. Plan standalone experiments around AGC mode. AI mode is only realistic if you duplicate the physical SCARP layout, including the audio filtering described in C.4.

●      Reception quality, Waterfall usefulness, scan behavior, and audio usability may all be impacted by your integration choices — power, grounding, wiring, and layout.

C.8 Source Code and Further Reading

Radiant will provide source code to qualified users who sign the Radiant source code agreement. Send requests to info@radiantinstruments.com with your contact information and a short description of the intended use.

For receiver architecture detail — the dual-conversion signal path, local oscillators and calibration, and the hardware/firmware interaction behind AGC and AI squelch — see the companion document “SCARP Hardware Theory of Operation.”