DIY Airband Receiver with Hybrid RF Front-End

The front-end filter of this receiver originally used a two-stage staggered configuration, but I switched to a three-stage Chebyshev design to improve the skirt characteristics. This resulted in better attenuation performance in the FM broadcast band and the upper image frequency range.

https://nobcha23.hatenadiary.com/entry/2024/08/22/185607

Detailed source code, assembly manual, and full schematic analysis are archived on my tech blog: [https://nobcha23.hatenadiary.com/entry/2024/09/04/201324],[https://github.com/Nobcha/R909-SDR/tree/main/R909_manual03_en.pdf],[https://github.com/Nobcha/R909-SDR/blob/main/R909-DSP3_test1.ino]

A compact receiver requires a simple but consistent user interface.

All parameters are updated immediately when adjusted, eliminating the need for an explicit ‘apply’ operation.

Frequency tuning resolution is defined by a dedicated step mode.

EEPROM storage is used only to restore parameters after power cycling.

The interface separates real-time control from persistence, keeping operation simple while maintaining flexibility.

1. Control Interface
 Input device: Rotary Encoder
 Actions:Rotate/Push/Double Push

2. Global Behavior
 All parameters are updated immediately when changed (live behavior)
 No explicit “apply” or “confirm” operation exists
 Parameter values are always active once adjusted
 Store current parameters to EEPROM for restoration after power cycling

3. Mode Switching Behavior

 Push→ Switch to Select Mode
 Double Push→ Store current parameters to EEPROM
  → Parameters are restored after power cycle
  → Switch to Select Mode
  → When MEM mode, store the frequency in the MEM channel
  → When SCN mode, going into Automatic scan mode

4. Mode List
 FREQ : Frequency
 STEP : Frequency Step
 MEM : Memory Channel
 SCN : Scan Channel
 VOL : Volume
 SQU : Squelch Level
 BAND : AM (air band) / FM
 B_W : AM band width
 F_COR : Crystal Frequency Calibration
 ACA : Automatic scan mode

5. Mode Specifications
 FREQ Mode
  Parameter : Receiving frequency
  Range : 118 – 136 MHz (AM) / 76 - 109 MHz (FM)
  Resolution: Defined by STEP mode
  Behavior : Changes immediately (live)

 STEP Mode
  Parameter : Frequency step size
  Options : 10Hz / 1kHz / 10kHz / 100kHz / 1MHz / 25 kHz
  Behavior : Defines increment/decrement step in FREQ mode

 VOL Mode
  Parameter : Audio volume
  Range : 0 - 63 
  Behavior : Immediate effect on audio output

 SQL Mode
  Parameter : Squelch threshold
  Range : 0 - 63
  Behavior : Audio muted below threshold level

 MEM Mode
  Parameter : Memory channel
  Range : CH0 – CH49
  Behavior : Select memory channel number / assign current frequency

 SCN Mode
  Parameter : Memory channel
  Behavior : Assign memory channel frequency

 BAND Mode
  Parameter : AM (air band 118 - 136 MHz) / FM (76 - 109MHz)

 B_W Mode
  Parameter : Si4732 AM band width
  Unit : 0 - 6 (0:6kHz,1:4kHz, 2:3kHz, 3:2kHz, 4:1kHz, 5:1.8kHz, 6:2.5kHz)

 F_COR Mode
  Parameter : Crystal frequency correction
  Unit : Frequency offset (±Hz for 25MHz of Si5351a)
  Behavior : Adjusts local oscillator accuracy

6. Frequency Control Rule
  Frequency increment/decrement is quantized by STEP setting
  Changing STEP immediately affects FREQ tuning resolution

7. Persistence Behavior
  EEPROM stores:
  Current parameters (mode-dependent)
  Stored values are:→ Restored automatically at power-on

  Automatic scanning
  Double Push on SCN mode → Automatic scanning mode
  Sweep MEM channels every 200mS, if there is signal, stay 1 Sec more.
  Signal detection (squelch open / RSSI threshold)

The RF front-end filter plays a critical role in airband reception, where strong out-of-band signals such as FM broadcast (88–108 MHz, 76-100MHz in Japan) and image frequencies can significantly degrade receiver performance.

Initial Approach

At the beginning of this project, a 5-pole Chebyshev band-pass filter was considered in order to achieve high selectivity and strong out-of-band rejection. However, practical constraints quickly became apparent.

A higher-order filter requires:

Larger PCB area
Careful control of inter-stage coupling
Complex tuning and adjustment

In a compact DIY implementation, these factors make the design difficult to realize reliably. As a result, a simpler and more practical 3-pole configuration was selected.

3-Pole Filter Implementation

The implemented filter is a compact 3-pole band-pass design using air-core inductors and discrete capacitors. This approach prioritizes:

Simplicity
Small PCB footprint
Ease of construction

Air-core inductors were chosen for the resonant elements due to their stability and suitability for small inductance values in the VHF range.

Measurement Results

The filter response was measured using a nanoVNA. The results are summarized as follows:

Insertion loss (passband around 120 MHz): approximately -9 dB
FM broadcast band (~100 MHz): approximately -30 dB attenuation
Image frequency region (~160 MHz): approximately -27 dB attenuation

These results confirm that the filter provides effective suppression of strong out-of-band signals, particularly in the FM broadcast band, which is critical for practical airband reception.

Observations and Limitations

While the out-of-band rejection is satisfactory, the insertion loss in the passband is higher than expected.

This led to an important realization:

In VHF filter design, practical factors such as component Q and PCB layout can dominate over the theoretical filter response.

In this implementation:

Small inductance values (e.g., single-turn coils) limit achievable Q
Parasitic effects from PCB layout and component leads contribute to loss
Strong coupling between stages can reduce filter sharpness

As a result, increasing filter order alone does not guarantee better performance in a real-world design.

Design Trade-offs

This filter represents a deliberate trade-off:

Pros
Good suppression of strong FM interference
Compact and simple construction
Stable and reproducible behavior
Cons
Higher insertion loss than ideal
Moderate selectivity compared to higher-order filters

Rather than pursuing a more complex topology, the design prioritizes practical usability and robustness.

Next Steps

Based on these observations, the next step is to improve the filter performance by focusing on component quality and implementation details rather than increasing filter order.

Conclusion

Although a higher-order filter was initially considered, a simpler 3-pole design proved to be more practical for this project. Measurement results highlight that, in VHF designs, real-world implementation factors often outweigh theoretical advantages.

This reinforces an important lesson:

Improving component quality and layout can be more effective than increasing filter complexity.