Խոռոչ ընդդեմ պարուրաձև զտիչներ անլար վիդեո հղումներում
In wireless video transmission systems—especially COFDM-based transmitters and receivers used for drones, իրավապահ, and surveillance—the RF filter is one of the most important components that determines the overall signal quality. Two of the most common types used are cavity filters իսկ helical filters. Both serve to pass the desired frequency band and suppress out-of-band interference, but their structure, կատարումը, and suitable applications differ significantly.
1. Appearance and Structural Differences
Cavity Filters (腔体滤波器) use one or more resonant metal cavities as frequency-selective elements. The resonant frequency depends on the physical dimensions of each cavity.
- Արտաքին տեսք: Cavity filters usually look like metal blocks or rectangular enclosures with connectors (Ավագ դպրոց, N- տիպ, or TNC) on each side. They often include tuning screws on top for fine frequency adjustment.
- Size and weight: Because they rely on physical cavities, they are relatively large and heavy, especially at lower frequencies.
Helical Filters (螺旋滤波器) are built from coiled conductors—metallic helices—mounted inside a metal shield. Each helix acts as a resonator, and coupling between adjacent helices determines the bandwidth.
- Արտաքին տեսք: Small, cylindrical or rectangular modules with tuning screws at the top. Often shielded in aluminum or brass housings.
- Չափը: Compact and lightweight, ideal for portable or airborne devices.
From the outside, cavity filters look like solid metallic boxes used in base stations, while helical filters are smaller modules that can be directly mounted on a PCB or inside lightweight transmitters.
2. Performance Comparison
| Առանձնահատկություն | Cavity Filter | Helical Filter |
|---|---|---|
| Հաճախականության տիրույթ | 100 ՄՀց – 6 ԳՀց | 100 ՄՀց – 1 ԳՀց (typ.) |
| Insertion Loss | 0.3–1 dB (low) | 1–2 dB (medium) |
| Selectivity / Rejection | Very high | Moderate |
| Power Handling | Բարձր (up to tens of watts) | Ցածր (below 5 Վ) |
| Չափը & Քաշը | Larger, heavier | Կոմպակտ, light |
| Temperature Stability | Excellent | Moderate |
| Cost | Higher | Իջնել |
Cavity filters are chosen when high isolation, low loss, իսկ temperature stability are crucial—such as in professional ground stations or long-range receivers.
Helical filters are used when compactness and weight are the main concerns, like in drone-mounted transmitters or portable receivers.
3. Application in Wireless Video Systems
In COFDM wireless video transmission systems, filters are installed both on the հաղորդիչ եւ ստացող to improve spectral purity and system stability.
• Transmitter Side
After the power amplifier (PA), էունք band-pass filter is used to eliminate unwanted harmonics and spurious emissions.
- For compact drone transmitters, որ helical filter is preferred because it is small, light, and provides sufficient harmonic suppression.
- For high-power base transmitters, որ cavity filter offers better power handling and higher isolation.
• Receiver Side
At the receiver front end, a pre-select filter blocks strong out-of-band signals before they enter the LNA (low-noise amplifier).
- Helical filters are adequate for portable receivers.
- Cavity filters are ideal for professional or fixed ground receivers where interference rejection and long-term stability are critical.
4. Which to Choose for a ±2 MHz Bandwidth
When the system requires a narrow passband, ինչպիսիք են ±2 MHz (total 4 ՄՀց), both filter types can work, but the choice depends on performance goals.
- Helical Filter: Suitable for narrowband frequencies (Է.Գ., 520–524 MHz). It provides moderate insertion loss and adequate rejection in small size.
- Cavity Filter: Recommended if the system demands sharp selectivity (≥40 dB rejection) or operates under wide temperature ranges. Cavity filters offer better long-term stability for narrowband COFDM systems.
Օրինակ:
- Drone transmitter at 522 MHz ±2 MHz → Helical filter (կոմպակտ, light).
- Ground receiver → Cavity filter (high isolation, reliable in outdoor conditions).
5. Customization and Design Options
When customizing filters for a wireless video transmitter or receiver, the following specifications are usually required:
- Center Frequency: Է.Գ., 522 ՄՀց
- Թողունակություն: Է.Գ., ±2 MHz (4 MHz total)
- Insertion Loss: Է.Գ., ≤1.0 dB for cavity, ≤2.0 dB for helical
- Out-of-Band Rejection: Է.Գ., ≥40 dB at 460 ՄՀց & 590 ՄՀց
- Դիմադրություն: 50 Ω input/output
- Միակցիչի տեսակը: Ավագ դպրոց, N- տիպ, or customized interface
- Power Handling: Depending on output power (Է.Գ., 1 Վ, 5 Վ, 10 Վ)
- Գործառնական ջերմաստիճանը: -40°C-ից +85 °C
- Mechanical Dimensions: For PCB or external mount
Manufacturers can adjust:
- Այն number of resonators (typically 2–6 stages).
- Այն cavity size or helix pitch to match desired bandwidth and rejection.
- Այն housing material (aluminum, brass, or stainless steel) for different applications.
6. Summary Table
| Դիմում | Recommended Filter | Reason |
|---|---|---|
| Drone video transmitter | Helical filter | Կոմպակտ, թեթև քաշ |
| Portable ground receiver | Helical filter | Balanced performance |
| Base station receiver | Cavity filter | High selectivity, low loss |
| High-power transmission | Cavity filter | Superior power handling |
| Narrowband ±2 MHz system | Cavity or Helical | Depends on size vs. rejection trade-off |
Both filters play a vital role in ensuring clean and reliable video transmission. Choosing the right one depends on whether your priority is size and weight կամ selectivity and stability.
7. ՀՏՀ
Q1: Can a filter be retuned if the frequency changes?
Այո. Both cavity and helical filters can be fine-tuned by adjusting the tuning screws. Սակայն, the tuning range is limited—typically ±5 MHz. For larger shifts, a redesign is recommended.
Q2: How to identify which type is installed in my system?
If the filter is a rectangular metal block with deep screw holes, it is likely a cavity filter. If it looks like a small can-shaped module on a PCB, it is a helical filter.
Q3: What if my application requires multiple channels?
Multi-channel COFDM systems can use switchable filter banks կամ broadband cavity filters designed for multiple center frequencies.
Q4: Can the filters be waterproof or outdoor-rated?
Այո. Cavity filters can be sealed and anodized for outdoor use. Helical filters can be enclosed in waterproof housings if needed for drone or vehicle applications.
Q5: Is there a big cost difference on Cavity vs Helical Filters?
Cavity filters are generally more expensive due to their machining precision and larger size, while helical filters are cost-effective for mass-produced transmitters and receivers.
Վերջնական մտքեր
Selecting between a cavity filter եւ ա helical filter is a balance between performance and practicality. For long-range, high-power, or professional receiving systems, cavity filters deliver unmatched stability and selectivity. For compact, airborne, or low-power COFDM transmitters, helical filters remain the most efficient and economical choice.
A properly chosen and tuned RF filter not only ensures compliance with local frequency regulations but also guarantees stable, interference-free wireless video transmission—whether on the ground or in the air.
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