21.4MHz Filter in Two-Way Radio Applications

The 21.4MHz filter is commonly used as an Intermediate Frequency (IF) filter in the receiver of two-way radios. Its performance directly determines two core specifications of the radio: selectivity and sensitivity.

Let's delve deeper into the details from several perspectives:

1. What is the "Intermediate Frequency" in a Two-Way Radio?

To achieve good performance, two-way radios (and other superheterodyne receivers) do not directly amplify and demodulate the received Radio Frequency (RF) signal (e.g., 400-470MHz). Instead, they employ the following steps:

① Mixing: The received high-frequency RF signal is mixed with a signal generated by a local oscillator.

② Generating the IF: This mixing process produces a new signal with a fixed frequency, which is the Intermediate Frequency (IF).

③ IF Amplification and Filtering: This fixed IF signal undergoes significant amplification and stringent filtering.

The advantage of the superheterodyne architecture is that the majority of the gain and selectivity (the ability to reject adjacent channel interference) are handled by this fixed IF section. This makes the circuit design stable and high-performing.

2. Why 21.4MHz?

21.4MHz is a very popular standard IF frequency in the VHF/UHF bands (including those used by two-way radios). 

The historical and technical reasons include:

Mature Supply Chain: There is a wide availability of mature, stable, and low-cost filters, amplifiers, and related components designed for the 21.4MHz frequency.

Performance-Cost Balance: This frequency is high enough to effectively suppress image frequency interference (a common reception issue), yet low enough to allow for the relatively easy design and manufacture of high-Q (high-quality factor) filters, such as crystal or ceramic filters.

Historical Precedent: This frequency has been used in the communications industry for decades, and many classic two-way radio chips and solutions are designed around this IF.

3. The Core Role of the 21.4MHz Filter in Two-Way Radios

Within the 21.4MHz IF section, the filter acts as the crucial "gatekeeper." Its role is vital:

① Improving Selectivity

Function: Selectivity refers to the receiver's ability to distinguish the desired signal from many signals and reject interference from adjacent channels.

How it works: The 21.4MHz filter possesses a very steep frequency response curve (often resembling a "brick wall"). It only allows a very precisely defined bandwidth of signals centered around 21.4MHz to pass (e.g., common channel spacings like ±6.25kHz, ±7.5kHz, ±12.5kHz).

Practical Effect: When two radio stations with very close frequencies are transmitting simultaneously, a two-way radio equipped with a high-performance 21.4MHz filter can clearly receive the target station's audio while effectively suppressing the signal from the adjacent channel, preventing crosstalk. Without this filter, or with a poor-performance filter, you would hear interfering noise from the other channel ("cross-talk").

② Improving Sensitivity

Function: Sensitivity refers to the receiver's ability to receive weak signals.

How it works: The filter removes out-of-band noise and interference before amplifying the IF signal. This means that primarily the "clean" desired signal itself is amplified. If amplification occurred without filtering, out-of-band noise would also be amplified, potentially overwhelming the weak desired signal.

Practical Effect: Enables your two-way radio to maintain intelligible communication over longer distances or when the signal is very weak.

4. Types of 21.4MHz Filters

Common types of 21.4MHz filters used in two-way radios include:

Ceramic Filters:

Characteristics: Low cost, small size, high reliability, no need for adjustment.

Application: Widely used in mid-range and low-end two-way radios. Their performance is sufficient for most general requirements. Their bandwidth and shape factor (a measure of the steepness of the filter's edges) are typically slightly inferior to crystal filters but significantly better than LC filters.

Crystal Filters:

Characteristics: Feature a very high Q factor, resulting in excellent selectivity and a shape factor very close to ideal.

Application: Typically used in high-end professional two-way radios or applications requiring extremely high interference immunity. The cost is higher than ceramic filters.

Summary

Suzhou Hangjing can provide SMD and DIP series crystal filters for customers' different requirements.

Please feel free to contact Hangjing sales or engineers for specific specifications and parameters.