By Jim Goings
In your RF system, your receiver’s ability to accurately demodulate the desired RF data packet can be significantly impacted by unwanted RF signals at or near the desired operating frequency. Fortunately, there are some techniques that you can apply to reduce interfering near-band, wide-band and in-band signals.
To address near and wide-band disturbers, enhance the radio’s selectivity and blocking characteristics, particularly when you are selecting your radio. You can also implement a SAW filter between the receiver’s antenna and the RF front end. This technique produces a bandpass effect that enables the desired signal to enter the radio with very little attenuation, while at the same time subjecting the disturber to increased attenuation.
If the SAW filter doesn’t adequately block the interference, take a look at the bandwidth of your radio’s intermediate frequency (IFBW). As an example, as shown in Figure 1, consider noise appearing 200kHz below the desired operating frequency. Here, an IFBW of 366kHz would only attenuate the disturber by 10dB at its corner frequencies. But when using a 25kHz IFBW, as shown in Figure 2, the disturber is attenuated by 56dB. High-performance RF devices now enable adjustments to the IFBW through an EEPROM-based configuration table. During the optimization process, be sure that the selected IFBW remains wide enough to account for variations in the RF frequency of the receiver and transmitter that result from modulation and tolerance of internal frequency references.
Figure 1: Blocking characteristics of the ATA5830 at 433.92MHz, IFBW = 366kHz
Figure 2: Blocking characteristics of ATA5830 at 433.92MHz, IFBW = 25kHz
To mitigate the effects of in-band disturbers, you can look to two methods: time domain redundancy or time and frequency domain redundancy. Because it is relatively simple and inexpensive, time domain redundancy is the most common architecture used today. This method mitigates intermittent sources of interference by sending multiple copies of the same RF data packet, delayed by a finite amount of time. As a result, you can use a single RF carrier frequency for both the transmitter and receiver.
With time and frequency domain redundancy, you can completely avoid a continuous RF disturber if the disturber’s spectrum occupies a small frequency range. This method can substantially improve radio performance. Learn more about RF system design considerations from our in-depth article.
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