Understanding Receiver Blocking and Desensitization

This article explains the concept of receiver blocking, a type of interference that can desensitize radio receivers. Receiver desensitization refers to the reduction in a receiver’s sensitivity, particularly when a strong, off-tune signal is present. In simpler terms, it means the desired signal appears weaker when a powerful interfering signal is nearby compared to when that interfering signal is absent. This effect is most noticeable when the desired signal is already weak.

Figure 1 illustrates receiver desensitization due to blocking interference.

The ability of a receiver to withstand strong, off-tune signals – often around 20 kHz or 100 kHz away from the desired signal – is a key specification. When a strong interfering signal is present, it consumes a significant portion of the receiver’s resources, leaving less power available for weaker, desired signals. Conversely, when the undesired signal is removed, those weaker signals receive the full power budget.

Receiver Blocking Testing

To measure receiver blocking performance, a common approach involves feeding two input signals to the receiver:

1. The Desired Signal: The signal the receiver is intended to pick up.
2. An Interfering Signal: A much stronger signal at a specific frequency spacing (e.g., 20 kHz or 100 kHz) from the desired signal.

The interfering signal’s strength is gradually increased until the blocking desensitization causes a 3 dB drop in the desired signal output. This point indicates the receiver’s blocking performance limit.

Mitigating Receiver Blocking

The effect of blocking can be minimized using front-end RF attenuators. While this reduces the overall gain of the receiver and therefore the signal level, it helps prevent desensitization by managing the power levels of incoming signals.

The following image depicts a typical setup used in receiver blocking testing:

As illustrated, both the desired and unwanted signals are combined using a power combiner and fed to the device under test (DUT), which is the receiver. For specific testing procedures, refer to z-wave conformance testing documentation, which includes receiver blocking tests for evaluating z-wave device performance.