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Application Note
Vishay RFWaves

Fast Discharge of RFW-122 Receiver Reference Capacitor
ABSTRACT This document describes a procedure for fast discharge of the receiver reference capacitor in the RFW122 module series. The purpose of fast discharge is to enable more efficient operation of the Vishay RFWaves receiver. An implementation of a scheme for capacitor discharge will improve the robustness of the communication system in various scenarios. Specifically, it enables better coexistence with other networks or other interference sources, as well as better coexistence inside the Vishay RFWaves network. WHY IS FAST DISCHARGE NEEDED? The RFW-122 RF module series includes an ASK receiver. A reference capacitor is used to hold a reference voltage at the receiver. The reference voltage enables differentiating between a receiving a "1" or a "0". It is charged according to the average power of the received RF signal. The capacitor has a slow self- discharge rate. This rate is much slower than the actual bit-rate (1 mbps). Due to the OOK modulation method, the capacitor is charged when "1" is received a
nd discharges when "0" is received (there is no input signal for "0"). Hence the slow discharge rate enables a relatively constant voltage level when regular data is received, and it can therefore be used as a reference. However, the natural capacitor discharge rate is not sufficient in the following cases: a. Avoiding External Network Blocking There is a strong interferer in the area of the receiver (see figure 1). The receiver does not sense the interfering transmissions, since a preamble is not recognized, but the reference capacitor could nevertheless be charged by their electromagnetic power. A Vishay RFWaves packet that was transmitted right immediately after the end of the interfering transmission, would not be received, since the normal discharge of the capacitor is relatively slow. The effect is for the receiver to be temporarily "blinded" by the strong transmission. This of course happens only if the interfering signal was stronger than the current Vishay RFWaves transmission (for example, in a cas
e where it was much closer to the RFW122 receiver or used very strong output power). In such a case, the capacitor would have to be discharged quickly at the end of the interferer transmission, in order to enable the subsequent Vishay RFWaves packet to be received immediately.
Document Number 84761 Rev. 1.0, 26-Jan-06

RFW122 Transmitter RFW122 Receiver

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Strong Interference Source

Figure 1. External network blocking

b. Avoiding Intra-Network Blocking Two consecutive packets received from different sources with different signal strengths (see figure 2). This circumstance can cause problems when we are dealing with a multi-device network and the distances of two devices from the receiver are considerably different. The difference between a distance of 1 m and 2 m can translate into a difference in receiving power of more than 6 dB. The second packet of the two consecutive packets, if its received signal is weaker, will probably not be received correctly. As explained in the previous paragraph, this occurs because of the time interval the reference capacitor requires to discharge to a level where it is sensitive enough to detect the second packet. Beyond distance, a number of other environmental conditions can cause similar problems. For example, reflections caused by a nearby object can significantly effect the relative levels of the received signals.
RFW122 Transmitter #1

RFW122 Receiver

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RFW122 Transmitter #2

Figure 2. Intra-network blocking

The company's products are covered by one or more of the following: Taiwan Patent No. 155994, Taiwan Patent No. 176767 USA Patent No. 6,535,545. Other patents pending.

For more information please contact: RFTransceivers@vishay.com

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