- Buyers Guide
Ultraminiature, High Reliability RF Bypass Relays
Double-pole double-throw (DPDT) relays have long been used as the switching mechanism for applications such as attenuators, circuit and component bypassing and other specialized circuits such as selecting between unity and a preset amplifier gain. These applications are all loosely described as bypass circuits. To create the bypass, circuit designers traditionally have externally joined either the normally open (NO) or normally closed (NC) positions of a DPDT relay on the circuit board. However, when applied to higher frequencies and decreasing signal strength, this technique causes unwanted additional signal attenuation or fidelity loss due to the additional circuit connections, added circuit lengths and circuit materials.
Given a sufficiently small signal, these factors may cause signal attenuation to the point of rendering the signal useless. An example of this condition is in cellular communications where the transmit signal generated by the base station may be many watts in strength. The remote hand-held unit has no trouble receiving this signal, yet it must transmit back through the antenna to the base station. The remote unit's transmit signal is weak comparatively, requiring the base station to amplify the signal directly at the receiving antenna to counteract further signal loss through lengths of cabling to the final amplifiers. Mounting an amplifier directly at the receive antenna has many disadvantages. If this amplifier becomes nonfunctional, the process of finding a replacement is extremely time-consuming. Under these conditions, a relay is placed parallel with the amplifier to route the signal around the faulty amplifier in the event of an amplifier malfunction. Constructing this bypass circuit is critical to preserve the weak signal's amplitude and minimize the circuit losses.
With these problems in mind, a family of high repeatability, ultraminiature bypass relays has been developed. These new TO-5 bypass relays are closely related to the high repeatability models RF300 and RF303 DPDT relays. The model RF310 and its sensitive model RF313 counterpart feature NC contacts that are internally connected, while the model RF320 and its sensitive model RF323 counterpart feature NO contacts that are internally connected, as shown in Figure 1 .
This configuration offers the circuit designer the ability to eliminate much of the length of the circuit path that normally would exist through a standard DPDT relay wired for bypass operation. The bypass path eliminates the need for the signal to exit the relay to an external bypass path and then re-enter the relay for further routing. By providing the bypass path internally, the signal has a shorter distance to travel through the relay. Reducing this signal path eliminates much of the material through which the signal must pass. Any material in the conductive path contributes to the bulk resistance of the circuit, which, in turn, causes signal attenuation. Three material changes are eliminated, thereby eliminating the impedance mismatches and, thus, the return losses resulting from these material transitions. In addition, four abrupt changes in direction (90° or more) and four interconnections are eliminated. Both changes in direction and interconnections represent discontinuities and, hence, signal reflections and loss. The new relay configurations with the internal jumper connections eliminate much of this signal attenuation.
These new relays also exhibit good RF insertion loss repeatability. Like their predecessors, the RF insertion loss repeatability is less than +/-0.1 dB across the characterized frequency range from 0.3 MHz to 3 GHz.
Figure 2 shows average RF insertion loss and SWR measurements taken in a 50 W test circuit utilizing a 50 W source for the model RF310 relay. The coil-energized data address the RF characteristics applicable to the contact portion of the relay, and apply to one contact set for one pole. The coil-de-energized insertion loss and SWR data address the RF characteristics applicable to the bypass path of the relay, and apply to the full length of the bypass path. Therefore, these graphs represent the entire switching circuit path. In comparison, the same data taken for a standard DPDT relay would have to take into account all of the effects of the various interconnections and abrupt changes in direction outlined previously. Figure 3 shows the isolation average measurement for both the RF310 and RF313 relays. The isolation data for the energized coil case represent the isolation between the two open stationary contacts connected to their respective moving contacts (referred to as pole-to-pole isolation). The case of the de-energized coil represents the isolation of the open stationary contacts to the bypass path. The data for the RF320/RF323 relays are similar.
These ultraminiature RF relays are housed in an industry-standard TO-5 package measuring 0.370" diameter by 0.275" high for the RF310 and RF320 relays and 0.385" high for the RF313 and RF323 relays. The available coil voltages are 5 and 12 V DC. The internal welded construction and Uniframe design offer high motor magnetic efficiency and good mechanical ruggedness. The contacts are constructed of gold-plated precious metal alloy for low contact loss and reliable switching. More information on these relays can be obtained from the company's Web site at http:\\www.teledynerelays.com.
Teledyne Relays, Hawthorne,
CA (800) 594-0855.