As discussed in the November Technical Corner Article, at frequencies common to two adjacent channels of a reactively coupled, contiguous (crossover) multiplexer, the minimum loss that can be experienced is 3 dB. At the common frequency, half the power will go into one channel and half into the other. If this loss is not acceptable, a price must be paid. This price is a loss of the ability to simultaneously view the outputs of both channels. In short, one must switch between the two channels. In most cases the data output from a wide band multiplexer channel must be processed through some digital data device. The time to perform such processing is usually much greater than the time required to switch between two channels. Thus, although simultaneity is lost, effectively it is maintained in the switched filter because one channel is being processed while the switching is occurring.
Another interesting feature of the switched filter is the relative ease with which higher order (more than three channels) structures may be constructed as compared to a multiplexer. The combination of blind-mate or drop-in filters, channelized microstrip interconnections, miniature switch modules and integrated drivers can form compact switched filter assemblies that will compete effectively with multiplexer techniques in wide band receiver applications. For higher power situations, the switches represent a power/switching speed limitation.
Switch-filter assemblies have characteristic implementation difficulties. These difficulties center around the problem of eliminating crosstalk between channels. Crosstalk occurs outside of the switch or filter, simply due to excess radiative or conductive coupling between the lines leading up or away from the switch or filter. The problem is usually made even more serious because conventional switched-filters employ drop-in switches and drop-in filters. Thus, the external line required to interconnect the switches and filters is at least 0.2” long. With filter and switch wall thickness of 0.100”, a total interconnection length of 0.400” results (click to see Fig. SW-1). Perhaps equally as severe is the fact that the external interconnection line is not well-defined in terms of impedance or electrical length, and thus creates uncertainty in the simulation of the composite switch and filter response. At very low frequencies, such a short line will not radiate enough to cause problems. However, at frequencies as low as 100 MHz, the short line, an inefficient radiating element, radiates enough energy to adjacent regions within the assembly that circuit isolation is reduced to less than the intrinsic switch isolation. Thus, no matter what isolation properties are displayed by the switch, the crosstalk problem reduces channel-to-channel isolation, frequently below specification.
At RS Microwave, we have virtually eliminated the aforementioned problem, using our proprietary blind-mate interconnection technique. The switches are provided with blind mate shrouds, filters with proprietary but GPO-compatible blind-mate fingers. Thus, the filters “plug-in” to the switches. The total interconnection length, including filter and switch walls, does not exceed 0.260” (click to see Fig. SW-2). No interconnecting line is required. Isolation is intrinsically at least 85 dB to 18 GHz, and thus the switch isolation is not compromised by interconnection problems. The interconnection is well-defined (i.e. 0.260” of 50 ohm line) and thus can be easily incorporated into simulation data. The reduced length of the interconnection (0.260” versus 0.400”) also helps to eliminate parasitic responses.
Switches are sometimes more expensive than passive filters, sometime display more insertion loss and certainly consume DC power. If filter networks can be substituted for switches, some reduction in loss, size and cost can ensue. In the cases where filter passbands are separated by some guardband, filters can be multiplexed at a common end and switched at the other end (click to see Fig. SW-3). This application is useful in generation of a series of local oscillator tones (“comb-generator”), because the filter passbands are narrow and well-spaced. If the input is multiplexed and the output switched, the RF power at the switch has already been reduced by the filter insertion loss, and the harmonics and spurious products generated by the switch are consequently reduced.
The switched filter technology is becoming increasingly more affordable, as MMIC based FET switches increase in frequency range. For applications below 6 GHz, this FET based technology is the low power approach of choice (70841A-3, 61461A-3). It is likely that the FET circuitry will be practical at frequencies up to 18 GHz or so in the next few years. Presently, PIN diodes remain the main alternative to the FET approach for fast switching, and the only solid-state technology available for high power switched filters.
RS Microwave has developed many such high power assemblies (50703-4). Such assemblies frequently contain bandpass filtering or remove harmonics or to protect against lightning by providing a DC short circuit at the input to the assembly. PIN diode switches require more driving current than FET based switches, but can provide low loss and high isolation when properly combined with RS Microwave’s blind-mate approach to integration. For slower switching, it is still possible to select filters using mechanical relay type switches. Such relays switch in milliseconds, not microseconds, but the switches only contribute 0.1 to 0.3 dB insertion loss, rather than losses of 1 to 2 dB as found in most solid state switches. It is possible to combine filter and switch assemblies with gain blocks, however, to reduce or eliminate insertion loss, if noise figure and compression do not become major concerns. As technology improves, RS Microwave will continue to integrate the best switches and gain blocks with the best filters,
