Switched Multiplexer
A switched multiplexer is a two port adaptive filtering device whose filtering characteristics may be electronically controlled to suit the operational environment. A typical application may be to precede an IFM to selectively reduce the incident signal spectrum.
The switched multiplexer splits the input frequency band into a number of channels, each of which may be switched into a bandpass or bandstop state. For a device comprising "n" channels it is therefore possible to have 2n switched states. This capability allows the device to be instantaneously configured as a tunable bandstop filter with single or multiple bandstop channels, or conversely as a tunable bandpass or multiple bandpass filter.
Sage Laboratories switched multiplexers are realised by power splitting the broadband input equally between two filterbanks. Each filterbank comprises multiplexed, switched bandpass channels forming "odd" and "even" channels. The outputs are recombined in a resistive combiner.
Splitting the full frequency in this manner allows the channel bandwidths to be designed to achieve the correct overlap at the adjacent band edges to ensure recombined amplitude and phase characteristics. Sage Laboratories has developed a technique for realising the filterbanks which permit many channels to be multiplexed with no channel interactions and giving uniform channel characteristics.
The filterbanks comprise a cascade of pseudo-complimentary diplexers multiplexed along a lossy manifold. This structure ensures that all the power on the manifold is delivered to each channel filter while preserving a broadband match on the manifold.
The channelizing network formed is connected to a similar output network through custom non-reflective GaAs FET or PIN diode switches which also incorporate an adjustable attenuator network. The non-reflective switching ensures that the multiplexer match is maintained whatever the switch state and allows the energy in the filter structure to quickly dissipate in the "off" state. The adjustable attenuator allows the amplitude to be equalised over all channels.
To ensure that the overall recombined performance is acceptable under all environmental conditions, the bandedge amplitude and phase characteristics of each filter must exhibit slowly varying functions of frequency. A proprietary "reduced sensitivity" filter prototype has been developed to yield flat amplitude and group delay without sacrificing selectivity.
Devices using this prototype have been produced with passband amplitude ripple of less than 3 dB over full military airborne temperature range with up to 32 channels selected.
Switched multiplexers are essential building blocks in wideband receivers which are expected to operate in dense signal environments. When combined with Sage Laboratories frequency and amplitude measurement products a switched multiplexer forms the heart of the Self Adaptive Channelized Receiver.
| Frequency Range |
2 - 4 GHz |
2.5 - 5.3 GHz |
2 - 6 GHz |
| Number of Channels |
20 |
16 |
16 |
| Nominal Bandwidth |
100 MHz |
175 MHz |
250 MHz |
| Rejection at Adjacent Band Center |
60 dBc |
50 dBc |
50 dBc |
| Nominal Insertion Loss |
20 dB |
20 dB |
16 dB |
| Insertion Loss Ripple |
< 3 dB |
< 3 dB |
< 3 dB |
| Isolation |
> 60 dB |
> 60 dB |
> 60 dB |
| Switching Speed |
40 ns |
40 ns |
70 ns |
One of our Switched Multiplexers:
FSM6420-1.pdf.
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