Frequency Activity Detector (FAD)
Activity Detector Unit (ADU)
Many advanced EW receivers, particularly those associated with ECM jamming systems do not require the high resolution frequency information associated with a DFD or IFM based receiver. This is typically the case where identification of a single pulse present within a broad frequency band is required to be detected and instantaneously down-converted to an IF band. An example of such a requirement would be the RF front end of a Digital Radio Frequency Memory (DRFM). In this situation the receiver is serving merely to monitor activity and very rapidly flag any activity detected by giving an accurate but not necessarily high resolution indication of frequency.
Sage manufactures a range of Frequency Activity Detectors (FAD) which perform frequency measurement of an RF pulse and output data in as little as 15 ns. The basis of these products is Sage's ability to produce high performance broadband channelizers and integrate detectors. These devices are ideal for receivers which must operate in very high pulse density environments.
The Sage FAD configuration consists of an RF input,which may be fed by a signal conditioning network as in the case of the IFM, followed by a detector integrated into each channel and a combination of video and logic circuitry whose function is to detect when activity in particular channel has crossed a predetermined threshold. A crucial element in the design is the channel filter characteristics and their contiguous frequency coverage.
The contiguous coverage is achieved by resistively splitting the input signal between two non contiguous bandpass channel multiplexers. This allows less than a 3 dBc crossover to be achieved and simplifies the multiplexing requirement. The multiplexers themselves are designed as individual lossy or pseudo-lossy manifold multiplexers in which each channel filter is coupled to the manifold feed. This technique is well proven and is extensively used throughout Sage's Switched Filter and Switched Multiplexer Products.
The physical realization is SSS printed combline filters inductively coupled to a printed manifold.The SSS technology allows for extremely accurate channel crossover frequencies to be achieved with very low temperature drift. Such implementation ensures excellent unit to unit performance tracking and tracking with Sage's Switched Multiplexer and Frequency Synthesizers. The filter networks are optimized for low propagation delay and often incorporate non-adjacent resonator coupling to give optimum passband performance in conjunction with high selectivity. The combination of accurate, reproducible crossovers and narrow crossover regions reduces the ambiguity zone around the crossover.
Each channel has its own detector. These can be either Tunnel or biased Schottky diode based. Since in most applications sensitivity is not an issue and the unit need only operate over a relatively narrow dynamic range the Tunnel detector is favoured. This also has the distinct advantage of providing stable outputs over temperature whilst requiring no bias networks and presenting a more tolerable RF match for the filter.
Sage integrates the detector diode and video choke network directly on the SSS circuit. The correct impedance match is presented to the output of the channel filter by a direct 'tap' to the last resonator. The video bandwidth of the detector circuit is chosen to ensure optimum probability of detection for a given minimum pulse width False Alarm Rate (FAR) requirement and throughput delay.
The video voltage from each detector is checked against a threshold level by a bank of high speed comparators. Additional comparisons may be made to allow correct operation in the presence of signal harmonics or simultaneous signals before a single channel output is indicated. Output data maybe in any format and ECL circuits are used where appropriate.
Sage can provide both fixed and customer variable thresholds within a single design. Great care must be exercised when setting threshold levels. A high threshold will yield low FAR but poor Pd whilst a low threshold will give excellent Pd but be prone to trigger on noise. Varying the video bandwidth helps to trade these parameters somewhat but given that such devices must normally be able to operate with small PW and have low throughput delay simply reducing the video bandwidth to reduce the noise level is not always practical. Careful thought must also be given to the propagation delay of the filters and the logic circuits when looking at the overall performance requirements.
| Frequency Range |
2.7 - 5.3 GHz |
2.5 - 5.3 GHz |
2 - 6 GHz |
| Number of Channels |
8 |
8 |
16 |
| Channel Bandwidth |
100 MHz |
350 MHz |
250 MHz |
| Input Power Level |
+5 to +10 dBm |
+12 to +17 dBm |
+10 to +16 dBm |
| Response Time |
40 ns |
20 ns |
30 ns |
| Threshold |
Programmable |
Fixed |
Programmable |
| Detection Probability |
0.99 |
0.99 |
0.99 |
| False Alarm Rate |
1/second |
1/second |
1/second |
| Output Format |
TTL |
Differential ECL |
Differential ECL |
| Power |
2W |
8W |
7W |
One of our FADs:
FDFD6420-1.pdf.
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