Super Heterodyne (SH) Receiver in RESM
The main figures of a receiver made for surveillance application are the Total bandwidth f, the Probability of Intercept PoI, the total dynamic range DR, and the sensitivity Smin.
These figures are generally related to each other: for example a high sensitivity and a wide bandwidth cannot be obtained at the same time.
A Super Heterodyne receiver is a tuneable receiver that has a relatively narrow band, this band is called Instantaneous Bandwidth (IBW).
The narrow band allows reaching high sensitivity and good Instantaneous Dynamic Range (IDR) but requires a sequence of tuning to cover the f.
This causes a reduction in the PoI value that must be mitigate by selecting accurately the sequence of tuning.
Figure 1 shows a sequence of tuning for a SH receiver, the law according to which the tuning is moved is the Receiver Search Strategy (RSS) and shall guarantee a full spectrum coverage with an acceptable PoI.
The attention required to achieve a good RSS is rewarded by an improvement in the other features as sensitivity, dynamic range and protection from the interferences.
In addition a narrow IBW allows using the digital signal processing techniques (DRx) so making the receiver a “multi-signal” receiver i.e. capable of processing contemporary signals (co-pulse).
In addition, tuneable receivers can protect themselves from noise and interferences by shifting in frequency.
The super heterodyne receiver uses frequency mixing or “heterodyning” to convert the RF signal to a fixed frequency band in which the analogue processing or the sampling is easier to be made than in the original RF band.
This band is called IF band, the centre of this band is the IF frequency, typical values are currently from 1 to some GHz for the IF Frequency and 1 to 4 GHz for the IF bandwidth.
The frequency conversion involves the following step:
- a coarse selection of the desired band in order to clean the signal from noise and far interferences
- one or more stages of mixing in each of which the following operation are performed:
- the signal is combined with a sinusoidal local oscillator (LO) to produce a number of intermodulation products,
- then the signal is filtered to extract the wanted intermodulation product,
- finally amplification may be required to compensate the losses.
The output of the last mixing stage is the IF signal, the last filter is the IF filter that is responsible for selectivity of the receiver, the nearest interferences are suppressed by the IF filter.
In order to achieve the best protection form the interference the whole frequency conversion chain must be in condition of linearity so increasing the required dynamic range.
In case of very strong interferences the required dynamic range could be excessive, then a programmable attenuator is added at the input of the receiver to avoid non linearity.
The attenuator “moves” the dynamic range of the receiver creating two kinds of dynamic range:
- The instantaneous dynamic range is that of the receiver without attenuator
- The total dynamic range is that obtained including the attenuator and is greater than the instantaneous one.
- The Instantaneous dynamic range is the ratio between saturation and noise level at the detection stage and is conditioned by those at both IF and Video stage.
In case of SH Rx with detection at IF stage the dynamic range is conditioned by the Detector Noise/Saturation.
For an SH with digital processing, the dynamic range is probably conditioned by the ADC.
As shown in Figure 2 the SH architecture is based on successive filtering and conversions (single or double conversion stages are often used).
The first stage includes a BRF filter that selects the total band and a variable attenuator that selects the dynamic range.
The conversion stage includes the anti-image filter, the OL generator and the mixer.
The IF stage extract the wanted signals with the BIF filter and amplifies them.
Finally signals are translated in the base band and processed for detection and parameter measurement.
The final processing in Base Band could be capable of coherent processing (based on I & Q) eventually digitally and in this case the phase is preserved up to the base-band, in alternative an envelope extractor can be used for incoherent signal processing.
Receiver Search Strategy (RSS)
As a Super Heterodyne receiver has limited instantaneous RF coverage, i.e. it can only look at a limited portion of the total RF band at any given time, a Receiver Search Strategy (RSS) must be implemented.
RSS is a list that details the order in which the instantaneous band is scanned and for how long.
The search strategy is programmed according to the pre-loaded Mission-Dependent Data (MDD)
The search strategy expresses how the scenario is analysed in terms of frequency sub-bands (dwells) over a certain time interval in order to satisfy the POI (Probability of Intercept) and TTI (Time to Intercept) requirements.
The strategy consists of rules, which, starting from the scenario of emitters described in the MDD, builds the best way to discover and analyse the surrounding emitters in terms of frequency band and persistence time for each identified band.
The search strategy is therefore a function having as domain the set of emitters of the scenario characterizing the mission and as co-domain the list of frequency intervals on which the receiver must insist for a time such as to be able to analyse and classify the emitters.
The search strategy is a compromise between how many times a certain dwell must be analysed and the time to devote to each dwell for collecting pulses.
These needs obviously clash with the complexity of the operational scenarios and with the instantaneous narrow band of the Super Heterodyne receiver.
Designing the search strategy a SH receiver means checking:
- Frequency ranges to search for
- Length of time intervals to search.
- Control of the dynamics of the signal of the emitters in the scenario (to adjust the instantaneous dynamic range of the receiver).
- Sequence of dwells to optimize search.
- The Signal-to-Noise ratio in Video depends only on the (last) IF Band: therefore the sensitivity can be (theoretically) improved using increasingly narrow filters (based on the minimum PW).
- The frequency selectivity makes processing possible in the presence of multiple signals (the probability of “co-pulse” is low if IF band << RF band)
- A coherent conversion (i.e. with I & Q) to Base Band makes accurate Phase measurements possible.
- The architecture is more complex than a WO based on Full-band Crystal-Video → sizing can become quite complicated (RF BW, IF BW, LO number, spurious etc.).
- POI <1 → the scanning strategy (in the absence of smart mission data to “guide” it) can become extremely critical.