Bandwidth can be a complex issue because of SysCalc's filtering capability. The following list of system parameters are affected by filtering.
Noise Figure
Noise Floor
Sensitivity
Spurious Free Dynamic Range
Effective Intermodulation Performance
Spurious Frequency Rejection
Simple vs Complex Filters
Each filter can be designated as being simple or complex (see Filter Specifications).
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Simple |
The simple model has no bandwidth provisions and exists solely as a concept block. In affect, the simple model has infinite bandwidth. It does, however, affect the system due to its loss. |
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Complex |
The complex model provides frequency dependent behavior by virtue of the defined loss and filter type. In general you will want to use the complex model. |
Note: SysCalc does not allow both simple and complex filter models in the same system design.
Projects with No Filtering or Simple Filters Only
A project does not have to include filters or may use the simple filter model exclusively. In such a case, there is nothing to limit the system bandwidth unless SysCalc's probe (see Probe Settings), provides bandwidth limiting. Without bandwidth limiting, noise dependent system parameters (e.g., MDS) will appear excessively large.
Typically, receiver bandwidth decreases as you progress through the stages from input to output because of decreasing filter bandwidths. As a consequence, the final filter is often the most limiting. This does not have to be true; when it is not true the following issue needs to be considered. Each component's noise is limited by the subsequent narrowest filter. The source, for example, will basically see the narrowest filter in the entire system whether it is the final component or not. If a filter is followed later in the system by a wider filter, any noise contributed by intermediate components will see the wider filter while the source sees the narrower filter. This will result in the noise floor being higher than anticipated because of the comparison between disparate bandwidths. If this situation truly represents the actual system them the results will be correct.
Note - If the probe is not bandlimited, SysCalc does not allow a system to be terminated with infinite bandwidth components, such as a gain block, attenuator or mixer unless the component has an attached data file that effectively limits its gain over frequency (see the example file, Filters & Noise).
Double Side Band Mixer Noise
SysCalc mixers are considered infinitely wide. Broadband noise coming into the mixer from prior stages will be 'doubly' mixed into the receive channel due to the LO image. It is always advisable to place a filter ahead of the mixer to reduce the incoming noise at the LO frequency. The best that can be achieved with the filter is a 3 dB improvement in NF; this is only possible, however, in the noise levels coming into the mixer are well above the inherent noise in the mixer itself (see the example file, Filters & Mixers & Noise).