The techniques of using simulation and connect-and-go applications boards can provide rapid RF prototyping with minimum risk.
Hardware developments today are required to be completed within ever diminishing timescales. Clients want prototype products fast, whilst being low cost and low risk - the race is always a challenge to win. There may be a need to develop and test a new system or simply to provide a functional prototype for marketing purposes. Time is always of the essence and fast delivery is paramount.
In the RF and high speed analogue domain the challenges are even tougher for fast prototyping. Often these circuits are highly layout critical and some behaviours are hard to predict and model.
Generally, upfront simulation is performed before committing to a full PCB design and layout. This is costly, takes a long time to make accurate models and at best gives an indication of performance for complex systems. Going too early to full PCB layout and assembly is also a high risk move and will undoubtedly lead to further turns or re-spins of the board before a satisfactory system can be achieved.
This approach is costly, takes a long time to deliver and is potentially high risk. The faster the system, the greater the risk.
How do we improve the confidence in our upfront designs and perform real world measurements before committing to a full PCB?
The use of connect-and-go application boards allows a high degree of functionality to be realised in a short time. The performance of devices can be characterised, long complicated line-ups can be assembled and overall confidence grown.
This method is extremely powerful as a number of systems can be trialled and optimised without inherent issues associated with layout at high frequencies. By complementing this approach with simulation, lower risk higher performance systems can be delivered ready for final layout and PCB production. This flexible method can enable very rapid development timescales to be realised.
System simulation up front is still important
For RF design of even simple systems, some simulation is a must when developing system line-ups. There are specific design tools that can help here, such as SYSCALC, ADS, MWO. These tools require a capital investment and in many cases a simple version of the tried and trusted Excel with appropriate macros meets the basic needs. Care needs to be taken when using any simulations or design assistance software - this is where an intuitive feel backed up by experience is necessary.
Simulate new components at circuit level
When trialling a new circuit topology - something for which there is no applications PCB - simple datasheet results can be used in the overall system line-up for a first pass. Where possible, build in margins of uncertainty where real life performances cannot be measured first.
Perform subsystem measurements in the real world
This is where real life measurements are performed on application PCBs or fast turnaround small PCBs designed specifically for a device or function.
Most manufacturers will supply application boards for their products. It is best to convert existing application PCBs to meet the immediate need or use fast prototyping on copper clad boards. This method allows real life measurements to be taken long before the circuits and subsystems go for PCB fabrication. Confidence is built in the design as real world figures to be factored back into simulations.
Replicate the real system in real life
By using application and fast turn boards, a full or part system can be constructed and tested. This approach can determine the performance of the system based on real measurements instead of being totally reliant on simulation.
Rapid real world measurements allow a fast insight into how the system will work in practice with the opportunity to identify any deficiencies or areas for improvement. Connectorised boards allow optimisation of the line-up as fast changes for different scenarios and components can be achieved in an instant.
In addition, the effect of close proximity interactions can be observed to determine if screens or physical spacing are an issue. Performance over varying temperature and power supply voltages can be performed, plus more advanced RF testing can be done, including monitoring gain, noise figure, stability, compression and third order intercept points. These parameters are very hard to reliably model in purely theoretical simulations.
Do not forget to use connect-and-go on the real PCB design
When confidence has grown in the system then the development can proceed to a full PCB layout. At this stage as many degrees of freedom should be built into the design. This can be achieved by adding connectors (fitted for development) that allow RF breakouts to reroute the key signals or add in additional subcircuits.
For example, providing a breakout to trial external filters, phase up line length or provide test connection for key diagnostics. This also allows subsections to be linked out, or even drop-in sub assemblies from third parties to be used.
The key is rapid development with in-built flexibility to deliver fast prototypes.
Much of the traditional risk in RF design can be reduced by the use of this approach of performing initial real life measurements on assembled systems.
By using application PCBs in addition to simulation, it is possible to build and analyse complex systems without the need to commit to full PCB layout and reduce reliance on software simulation. This provides the fastest route to RF and analogue prototyping while minimising risk and maintaining flexibility.
Tim Fergus is a principal consultant in the Wireless Technology Practice of PA Consulting Group.