In addition to the general spec, there are also a range of considerations that need to be laid out to guide the design. These considerations relate to the other functionality that the receiver needs to provide. Some are electrical characteristics, like the type of interface for the demodulated signal. Others are purely mechanical, like the allowance for space, the form factor, the board and so on.
Another issue to contend with here is that the first board built will almost certainly be a prototype. For this reason, I'll try to define as much as I can of the final requirement, but divide those requirements into two categories. The first categories are the mandatory capabilities in the prototype for proof of concept, the second are only mandatory in the final build.
Clearly the most difficult aspect of the requirement is the RF spec already outlined. This is separate because it is the most important thing and it's ambiguous. Obviously it falls into the proof of concept category above. On this page we can set out what we want to do with the radio, once we know we can depend on the RF spec. Equally some of the final capabilities are useful to the proof of concept work.
Requirements Matrix
Prototype
Final Unit
• Meets the requirements of the RF spec previously described
• Antenna sub-assembly to be seperable from the main unit, to allow ideal positioning and orientation
• Antenna interface to be balanced line for superior common mode rejection
• Provides capability to;
Receive MSF radio signal
Indicate signal activity with an LED indicator
Provide an "open collector" output suitable for connection to an RS232 level converter
Allow decoding of received signal on a typical PC using suitably crafted test software
Provide a separate test harness capable of;
Modulating RF from a signal generator to the MSF standard via RS232
Converting an "open collector" output to RS232 levels
• Provides capability to;
Receive MSF radio signal
Decode timing information
Maintain a parallel, crystal referenced, real time clock to allow for radio signal interruptions
Display date, time and other meta information on a dedicated display panel
Provide a BNC terminated high stability frequency reference output
Provide RS232 or USB, and Ethernet interfaces
Provide a basic NTP server capability
Provide a HTTP server capability
Provide comprehensive unit control via front panel, and digital interfaces using HTTP on ethernet
• Printed circuit assemblies designed around a standard "vero" style case
• Uses "generic" analogue and digital components where possible (as before, but also a gate array - with suitable line drivers for display and ethernet)
• Minimise the use of polarised capacitors, and variable components - pots, trim caps, slug coils. Where unavoidable slug coils preferred over trim caps
• Uses "through hole" technology throughout for low production cost and ease of prototyping and debug
• Uses "surface mount technology" where possible and through hole where necessary for, component density, compactness, and ease of manufacture
• Operates from external regulated 9v isolating power supply, consistent with the general form and function of a Class III device operating from a Class II double insulated supply, SELV - IEC 61140
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That then, outlines the general specification. On the next page we'll look at the start of circuit design
