Kit Design - One Approach

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Introduction

Designers need to consider their user market (commercial and/or consumer), the technical and environmental requirements, competition, presentation, flexibility, reliability, documentation, expected operational lifetime, component availability and costs. These are never easy, they often conflict with each other and there is always a human tendency to over complicate so some form of compromise is always required. Niche markets like Amateur Radio add extra complexity because of the multiple modes of operation, the wide possible skill set of the end user and the use of high volume low cost manufacturing of the final product is not an option as it would be with a conventional consumer electronics product.

The purpose of all radio equipment is to enable the highest possible quality communications in both good and bad environments. This may sound simple but quality is very subjective and the equipment must work in the presence of other signals, local and remote interference, varying temperatures and supply voltages - the list of performance parameters can be endless.

Designers must also take into account how the particular product will be used - as a built and tested standalone product, as part of a larger product using other suppliers units or as a kit for self assembly by the end user. The accompanying instructions must be clear but detailed which also raises a cost issue.

Each unit must be capable of providing the modes of operation required by the target user. In amateur radio, the most common modes on the HF bands are SSB and CW with AM required for broadcast radio listening and for VHF operation FM must also be considered. Modules should therefore be designed with some form of future flexibility in mind.

Home constructors can start with something simple and add to that as they increase their skills and become more confident in their abilities.

This article considers some of the high level design requirements for kits intended for the Amateur Radio market and seeks to encourage discussion on this topic.

Receivers

Designs generally fall into one of the following categories:

• Direct Conversion - using a diode, super-regerative or reaction detector or a single frequency conversion, possibly with some form of signal and audio amplification.
• Superheterodyne 1 - using one or more frequency conversion stages and conventional amplifiers, filters and detectors for AM, CW, SSB and FM etc.
• Superheterodyne 2 - similar to the previous type but using some form of Digital Signal Processing (DSP) to provide a combination of selectivity, gain control and detection.
• Direct DSP - this is a new concept that requires very specialised digital integrated circuits (ICs) and related software to digitally process the incoming signal to audio.

Transmitters

Designs generally fall into similar categories to the above receivers:

• Crystal or variable frequency oscillator followed by an optional series of multipliers and amplifiers to reach the desired output frequency and power.
• Superheterodyne 1 and 2 - this type allows the transmitter to be linked to the receiver in terms of frequency and mode (a common feature of modern Amateur Radio operation) so the transmitter will reflect a number of the receiver design features. Type 2 - uses some form of DSP to generate a suitably modulated signal to be transmitted in all standard modes.
• Direct DSP - in a reverse of the receiver equivalent, this digitally generates the modulated signal to be transmitted at the final frequency and requires subsequent stages of amplification to reach the desired output power level.

Other pages on this website already contain information on a range of crystals and ceramic resonators for filters and data on modules that have been developed as individual projects that could be combined into more complex functions.

Readers are invited to send their comments and thoughts to my email address on the main page and I will collate these and extend this article appropriately.

To be continued and revised (as time and inspiration allow)…........

Module Design

A basic receiver or transmitter may be broken down into a number of discrete modules that would share certain design features and therefore control the costs - too many different designs will increase the overall costs. Each module must be specified in ‘black box’ terms - typically gain, selectivity, signal handling levels, input and output impedances, frequency range, supply voltage etc and these parameters should not be dependant on any other interconnected modules.

Readers will see a pattern emerging in terms of blocks of functionality that could be translated into electronic designs and printed circuit boards (PCBs) but this is not without its problems. Discrete and linear IC component availability is becoming an increasing issue and a reasonable lifetime for a printed circuit board (PCB) design is required to make it cost effective and maintainable into the future. Tuned circuit assemblies and mode filters are more design problems to be resolved.

What is required is some joined up thinking in the design of a family of communications modules that could be used standalone where appropriate or interconnected as required by the user to provide more complex functions. Ideally they should fit into easily obtainable screened enclosures made from low cost tin or die cast boxes.

Each module must be specified in terms of its overall functionality and if this is done correctly then it will work regardless of the internal circuit details. In other words the designer is thinking about a black box approach so that if circuit changes become necessary in the future, say due to component availability issues, the overall functionality is not affected. The designer must therefore specify how each module will work in terms of gain, input and output impedances, power supplies, control lines etc.

This approach has its problems which the designer must overcome but results in a more consistent and predictable product. A balance between functionality and cost is important because PCB production is an expensive process so a PCB with just a few components present will be relatively expensive. High packing densities and surface mount components can also generate problems in construction and future maintenance. These are all interesting but solvable challenges that the designer must overcome.

The following links will take you to block diagrams or suggested circuits of typical modules that may be used on their own or as part of a more complex assembly:

The following links will take you to block diagrams or suggested circuits of typical modules that may be used on their own or as part of a more complex assembly:

Typical Test Equipment

• Test Meter
• SWR Meter.
• RF Power Meter
• Two Tone Oscillator
• Return Loss Bridge
• Grid Dip Oscillator (GDO)
• Frequency Counter
• Signal Generator
• Absorption Wavemeter