We designed Breadboard One as the first BitScope Ed project because it’s a simple mixed signal circuit which explains the key elements of modern mixed signal systems.
A Mixed Signal Circuit is one which has both analog and digital components. In analog circuits the currents and voltages vary continously whereas in a digital circuits they switch between discrete values (high and low) to represent logical or numerical values.
Almost all modern electronic systems comprise mixed signal circuitry but the development methods and diagnostic tools you need to design and debug the analog and digital components of these circuits are quite different.
The block diagram above shows Breadboard One which comprises four primary circuit components:
|COUNTER||Digital||4 bit UP/DOWN Counter (CD4029)|
|DAC||Mixed||4 bit R/2R Digital to Analog Convertor|
|FILTER||Analog||Passive RC Low-Pass Filter|
|SCHMITT||Mixed||Inverting Schmitt Trigger|
We will explain the design, operation and diagnosis of each of these components in forthcoming posts but for now, take note of each component’s type.
The COUNTER is exclusively a digital component. It accepts a digital clock (CLK) input and produces a 4 bit binary output on 4 separate digital signals.
By contrast the FILTER is an analog component. It accepts an analog signal at its input and produces another (filtered) analog signal at its output.
The other two components are mixed signal devices. The DAC accepts the 4 bit binary encoded digital signals from the COUNTER and produces an analog sawtooth output. The SCHMITT trigger comparator accepts the analog output of the FILTER and produces a digital output signal which feeds back into the COUNTER Up/Down input.
Tools for Mixed Signal Analysis and Debugging.
To analyze and debug these components as well as the circuit as a whole requires a range of test equipment.
For analog signals an oscilloscope and sometimes a spectrum analyzer are invaluable tools. They show continuous changes in voltage and current over time or the frequency characteristics of signals in a circuit.
For digital signals these instruments can be useful but more often you will be need a logic analyzer which can show the the relative timing of the high and low transitions of multiple digital signals at once or decode a sequence of transitions to show serial data.Of course one usually needs a power supply and in many cases a waveform generator or clock generator. More specialized tools like a frequency meter can be useful and a simple voltmeter or current meter are often handy.
All these test instruments and many of the associated tools are what BitScope provides, all in single compact device with connections made via its Smart Port Interface. Breadboard One connects directly but logic probes or analog probes like DP01 can be used too. Some BitScopes have BNC inputs for use with standard oscilloscope probes.
For more detail: Breadboard One | A typical Mixed Signal Circuit.