Mixed Signal Circuits

Breadboard One | A typical Mixed Signal Circuit.

Breadboard One Block Diagram

Breadboard One | Mixed Signal Circuit | Block Diagram

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:

ComponentTypeDescription
COUNTERDigital4 bit UP/DOWN Counter (CD4029)
DACMixed4 bit R/2R Digital to Analog Convertor
FILTERAnalogPassive RC Low-Pass Filter
SCHMITTMixedInverting Schmitt Trigger

Breadboard One | Circuit Components

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.

BitScope Smart Port Interface.

BitScope Smart Port Interface

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.

Electronic Projects Lab for Raspberry Pi with BitScope DSO.

For a bit of fun, we built an Electronic Projects Lab for Raspberry Pi based on Breadboard One to test the circuit (as shown in this screenshot taken from a monitor connected to a Raspberry Pi).

Why a triangle waveform?

Why is the waveform produced by the circuit at the output of the FILTER (yellow trace) a triangle?

To answer this question we need to understand the operation of the entire circuit, its digital, analog and mixed signal components. We'll explain each of the components individually in future posts.

For now, consider how they work together.

BitScope's waveform generator is connected to the COUNTER clock (CLK) input. It provides the clock that drives the entire circuit. The counter counts UP producing a linearly stepped increasing number.

This number, encoded as a set of 4 binary signals, drives the DAC which produces an increasing voltage for each new value it receives. This analog output signal from the DAC is passed through low pass FILTER to smooth it before appear at the (inverting) SCHMITT trigger input.

The SCHMITT output (a digital signal) switches from HIGH to LOW when the voltage passes a high threshold (a voltage of about 2/3 of the power rail) which flips the COUNTER to count DOWN instead of up. This reverses the DAC voltage producing a downward ramp until the SCHMITT switches again (at about 1/3 of the power rail voltage) and the process repeats so long as the clock continues to run.

This produces the triangle shaped waveform seen at the FILTER output.

The real-world is a little more complicated!

This previous description outlines the fundamental operation of the Breadboard One mixed signal circuit. In theory its operation is reasonably straightforward but in practice the real world can get in the way. It's a triangle waveform but it's a little distorted. Why? There seems to be some noise on some of these signals. Where does it come from?

We hope to address questions and many others down the track in BitScope Ed, so stay tuned!

Related Posts

BitScope
Build a powerful mixed signal workstation with one simple 3D print !Sep 3
Raspberry
Raspberry Pi 3 Launch and BitScope BladeFeb 29
BitScope
Physical Computing with Raspberry Pi and BitScope!Dec 01
Raspberry
BitScope, Ozberry & Pimoroni at Sydney Mini Maker Faire!Aug 12
Education
Picademy Workshop with Carrie Anne PhilbinJul 27
Raspberry
BitScope Blade for Raspberry Pi at Hannover Maker Faire!Jun 05
Software
BitScope DSO 2.9 Now Available.Jun 02
BitScope
Getting Started with BitScopeMay 08
BitScope
BitScope Micro In-Line Data AcquisitionApr 02
Education
BitScope Micro & Raspberry Pi in Education at BETTJan 21
Education
Electronic Measurement with BitScope & Raspberry PiNov 12
BitScope
Hands on with BitScope Micro at electronica 2014Nov 11
BitScope
BitScope Micro Video Tutorial with Farnell element14Nov 10
BitScope
BitScope Micro and Raspberry Pi at electronica 2014!Nov 06
BitScope
BitScope Micro element14 Webinar Review.Nov 03
BitScope
BitScope Micro Webinar with element14!Oct 29
BitScope
element14 launches BitScope Micro for Raspberry Pi!Oct 27
BitScope
BitScope Micro Diagnostic Port Explained.Aug 16
BitScope
BitScope Model 10 or BitScope Micro?Apr 25
BitScope
BitScope Micro, what's in the box?Apr 17
Raspberry
BitScope Micro Questions & AnswersApr 12
Raspberry
BitScope Micro (New Product) for Raspberry Pi!Apr 11
Education
BitScope Workshop at OzBerryPiApr 8
Education
BitScope Ed at the Sydney Mini Maker Faire!Nov 20
Education
Breadboard One, the 4 Bit Up/Down CounterNov 07
Education
BitScope Ed at OzBerryPi, Presentation Review.Oct 21
Raspberry
How to Install BitScope DSO on Raspberry PiOct 18
Raspberry
Electronic Projects Lab for Raspberry PiOct 17
BitScope
Dual Channel Active Differential Probe (New Product).Oct 15
BitScope
BitScope Smart Port. The clever connection.Oct 12
Education
BitScope Ed, an Educational Blog about Electronics.Oct 09
Raspberry
BitScope Software for Raspberry Pi OscilloscopeSep 27
Raspberry
BitScope Raspberry Pi OscilloscopeSep 25
More...