dsp.DigitalDownConverter

Translate digital signal from intermediate frequency (IF) band to baseband and decimate it

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Description

The dsp.DigitalDownConverter object translates digital signal from intermediate frequency (IF) band to baseband and decimates it.

To digitally downconvert the input signal:

  1. Create the dsp.DigitalDownConverter object and set its properties.

  2. Call the object with arguments, as if it were a function.

To learn more about how System objects work, see What Are System Objects?

This object supports C/C++ code generation and SIMD code generation under certain conditions. For more information, see Code Generation.

Creation

Syntax

dwnConv = dsp.DigitalDownConverter
dwnConv = dsp.DigitalDownConverter(Name=Value)

Description

dwnConv = dsp.DigitalDownConverter returns a digital downconverter (DDC) System object™, dwnConv.

dwnConv = dsp.DigitalDownConverter(Name=Value) sets properties using one or more name-value arguments. For example, to specify a decimation factor of 50, set DecimationFactor to 50.

example

Properties

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Unless otherwise indicated, properties are nontunable, which means you cannot change their values after calling the object. Objects lock when you call them, and the release function unlocks them.

If a property is tunable, you can change its value at any time.

For more information on changing property values, see System Design in MATLAB Using System Objects.

Set this property to a positive integer scalar, or to a 1-by-2 or 1-by-3 vector of positive integers.

When you set this property to a scalar, the object automatically chooses the decimation factors for each of the three filtering stages.

When you set this property to a 1-by-2 vector, the object bypasses the third filter stage and sets the decimation factor of the first and second filtering stages to the values in the first and second vector elements respectively. Both elements of the DecimationFactor vector must be greater than one.

When you set this property to a 1-by-3 vector, the i th element of the vector specifies the decimation factor for the ith filtering stage. The first and second elements of the DecimationFactor vector must be greater than one, and the third element must be 1 or 2.

Data Types: double

When you set this property to true, the object designs filters with the minimum order that meets the passband ripple, stopband attenuation, passband frequency, and stopband frequency specifications that you set using the PassbandRipple, StopbandAttenuation, Bandwidth, StopbandFrequencySource, and StopbandFrequency properties.

When you set this property to false, the object designs filters with orders that you specify in the NumCICSections, SecondFilterOrder, and ThirdFilterOrder properties. The filter designs meet the passband and stopband frequency specifications that you set using the Bandwidth, StopbandFrequencySource, and StopbandFrequency properties.

Data Types: logical

Number of sections of CIC decimator, specified as a positive integer scalar.

Dependencies

This property applies when you set the MinimumOrderDesign property to false.

Data Types: double

Order of CIC compensation filter stage, specified as a positive integer scalar.

Dependencies

This property applies when you set the MinimumOrderDesign property to false.

Data Types: double

Order of third filter stage, specified as an even positive integer scalar. When you set the DecimationFactor property to a 1-by-2 vector, the object ignores the ThirdFilterOrder property because the third filter stage is bypassed.

Dependencies

This property applies when you set the MinimumOrderDesign property to false.

Data Types: double

Two-sided bandwidth BW of the input signal, specified as a positive integer in Hz or in normalized frequency units (since R2024b). The object sets the passband frequency of the cascade of filters to one-half of the value that you specify in the Bandwidth property. Set the value of this property to less than SampleRate/DecimationFactor. If you set NormalizedFrequency to true, set the value of this property to less than 2/DecimationFactor. (since R2024b)

Data Types: double

Specify the source of the stopband frequency as Auto or Property.

When you set this property to Auto and set:

  • NormalizedFrequency to false –– The object places the cutoff frequency of the cascade filter response at approximately Fc = Fs/(2M) Hz, where M is the total decimation factor that you specify in the DecimationFactor property. The object computes the stopband frequency as Fstop = Fc + (TW/2). TW is the transition bandwidth of the cascade response computed as 2×(Fc–Fp), and the passband frequency Fp equals BW/2, where BW is the two-sided bandwidth of the input signal.

  • NormalizedFrequency to true –– The object places the cutoff frequency of the cascade filter response at approximately Fc = 1/M, where M is the total decimation factor that you specify in the DecimationFactor property. The object computes the stopband frequency as Fstop = Fc + (TW/2). TW is the transition bandwidth of the cascade response computed as 2×(Fc–Fp), and the passband frequency Fp equals BW/2, where BW is the two-sided bandwidth of the input signal.

When you set this property to Property, you can specify the stopband frequency value using the StopbandFrequency property.

Stopband frequency Fstop, specified as a positive scalar in Hz or in normalized frequency units (since R2024b).

Dependencies

To enable this property, set the StopbandFrequencySource property to Property.

Data Types: double

Passband ripple of cascade response in dB, specified as a positive scalar. When you set the MinimumOrderDesign property to true, the object designs the filters so that the cascade response meets the passband ripple that you specify in the PassbandRipple property.

Dependencies

To enable this property, set the MinimumOrderDesign property to true.

Data Types: double

Stopband attenuation of cascade response in dB, specified as a positive scalar. When you set the MinimumOrderDesign property to true, the object designs the filters so that the cascade response meets the stopband attenuation that you specify in the StopbandAttenuation property.

Dependencies

To enable this property, set the MinimumOrderDesign property to true.

Data Types: double

Type of oscillator, specified as one of these options:

  • Sine wave –– The object frequency down converts the input signal using a complex exponential obtained from samples of a sinusoidal trigonometric function.

  • NCO –– The object performs frequency down conversion with a complex exponential obtained using a numerically controlled oscillator (NCO).

  • Input port –– The object performs frequency down conversion using the complex oscillator signal, z, that you pass as an input to the object.

  • None –– The mixer stage in the object is not present and the object acts as three stage cascaded decimator.

Center frequency of the input signal Fc, specified as a positive scalar in Hz or in normalized frequency units (since R2024b). The value of center frequency must be less than or equal to half the value of the SampleRate property. The object downconverts the input signal from the passband center frequency you specify in the CenterFrequency property to 0.

Dependencies

To enable this property, set the Oscillator property to Sine wave or NCO.

Data Types: double

Number of NCO accumulator bits, specified as a positive integer in the range [1 128].

Dependencies

To enable this property, set the Oscillator property to NCO.

Data Types: double

Number of NCO quantized accumulator bits, specified as an integer in the range [1 128]. The value you specify in this property must be less than the value you specify in the NumAccumulatorBits property.

Dependencies

To enable this property, set the Oscillator property to NCO.

Data Types: double

When you set this property to true, a number of dither bits specified in the NumDitherBits property will be used to apply dither to the NCO signal.

Dependencies

To enable this property, set the Oscillator property to NCO.

Number of NCO dither bits, specified as a positive integer smaller than the number of accumulator bits that you specify in the NumAccumulatorBits property.

Dependencies

To enable this property, set the Oscillator property to NCO and the Dither property to true.

Data Types: double

Since R2024b

Option to set frequencies in normalized units, specified as one of these values:

  • true –– The center frequency, stopband frequency, and bandwidth must be in the normalized frequency units (0 to 1).

    When you set the NormalizedFrequency property to true while creating the object and you do not set the frequency specifications, the object automatically sets the default values to normalized frequency units using the default sample rate of 30 MHz.

    ddc = dsp.DigitalDownConverter(NormalizedFrequency=true)

    ddc = 
  dsp.DigitalDownConverter with properties:

           DecimationFactor: 100
         MinimumOrderDesign: true
                  Bandwidth: 0.0133
    StopbandFrequencySource: "Auto"
             PassbandRipple: 0.1000
        StopbandAttenuation: 60
                 Oscillator: "Sine wave"
            CenterFrequency: 0.9333
        NormalizedFrequency: true

    When you set the NormalizedFrequency property to true after you create the object, you must specify the center frequency, stopband frequency, and bandwidth in normalized units before you run the object algorithm. 

    ddc = dsp.DigitalDownConverter
    ddc = 
  dsp.DigitalDownConverter with properties:

           DecimationFactor: 100
         MinimumOrderDesign: true
                  Bandwidth: 200000
    StopbandFrequencySource: "Auto"
             PassbandRipple: 0.1000
        StopbandAttenuation: 60
                 Oscillator: "Sine wave"
            CenterFrequency: 14000000
        NormalizedFrequency: false
                 SampleRate: 30000000

    To specify the normalized frequency values, set NormalizedFrequency to true and manually convert the frequency values in Hz to the normalized values using the input sample rate in Hz. For example, if the input sample rate Fs is 30 MHz, the corresponding bandwidth value in normalized units is BWHz/(Fs/2), the corresponding center frequency in normalized units is FcHz/(Fs/2), and the corresponding stopband frequency in normalized units is FstopHz/(Fs/2).

    ddc = dsp.DigitalDownConverter;
ddc.NormalizedFrequency = true;
ddc.Bandwidth = 200000/(30e6/2);
ddc.CenterFrequency = 14e6/(30e6/2)

    ddc = 
  dsp.DigitalDownConverter with properties:

           DecimationFactor: 100
         MinimumOrderDesign: true
                  Bandwidth: 0.0133
    StopbandFrequencySource: "Auto"
             PassbandRipple: 0.1000
        StopbandAttenuation: 60
                 Oscillator: "Sine wave"
            CenterFrequency: 0.9333
        NormalizedFrequency: true

  • false –– The bandwidth, stopband frequency, and center frequency values are in Hz. You can specify the input sample rate through the SampleRate property.

Data Types: logical

Sample rate of the input signal Fs, specified as a positive scalar value greater than or equal to twice the value of the CenterFrequency property.

Dependencies

To enable this property, set NormalizedFrequency to false. (since R2024b)

Data Types: single | double

Fixed-Point Properties

Specify the data type at the input of the first, second, and third (if it has not been bypassed) filter stages as one of Same as input | Custom. The object casts the data at the input of each filter stage according to the value you set in this property.

Specify the filters input fixed-point type as a scaled numerictype (Fixed-Point Designer) object with a Signedness of Auto.

Dependencies

This property applies when you set the FiltersInputDataType property to Custom.

Specify the data type of output as Same as input | Custom.

Specify the output fixed-point type as a scaled numerictype (Fixed-Point Designer) object with a Signedness of Auto.

Dependencies

This property applies when you set the OutputDataType property to Custom.

Usage

Syntax

y = dwnConv(x)
y = dwnConv(x,z)

Description

y = dwnConv(x) takes an input x and outputs a signal, y that is frequency downconverted and downsampled.

example

y = dwnConv(x,z) uses the complex input, z, as the oscillator signal used to frequency down convert input x when you set the Oscillator property to Input port.

Input Arguments

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Data input, specified as a column vector or a matrix. The length of input x must be a multiple of the decimation factor. When the data type of x is double or single precision, the data type of y is the same as that of x. When the data type of x is of a fixed-point type, the data type of y is defined by the OutputDataType property.

The input can have multiple channels only if its data type is double or single. The input can be of data type double, single, signed integer, or signed fixed-point (fi objects).

Data Types: single | double | int8 | int16 | int32 | int64 | fi
Complex Number Support: Yes

Oscillator signal used to frequency down convert the input signal, specified as a column vector or a matrix. This input must be complex. The length of z must be equal to the length of x. z can be double, single, signed integer, or signed fixed-point (fi objects).

Dependencies

This input applies when you set the Oscillator property to Input port.

Data Types: single | double | int8 | int16 | int32 | int64 | fi
Complex Number Support: Yes

Output Arguments

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Down converted and down sampled signal, returned as a column vector or a matrix. The length of y is equal to the length of x divided by the DecimationFactor. When the data type of x is double or single precision, the data type of y is the same as that of x. When the data type of x is of a fixed point type, the data type of y is defined by the OutputDataType property.

Data Types: single | double | int8 | int16 | int32 | int64 | fi
Complex Number Support: Yes

Object Functions

To use an object function, specify the System object as the first input argument. For example, to release system resources of a System object named obj, use this syntax:

release(obj)

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getDecimationFactorsGet decimation factors of each filter stage of a digital down converter
getFilterOrdersGet orders of digital down converter or digital up converter filter cascade
getFiltersGet handles to digital down converter or digital up converter filter cascade objects
groupDelayGroup delay of digital down converter or digital up converter filter cascade
visualizeDisplay response of digital down converter or digital up converter filter cascade
filterAnalyzerAnalyze filters with Filter Analyzer app
stepRun System object algorithm
releaseRelease resources and allow changes to System object property values and input characteristics
resetReset internal states of System object

Examples

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Open Live Script

Create a digital up converter object that up samples a 1 KHz sinusoidal signal by a factor of 20 and up converts it to 50 KHz. Create a digital down converter object that down converts the signal to 0 Hz and down samples it by a factor of 20.

Create a sine wave generator to obtain the 1 KHz sinusoidal signal with a sample rate of 6 KHz.

Fs = 6e3; % Sample rate
sine = dsp.SineWave(Frequency=1000,...
    SampleRate=Fs,...
    SamplesPerFrame=1024);
x = sine(); % generate signal

Create a DigitalUpConverter object. Use minimum order filter designs and set passband ripple to 0.2 dB and the stopband attenuation to 55 dB. Set the double sided signal bandwidth to 2 KHz. 

upConv = dsp.DigitalUpConverter(... 
 InterpolationFactor=20,...
 SampleRate=Fs,...
 Bandwidth=2e3,...
 StopbandAttenuation=55,...
 PassbandRipple=0.2,...
 CenterFrequency=50e3);

Create a DigitalDownConverter object. Use minimum order filter designs and set the passband ripple to 0.2 dB and the stopband attenuation to 55 dB.

dwnConv = dsp.DigitalDownConverter(...
  DecimationFactor=20,...
  SampleRate=Fs*20,...
  Bandwidth=3e3,...
  StopbandAttenuation=55,...
  PassbandRipple=0.2,...
  CenterFrequency=50e3);

Create a spectrum estimator to visualize the signal spectrum before up converting, after up converting, and after down converting.

window = hamming(floor(length(x)/10));
figure; pwelch(x,window,[],[],Fs,"centered")
title("Spectrum of baseband signal x")

Figure contains an axes object. The axes object with title Spectrum of baseband signal x, xlabel Frequency (kHz), ylabel Power/frequency (dB/Hz) contains an object of type line.

Up convert the signal and visualize the spectrum

xUp = upConv(x); % up convert
window = hamming(floor(length(xUp)/10));
figure; pwelch(xUp,window,[],[],20*Fs,"centered");
title("Spectrum of up converted signal xUp")

Figure contains an axes object. The axes object with title Spectrum of up converted signal xUp, xlabel Frequency (kHz), ylabel Power/frequency (dB/Hz) contains an object of type line.

Down convert the signal and visualize the spectrum

xDown = dwnConv(xUp); % down convert
window = hamming(floor(length(xDown)/10));
figure; pwelch(xDown,window,[],[],Fs,"centered")
title("Spectrum of down converted signal xDown")

Figure contains an axes object. The axes object with title Spectrum of down converted signal xDown, xlabel Frequency (kHz), ylabel Power/frequency (dB/Hz) contains an object of type line.

Visualize the response of the decimation filters

visualize(dwnConv)

Figure contains an axes object. The axes object with title Magnitude Response (dB), xlabel Frequency (Hz), ylabel Magnitude (dB) contains 4 objects of type line. These objects represent CIC decimator, Decimation factor = 5, CIC compensator, Decimation factor = 2, Halfband decimator, Decimation factor = 2, Cascade response.

Open Live Script

Get decimation factors of each filter stage of the dsp.DigitalDownConverter System object™.

Create a dsp.DigitalDownConverter System object with the default settings. Using the getDecimationFactors function, obtain the decimation factors of each stage of the object.

dwnConv = dsp.DigitalDownConverter
dwnConv = 
  dsp.DigitalDownConverter with properties:

           DecimationFactor: 100
         MinimumOrderDesign: true
                  Bandwidth: 200000
    StopbandFrequencySource: 'Auto'
             PassbandRipple: 0.1000
        StopbandAttenuation: 60
                 Oscillator: 'Sine wave'
            CenterFrequency: 14000000
        NormalizedFrequency: false
                 SampleRate: 30000000

  Show all properties


M = getDecimationFactors(dwnConv) %#ok
M = 1×3

    25     2     2

The DecimationFactor property of the object is set to 100. The output M is by default a 1-by-3 vector, where each element in the vector is a factor of the overall decimation factor.

When you set the DecimationFactor to a 1-by-2 vector, the object bypasses the third filter stage and sets the decimation factor of the first and second filtering stages to the values in the first and second vector elements respectively.

dwnConv.DecimationFactor = [10 10]
dwnConv = 
  dsp.DigitalDownConverter with properties:

           DecimationFactor: [10 10]
         MinimumOrderDesign: true
                  Bandwidth: 200000
    StopbandFrequencySource: 'Auto'
             PassbandRipple: 0.1000
        StopbandAttenuation: 60
                 Oscillator: 'Sine wave'
            CenterFrequency: 14000000
        NormalizedFrequency: false
                 SampleRate: 30000000

  Show all properties


M = getDecimationFactors(dwnConv)
M = 1×2

    10    10

The output of the getDecimationFactors function is now a 1-by-2 vector.

More About

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Algorithms

The digital down converter downconverts the input signal by multiplying it with a complex exponential that has the specified center frequency. The algorithm downsamples the frequency downconverted signal using a cascade of three decimation filters. In this case, the filter cascade consists of a CIC decimator, a CIC compensator, and a third FIR decimation stage. The following block diagram shows the architecture of the digital down converter.

The scaling section normalizes the CIC gain and the oscillator power. It can also contain a correction factor to achieve the desired ripple specification. When you specify an oscillator signal through the input port, the normalization factor does not include the oscillator power factor. Depending on how you set the decimation factor, the block bypasses the third filter stage. When the input data type is double or single, the algorithm implements an N-section CIC decimation filter as an FIR filter with a response that corresponds to a cascade of N boxcar filters. The algorithm emulates a CIC filter with an FIR filter so that you can run simulations with floating-point data. When the input data type is fixed-point, the algorithm implements a true CIC filter with actual comb and integrator sections.

This block diagram represents the DDC arithmetic with single or double-precision, floating-point inputs.

For details about fixed-point operation, see Fixed Point.

Extended Capabilities

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Version History

Introduced in R2012a

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See Also

Functions

Objects

Blocks

Topics