dsp.DigitalUpConverter

Interpolate digital signal and translate it from baseband to IF band

Description

The dsp.DigitalUpConverter System object™ interpolates a digital signal, and translates it from baseband to intermediate frequency (IF) band.

To digitally upconvert the input signal:

1. Create the dsp.DigitalUpConverter 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?.

Creation

Syntax

upConv = dsp.DigitalUpConverter

upConv = dsp.DigitalUpConverter(Name,Value)

Description

upConv = dsp.DigitalUpConverter returns a digital up-converter (DUC) System object, upConv.

upConv = dsp.DigitalUpConverter(Name,Value) returns a DUC System object with the specified property Name set to the specified value Value. You can specify one or more name-value pair arguments in any order as (Name1,Value1,...,NameN,ValueN). Enclose each property name in single quotes. For example, create an object that upsamples the input signal by a factor of 20, using a filter with the specified qualities.

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

Properties

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.

SampleRate — Sample rate of input signal

30000000 (default) | positive scalar

Set this property to a positive scalar value. The value of this property multiplied by the total interpolation factor must be greater than or equal to twice the value of the CenterFrequency property.

Data Types: single | double

InterpolationFactor — Interpolation factor

100 (default) | positive integer | vector of positive integers

Interpolation factor, specified as a positive integer, or 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 interpolation factors for each of the three filtering stages.

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

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

Data Types: double

MinimumOrderDesign — Minimum order filter design

true (default) | false

Minimum order filter design, specified as true or false.

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 FirstFilterOrder, SecondFilterOrder, and NumCICSections properties. The filter designs meet the passband and stopband frequency specifications that you set using the Bandwidth, StopbandFrequencySource, and StopbandFrequency properties.

Data Types: logical

SecondFilterOrder — Order of CIC compensation filter stage

12 (default) | positive integer

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

Dependencies

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

Data Types: double

FirstFilterOrder — Order of first filter stage

10 (default) | positive even integer

Order of first filter stage, specified as a positive even integer.

Dependencies

To enable this property, set the MinimumOrderDesign property to false. When you set the InterpolationFactor property to a 1-by-2 vector, the object ignores the FirstFilterOrder property, because the first filter stage is bypassed.

Data Types: double

NumCICSections — Number of sections of CIC interpolator

3 (default) | positive integer

Number of sections of CIC interpolator, specified as a positive integer.

Dependencies

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

Data Types: double

Bandwidth — Two-sided bandwidth of input signal in Hz

200000 (default) | positive integer

Two-sided bandwidth of input signal in Hz, specified as a positive integer. The object sets the passband frequency of the cascade of filters to half of the value that you specify in this Bandwidth property.

Data Types: double

StopbandFrequencySource — Source of stopband frequency

Auto (default) | Property

Source of stopband frequency, specified as Auto or Property. When you set this property to Auto, the object places the cutoff frequency of the cascade filter response at approximately F_c = SampleRate/2 Hz and computes the stopband frequency as F_stop = F_c + TW/2. TW is the transition bandwidth of the cascade response, computed as 2×(F_c–F_p). F_p is the passband frequency computed by Bandwidth/2.

StopbandFrequency — Stopband frequency in Hz

150000 (default) | positive scalar

Stopband frequency in Hz, specified as a positive scalar.

Dependencies

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

Data Types: double

PassbandRipple — Passband ripple of cascade response in dB

0.1 (default) | positive scalar

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 this PassbandRipple property.

Dependencies

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

Data Types: double

StopbandAttenuation — Stopband attenuation of cascade response in dB

60 (default) | positive scalar

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 this StopbandAttenuation property.

Dependencies

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

Data Types: double

Oscillator — Type of oscillator

Sine wave (default) | NCO

Type of oscillator, specified as Sine wave or NCO. When you set this property to Sine wave, the object frequency-upconverts the output of the interpolation filter cascade by using a complex exponential signal obtained from samples of a sinusoidal trigonometric function. When you set this property to NCO, the object frequency-upconverts the output by using a complex exponential obtained from a numerically controlled oscillator (NCO).

CenterFrequency — Center frequency of output signal in Hz

14000000 (default) | positive scalar

Center frequency of output signal in Hz, specified as a positive scalar. The value of this property must be less than or equal to half the product of the SampleRate property and the total interpolation factor. The object up converts the input signal so that the output spectrum centers at this frequency you specify in the CenterFrequency property.

Data Types: double

NCO Properties

NumAccumulatorBits — Number of NCO accumulator bits

16 (default) | integer in the range [1, 128]

Number of NCO accumulator bits, specified as an integer in the range [1, 128]. For more details, see the dsp.NCO System object.

Dependencies

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

Data Types: double

NumQuantizedAccumulatorBits — Number of NCO accumulator bits

12 (default) | integer in the range [1, 128]

Number of NCO accumulator bits, specified as an integer in the range [1, 128]. The value you specify for this property must be less than the value you specify in the NumAccumulatorBits property. For more details, see the dsp.NCO System object.

Dependencies

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

Data Types: double

Dither — Dither control for NCO

true (default) | false

Dither control for NCO, specified as true or false. When you set this property to true, the object uses the number of dither bits specified in the NumDitherBits property when applying dither to the NCO signal. When this property is false, the NCO does not apply dither to the signal. For more details, see the dsp.NCO System object.

Dependencies

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

Data Types: logical

NumDitherBits — Number of NCO dither bits

4 (default) | positive integer

Number of NCO dither bits, specified as a positive integer scalar smaller than the number of accumulator bits that you specify in the NumAccumulatorBits property. For more details, see the dsp.NCO System object.

Dependencies

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

Data Types: double

Fixed-Point Properties

FiltersOutputDataType — Data type at output of each filter stage

Same as input (default) | Custom

Data type at the output of the first (if it has not been bypassed), second, and third filter stages, specified as Same as input or Custom. The object casts the data at the output of each filter stage according to the value you set in this property. For the CIC stage, the casting is done after the signal is scaled by the normalization factor.

CustomFiltersOutputDataType — Fixed-point data type at output of each filter stage

numerictype([],16,15) (default) | numerictype object

Fixed-point data type at output of each filter stage, specified as a scaled numerictype (Fixed-Point Designer) object with the Signedness property set to Auto.

Dependencies

To enable this property, set the FiltersOutputDataType property to Custom.

OutputDataType — Data type of output

Same as input (default) | Custom

Data type of output, specified as Same as input or Custom.

CustomOutputDataType — Fixed-point data type of output

numerictype([],16,15) (default) | numerictype object

Fixed-point data type of output, specified as a scaled numerictype object the Signedness property set to Auto.

Dependencies

To enable this property, set the OutputDataType property to Custom.

Usage

Syntax

y = upConv(x)

Description

y = upConv(x)returns an upsampled and frequency-upconverted signal y, for a real or complex input column vector x.

Input Arguments

x — Input signal

real or complex column vector

Input signal, specified as a column vector of real or complex values. The length of input x must be a multiple of the decimation factor. When the data type of x is double or single, 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

Output Arguments

y — Upconverted and upsampled signal

column vector

Upconverted and upsampled signal, returned as a column vector. The length of y is equal to the length of x divided by the InterpolationFactor. When the data type of x is double or single, 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)

Specific to dsp.DigitalUpConverter

getInterpolationFactors	Get interpolation factors of each filter stage of digital upconverter
getFilterOrders	Get orders of digital down converter or digital up converter filter cascade
getFilters	Get handles to digital down converter or digital up converter filter cascade objects
fvtool	Visualize frequency response of digital down converter or digital up converter filter cascade
groupDelay	Group delay of digital down converter or digital up converter filter cascade
visualizeFilterStages	Display response of digital down converter or digital up converter filter cascade
generatehdl	Generate HDL code for quantized DSP filter (requires Filter Design HDL Coder)

Common to All System Objects

step	Run System object algorithm
release	Release resources and allow changes to System object property values and input characteristics
reset	Reset internal states of System object

Examples

Upconvert Sine Wave Signal

Create a DUC System object™ that upsamples a 1-kHz sinusoidal signal by a factor of 20 and upconverts it to 50 kHz.

Note: This example runs only in R2016b or later. If you are using an earlier release, replace each call to the function with the equivalent step syntax. For example, myObject(x) becomes step(myObject,x).

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 DUC System object. Use minimum order filter designs and set the passband ripple to 0.2 dB and 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 spectrum estimator to visualize the signal spectrum before and after upconverting.

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

Upconvert the signal and visualize the spectrum.

 xUp = upConv(x); 
 window = hamming(floor(length(xUp)/10));
 figure; pwelch(xUp,window,[],[],20*Fs,'centered')
 title('Spectrum of upconverted signal xUp')

Visualize the response of the interpolation filters.

 visualizeFilterStages(upConv)

Algorithms

The object up samples the input signal using a cascade of three interpolation filters. This object frequency-upconverts the upsampled signal by multiplying it by a complex exponential with center frequency equal to the value in the CenterFrequency property. In this case, the filter cascade consists of a FIR interpolation stage, a second stage for CIC compensation, and a CIC interpolator. The block diagram shows the architecture of the digital up 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. Depending on the setting of the InterpolationFactor property, you might be able to bypass the first filter stage. When the input data type is floating point, the object implements an N-section CIC interpolation filter as a FIR filter with a response that corresponds to a cascade of N boxcar filters. The CIC filter is emulated with a FIR filter so that you can run simulations with floating-point data. When the input data is fixed-point type, the object implements a true CIC filter with actual comb and integrator sections.

The diagram represents the DUC arithmetic with floating-point inputs.

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

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

See System Objects in MATLAB Code Generation (MATLAB Coder).

Fixed-Point Conversion

Design and simulate fixed-point systems using Fixed-Point Designer™.

The block diagram represents the DUC arithmetic with signed fixed-point inputs.

• WL is the word length of the input, and FL is the fraction length of the input.

• The output of each filter is cast to the data type specified in the FiltersOutputDataType and CustomFiltersOutputDataType properties. The casting of the CIC output occurs after the scaling factor is applied.

• The oscillator output is cast to a word length equal to the FiltersOutputDataType word length plus one. The fraction length is equal to the FiltersOutputDataType word length minus one.

• The scaling at the output of the CIC interpolator consists of coarse-gain and fine-gain adjustments. The coarse gain is achieved using the reinterpretcast (Fixed-Point Designer) function on the CIC interpolator output. The fine gain is achieved using full-precision multiplication.

The figure shows the coarse-gain and fine-gain operations.

If the normalization gain is G (where 0<G≦1), then:

• WL_cic is the word length of the CIC interpolator output, and FL_cic is the fraction length of the CIC interpolator output.

• F1 = abs(nextpow2(G)), indicating the part of G achieved by using bit shifts (coarse gain).

• F2 is the fraction length specified by the FiltersOutputDataType and CustomFiltersOutputDataType properties.

• fg = fi((2^F1)*G,true,16), indicating that the remaining gain cannot be achieved with a bit shift (fine gain).