Challenge 4

Multiply by a complex sine wave to slide a signal down to 0 Hz -- the move every SDR receiver makes.

Author GNU Radio World
Category Gnuradio World Challenges
Blocks 14
Connections 7
File _gnuradio-world-challenges/challenge_4.grc

Press Load to open this flowgraph in an editor embedded above, then ▶ to run it. Nothing is downloaded until you do.

Open in the full editor ▶ Download the .grc

Blocks used

Block	Module	Used
Note	`GNU Radio`	3
Signal Source	`GNU Radio`	3
Add	`GNU Radio`	1
Challenge	`GNU Radio`	1
GUI Layout	`GNU Radio`	1
Multiply	`GNU Radio`	1
Noise Source	`GNU Radio`	1
Spectrum Analyzer	`GNU Radio`	1
Throttle	`GNU Radio`	1
Variable	`GNU Radio`	1

Notes in this flowgraph

Two signals are on the air: the one you want at +250 kHz, and a neighbor at -180 kHz. Filtering can throw the neighbor away, but it cannot move your signal to the middle -- and every receiver wants its signal at 0 Hz, where it can be filtered narrowly and decimated hard.
Multiplying two sine waves adds their frequencies. With complex samples that is all there is to tuning: multiply by a complex sine wave at frequency f and every signal in the band moves by f. That multiply is called a mixer, and the sine wave feeding it is the local oscillator. The LO is at 0 Hz right now, which is a sine wave of a constant 1 -- multiplying by it changes nothing.
Challenge: 1) Run it and confirm the two signals sit at +250 kHz and -180 kHz 2) Stop, and set the local oscillator's Frequency so the wanted signal lands at 0 Hz. To move a signal down, the LO frequency is negative 3) Run it again. The neighbor moved by the same amount, to -430 kHz -- a mixer shifts the whole band, not one signal Tune, filter, decimate: that is a receiver.

Flowgraph source

_gnuradio-world-challenges/challenge_4.grc

options:
    parameters:
        author: GNU Radio World
        comment: ''
        copyright: ''
        description: Multiply by a complex sine wave to slide a signal down to 0 Hz -- the move every SDR receiver makes.
        generate_options: qt_gui
        id: Challenge_4
        title: Challenge 4
    states:
        coordinate: [10, 10]
        rotation: 0
        state: enabled
blocks:
-   name: samp_rate
    id: variable
    parameters:
        comment: ''
        value: '1000000'
    states:
        coordinate: [590, 10]
        rotation: 0
        state: enabled
-   name: analyzer
    id: wasm_spectrum_analyzer_sink
    parameters:
        affinity: ''
        alias: ''
        average: '0.2'
        center_freq: '0'
        comment: Finds and measures signals
        db_per_div: '10'
        fftsize: '4096'
        level_offset_db: '0'
        level_unit: dBFS
        name: After the mixer
        reference_level: '0'
        samp_rate: samp_rate
        trace_mode: average
        type: complex
        update_time: '0.1'
        wintype: window.WIN_BLACKMAN_hARRIS
    states:
        coordinate: [1210, 490]
        rotation: 0
        state: enabled
-   name: challenge_4
    id: wasm_challenge
    parameters:
        affinity: ''
        alias: ''
        challenge_id: challenge_4
        comment: ''
        criteria: "[\n {\"kind\": \"param\", \"block\": \"lo\", \"param\": \"freq\", \"equals\": -250000, \"tolerance\": 5000, \"goal\": \"Set the local oscillator to the frequency that slides +250 kHz down to 0 -- mind the sign\"},\n {\"kind\": \"detected_signal\", \"center_freq\": 0, \"center_tol\": 15000, \"min_peak_level\": -40, \"goal\": \"Run it: the wanted signal now sits at 0 Hz\"},\n {\"kind\": \"detected_signal\", \"center_freq\": -430000, \"center_tol\": 25000, \"goal\": \"...and the other signal slid down by exactly the same amount, to -430 kHz\"}\n]"
        requires: challenge_3
        title: Mix it down to baseband
    states:
        coordinate: [760, 10]
        rotation: 0
        state: enabled
-   name: channel
    id: blocks_add_xx
    parameters:
        affinity: ''
        alias: ''
        comment: What the radio hears
        maxoutbuf: '0'
        minoutbuf: '0'
        num_inputs: '3'
        type: complex
        vlen: '1'
    states:
        coordinate: [370, 540]
        rotation: 0
        state: enabled
-   name: gui_layout
    id: wasm_gui_layout
    parameters:
        affinity: ''
        alias: ''
        columns: '12'
        comment: ''
        layout: '{"analyzer":[0,0,12,8]}'
        row_height: '60'
    states:
        coordinate: [300, 10]
        rotation: 0
        state: enabled
-   name: lo
    id: analog_sig_source_x
    parameters:
        affinity: ''
        alias: ''
        amp: '1'
        comment: "The local oscillator:\na complex sine wave"
        freq: '0'
        maxoutbuf: '0'
        minoutbuf: '0'
        offset: '0'
        phase: '0'
        samp_rate: samp_rate
        type: complex
        waveform: analog.GR_COS_WAVE
    states:
        coordinate: [610, 690]
        rotation: 0
        state: enabled
-   name: mixer
    id: blocks_multiply_xx
    parameters:
        affinity: ''
        alias: ''
        comment: "Multiplying by a sine wave\nshifts every frequency"
        maxoutbuf: '0'
        minoutbuf: '0'
        num_inputs: '2'
        type: complex
        vlen: '1'
    states:
        coordinate: [970, 570]
        rotation: 0
        state: enabled
-   name: neighbor
    id: analog_sig_source_x
    parameters:
        affinity: ''
        alias: ''
        amp: '0.25'
        comment: Someone else's signal
        freq: '-180000'
        maxoutbuf: '0'
        minoutbuf: '0'
        offset: '0'
        phase: '0'
        samp_rate: samp_rate
        type: complex
        waveform: analog.GR_COS_WAVE
    states:
        coordinate: [10, 680]
        rotation: 0
        state: enabled
-   name: note_0
    id: note
    parameters:
        affinity: ''
        alias: ''
        bgcolor: '#4dff6a'
        comment: ''
        note: 'Two signals are on the air: the one you want at +250 kHz, and a neighbor at -180 kHz. Filtering can throw the neighbor away, but it cannot move your signal to the middle -- and every receiver wants its signal at 0 Hz, where it can be filtered narrowly and decimated hard.'
    states:
        coordinate: [1190, 10]
        rotation: 0
        state: enabled
-   name: note_1
    id: note
    parameters:
        affinity: ''
        alias: ''
        bgcolor: '#4dff6a'
        comment: ''
        note: "Multiplying two sine waves adds their frequencies. With complex samples that is all there is to tuning: multiply by a complex sine wave at frequency f and every signal in the band moves by f. That multiply is called a mixer, and the sine wave feeding it is the local oscillator.\n\nThe LO is at 0 Hz right now, which is a sine wave of a constant 1 -- multiplying by it changes nothing."
    states:
        coordinate: [10, 210]
        rotation: 0
        state: enabled
-   name: note_2
    id: note
    parameters:
        affinity: ''
        alias: ''
        bgcolor: '#4dff6a'
        comment: ''
        note: "Challenge:\n1) Run it and confirm the two signals sit at +250 kHz and -180 kHz\n2) Stop, and set the local oscillator's Frequency so the wanted signal lands at 0 Hz. To move a signal down, the LO frequency is negative\n3) Run it again. The neighbor moved by the same amount, to -430 kHz -- a mixer shifts the whole band, not one signal\n\nTune, filter, decimate: that is a receiver."
    states:
        coordinate: [440, 210]
        rotation: 0
        state: enabled
-   name: receiver_noise
    id: analog_noise_source_x
    parameters:
        affinity: ''
        alias: ''
        amp: '0.002'
        comment: Gives the detector a noise floor
        maxoutbuf: '0'
        minoutbuf: '0'
        noise_type: analog.GR_GAUSSIAN
        seed: '7'
        type: complex
    states:
        coordinate: [10, 860]
        rotation: 0
        state: enabled
-   name: throttle
    id: blocks_throttle2
    parameters:
        affinity: ''
        alias: ''
        comment: "Limits the rate of IQ samples\nflowing through"
        ignoretag: 'True'
        limit: time
        maximum: '0.02'
        maxoutbuf: '0'
        minoutbuf: '0'
        samples_per_second: samp_rate
        type: complex
        vlen: '1'
    states:
        coordinate: [610, 550]
        rotation: 0
        state: enabled
-   name: wanted
    id: analog_sig_source_x
    parameters:
        affinity: ''
        alias: ''
        amp: '0.4'
        comment: The one you want
        freq: '250000'
        maxoutbuf: '0'
        minoutbuf: '0'
        offset: '0'
        phase: '0'
        samp_rate: samp_rate
        type: complex
        waveform: analog.GR_COS_WAVE
    states:
        coordinate: [10, 500]
        rotation: 0
        state: enabled
connections:
- [channel, '0', throttle, '0']
- [lo, '0', mixer, '1']
- [mixer, '0', analyzer, '0']
- [neighbor, '0', channel, '1']
- [receiver_noise, '0', channel, '2']
- [throttle, '0', mixer, '0']
- [wanted, '0', channel, '0']
metadata:
    file_format: 1
    grc_version: 3.11.0.0