Challenge 3

Filter out an interferer, decimate what is left, and tell the display about the new sample rate.

Author GNU Radio World
Category Gnuradio World Challenges
Blocks 13
Connections 6
File _gnuradio-world-challenges/challenge_3.grc

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Blocks used

Block	Module	Used
Note	`GNU Radio`	3
Signal Source	`GNU Radio`	2
Add	`GNU Radio`	1
Challenge	`GNU Radio`	1
GUI Layout	`GNU Radio`	1
Low Pass Filter	`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 tones arrive together: the one you want at 50 kHz, and an interferer at 300 kHz. A low-pass filter passes everything below its cutoff frequency and attenuates everything above it, over a transition band whose width you also choose. The cutoff is wide open at 400 kHz right now, so both get through.
Once a signal is narrow, carrying it at 1 MS/s is wasted work. Decimating by 4 keeps every 4th sample, and the stream leaving the filter is 250 kS/s. That is why you filter first: anything left above the new Nyquist rate of 125 kHz would fold back on top of your signal. A block that changes the sample rate does not tell the blocks after it. They take their rate as a parameter, and it is on you to keep it right.
Challenge: 1) Run it first and see both tones 2) Set the Low Pass Filter cutoff between the two -- well above 50 kHz, well below both 300 kHz and the 125 kHz Nyquist rate you are about to create 3) Set its Decimation to 4 4) Set the Spectrum Analyzer's Sample Rate to samp_rate/4, or the display will put the tone at the wrong frequency 5) Run it again: one tone, still at 50 kHz, in a span four times narrower

Flowgraph source

_gnuradio-world-challenges/challenge_3.grc

options:
    parameters:
        author: GNU Radio World
        comment: ''
        copyright: ''
        description: Filter out an interferer, decimate what is left, and tell the display about the new sample rate.
        generate_options: qt_gui
        id: Challenge_3
        title: Challenge 3
    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 filter
        reference_level: '0'
        samp_rate: samp_rate
        trace_mode: average
        type: complex
        update_time: '0.1'
        wintype: window.WIN_BLACKMAN_hARRIS
    states:
        coordinate: [1250, 560]
        rotation: 0
        state: enabled
-   name: challenge_3
    id: wasm_challenge
    parameters:
        affinity: ''
        alias: ''
        challenge_id: challenge_3
        comment: ''
        criteria: "[\n {\"kind\": \"param\", \"block\": \"lpf\", \"param\": \"cutoff_freq\", \"min\": 60000, \"max\": 100000, \"goal\": \"Set the Low Pass Filter cutoff to keep the 50 kHz tone and drop the 300 kHz interferer\"},\n {\"kind\": \"param\", \"block\": \"lpf\", \"param\": \"decim\", \"equals\": 4, \"goal\": \"Decimate by 4 in that same block -- 1 MHz of samples for a narrow signal is waste\"},\n {\"kind\": \"param\", \"block\": \"analyzer\", \"param\": \"samp_rate\", \"equals\": 250000, \"goal\": \"Tell the Spectrum Analyzer its input rate is now samp_rate/4\"},\n {\"kind\": \"detected_signal\", \"center_freq\": 50000, \"center_tol\": 15000, \"min_peak_level\": -40, \"goal\": \"Run it: the tone still reads 50 kHz, and the interferer is gone\"}\n]"
        requires: challenge_2
        title: Filter and decimate
    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, 620]
        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: interferer
    id: analog_sig_source_x
    parameters:
        affinity: ''
        alias: ''
        amp: '0.5'
        comment: The one you do not want
        freq: '300000'
        maxoutbuf: '0'
        minoutbuf: '0'
        offset: '0'
        phase: '0'
        samp_rate: samp_rate
        type: complex
        waveform: analog.GR_COS_WAVE
    states:
        coordinate: [10, 760]
        rotation: 0
        state: enabled
-   name: lpf
    id: low_pass_filter
    parameters:
        affinity: ''
        alias: ''
        beta: '6.76'
        comment: "Passes low frequencies,\nstops high ones"
        cutoff_freq: '400000'
        decim: '1'
        gain: '1'
        interp: '1'
        maxoutbuf: '0'
        minoutbuf: '0'
        samp_rate: samp_rate
        type: fir_filter_ccf
        width: '50000'
        win: window.WIN_HAMMING
    states:
        coordinate: [940, 590]
        rotation: 0
        state: enabled
-   name: note_0
    id: note
    parameters:
        affinity: ''
        alias: ''
        bgcolor: '#4dff6a'
        comment: ''
        note: 'Two tones arrive together: the one you want at 50 kHz, and an interferer at 300 kHz. A low-pass filter passes everything below its cutoff frequency and attenuates everything above it, over a transition band whose width you also choose. The cutoff is wide open at 400 kHz right now, so both get through.'
    states:
        coordinate: [1190, 10]
        rotation: 0
        state: enabled
-   name: note_1
    id: note
    parameters:
        affinity: ''
        alias: ''
        bgcolor: '#4dff6a'
        comment: ''
        note: "Once a signal is narrow, carrying it at 1 MS/s is wasted work. Decimating by 4 keeps every 4th sample, and the stream leaving the filter is 250 kS/s. That is why you filter first: anything left above the new Nyquist rate of 125 kHz would fold back on top of your signal.\n\nA block that changes the sample rate does not tell the blocks after it. They take their rate as a parameter, and it is on you to keep it right."
    states:
        coordinate: [10, 260]
        rotation: 0
        state: enabled
-   name: note_2
    id: note
    parameters:
        affinity: ''
        alias: ''
        bgcolor: '#4dff6a'
        comment: ''
        note: "Challenge:\n1) Run it first and see both tones\n2) Set the Low Pass Filter cutoff between the two -- well above 50 kHz, well below both 300 kHz and the 125 kHz Nyquist rate you are about to create\n3) Set its Decimation to 4\n4) Set the Spectrum Analyzer's Sample Rate to samp_rate/4, or the display will put the tone at the wrong frequency\n5) Run it again: one tone, still at 50 kHz, in a span four times narrower"
    states:
        coordinate: [440, 260]
        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, 940]
        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, 630]
        rotation: 0
        state: enabled
-   name: wanted
    id: analog_sig_source_x
    parameters:
        affinity: ''
        alias: ''
        amp: '0.5'
        comment: The one you want
        freq: '50000'
        maxoutbuf: '0'
        minoutbuf: '0'
        offset: '0'
        phase: '0'
        samp_rate: samp_rate
        type: complex
        waveform: analog.GR_COS_WAVE
    states:
        coordinate: [10, 580]
        rotation: 0
        state: enabled
connections:
- [channel, '0', throttle, '0']
- [interferer, '0', channel, '1']
- [lpf, '0', analyzer, '0']
- [receiver_noise, '0', channel, '2']
- [throttle, '0', lpf, '0']
- [wanted, '0', channel, '0']
metadata:
    file_format: 1
    grc_version: 3.11.0.0