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Bandwidth and SDR Reception

Discussion in 'Amplitude Modulation - AM Fans' started by KC3EPA, Mar 18, 2019.

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  1. KB3WFV

    KB3WFV Premium Subscriber QRZ Page

    Very cool
    Now add the ability for it to self adjust based on current band condition and it would be perfect.

    Imagine that? An audio processor that would automatically narrow when the band gets crowded and widen when the band opens up.

    This auto band width could even be added to web sdr receivers, automatically adjusting band pass filters to the transmitted signal width being received.

    ** reality contracts available upon request
    ** patent pending

  2. KB3WFV

    KB3WFV Premium Subscriber QRZ Page

    Stupid auto-complete. I meant ...

    ** royalty contracts available upon request
  3. K4KYV

    K4KYV Subscriber QRZ Page

    There isn't a one-size-fits-all bandwidth for any given band conditions. Sometimes with my 75A-4/SE-3 combination, even when the band is congested, an AM signal may be more intelligible when using the 8 kc/s mechanical filter than with the 6. This is particularly true in synchronous detection mode. Brute-force limitation of receiver bandwidth, filters out interference but also filters out intelligence-bearing articulation and sibilance from the voice. The synchronised product detector doesn't reduce the level of interference within the passband, but makes that interference more transparent.
  4. N2DTS

    N2DTS Ham Member QRZ Page

    All sdr rigs that I know of have sync detection of AM signals PLUS great filters, except maybe the Elecraft stuff, maybe.
    Sync detection of one sideband often does wonders for qrm reduction without degrading the audio quality.

    Plus, when one station is 2 or more Kc off frequency, you can see it and tune things or adjust the filter to suit.
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    If you are talking about Web SDR, The bandwidth may be limited.

    The audio will sound distorted if that is the case.

    Have Fun.
  6. WA3VJB

    WA3VJB Ham Member QRZ Page

    I've noticed that the passbands on most nodes of websdr max out at just under 8Kc.
    A few go beyond that, including Michael's K3FEF's in Pennsylvania. I wish I knew how to suggest to some of the other site sponsors how to open them up, including the marvelous SF Bay area one in California. That node really hears.
  7. W2NBC

    W2NBC Ham Member QRZ Page

    W8KHK likes this.
  8. W2NBC

    W2NBC Ham Member QRZ Page

    That processor sure looks like a fantastic way to "control" your audio signature! I hope you're thinking about producing kits and "built" units?
    Great looking board!
    W8KHK likes this.
  9. N2DTS

    N2DTS Ham Member QRZ Page

    I use one of the early units and it works well.
    I can see my TX audio on the sdr and adjust it to fit the space between signals.
  10. W8KHK

    W8KHK Ham Member QRZ Page

    The unit Brett, N2DTS, is using includes a differential input stage, a low-cut (high pass) filter, a single-stage SCAF (switched capacitor) variable bandwidth filter, and four stages of low pass filter prior to the differential (balanced) output stage. It also includes a utility output stage which may be used for headphone monitoring or a VU meter.

    Since our collaboration began, we have added microphone input, four-pole all-pass filter (aka phase rotator), 150 uS pre-emphasis, compressor, limiter, a clipper (configurable to +100 or +125 percent positive, 100 percent negative, to catch the occasional impulses that escape the limiter) and migrated to a two-stage SCAF to provide a much sharper cutoff slope (which may be clocked at several fixed calibrated bandwidths, or continuously variable bandwidth via a single potentiometer). The board includes circuitry to provide indication of positive and negative predominant modulation waveform, as well as normal and peak level indicators, virtually eliminating the need for a VU meter. Modulation and bandwidth monitoring is certainly NOT discouraged. (To paraphrase Don (KYV) Keep those Headlights Shining.)

    The unit now includes two microphone inputs, one for balanced or unbalanced low level microphones, the other has a 10 megohm impedance for microphones such as the Astatic D-104. Two separate balanced or unbalanced line-level inputs are also included, and all four inputs may be controlled via front panel gain controls, or trim-pots directly on the PC board.

    Two separate differential output amplifiers, in addition to the single-ended utility amplifier, allow us to interface to two separate transmitters, eliminating the need to switch cables or re-adjust modulation level. Each differential output includes a configurable 40 dB pad, with optional DC blocking capacitor, to allow direct connection to microphone input on the newer rigs, and the capacitor addresses the issue of DC on the microphone input line.

    I initially started out with a hand-wired proto board, but the circuit grew to complex to manage with hand-wiring, so we went through three revisions of PC boards as we added features and improved the design. As Clark mentioned, we are in the Alpha test stage, fine-tuning the levels and optimizing the default configuration. All modules listed above may be optionally enabled or bypassed as desired. I plan to use mine with the push-pull 250-TH rig, modulated by a pair of 810s, and my restored Johnson Viking II. When this project is completed, I will have time to work on my series-modulated 3CX3000 and my Class-E PDM rig.
    Last edited: Mar 22, 2019

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