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Automatic RF noise cancellation and audio noise measurement

Discussion in 'General Electronics' started by Jason Hsu, Aug 5, 2003.

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  1. Jason Hsu

    Jason Hsu Guest

    THE BIG QUESTION: How do you measure the audio noise level in a
    receiver? Does anyone here know of any circuits that show the overall
    audio noise level?

    Let me tell you what this is all about.

    SIDE NOTE: Thanks again to those of you who helped me with the
    SWR/wattmeter project I worked on last semester. This project taught
    me more about ferrite cores, op amps, and diodes than ANY class
    possibly could. I learned about stray capacitance and what rails
    mean. I also learned that not all 1N34A diodes are alike.

    In this coming semester, I will be working on an even more ambitious
    project - an automatic noise canceller for 160m to 10m. It will
    definitely be a control system, and it may even use DSP. The device
    will also require a motor for automatically adjusting the controls.
    The idea is to design and build an automatic version of the Timewave
    ANC-4 or the MFJ-1025/1026 RF noise cancellers.

    The ANC-4 and MFJ-1025/1026 are manual devices. There are automatic
    noise cancellation devices, but their adjustments are at the audio
    level rather than the RF level.

    The back panel of my device will contain:
    1. Connector for the signal antenna
    2. Connector for the transceiver
    3. Connector for the noise antenna
    4. DC power connector

    The front panel of my device will contain:
    1. Power switch and the obligatory idiot light to show that the power
    is on
    2. 2 audio connectors: one connects to the transceiver, and the other
    connects to an external speaker
    3. Frequency range control for choosing the inductance values, as the
    proper inductance values in the RF noise cancelling circuit varies
    with frequency
    4. Phase range knob so that the phase shifter can cover all 360
    5. Manual noise phase knob
    6. Manual noise gain knob
    7. LED display to show the noise level
    8. Noise phase adjustment push-button: Pressing this button activates
    the control system to adjust the phase shift of the noise.
    9. Noise gain adjustment button: Pressing this button activates the
    control system to adjust the gain of the noise.

    Other features:
    1. I need to design my automatic noise canceller so that parts won't
    blow up if I transmit 100W through it. This probably requires some
    type of automatic bypass circuitry.
    2. Transmitted power must not reach the noise antenna (at least not
    without a great deal of attenuation)
    3. It also needs a low insertion loss.
    4. A preamplifier would be desirable.

    How it would work:
    1. You manually turn the phase and gain controls to the minimum
    settings. (Or I could put in a reset button to do that. No, that
    would add too much complexity to the circuit.)
    2. You select the band using the band inductance switch.
    3. You move the phase polarity switch to the negative setting. If
    the noise cancellation procedure does not work, it probably means the
    opposite setting is required.
    4. Press the noise gain adjustment button. The device adjusts the
    noise gain until a change in the signal level (or the S meter) is just
    noted. (Some type of derivative function would come into play.)
    5. Press the noise phase adjustment button. The device adjusts the
    noise phase shift so as to produce a null in the signal level (or the
    S meter).
    6. Steps 4 and 5 could be repeated to make further adjustments.
    There might be away to activate the control system with just one

    Does anyone know of circuits that measure audio noise? Making the RF
    noise canceller automatic requires an audio circuit to monitor the
    noise level. This noise level needs to be converted into a large DC
    voltage, which feeds the motors that turn the variable capacitor and

    Jason Hsu, AG4DG
    [email protected]
  2. Tweetldee

    Tweetldee Guest

    Allow me to infuse a different idea. Think about noise as a fairly
    broadband phenomenon. The same atmospheric noise that you hear on 21.500
    Mhz will also be heard on 21.510 Mhz. Consider the possibility of biulding
    two identical receivers, the main receiver tuned to the desired reception
    frequency, the other tuned just a few Khz away, but out of the received
    signal's bandwidth. Then, you detect the noise on the second receiver,
    invert it, and sum it into the first receiver's signal chain. This concept
    is used in ultra-low noise, high sensitivity deep space receivers to almost
    totally eliminate noise in the received signal.

    Now, as to your concern to keep 100W of RF from blowing up your noise
    canceller. That's what antenna changeover relays are for. They normally
    connect the antenna to the receiver, but when the transmitter is keyed up,
    the relay connects the antenna to the transmitter output, and disconnects it
    from the receiver. Simple, but effective, and has been used for many many
    years in amateur as well as commercial radio equipment.
    Tweetldee at att dot net (Just subsitute the appropriate characters in the

    Time is what keeps everything from happening all at once.
  3. Dave Shrader

    Dave Shrader Guest

    There are numerous sources of noise at the input to a receiver:

    1) Background HF noise that exhibits a 1/f rolloff characteristic. This
    has seasonal and diurnal variations.

    2) Atmospheric noise caused by local or near local weather. Remember
    there are 10,000 thunderstorms in the world at any time.

    3) Solar flare noise. During an solar X-ray event the HF noise, #1, can
    increase by over 60 dB for short periods of time.

    4) Local line interference noise.

    5) In the future, BPL noise.

    6) I'm sure there are other noise sources such as defective insulators
    on power lines. ;-)

    The ANC-4 and MFJ 1025/1026 ONLY operate on #4.

    So, your first task is to become familiar with the spectral
    characteristics of each so you can design a basic filtering circuit for
    the undesired noise source. My ANC-4 does not respond to the 1/f HF
    solar noise sources, atmospheric induced noise or solar flare noise.

    Weather induced noise contains frequency components that are in the
    audio passband, you can hear lightning induced static crashes in your
    SSB/AM radio. How will you discriminate between desired audio and static

    Solar flare noise sounds like a slowly rising rushing hiss in the
    receiver. It has a slow rise time generally in the range of tens of
    seconds to minutes. It is broadbanded and will easily over ride any low
    level desired signal. The noise to signal, NOT signal to noise, ratio
    can easily exceed 40 dB making detection almost impossible. These
    characteristics are proportional to the intensity of the solar flux and
    also the wavelength of the solar flux. Detecting desired signals in this
    environment is extraordinarily difficult.

    Manmade noise, these are processed by the ANC-4 and MFJ units, contains
    basically repetitive waveforms from TVs, VCRs, Remotes, Routers,
    Computers, etc.

    A design approach would be to use digital filtering and a series of
    Kalman filters [digital filter algorithms], assuming they are available
    to the general public. The Kalman filters would be tailored to the
    spectral content of the noise source you are attempting to cancel. A
    second design approach would be a fast Fourier transform that
    discriminates N/S from S/N. You want to keep the noise OUT of the FIRST
    RF STAGE to avoid pumping the AGC and getting false signals. For HF
    digital filtering this will require a very fast processor [possibly GHz

    How to measure the AF noise from a receiver? I'd start by looking into a
    Bolometer circuit with AC coupling. This would give a pseudo RMS reading
    that could be used as a reference for a series of servo based op-amps to
    control your gain and phase circuits.

    Depending on how far you want to pursue the science it could be a very
    interesting project.

    With a statement of principles it is YOUR Project, NOT the project for
    the members of this list. So, go and have fun!

    Deacon Dave, W1MCE

    Among the many components of my background was 7 years as Project
    Engineer designing special test equipment on contract for the USAF.
  4. Hi Jason:

    I think the "signal antenna" and "noise antenna" are both just
    antennas. I can't see how you can make an antenna that only hears noise
    and another that hears the same noise and signal. If that's the case
    then by adjusting the relative phase and amplitude of the two "antennas"
    what you are doing is changing the antenna pattern. This will improve
    the overall s/n if there is a source of noise that's coming form some
    direction more than from everywhere.

    In "Beam and null switch step steerable antenna system" U.S. patent
    <> 4,063,250
    <> Fenwick has an
    antenna system where the main lobe is steered using time delay beam
    steering (this is a frequency independent version of a phased array) and
    in addition can steer a null independent of the main lobe. He used
    mechanical relays and coax line, but a more modern version could use PIN
    diodes and a computer to try all the possible main and null lobe
    possibilities and use the best ones.

    To measure the sensivity of an AM receiver you measure the RMS audio
    output voltage with a signal present and without a signal present.
    For a more detailed look at the noise you need a spectrum analyzer, like
    the HP-Agilent 4395A that can make true RMS noise measurements, see:

    Have Fun,

    Brooke Clarke, N6GCE
  5. N. Thornton

    N. Thornton Guest

    Hi Jason

    Theres a whole lot of unworkable ideas on this thread. I shouldnt rush
    in without reading up well first.

    I'll just address a couple of Dave's ideas.

    Lot of work was done on this in the 30s, the cnoclusion was they
    couldnt. However you can if you use the standard 405-line TV noise
    reduction method. Basically detect brief signals that are greater in
    amplitude than your wanted a.f., and zero or reduce the af output
    during that time.

    I dont see how you can manipulate signals that havent even reached the
    first rf amp. You cant even read them as theyre microvolts. Putting
    them thru any kind of digital filter - you work it out.

    Jason your idea of mixing different noise sources is also a non
    runner. Sorry, but youve got to consider _exactly_ what is in those
    noise sources and how they differ. It will soon become apparent that
    by the time you add them youve got 2 different things. The noise f
    distribution might be the same, but the noise signal will be quite

    Regards, NT
  6. Ideally you point the noise antenna at the noise, and the signal antenna at
    the signal. In the real world the noise antenna is usually fixed, but might
    be pointed at something like a power line. In any case, the noise antenna
    is usually quite low, and you hope it won't pick up much of the desired

    BTW, way back when, either Radio Shack or Lafayette sold a CB radio that
    used a separate noise channel that was tuned to just outside the CB band. No
    idea how well it worked.

    Hi Jason:

    I think the "signal antenna" and "noise antenna" are both just antennas. I
    can't see how you can make an antenna that only hears noise and another that
    hears the same noise and signal.
  7. Jerry Avins

    Jerry Avins Guest

    The line from the change-over relay to the receiver should be shorted
    when the receiver is disconnected and either short or a half wave, or
    else open when the receiver is disconnected and a quarter wave long.

  8. Precious Pup

    Precious Pup Guest

    It even works for _on-channel_ jamming TX'ers off in another direction from the desired signal (cable co's use
    the technique). Separate directional antennas are used for the desired and the undesired signals. The
    undesired signal (jammer) is gain adjusted and phase shifted and then summed into the "main" receiver signal
    path at 180 degrees (and equal amplitude) from the same jammer entering into the main antenna/rx'er path. The
    obvious null for the undesired signal occurs. Nulling of the desired signal is highly improbable given the
    directional aspects of the antennas. The sum can occur in the rx'er front end. Yes, this is signal
    processing down at the microvolt level. One problem is tuning drift.
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