Synchronous AM reception buildout on Kiwi SDRs

Discussion in 'Amplitude Modulation' started by WA3VJB, Jun 9, 2020.

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  1. WA3VJB

    WA3VJB Ham Member QRZ Page

    A few months ago the developers of the Kiwi SDR receiver network rolled out a firmware update that activated synchronous detection AM (SAM) among available reception modes.

    In recent weeks, that now has been refined further to allow selectable synchronous AM on either the lower sideband or the upper sideband. It's helpful when there's congestion off to one side or the other, but not that much different than sliding the receiver passband to the preferred chunk of transmitted signal, with or without the synch detector selected.

    The operators of some of the individual nodes around the world have yet to update their systems to offer these improvements, so you may see a grayscale button that does not do anything. Those nodes that have been updated will have a fully functional SAM softkey on the screen. Toggling through the button selects SAM, SAL, and SAU.

    Really improves the listening experience now that summer static season is underway.
    K5UJ likes this.
  2. K4KYV

    K4KYV Premium Subscriber QRZ Page

    There is a misunderstanding by many, who think synchronous detection inherently selects one sideband and suppresses the other. I believe synch mode on some receivers actually does that: forces you to choose USB or LSB. But synch detection was inherently designed for receiving both sidebands of a DSB signal. All that an AM synch detector is, is a BFO (local oscillator) controlled by a PLL, that locks the BFO exactly on frequency and in phase with the original AM carrier. With SSB, all that matters is that the BFO be on the exact same frequency as the original carrier. The synch detector can be useful to eliminate the frequency tuning error with SSB, but the phase of the inserted carrier doesn't matter.

    OTOH, for proper DSB reception, the inserted carrier must be precisely in phase with the original, or else the two sidebands won't properly reinforce each other at the detector. In fact, if the inserted carrier is shifted 90° from the original, the sidebands will cancel each other and there will be no audio output at the detector!

    DSB reception inherently has a 6 dB improvement over SSB at the detector, since the vector addition of the two sidebands working together doubles the recovered audio voltage at the detector. That's 6 dB gain, whereas the power transmitted in those two sidebands is only double, or 3 dB more than that of each one of the sidebands. So, DSB reception inherently gives an additional 3 dB of "free" signal gain. IOW, you get a six dB of signal boost for only three dB of additional power.
    W1TRY and K5UJ like this.
  3. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    electronicsnotes has a good summary of SD:

    as does this QST article from July 1993:

    WA3VJB likes this.
  4. K5UJ

    K5UJ Ham Member QRZ Page

    Sync. detectors help with QRN and QRM. Another thing that helps a bit with QRN are the small loop antennas with preamps. They seem to offer a bit better S/N over the static. But, nothing beats power. How does it go: Mo fiya in a hiya wiya. Summer is not kind to piss weakers.
  5. WA3VJB

    WA3VJB Ham Member QRZ Page

    At the risk of enabling the piss weakers, I notice a significant improvement trying to pull out such stations using a synch detector, especially when they are running thin audio, like 40% modulation.

    Some poor devil in Virginia who I encounter on 75m got himself a nice Flex transceiver over the Winter, and now has no money to get an amplifier. But boy will he tell you his audio sounds great. He must tell you this, because chances are poor you can hear it on your own.
    K5UJ, K4KYV, AG5CK and 1 other person like this.
  6. WZ5Q

    WZ5Q Ham Member QRZ Page

    I'm DYING over here, LMAO !!!! :p:D;)
  7. N2DTS

    N2DTS Ham Member QRZ Page

    Some programs call it ecss. They allow one, the other, or both sidebands as you click the ecss button on the screen.
    One of my sdr programs has both, sync AM (no sideband selection) and also has the exss which does.

    My flex 5000 on the ke9ns software shows being able to select one, the other or both sidebands in sync AM but it actually does not do anything.
    Maybe a newer version has it working but I have not looked into it yet.

    My small receiving loop works quite well at reducing noise but it IS directional, it needs a rotor.
    Maybe it seems to do so much better because it IS directional...

    I have not found sync or ecss detection to really help on weak signal stuff on any of the radios/programs I have tried, the homebrew rx
    seems to work best on weak in the noise signals.

    My 756 pro2 noise blanker seems to work VERY well, as did the one in the Elad fdm duo.
    None of the sdr programs seem to be very good at that, the flex software has loads of settings, maybe it would work if you got
    the right combo but I never found it.
  8. AG5CK

    AG5CK Ham Member QRZ Page

    I don't see much benefit of the sync on my Anan SDR for weak signals. There is a difference in tone between AM and AM sync that is helpful in some situations.

    As mentioned the ability to select one sideband or the other works very well for QRM and doesn't degrade the fidelity as much as narrowing up the filter. I often forget that I'm only listening to one side band.

  9. K4KYV

    K4KYV Premium Subscriber QRZ Page

    The advantage of the sync detector is that it's a product detector. That's mostly what accounts for the alleged superiority of SSB; you receive it with a product detector. Even with a diode detector in a classic receiver, you turn the RF gain way down for distortion-free SSB reception, which makes it act much like a product detector, since the inserted carrier is way above the desired signal, practically eliminating the quadrature distortion. (If you don't know what quadrature distortion is, look it up).

    Receiving AM with an envelope detector (usually a diode detector), every signal within the passband of the receiver's i.f. filter (i.f. selectivity) intermodulates with every other signal. For example, think of a heterodyne 1 kc. off frequency from the AM station transmitting. With the diode detector, the AM carrier heterodynes against the rogue carrier producing the squeal. But every element of the USB and LSB of the AM signal also each heterodyne against the rogue carrier. That produces additional intermodulation products. Let's say there is a static crash. That static crash beats against the AM carrier, the rogue carrier, every element of the USB of the AM signal, and every element of the LSB of the AM signal. All those intermodulation products produce a mish-mash of garbage that sometimes renders the desired signal unintelligible.

    With the sync detector, we use what is sometimes called exalted carrier reception. The BFO is synchronised precisely with the original AM carrier. Since we are receiving with a product detector, the only audio products resulting from the demodulation at the detector are the USB and LSB of the AM signal beating with the BFO, the rogue carrier beating with the BFO, and the wide spectrum of noise from the static crash beating with the BFO. The product detector doesn't respond to the USB and LSB mixing with the rogue carrier, the rogue carrier mixing with the static crash, nor the USB and LSB mixing with the static crash. Yes, you still hear the heterodyne from the rogue carrier and you still hear the static crash, but all that other garbage from every element of signal within the receiver's pass-band intermodulating with every other other element of signal, is not heard. You hear only what is beating against the inserted carrier. That makes the heterodyne and static crash much more transparent, so you can largely ignore the QRM and hear the desired signal. Kind of like being in a small room with a party going on, trying to converse with another person. You still hear all the other chit-chat and the music going on in the background, but the human brain allows you to mentally select the voice of the person you are talking to and reject all the rest, and you can understand that person above all the other noise. The sync detector does the same thing with all the undesirable noise within the receiver passband.
    K5UJ likes this.
  10. K5UJ

    K5UJ Ham Member QRZ Page

    This is why, to some extent, I advise people intent on running a rig like a Ranger into an antenna, to not do a great deal to make it extremely wide and flat like a lot of the modifications to the audio attempt to achieve. Increasing positive peak headroom and lowering distortion is fine; passing everything down to 20 cycles is pretty much a waste.

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