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Direct conversion SSB transmitters

Discussion in 'Discussions, Opinions & Editorials' started by KL7AJ, Jan 29, 2012.

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  1. KL7AJ

    KL7AJ Ham Member QRZ Page

    Back in the late 1970s, there were countless projects for DC (Direct Conversion) receivers that had surprisingly good performance. Most folks don't realise that this method is capable of generating perfectly good SSB signals as well, without the need for crystal filtering OR persnickety phasing adjustrments. The only tricky part is coming up with a very clean synthesizer over the entire HF range. However, there are some good low-cost DDS chips available now that do the job nicely. I'd like to hear from any other hams who have experimented with this method.

  2. K0RGR

    K0RGR Premium Subscriber QRZ Page

    There are a number of DSB transmitter designs out there that apply an RF signal to one port of a balanced mixer, and apply audio to the IF port, resulting in a DSB suppressed carrier signal at the output. These designs can be quite simple - the port-to-port isolation of the mixer determines the level of carrier suppression, so the quality of the mixer is important.
  3. KL7AJ

    KL7AJ Ham Member QRZ Page

    But you still have to deal with the opposite sideband. The DC system requiires that the local oscillator is at the final carrier freq. :

  4. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Are you referring to what was often called "The Third Method" of generating SSB?
  5. K0RGR

    K0RGR Premium Subscriber QRZ Page

    Well, I don't think DC receivers are 'single signal', you will hear a CW signal either side of zero beat on one, and if you listen to a DSB signal with or without the carrier, you can copy either sideband. I'm aware of techniques to make DC RX truly single signal, and I think that is really how SDR's work, so the principles should apply to transmitters somehow - I just don't know how. The phasing method is also called 'Hartley Moudulation', and it resembles the quadrature technique used with SDR's quite a bit. Perhaps it's the same thing. I'm always amazed at how ancient and forgotten technology keeps getting resurrected. I plan to corner the market on 5.25" floppy drives!

    It's been a while since I looked at the phasing method of SSB generation, but I think that was typically done at a fixed frequency and then up or down converted, too, because of the difficulty of achieving a real 90 degree phase shift over a broad frequency range. I believe the old Central Electronics 10B and 20A were phasing exciters, and I would not call them simple. I remember having to adjust the 'phasing' control on the front panel from time to time.

    Lots of people actually run DSB with suppressed carrier. Yes, it wastes bandwidth, but so does AM, and it doesn't require a modulator for AM - the mixer works as a balanced modulatior. If you search for 'DSB Transceiver' on YouTube, you will see lots of them.
  6. AC0FP

    AC0FP Ham Member QRZ Page

    This is the way I made my SSB rig back in the seventies using a Minicircuit labs DBM. I followed that with a crystal filter and the carrier was 70 dB down.

  7. KA5S

    KA5S Subscriber QRZ Page

    DA1GI wrote to QST's Technical Correspondence page in 1982 discussing the idea of an all-phasing transceiver. See the QST Archive (members only) for June 1982, Pg. 51;

    In 1993 there were articles about KK7B's "R2" phasing receiver and a matching design T2 transmitter; K0JD mentions them on his Web page

    (emphasis added)

    in 2004 you could buy R2Pro's from Kanga, and there were board templates on the ARRL site.

    The links are outdated and don't work, but search for the articles or author in the QST ARchive.

    FWIW, I saw an R2 for sale at a reecent hamfest Not many folks recognized it.

    Last edited: Jan 30, 2012
  8. K5UOS

    K5UOS Premium Subscriber QRZ Page


    The T2 series transmitters were in QST more recently, too:

    Dec 2006 - QST (Pg. 28)

    The MicroT2 -- A Compact Single-Band SSB Transmitter
    Author: Campbell, Richard, KK7B
    Article: QST Archive [PDF]

    I use a similar circuit to the R2 as the product detector and audio section for a couple of transceivers with superhet receivers. I also a similar circuit as the DC RX portion of a DSB Transceiver.

    Rick has some TX phasing information in Experimental Methods in RF Design as well as many simple low noise and BC band resistant RX designs.

    The RX circuits are a lot of fun to experiment with. I have not built a T2.

    One observation is that the circuits can be as complex as simple filter designs.

  9. K5UOS

    K5UOS Premium Subscriber QRZ Page

    For clarification - You can't really listen to a DSB signal on a DC receiver unless it is designed for single signal reception. You can copy SSB. Even with a fair to good DC receiver the audio can be really amazing.
  10. K5UOS

    K5UOS Premium Subscriber QRZ Page

    Hi Eric,

    I bought several 7.195 and 7.285MHz crystals years ago to experiment with a direct conversion SSB filter type transmitter.

    It would be limited to one spot frequency.

    I would have used an SBL-1 with a simple audio OP amp and crystal carrier oscillator as inputs. A 3 or 4 pole crystal filter at the same nominal frequency as the carrier oscillator to reject the unwanted sideband.

    A 2 or 3 stage transmitter chain to get the SSB output up to 1W and a IRF510 PA to approx. 10W.

    The carrier could be used as a VBFO for a DC receiver. A real minimalist transceiver could be built.

    These days I am more likely to try it on 10 or 15M. Limited but acceptable to those of us who started with crystal control.

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