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MOBO 3.6 Q/A

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  • strawbob2
    Some questions below are mine about some of Alex s circuits, which he has graciously provided answers to off-line. Shared here in the event that others have
    Message 1 of 1 , Apr 30, 2009
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      Some questions below are mine about some of Alex's circuits, which he has graciously provided answers to off-line. Shared here in the event that others have the same or similar questions.

      A huge thank you to all of you who share your time and talents to the development and evolution of Tony's amazing Softrocks. The cluster of circuits on my bench is nothing short of synergy.

      An old Jerry Jeff Walker tune SDR rendition...

      Click a little, talk a little,
      Click a little, talk a little,
      Click, click, click, talk a lot, click a little more...

      I'm lovin it!

      73's, Robert NQ0T
      I got the new code for controlling K5OOR's filter bank installed on the UBW, built a relay driver board and everything is working well.  Thank you!  Pretty cool to hear the click, click, click as I change bands.

      I was going to build your SWR protection circuit and relay driver board and had some questions.  Are these circuits your own or can you point me at a reference?  Not sure how many others are following your design.  If you would like me to post this stuff on the reflector, so you don't have to repeat yourself, that's fine too.

      > The SWR protection circuit and relay driver circuit are my own
      > adaptation of the circuits used by Helge Granberg for his 2kw
      > experimental linear PA. I have used my adapted circuits for 3 of the
      > linear PA's I've built. All the 3 are based on kits from
      > Communications Concepts, Inc. They are:

      1. 300W AR305
      2. 600W EB104
      3. 1kW AR347

      The SWR and relay driver circuits have worked well for my 3 PA's. But they certainly cannot provide 100% protection under all adverse conditions - so use at your own risk! The SWR circuit can be improved
      - see an improved version of the circuit in my prelim mobo v4.0 design, for example. (FILES > 9V1AL > Motherboard > v4.x > prelim v4.0 design). However, the mobo design has progressed to v4.3 now, and we will be using a digital uController based SWR protection and ALC control.

      I gather the directional coupler is in a shielded box?  Just to keep RF leakage down?  What is the purpose of the 25p trimmer capacitor? This circuit puts a DC voltage on the LM358 that is proportional to the reflected voltage?  There are two potentiometers for sensitivity and threshold.  How are these adjusted?  If I recall, you adjust the bias on the amp with the output of the LM358, correct?

       > Yes, the directional coupler is in a shielded box to keep RF leakage down. The 25p trimmer is adjusted such that at SWR of 1:1, zero DC voltage comes out through the feed-thru capacitor. This occurs when the current side from the 20t:1t transformer develops the same voltage through the 50ohm resistor (100 ohm x 2 parallel) as the capacitor divider (1p:20p from the trimmer). The threshold potentiometer is adjusted to about 2.5v usually, and the sensitivity set to highest initially with the AMP output to a dummy load. The slightest SWR mismatch will then trigger the protection (which can be heard as audio distortion in a receiver, and reduced RF power out on the power meter). The sensitivity is then slowly reduced to a level of your liking. The LM358 acts as a comparitor with a small amount of hysterisis (from the 33k feedback resistor). The output of LM358 is then used for the PA bias supply.

      I am following the T/R Relay Driver circuit, but not understanding the purpose of the hex Schmitt trigger inverters.

      > The heart of the T/R Relay Driver is the diode-resistor-capacitor
      > configuration to setting the Rx and Tx delays, such that on key down
      > (Tx), the Tx relay is instantly turned on, but the Rx relay is
      > delayed. On key up (Rx), the Rx relay is instantly turned off, but
      > the Tx relay stays on for a few milliseconds. The inverters buffer
      > the signals to provide a repeatable voltage/current source/drain for
      > the resistor-capacitor network. The Schmitt trigger inputs provides
      > some measure of noise immunity in the RF rich environment. We don't
      > want any misfiring of the relays!

       > My experience has been that shielding of the direction coupler portion of the SWR protection circuit is not necessary for the 300W PA provided that FB's are used for any long connecting wires coming in and out of the circuits. For the 1kW PA, I put in the shielding. My 600W PA has no shielding but it has worked OK so far. For my SDR with the PA-100 putting out about 50W, no shielding is used.

      The MOBO is working good; I pretty much talk to whomever I call. Using Rocky on CW.  Is there a version of PowerSDR that works with the UBW for TX and RX?  I would like to do SSB at some point, after the amp is integrated.

       > PowerSDR-sr40 and PowerSDR-IQ both work with the UBW for Rx and Tx in SSB. PowerSDR-IQ is by far the best SDR software to use for SSB Rx and Tx, with it's automatic I/Q balancing feature.

      > However, I cannot get CW transmitting to work with PowerSDR-IQ. With
      > PowerSDR-sr40, I can use the CWX keyboard console for CW Tx using the
      > Softrock mode, but not with the SDR1000 mode.
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