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T/R switching and Solid State or SDR receivers

Discussion in 'Amplitude Modulation' started by KC2ZFA, Jan 14, 2021.

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  1. KE0ZU

    KE0ZU Ham Member QRZ Page

    I use a bench audio amp and a audio select / conbiner panel. The conbined Rx audio is muted as part of the transmit sequence.


    Once you get beyond plugging the mic into a couple radios, things start getting fairly complex, with muting, and speaker audios etc. Don't know how/if you are going to switch your mic audio, but I put together a 6 station controller for distributing the mic audio, and Key line, along with a universal mute line, with minimal/reversable mute cktry in each receiver.

    The basic mic audio distribution is a pretty simple concept, but it, along with the Key line, and receiver mute functions, that can interface any and all comers requires a few switching relays and a fair amount of cabling.

    Unlike you, I only have 6 "rigs" on the bench at a time.

    [​IMG]
     
    Last edited: Jan 23, 2021
  2. W1BR

    W1BR Ham Member QRZ Page

    Receive muting in the HQ-150 involved keying the B Plus. That seems to be a poor approach to maintaining receiver stability. Keying the speaker works well, and going beyond using a TR switch I'm thinking of using
    a separate wire antenna for RX on 160 through 40 meters. The audio biasing cutoff used in the 75A4 and
    NC-303 avoids idling the LO, but it still results in a thump when keying and unkeying. I'm now adding relay
    contacts to handle the speakers.... another project.
     
  3. K4KYV

    K4KYV Premium Subscriber Volunteer Moderator QRZ Page

    I kill the audio by using a relay to short the line between the last speech amplifier stage and the BL-40 compressor/limiter. That way the compressor isn't hammered with speaker audio or other sounds in the shack when not transmitting. The compressor uses a small module consisting of an incandescent lamp next to a phototransistor to regulate the gain of the stage, and as with any incandescent lamp it has a finite life. That module is proprietary, although replacements are available and I have a spare; I want it to last as long as possible before I have to replace it. So by killing the audio during receive I protect the transformer and don't needlessly expend life of the module.

    If the following stage is class-A, it doesn't draw grid current, so running it with no load in receive shouldn't make any difference. But still, loud noises in the shack such is running power tools while the rig is on but in stand-by could overdrive whatever is in the transmitter audio chain that isn't shut off during receive. Better safe than sorry.
     
  4. K5UJ

    K5UJ Ham Member QRZ Page

    I was vague or inaccurate about this in my description but in my case the relay/resistor in the tx audio line follow the mic. preamp just ahead of processing and everything else downstream so everything at line level is dead.

    What some guys do in your case is get a distribution amp so a single audio source can go out to several different loads and you can set the proper level for each one.
     
  5. W9JSW

    W9JSW Ham Member QRZ Page

    I am using a commercial Oxmoor dist amp. 2 inputs and 6 outputs, all XLR. Was quite inexpensive on the 'bay at around $50 to $70. I feed my mic into a Shure mixer. From the mixer I am using a DBX PA2 to provide bandwidth control, parametric equalization and limiting. Then to the Oxmoor and on to the rigs.

    MDA-26T Distribution Amplifier - Oxmoor Professional Audio Products

    For my SDR, the rig itself separates internal PTT and external PTT with a delay. External PTT goes to my exciter amp. In that amp there is a T/R board that mutes the RX during TX.

    When lashed up to the 813 rig I have a sequencer that keys the sdr and the antenna on a 300ms delay to make sure that the sdr never sees any residual power on RX input. Also on the RX feed, I have a BC HPF with back to back diodes. In the SDR there are TVS diodes as well on RX.

    So basically the 813 is in control. It sequences itself (audio/mod/plate/screen and drive) and tells the SDR and exciter amp to go into CW mode by closing the key input. This causes a TX CW drive feed that the exciter amp boosts to 40W. When the 813 goes to RX, it sequences in reverse, drops the drive and then 300ms later drops the ANT relay.

    I am just now bringing this rig online.
     
  6. KE0ZU

    KE0ZU Ham Member QRZ Page

    Since all transmitters/transceivers have their own Mic Gain and other audio controls, I just keep the Tx audio line level constant and adjust each rig as necessary, once and done. The mic audio is switched along with the Key line, to the desired exciter. Any compression, band pass shaping etc., is done by what ever provisions are available in any particular transmitter. I don't do any "tweaking" to make me sound like the Big Dog on the block, I just keep it simple.
     
  7. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    I have 1N34A’s...they clamp at lower voltage...safer with them ?
     
  8. KE0ZU

    KE0ZU Ham Member QRZ Page

    Give them a number of test cycles to verify their ability to do the job. I don't think there would be a problem. I use a pair of germanium 1N60s, but after a 16dB pad, I have a pair of 914s right at the Johnson T/R switche's output connector.

    [​IMG]
     
  9. W1BR

    W1BR Ham Member QRZ Page

    Adding 16 dB loss increases the RX receiver noise figure by the same amount... that is pretty horrible
    for any band much above 80 meters.

    Simplest solution is to run a separate wire antenna for receiving.
     
  10. KC2ZFA

    KC2ZFA Ham Member QRZ Page

    I settled on this plumber’s delight...and also ordered the Array Solutions box. Box 2 also provides rx mute. The sampler on the left feeds rf and audio to the scope.
     

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