75 meter AM Transceiver

  Inside View  

 Warning!!!   The linked page displays naked components! Viewer discretion is advised.  Warning!!!

  General Highlights
  • Frequency range of 3.75MHz → 4.00MHz with about 20 KHz of overcoverage at band ends
  • Front panel layout
    • Backlit S/RF meter
    • Backlit Frequency Readout
    • Controls  (L→R)
      • IF Bandwith Selector
      • RIT Enable
      • RIT Tune
      • PWR/AF Gain
      • Main Tuning
    • LED Lamps above Main Tuning  (L→ R)
      • Transmit indicator
      • RIT enabled indicator
      • 4KHz filter selection
      • 8KHz filter selection
  • Rear panel connections
  • Case interior is fan pressurized and cooled
  Receiver
  • Basic scheme − Single conversion to the 455KHz IF frequency with high side injection
  • Selectivity −  Provided by either a 4KHz or 8KHz Collins mechanical filter
  • S-meter response from an unmodulated carrier input
    • S0 − No input signal
    • S¼ − 0.3µV (just above AGC threshold)
    • S1 − 1µV
    • Center scale − 30µV
    • S9 − 100µV
    • Full scale − 30mV
    • Off scale and almost pegged − 100mV
  • Image rejection  −  Better than 75db from two fixed tuned bandpass filters
  • Digital frequency display resolution − 100Hz
  • RIT tuning range −  ±4KHz
  • Tuning rate − Less than 25KHz per turn
  • AF output −  3.5 watts RMS into 8Ω
  Transmitter
  • Basic scheme − Heterodyne transmitter
  • The output amplifier − A single-ended Motorola MRF148 power FET biased for class AB linear operation
  • Method of modulation − Linear Series Modulation applied to the driver stage
  • Power output − 1 watt carrier into 50Ω (4W PEP @100% modulation)
  • Spectral purity − Mixed products and harmonic energy output less than -50dbc, except second harmonic which is less than -45dbc
  Simplified Block Diagram

      

  Data Sheets

  Why AM?

  • Why not?  It's no secret that SSB and CW are more effective when the going gets tough.  I operate those modes, too.  I enjoy AM operation because it sounds so good! Also, the folks who operate AM are typically a friendly and courteous bunch, willing to share their considerable technical expertise with fellow hams.
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