Saturday, April 18, 2015

BITX Modular The Backplane

     I had been looking around for a QRP transceiver to build, and decided to do something based on the BITX .  The simple design with the use of a common bi-directional amplifiers for several stages makes it ideal for building a modular system.  Except for the final amplifier, I broke the basic transceiver  down to 7 basic assemblies.

1.  Band pass filter and Driver Amplifier
2.  Bi-directional RF Amplifier
3.  Mixer
4.  Bi-directional IF Amplifier
5.  Crystal Filter
6.  Bi-directional IF Amplifier
7.  Modulator\Product Detector  Audio\Mic Amplifier.

      With these modules in mind I looked at what common signals would be used by each module.  The BITX  amplifiers switch direction by supplying 12 volts to one of the two amplifiers, and grounding the supply line on the inactive amplifier.  These signals are labeled  as R and T in the BITX schematic, and are controlled by either a relay or switch.  Other modules also need switched 12 volts for both receive and transmit. Most  modules require an input and output which are swapped when going from receive to transmit.
   While the individual modules could just be mounted in a chassis, I  decided that I would build a back-plane similar to what you would find in a computer, and what I have seen in some instrumentation systems I worked on.  I incorporated relay switching of the 12 volts for the R and T signals.  I brought out all the signals to two  connectors , one that contains signals related to the receive path and the other the transmit path.  I located the un-switched  12 volts along with the transmit connector.  I  placed a ground between each signal and added bypass capacitors on the supply lines to minimize noise.
Schematic
Layout and Etched boards








Assembled Backplane
Backplane with Balanced Modulator- Audio board














 For the modules I am going with a 'Muppet' type construction using double sided circuit boards.  Hopefully the grounded solid copper layer on the back of the boards will provide adequate shielding.  For the connectors I am using  .1in. right angle headers on the modules and pins on the Backplane. 

I etched up a Backplane and the Balanced Modulator - Audio Baord using toner transfer.  After etching I used a new 'Liquid Tin' solution  form MG Chemicals that I wanted to try.  It is fairly expensive, but the results are great. The boards look nice and  solder better than just bare copper.

Now just need to check out the 12 volt and R/T switching and I can move on and build and test the Balanced modulator Audio Board.











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