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Saturday, May 9, 2015

BITX DIGI-TIA Build Update #1 Building the First 2 TIA Amplifiers



I started building the new transceiver today.   I am tentatively calling it the BITX DIGI-TIA.  Digi because it will have at its heart an Si5351 for BFO and VFO signal generation, and TIA because it will use Termination Insensitive Amplifiers (TIAs).  These TIA amps are designed to present a 50 ohm impedance not matter what impedance you connect to the other end.  This is a very useful feature when you are trying to get a certain impedance for a crystal filter in a bidirectional rig -- you want the same termination impedances in  both directions.
 
I built the "top" halves of two of the TIAs, using CNC-cut boards given to me by Pete Juliano.  Thanks Pete!  I like the boards, and I no longer have a thick coating of dried SuperGlue on my fingers.
 
Both amps seem to be performing  very well. Some numbers:
 
Current draw at 12 V (no signal) 30 ma.
 
Gain at 9 MHz (no load):  Amp 1:  27.2 db    Amp 2: 26.5 db.
 
I then took a 110 ohm load and put it across the output of Amp 1.  Gain dropped to 23.9 db (as you'd expect).  BUT HERE'S THE GOOD PART:  The input voltage from my homebrew sig generator stayed exactly the same.    Termination insensitivity. 
 
More on these amps (with a link to Wes's article) here:
 
 
Next  I'll put the 9 MHz filter between these amps and measure the shape of the filter passband.
 


 
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