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Sunday, September 29, 2013

BITX Build Update #11 -- Peakin' and Tweakin'

Oh how I love the sound of a newly built receiver!  I'm sitting here listening to G0MJS on 17 meters.  Lots of other stations from across the pond coming in very nicely. 

Earlier in the week I had some sensitivity problems.  I could hear the noise floor, but just barely.  And the receiver just seemed to have trouble inhaling.  So I started poking around.  It seemed that each poke improved things a bit.  I had used Farhan's original schematic (mostly).   Later versions put an additional transistor in the IF amp.  So I went ahead and added that mod.  That helped a bit.  Then I noticed that BFO energy was getting into the AF amp.   So I put a .1uF cap to ground at the input to the AF preamp.  That took care of the RF and did no damage to the AF.  

But the rig still seemed a bit hard of hearing.  This morning Farhan advised me to take a look at the mixers.  I used some junk-box diodes that I didn't know too much about...  I measured the forward resistance and found it to be quite a bit higher than the usual 1N914s.  So I switched all 6 mixer diodes.  That helped noticeably.

I also checked the input bandpass filter.   It seems OK.   In the process I learned to use the EMRFD filter programs (thanks Wes) and the ELSIE filter program (also very nice).

Anyway, the rig sounds great now.   On to the transmitter.  

Farhan advises building the power amplifier on a separate copper clad board.  But I have room for it on my main board.  Should I live dangerously?   


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3 comments:

  1. Hi Bill - me again.
    It looks like you have separate 'islands' per section on your main board. Is it double-sided? If so, I'd use that, but if not there may be benefit in using that as ground-plane for separate board. Bear in mind 'single-ground-point' either way in your layout.
    73 de ZL2DEX

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  2. "Should I live dangerously?"


    - What sort of frequencies at what magnitude will this create?

    What sort of amplifier design are you going for? Hundred watt class-D beast? A single op-amp?


    - Can you shield it?
    If you purchase thick aluminium (easily hand moulded) in rolls as 'gutter flashing' from hardwares you can do it yourself. A hammer and some right angle corners (eg desk edges) will suffice for shaping.

    If you are going tiny (eg an op-amp) a neat valve-esque container can be made from an "aluminium pill container" off ebay. If you cut the threaded section of the cap off to make a ring and solder this to your pcb you have a seat. Then make the whole circuit stand vertically up inside this seat, and screw on the rest of the container to hide it.


    - Is there enough space for cooling?
    Active? Passive? So small none is even necessary?


    Isolating the position of the amplifier geographically on the PCB will require longer wires. Small diam coax stolen from RCA cables might help

    Looking great! Waiting eagerly to see how this progresses.

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