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Monday, September 23, 2013

BITX Build Update #9 -- Discretion

With the exception of the PA, all of the stages of my BITX 17 are built.  Over the weekend I put in the DC wiring for the receiver and the inter-stage connections (using the Belden coax with the exposed shield and Teflon di-electric).  It looks nice. 

In my experience, almost all new superhet receivers require a certain amount of debugging and coaxing before they will work.  This one is no exception.  The VFO and the BFO work fine, and all three RF and IF amp stages are also good.  The bandpass filter  that I built passes the desired band and tunes up nicely on the right frequency.  The product detector was acting weird and wasn't balancing out properly, but I got that all sorted.  

I can put an 18.110 MHz signal at the antenna connection and see the signal go through the bandpass filter (with loss), on to the RF amp stage, to the first mixer where it meets the 23 MHz energy from the VFO.  A very messy mixture goes from the mixer to the first IF amp which sends it to the 5 MHz Cohn filter.  The filter works, but it has a lot of ripple, so I need to work on the termination impedances.  Second IF works fine, then the signal goes to the product detector.  AF comes out.  

Here's where the discretion comes in.  Instead of the LM386 chip, I built a 40db two transistor direct coupled AF amplifier. 

So it all works, but the receiver is quite deaf.  I think I just don't have enough gain in the whole system.  I looked at the schematic for the BITX-17 kit.  It very helpfully has total (net) gain figures for the RX.  I can see that my current configuration comes up short.   

Here is what I'm thinking of doing:  I might replace the 40db direct-coupled AF amp with a 100 db Darlington pair.   I really like the discrete Darlington AF amp that KD1JV has in his "all discrete" transceiver:
http://kd1jv.qrpradio.com/ADC/ADC-40.htm  Nice.  3 2N3904's driving a speaker.  I may use that.  
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  1. have a look at:

  2. Thanks for having such a great site! I always look forward to the podcast too.

    Here's a link to a discrete amp replacement from the ARRl. I've built this amp. It's on page 1.12.

    I also have an IF amp I've used on my bitx, but I'll try to find that link later.



  3. I understand wanting to ditch the LM386 and having a discrete output stage.

    But why not use an op-amp for audio gain.

    Something like one of the amps on Todd's site: http://www.qrp.pops.net/af-amp-2008.asp

  4. I agee it looks nice Bill. 73 de chas ai4ot

  5. Yikes! 100dB is direct-conversion receiver territory! That receiver certainly should not require that much gain. My opinion: Go for less audio gain and the least distortion. Put the gain somewhere earlier in the chain. I learned a lot about getting relatively simple receivers to really perform when listening to Wayne Burdick (Elecraft) speak on the subject (specifically the design of the K1) at Pacificon one year. Bottom line: Gain distribution, i.e. don't put the majority of your gain in one stage.

    Regarding the IF filter, have you tried out "AADE FILTER DESIGN AND ANALYSIS" from Almost All Digital Electronics?


    Works great and it's freeeeee!"

    Best of luck with the project and

    73.......Steve Smith WB6TNL
    "Snort Rosin"


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