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Thursday, September 26, 2013

BITX Build Update #10 -- Darling(ton)

Having concluded that I was significantly short on overall receiver gain, I went in yesterday and changed my AF amp from a 40db direct-coupled circuit to a 100db Darlington pair.  I immediately noticed a big increase in audio output. 

I did a quick receiver alignment using my Arduino/AD9860 sig generator.  First I determined the actual bandpass of the crystal filter: 4.998170 MHz -- 5.000960 MHz.   Using a freq counter, I set the BFO at 5.00126.   I immediately started hearing 17 meter SSB signals from the West Coast.  That's always a nice moment:  first signals through a new receiver.  Kind of like "first light" in a new telescope.  Even with the filter ripple, it sounds great.

I think I'm still significantly short of gain.  Audio is still faint. I notice that in the BITX17A they have added a second transistor (Q17) in the second RX receive amp.  Maybe I should try something similar.   Or should I add some gain in the audio chain?

I'm really enjoying this BITX project. 


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2 comments:

  1. To produce sufficient acoustic output you need to have sufficient gain and your amplifier has to be capable of producing enough voltage and current to drive the speaker. With an 8 ohm load and a 12V supply, the amplifier needs to produce around 800mA peak at 6V peak.

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  2. Hi Bill
    Both Bitx 20 and 17a kit diagrams show second transistor. Probably better drive to (de)modulator. I'd run with that for a start at least.
    73 de ZL2DEX

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