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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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.
ReplyDeleteHi Bill
ReplyDeleteBoth 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