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Improved audio from the Bitx40 (video)

 

(I think Bill might have mentioned this on Soldersmoke but I can't find the reference)


My Bitx has C102 as 47pF and no carrier frequency adjust trimmer (C103).?

It sounded very slightly bassy, lacking highs. ?The area around 2kHz is important.?


Most people wouldn't notice it but at QRP your audio band pass needs to be optimum to be heard on SSB.?

In short, I got better audio by removing C102's 47pF chip capacitor and substituting 27pF. ?Either a 27pF disc ceramic or 22 + 4.7pF. ?


Reports from other stations have been complimentary.?

This video discusses these topics in detail.?


73, Peter VK3YE
 

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Hi Peter,

Wouldn't it be better to remove the 47pf C102 all together and short the terminals together, then put a 2-40pf trimmer cap back in place where it is missing? Then you could adjust it while watching the audio analyzer and listening to the receive audio. I was already planning on doing it so I could accomplish the same thing you did.

Joel Caulkins
KB6QVI

On Jan 1, 2017, at 7:28 PM, parkerp@... [BITX20] <BITX20@...> wrote:

?

(I think Bill might have mentioned this on Soldersmoke but I can't find the reference)


My Bitx has C102 as 47pF and no carrier frequency adjust trimmer (C103).?

It sounded very slightly bassy, lacking highs. ?The area around 2kHz is important.?


Most people wouldn't notice it but at QRP your audio band pass needs to be optimum to be heard on SSB.?

In short, I got better audio by removing C102's 47pF chip capacitor and substituting 27pF. ?Either a 27pF disc ceramic or 22 + 4.7pF. ?


Reports from other stations have been complimentary.?

This video discusses these topics in detail.?


73, Peter VK3YE
 

Yes, could have done that. ?

But wasn't bothered to remove the board when the capacitors could just be soldered on the top side!?
 

Is it possible that the bandpass filter characteristics with the 47pF capacitor are a remnant of the BitX20's USB operation?? If so, would adding future bands like 60m or 20m sound better with the 47pF capacitor or would the characteristics be the same regardless of which sideband is used?

Cheers,
John VA7JBE
 

John - depends on if the filter's bandpass is symmetrical OR the mixing scheme is kept so the IF is not subtracted.?

With USB you'd need to drop the carrier oscillator approx 6 kHz. ?Provision is made on the board for a series inductor for that purpose. ?This would need to be carefully set to provide reasonable audio.?

One way around this (especially if using a DDS) is to subtract the IF by having the VFO on the high side. ?Eg for 5 MHz the DDS would be around 17 MHz and for 14 MHz around 26 MHz. ?Then you wouldn't need any USB/LSB switching as it would always be right due to the mixing.?
 

Thanks Peter,

I've ordered mine with the DDS so to simplify matters I'll probably do the work in the firmware and adjust C102 as you did (or add a variable capacitor and really dial it in.)

Cheers,
John VA7JBE

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