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Set frequency alignment

 

Roland,

That diagram has been wrong for a long time.? I used it at first to explain the sideband reversal when I was corrected by one of the original contributors to the BITX40 and later the uBitx.? Many people are using the original software without the modifications.? If there has been a sideband reversal, it is most likely the BFO is set above the SSB filter midpoint and reverses the sideband in the demodulator stage.

I have found that some of the documents for the BiTx series of transceivers have errors.? I try to verify with theory or inspection (of the circuit or code) to verify the information.? I had asked it to be corrected in the past.? The comments in the original v6 code agree with my prior post:


/**
?* The uBITX is an upconnversion transceiver. The first IF is at 45 MHz.
?* The first IF frequency is not exactly at 45 Mhz but about 5 khz lower,
?* this shift is due to the loading on the 45 Mhz crystal filter by the matching
?* L-network used on it's either sides.
?* The first oscillator works between 48 Mhz and 75 MHz. The signal is subtracted
?* from the first oscillator to arriive at 45 Mhz IF. Thus, it is inverted : LSB becomes USB
?* and USB becomes LSB.
?* The second IF of 11.059 Mhz has a ladder crystal filter. If a second oscillator is used at
?* 56 Mhz (appox), the signal is subtracted FROM the oscillator, inverting a second time, and arrives
?* at the 11.059 Mhz ladder filter thus doouble inversion, keeps the sidebands as they originally were.
?* If the second oscillator is at 33 Mhz, the oscilaltor is subtracated from the signal,
?* thus keeping the signal's sidebands inverted. The USB will become LSB.
?* We use this technique to switch sidebands. This is to avoid placing the lsbCarrier close to
?* 11 MHz where its fifth harmonic beats with the arduino's 16 Mhz oscillator's fourth harmonic
?*/

The one difference is the reason for the BFO frequency selection.

73
Evan
AC9TU
 

Thanks Evan.?

I finally took the time to work through the math, and now I see that the original software requires clk#0 to be set near the lower corner of the crystal filter passband.? If clk#0 gets set near the upper corner, the radio still works, but the sense of the LSB/USB buttons is reversed.? ?In my software I changed the clk#1 frequencies based on the diagram, so I need to set the clk#0 frequency near the upper passband corner to get the sense of the buttons correct. The radio seems to work fine anyway.

I've not tried to characterize the skirts of the crystal filter passband to see if there are any significant differences that might make one approach perform better than the other.? ?Might be a project for another day.??

73

Rolan

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