- peaberry CW ouput 10uS/div
- peaberryCW_txmit.jpeg (23.95 KiB) Viewed 14464 times
The modulation here is about 10uS period or, more precisely, 96kHz as determined from my station receiver. I first thought this was related to the 96kHz peaberry audio rate but this is not the case as the modulation frequency (but not the amplitude) changes as the difference between LO and tune frequency changes. I can change this now that I can change and rebuild the source code. In fact, the modulation frequency is 4x the difference between LO and tune (in the unmodified PeaberryCW this is 24kHz). I feel certain now that this is due to the way the SN74CBT3253 IQ modulator works. Each cycle of the LO clock samples the I and Q lines 4 times: I, Q, -I and -Q. The transmit signal starts with I=1 and Q=0 and then phase rotates toward I=0 and Q=1, and so on. As long as the phase is such that I or Q is near maximum, the output of the mixer will be maximum. However, when the phase is near 45deg, I will be cos(45)=0.707 and Q will be sin(45)=0.707. The output of the mixer will then be only 71% of the maximum. This happens for values of phase = 45, 135, 225, and 315deg; i.e., four times per audio cycle and leads to the modulation frequency of 4x the difference between LO and time. Here is a calculation of the mixer output for a LO of 10MHz:
- Calculation of mixer output with 24kHz audio
- Envelope.JPG (35.32 KiB) Viewed 14464 times
and an expansion of the low amplitude part:
- Expanded mixer output
- Envelope_binning.JPG (21.98 KiB) Viewed 14464 times
I thought maybe if I made the LO-tune freq very small then this modulation could be made with a longer period than typical keying dots and dashes but I could not get the Peaberry to xmit with LO-tune differences less than about 50Hz. With this you just get a monstrous 50Hz buzz on the CW tone. More work to be done on this as I get time.
73, Ken KS1H