Since I just messed with the Tiny 13A microcontroller in the previous installment, I decided to mess with it a little more and add the parts that produce the sidetone for CW and tune mode. The instructions say that, to enter tune mode, quickly press and release the PTT switch once, and then further presses of the PTT switch will key the transmitter with a 600 Hz tone, so you can read the SWR and adjust your ATU. To exit tune mode, quickly press and release the PTT switch again. CW mode is activated by sending the letter H with the PTT (or with a key you’ve plugged in). That will activate the sidetone as well. To reset the rig to SSB, turn the rig off and then on again.
The 600-Hz sidetone is produced in the Tiny 13A and is output as a square wave on pin 3. R33 attenuates the output a bit and then the signal passes through a low pass filter made up of L2, C46, and C47 to smooth it into more of a sine wave (C48’s purpose is to filter out any harmonics). R47 and R49 make up a voltage divider, and the signal from the junction of those two resistors is fed into the LM386 op amp.
The LM386 is designed specifically for audio applications and contains internal components that fix its gain at 20. While it’s possible to add external parts to modify the gain, the designer hasn’t done that here. The sidetone is fed into the inverting input of the 386 while the audio from the receive preamp is fed into the non-inverting input. As far as I can tell, there should never be a time when both are active at the same time. I did notice that there is no volume control for the sidetone, and mine sounds kind of loud, so I may need to adjust the values of R47 and R49 to try to attenuate that somewhat, but I’ll wait until the rig is done to make sure I still have a problem before trying to fix it (installing R50 and the other devices up the chain may take care of the problem).
The output of the LM386 appears to be sent through another low pass filter before being sent out to the speaker.
To build this section, install the following parts on the board:
- R33, R35, R47, R49
- C46, C47, C48, C56, C58, C62, C63, C64, C70, C72
- L1, L3
- V3 (the volume control potentiometer with the on/off switch)
Once again, I installed header pins for the connections to the volume control and the speaker. It might be possible to test this part of the circuit without V3, but it’ll be needed soon anyway so best to hook it up. Here’s what my board looks like after all that:
With the connectors for the volume control and speaker:
Obviously, the easy way to test this part of the circuit is to plug a speaker in, put the rig in tune mode, and see if you hear a tone. If you’re interested in more detail, you could use an oscilloscope to examine the audio oscillator signal at different places. I decided to haul out my old Tektronix 454 scope and do just that. First, I measured at U5 pin 3:
Next, at the junction of R33 and L3:
Next, at the junction of R47 and R49. Looks a lot more like a sine wave but is greatly attenuated (vertical scale is 50mV/div):
Finally, at the speaker terminal (vertical scale is 0.5V/div):