The AGC is too limited for HF. Strong signals are distorted.
I use the 1350p IF amplifier. It has 60db dynamic range that is not enough. So I added a voltage controlled attenuator to the input of the IF unit.
It is just a npn transistor. Collector is connected to the input. Emitter to ground .Voltage over the transistor is low so it works as a resistor.
By changing base current I can change the attenuation. I can reduce S9 +10db signals to S1signals.
Now when the AGC voltage for the 1350p exceeds 5.5 V the attenuator starts to work. It helps a lot.
Most distortion is gone. However I found the attack time of the AGC could be faster . That would help to increase audio quality.
It seems I found the problem in the software. I use the PIC 16F877. PORTB is configured as input. It is configured to generate an interrupt when one of the pins changes. Now according to the PIC datasheet I should not poll the port while it uses the interrupt. So I disabled interrupts while polling. This seems to help.
No comments:
Post a Comment