A
Anthony Fremont
- Jan 1, 1970
- 0
First off, big thanks to everyone for the overwhelming amount of help I
received.
I've been playing around with LC oscillators and other RF like things lately
and I decided to try my hand at making a superhet WWV receiver. Not being
an analog kinda guy I was plagued with too much drift at 9.545 MHz, amongst
other things. Being a PIC kinda guy, I thought it might be neat to apply
one here. ;-)
So I took a couple of variactor diodes and added them to the oscillator. I
tapped the oscillator buffer output, fed it thru a Schmidt trigger to square
it up and then into a PIC to clock TMR1. Over a 4mS (actually 4.096mS), I
count pulses with the 16 bit timer. This gives a decent count without
overflowing the 16 bit timer. Every 20mS I look at the counted frequency
and then "dink" the duty cycle of the hardware PWM. The PWM output is fed
thru a low-pass filter to smooth it out and then applied as reverse bias to
the varactors.
The end result is that the oscillator "locks" within a second or two of
power on and then stays within 100-200Hz of the correct frequency from there
on out. Not fantastic stability, but should be entirely usable for AM
reception. Of course there is lots of room for improvement, but I thought
it neat anyway. The PWM could be upped to 10bits and the frequency
counter gate period increased. This would allow _much_ more precise
frequency control than I presently have.
Here is a pic of the circuit constructed on breadboard (800KB):
http://i55.photobucket.com/albums/g143/afremont/DSC00885_edited.jpg
The top left is the oscillator, just to the right of that is the buffer
stage. Further to the right is the NE602 mixer that combines the signal
from the antenna/preselector and the LO. The detector is not finished yet,
but the receiver is hearing on 10MHz, just not very good yet. ;-) The
bottom half of the board contains the PIC which is clocked by a 4MHz crystal
and the 74HC14 gyrator...er..um hex inverting Schmidt trigger.
Just thought some of you all might get a kick out of it. Thanks for
looking.
received.
I've been playing around with LC oscillators and other RF like things lately
and I decided to try my hand at making a superhet WWV receiver. Not being
an analog kinda guy I was plagued with too much drift at 9.545 MHz, amongst
other things. Being a PIC kinda guy, I thought it might be neat to apply
one here. ;-)
So I took a couple of variactor diodes and added them to the oscillator. I
tapped the oscillator buffer output, fed it thru a Schmidt trigger to square
it up and then into a PIC to clock TMR1. Over a 4mS (actually 4.096mS), I
count pulses with the 16 bit timer. This gives a decent count without
overflowing the 16 bit timer. Every 20mS I look at the counted frequency
and then "dink" the duty cycle of the hardware PWM. The PWM output is fed
thru a low-pass filter to smooth it out and then applied as reverse bias to
the varactors.
The end result is that the oscillator "locks" within a second or two of
power on and then stays within 100-200Hz of the correct frequency from there
on out. Not fantastic stability, but should be entirely usable for AM
reception. Of course there is lots of room for improvement, but I thought
it neat anyway. The PWM could be upped to 10bits and the frequency
counter gate period increased. This would allow _much_ more precise
frequency control than I presently have.
Here is a pic of the circuit constructed on breadboard (800KB):
http://i55.photobucket.com/albums/g143/afremont/DSC00885_edited.jpg
The top left is the oscillator, just to the right of that is the buffer
stage. Further to the right is the NE602 mixer that combines the signal
from the antenna/preselector and the LO. The detector is not finished yet,
but the receiver is hearing on 10MHz, just not very good yet. ;-) The
bottom half of the board contains the PIC which is clocked by a 4MHz crystal
and the 74HC14 gyrator...er..um hex inverting Schmidt trigger.
Just thought some of you all might get a kick out of it. Thanks for
looking.