On 13 Mar, 17:52, "colin" wrote:
"Anthony Fremont" wrote in message
...
Hello all,
I was playing around and saw that my junk box had all the parts so I
started tossing this together:
http://newenglandqrp.org/files/w1aw-receiver.jpg
The problem is (well I think it's a problem) is that I'm all the way down
to a 10pF cap for the crystal trimmer and the highest frequency I can get
out of it is still less than 3580kHz. Pleae correct me if I'm wrong, but
I'm thinking that the 20uH inductor is supposed to pull the colorburst
crystal high in frequency then the adjustable cap should be able to tweak
it back down. Higher capicitance does result in even lower frequency. I
made the 20uH inductor by wrapping 17 turns on an old amidon FT50-61
ferrite toroid. I don't have any way to measure the inductance, but by my
calculations it should be approximately 20uH. Is the ferrite saturating
and messing things up? I see that touching the 20uH inductor it will
cause the frequency to shift down as much as 4-5kHz.
I believe this is direct conversion and that the crystal should exactly
match the desired station (3581kHz) I want to recieve, that's right isn't
it? Maybe this is close enough? I have to wait til tonight to see if I
can actually hear anything.
I mostly tinker with PIC chips so this is pretty much out of my league. I
just wanted to throw it together and listen to the nightly report. It
seems to be working as I can touch the 7.8uH coil (40 turns on a T50-2
toroid) and I get increased noise out of pins 4 and 5 on the NE602 (hey
it's what I had in the box ;-). I haven't added the 386 yet, I just
wanted to test the front end first. I'm cramming it all on a RS 276-259
pc proto board, but the layout is coming along pretty well for not
planning it out. ;-) Thanks for helping.
maybe the Q of you inductor is too low, try an air core,
needs lots more turns, a trimmer core as a core is good too.
Also you should test it with the crystal shorted,
then when you get close to the frequency you want put the crystal back in.
Colin =^.^=
My W1AW receiver uses the crystal with just a 5-47 pf variable
capacitor...no inductor, and it nets right on frequency. Try shorting
the inductor and see if this gets you closer to the required
frequency. Not all color burst crystals were created equal. If you
have another crystal you might want to try it.
Your ferrite will not be saturating at the small amount of signal you
are sending through it as part of an SA-602 oscillator.
Arv - K7HKL
_._