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PostPosted: Sat Jan 19, 2013 5:13 pm 
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Joined: Tue Oct 02, 2012 4:58 pm
Posts: 38
Jim,
I have my receiver working on all 3 bands now and FINALLY have MPLAB able to build a project without spitting out an error about not finding a path to the tools. So in the meantime I was going to try to attack the off frequency problem while I wait for a replacement final RF transistor as I had 1 bad one I found when I started to bring up the transmitter. I noticed that Bill N4BKT measured a LO frequency of 39.3223 MHZ in his correspondence with the group. When I look at my LO on pin 4 of the SI570 with a digital oscilloscope it is about 500mv p-p and has a lot of jitter. Putting the scope in averaging the frequency is still bouncing around from 38.7-39.8Mhz. I am calling it 39.3MHz for a readout. This number is VERY close to the number you have in the spreadsheet for the 49.99 MHz LO. I guess my question is do you think my LO has a problem? The receiver seems to work fine but I wouldn't expect this much movement in the waveform. My radio is off frequency by about 12KHz low meaning I read for example 28.388 for a frequency that is really at 28.400MHz on 10 Meters and about the same amount on the other 2 bands. Any thoughts?
Thanks,
Eric W2EJC


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PostPosted: Sun Jan 20, 2013 12:16 pm 
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Joined: Tue Oct 02, 2012 4:58 pm
Posts: 38
Here is an example of what the output of the LO looks like. In this picture the freq measured by the scope is 39.1 MHz but it moves from this being about the low up to 39.8 MHz depending on just when the picture/sample is taken. It amazes me that Bill can read his LO frequency out to 4 decimal places which makes me feel there is something amiss with mine. The radio is receiving beautifully however. Any comment on the quality of the LO waveform would be welcome from other builders as I would like to know what they are seeing.
Thanks, Eric


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PostPosted: Sun Jan 20, 2013 5:06 pm 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
Hi Eric,

If the frequency was actually moving around over a MHz, your receiver would not be working very well, if at all. The waveform on your scope looks like a nice healthy SI570 output. With a good enough frequency counter, the frequency is stable to 7 or 8 digits. Your scope says signal amplitude low which means that for some reason the scope is have trouble counting the zero crossings, which shouldn't be the case given the fidelity of the input signal.

I suspect that there's nothing wrong with the 570 but that it's an issue with how you are measuring the frequency.

73's
Jim WA2EUJ


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PostPosted: Sun Jan 20, 2013 9:00 pm 
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Joined: Tue Oct 02, 2012 4:58 pm
Posts: 38
Thanks Jim,
Thanks to your spreadsheet, a few stabs at calculating the Fxtal frequency and getting the new base and steps loaded into the PIC, I now have a receiver that is dead on frequency for both USB and LSB on the 10 and 12M bands. I haven't tried it on 6M yet but have no reason to doubt that the frequency will be right on there as well :D It seems nice to be learning a little about programming. At least being able to read and write the code to the chips is a major step for me. There is a fair amount of jitter on the LO signal and it changes constantly on the scope and I didn't try to measure it with a freq counter but I will. I am just really pleased that I finally have the MPLAB software working. I am hoping to get the transmitter going this week when the new RF MOSFET gets here. I am sure that will bring more challenges and opportunities to learn more.
Thanks for all your help and patience on this project,
Eric


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PostPosted: Tue Jan 22, 2013 7:28 am 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
Hi Eric,

I'm glad that you're making good progress. Keep us posted.

Jim


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