Hey guys, I need some help! My Veronica PLL xmtr's frequency is rock steady, but it stays rock steady at 8khz low! This causes me absolutey NO problems whatsoever --- every radio I try locks in on it just fine, even in the fringe areas. But I wonder -- is there a certain number of khz we're supposed to stay from the center of an FM channel? Or are we allowed to roam all over the band, as long as we stay within the band, like in ham radio? In full-power FM you're allowed 2khz either way, but I can't find any reference to frequency tolerance in the "Understanding the Pt. 15 Rules & Regs" booklet from the FCC. Anybody know? Thanks!
No, there is no specified tolerance for part 15 FM as long as the signal and the sidebands remain in band.
If memory serves the tolerance for broadcast FM is +/- 5 kHz (you cited 2 kHz and I could be wrong since its been a while since I paid attention) and it is a good idea to try to stay within this so digitally tuned receivers can lock on properly. If you are satisfied with your -8 kHz then leave it be.
Neil
There should be a trimmer capacitor at the crystal to fine tune it to frequency. If not..add one. 8Khz is not a whole lot, but a concern that perhaps your PLL's crystal has swung off spec and will continue to do so...a slow and painless drift eventually causing the PLL oscillator to not function.
RFB
Thanks Radio8z for the freq. tolerance info.
RFB - Good observation! I already had to add extra inductance to an osscilator coil to get it to lock reasonably quick & easy. (Just stuck a screw in the coil. Presto!) Maybe I'll try a smaller screw, but I'll probably just leave it alone. It's not a problem.
Do frequency meters ever misread? What is the meter's tolerance, I wonder? Can't find it in the meter manual. It's a Radio Shack 22-306. Thanks again, guys!
Unless they (the fcc) changed it, the odd channels are usable as well under part 15 rules.
I don't know why i brought that up, must be lack of R.E.M Sleep lol.
Is this the meter your using rlkocher?
http://support.radioshack.com/support_meters/doc62/62270.pdf
My Uniden BC72 XLT sports a frequency counter if you press certain buttons on the scanner keypad.
Kind of a neat feature but no where near as accurate as what your using.
"Do frequency meters ever misread?"
It's known to happen. Older frequency counters can drift off calibration due to the same reason why a crystal oscillator would drift...the reference crystal in the counter drifts off spec, thus causing it to misread and display an inaccurate frequency count.
Ya I would not worry about the 8Khz offset on the Veronica unit. At least not until a digital receiver starts having problems locking onto the carrier when its modulated. Not sure about the Radio Shack meter's tolerances. At one time products from Radio Shack had not just specifications in the manuals, but a schematic too on the last page.
RFB
First make sure the counter locks on to the signal. If the frequency reading is steady it is OK, if it is bouncing around it is not locked.
The specs from the RS link state "initial accuracy 1 ppm" which doesn't say much and is not usable to determine the accuracy. Usually modestly priced counters without crystal ovens are +/-10 ppm. or thereabouts. This translates to about +/-1kHz in the FM band which is adequate for part 15 stuff.
Let everything stabilize before trusting the readings with a 10 minute "warm up" usually being satisfactory. The reading should not be drifting appreciably where drifting would indicate something is not stable. And don't connect the counter to the oscillator since this can pull the frequency. Connect it to the tx. output if it will lock on the signal.
Here I have a Phase Matrix counter with a crystal oven which is on all the time the unit is plugged in. I check it against WWV about once a year and it stays in spec. I also have a $100 el cheapo counter which, when compared to the PM unit, does drift but the drift from a cold start is usually less than 200 Hz at 10 MHz. Absolute accuracy after "warm up" is generally better than 10 Hz at 10 MHz at stable room temp but I periodically recalibrate it with the PM. This is good enough for general lab work and part15 and ham stuff.
Neil
I have my Motorola RD2012 and HP 8590 calibrated every year at a considerable expense. The RD2012 serves as both my FIM, modulation meter and frequency counter, amongst many other neat things it can do. And the HP serves as my spectrum monitor watching for any spurs and junk that may crop up.
I have a B&K frequency counter I salvaged from a closed down TV repair shop and I rarely use it, but so far it has kept its accuracy within a few ppm's of the specs. Good enough for quick checks and initial designing, but the Motorola RD2012 unit is my be all end all device that tells me what is truly going on.
RFB