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Author Topic: Frequency readout  (Read 1794 times)
W0WCA
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Posts: 38




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« on: April 16, 2014, 12:04:26 PM »

Recently participated in the ARRL frequency Measuring Contest.  It was an interesting and educational exercise and I enjoyed the experience.  Actually did better by far than I expected! 
My question for the forum is this though: since most, if not all current transceivers use an eight digit frequency readout (right down to the very last Hertz) how accurate is the last digit?  Example: on my 14 year old T/T Jupiter I find that the last digit (as zero beat against WWV) seems to be 4 Hz high (and this seems to vary a bit depending on which WWV frequency that I tune to).  I’m thinking this is not too bad for a 14-year-old rig.  Am wondering what your experience is?  Are the current radios “spot on” or are they just real close?  Are the $10,000 rigs better in this regard that the $2k to 3K radios?
Just wondering?
Regards,
Bill, W0WCA

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AA4PB
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« Reply #1 on: April 16, 2014, 12:28:37 PM »

No amateur grade receiver is "spot on" to the last Hz. If you are consistently 4Hz off I'd say that's pretty darn good. Oscillators drift with time and temperature so the dial reading can change over time. Some models have an optional TCXO (temperature compensated reference oscillator) that will improve stability and will be more accurate if properly calibrated. They still are not "spot on".

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K0OD
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« Reply #2 on: April 16, 2014, 01:16:12 PM »

I've used my Flex-5000a in FMTs and its last digit IS spot on, especially when the radio is calibrated using the built-in Flex method which takes just a few seconds. To estimate to 0.1 Hz I use the Flex's scope. Doppler and other factors complicate things. And just hearing all FMT beacons can be difficult. Flex radios are very popular in FMTs.

I see your results which are excellent. Two of your three readings were within 1/2 Hz.
http://www.b4h.net/fmt/fmtresults201404.php

Some older radios give the software option to turn on an additional digit on the readout. I think my old TS-850 and TS-440 can do that. But that digit is pretty close to random. Fifty years ago, the one-KILO-Hertz Collins readout was remarkable. Now one Hz accuracy is common for new models.

Quote
Are the $10,000 rigs better in this regard that the $2k to 3K radios?

I think $600 Flex-1500s and some other SDRs may be on the mark. Elecraft K3s are good to about 1 Hz. I'm reading that stock Anan SDRs aren't terribly accurate. How good is the new Flex 6XXX series? Those FMT Scores suggest it's no better than old Flex radios. 

You get better with FMTs with practice. Join the Yahoo FMT group. 
 
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M6GOM
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« Reply #3 on: April 17, 2014, 04:39:04 AM »

Most transceivers without a TXCO are accurate to +/-5ppm because of the variances in temperature between calibration and where you're operating. That converts basically to anything from 0Hz to +/-5Hz per 1MHz. So on 21MHz WWV they can be up to 105Hz out. A TXCO which compensates for drift typically brings this inaccuracy down to +/-0.5ppm giving an accuracy of 0Hz to +/-10.5Hz on the 21MHz WWV.

I will point this out if it isn't mentioned in the ARRL article: Transceivers drift with temperature. When checking calibration or calibrating it you basically want to have left the radio at room temperature ideally overnight and had it running for at least an hour before starting to calibrate. You will find that the next time you turned the radio on a day later that if you went back to WWV straight away it may be off frequency a little at first but an hour later be pretty much bang on. Many people make the mistake of calibrating a radio that is stone cold then next time they use it wonder why its drifted off frequency after a few hours use.

My TS480 is about 80Hz out on 10m. I don't worry about it as I've always tuned SSB so it sounds right rather than what is on the dial like many people do so the fact that someone I'm listening to has to be tuned to say 28.499.92 instead of a nice tidy 28.500.00 doesn't bother me. It bothers a lot of people for some reason and they seem to be under the illusion the last figures should be .00, .25,.50,.75 but the one thing I would like to say is they're assuming that the station they're listening to is dead on accurate. For all they know they might be talking to me where 28.500.00 on my dial would actually be 28.500.80 on a dead on radio......
« Last Edit: April 17, 2014, 04:43:57 AM by M6GOM » Logged
K2GWK
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« Reply #4 on: April 17, 2014, 09:47:02 AM »

I bet a K3 with the K3EXREF option and a GPSDO will be very close. The option allows you to phase lock the internal reference oscillator on a K3 with the 10MHz out of a GPSDO (GPS Disciplined Oscillator).
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W0WCA
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« Reply #5 on: April 17, 2014, 06:11:28 PM »

Gentlemen:
Thank you so much for your input.  You have provided me with an excellent frame of reference upon which to base my expectations.  Furthermore, the specific references to the calibration methodology of the Flex 5000 and the K3 with the K3EXREF are both excellent “foods for thought”!
For my own part, I have let the trimmer cap in the TCXO on my Ten Tec Jupiter get inside my brain.  Additionally, I have done my best to get into the mind and heart of the TCXO.  It took some courage and perhaps foolhardiness but after two hours of tweaking, I managed a perfect null or zero-beat on 10MHz WWV;  this null against the radios internal “spot” function.  We are two Hertz off on 5MHz and 15Mhz.  So, now it comes down to the accuracy of the “spot” tone in the Jupiter.  We will have to wait for the next FMT.
Again, Thanks for the input!
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M6GOM
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« Reply #6 on: April 19, 2014, 04:52:22 AM »

I personally wouldn't be bothered. Whilst you could be obsessed with getting it absolutely spot on frequency the majority of the amateurs you'll contact won't be so diligent. You'll just end up having to tune to their frequency anyway.

If you do digimodes then not having drift is important. If you're doing WSPR modes then accuracy is important. For all other modes "near enough" to ensure you don't TX out of band suffices. Hell look at some of the altoids tin kits - they have no frequency dial on at all!
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