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Author Topic: Drift  (Read 385 times)
K6GC
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« on: October 03, 2007, 08:41:06 AM »

What is an acceptable drift for a radio these days?  I have an Elecraft K2/100 and it is drifting about 100Hz dead cold to full heat in a long QSO.

I called the factory and they said this is "typical" for modern equipment.
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WB2WIK
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« Reply #1 on: October 03, 2007, 09:01:00 AM »

100 Hz from cold to warm is acceptable.

The only time 100 Hz drift would really matter is for PSK31 or one of the weak signal digital modes.  For SSB-CW work, who cares?

It's a whole lot better than the "best of the best" analog equipment of 25-30 years ago...

WB2WIK/6

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WA3SKN
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« Reply #2 on: October 03, 2007, 09:04:07 AM »

3 Parts-per-million is typical, with about 7 parts-per-million for FM only rigs.
Thats after 10 minute warm-up.  The "million is Mhz, so 14 Mhz freq would be 3 times 14, or 43 cycles.
Note there is usually a warm-up period before making the measurement.
Some radios are better, but this is typical.
This is MUCH better than "The Good Old Days"... some tube rigs drifted all over the place!

-Mike.
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KE3WD
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« Reply #3 on: October 03, 2007, 09:07:31 AM »

You could brew up a temperature controlled oven for the clock oscillator and yield instant-on cal, but that really wouldn't be worth the expense and bother for this purpose.  Just turn the rig on early.  

Years back we used to leave the rigs on 24/7 to avoid temp change recal issues.  Steady state, I think components lasted longer that way, too.  

You *may* be able to rig up a situation where only the xtal oscillator is powered at all times with a little study of the schematic and some thought.  That might work about as well as a xtal oven would.  

Me, I wouldn't be overly concerned about it at all, though.


KE3WD
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N3OX
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« Reply #4 on: October 03, 2007, 09:12:56 AM »

"The only time 100 Hz drift would really matter is for PSK31 or one of the weak signal digital modes"

Most people on PSK are using automatic frequency tracking anyway.

So scratch that one off.

Probably would be a problem on WSJT or something...
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AA4PB
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« Reply #5 on: October 03, 2007, 09:18:33 AM »

The IC-756PROIII is spec'd at +/- 0.5ppm after 1 min warm up over a temp of +32 to +122 deg F. That's 0.5Hz per MHz or +/- 15Hz at 30MHz.

Most modern transceivers lock everything to a TCXO or crystal reference oscillator. The K2 as I recall, uses a free-running VFO so I wouldn't expect it to be as stable. That's especially true with the 100W version because of the added heat. The K2 uses the crystal reference oscillator only during the VFO calibration process.
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AA4PB
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« Reply #6 on: October 03, 2007, 09:21:32 AM »

PSK31 AFC isn't going to handle 100Hz of drift. I think its somewhere around +/- 10Hz.
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AA4PB
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« Reply #7 on: October 03, 2007, 09:50:14 AM »

By the way, I don't see where anyone specs their radios drift from cold. To be fair you should measure it after a few minutes of operation.

Also note that most mfgs specs are over temperature and they are much more stable when setting on the desk in your air conditioned home. To test to specs you'd have to have a temperature chamber.
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G4AON
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« Reply #8 on: October 03, 2007, 10:03:52 AM »

If you post your question on the Elecraft Reflector you will get some comparison figures from other K2/100 owners. Has yours been modified with the PLL (thermistor) and BFO modifications? Elecraft part numbers E850138 and BFOMDKT.

My ten Watt K2 is "fairly stable", warm up figures on various bands are posted at:

http://www.astromag.co.uk/k2/

A typical "non TCXO" HF transceiver is stable to around +/- 10 parts per million, which equates to +/- 140 Hz at 14 MHz, often not specified over a temperature range. The specifications are usually vague, what they "don't" specify is often what you get!

Dave
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K7PEH
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« Reply #9 on: October 03, 2007, 11:50:36 AM »

I suppose there are a lot of experiments that can be performed to discover the actual source of the 100 Hz drift.  I would imagine that heat from warmup affecting some dimensional quality of a capacitor might be the first culprit to discover.

As a physicist to be, Dan should take this on as an experiment.  Take this guys rig and discover the actual cause of the drift and answer such questions as:   Is it temperature dependent?  Probably so but does the drift stop because the temperature has reached steady-state or would the drift increase with further increases in temperature.  If a pet cat sits on the rig to keep it warm then is the drift motion quicker to reach steady-state.  Is the drift amount frequency dependent or is it 50 Hz on some bands and 150 Hz on others?  What if you freeze the rig down to zero degrees centigrade, how does this affect drift and time to reach steady-state.
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AA4PB
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« Reply #10 on: October 03, 2007, 12:42:47 PM »

A radio with a free running VFO is never going to be as stable as one locked to a VCXO. On the other hand, a radio with a VFO locked to a VCXO is never going to have as low phase noise as one that has a free running VFO. That's where Elecraft chose to make the trade off.

When they told you it is as stable as the typical modern radio I'm sure they meant the typical modern radio with a free running VFO. Actually, the K2 is better than most in that category.
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K6GC
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« Reply #11 on: October 04, 2007, 02:43:25 PM »

I'd like to thank all you fellows for responding to my query !

I feel much more informed now and able to judge the situation more appropriately.

73,

TR, WB6TMY
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