How do you connect an external frequency reference to a FLEX-6000?

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  • Updated 6 years ago
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Are there any such indications for external disciplined sources?

Note: This topic was created from a reply on the How do I know that the GPSDO is providing frequency stability and accuracy for th... topic.
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philip.theis, Elmer

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Posted 6 years ago

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Tim - W4TME, Customer Experience Manager

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If you want to connect an external frequency reference, it needs to be a 10 MHz sinusoidal source whose signal strength is ~ +5 dBm at 50 ohms.

When the FLEX-6000 powers up it, if there isn't a GPSDO installed, it will look for a signal on the 10 MHz In connector. if it finds a clock signal, it will use that as opposed to the internal XO.

To answer your main question, there is not an indicator on the radio or in SmartSDR that the radio is using an external clock source.
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Steve - N5AC, VP Engineering / CTO

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Official Response
Phil,

On boot, the radio prefers references in this order:
1. External 10MHz reference
2. Internal GPSDO
3. Internal oscillator (OCXO or TCXO)

If the radio loses lock due to a reference disconnection, etc. the radio will search for any reference that will give it a lock. So if you are using an external reference and it is applied at power up, it will be used. But if you disconnect it, the radio will hunt for something else, find the internal OCXO/TCXO and then will lock to this reference. (I've attached the flow chart for those that are pedantically inclined)

Since this could be undesirable, provisions have been made to have a selection of reference. So you could select the external reference and if it goes away, the radio would just stop working until it was reapplied. The radio will not operate without a reference of some kind.

Today, the radio knows which reference it is using, but this is not sent to the GUI today. I would recommend adding an "idea" here for this purpose. We will be using the popularity of ideas here as input for the order to add features.

Steve

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John Stuart KM6QX

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I did some frequency stability tests on my 6700 (which doesn't have the internal GPSDO). I normally run my 6700 referenced to an external 10 MHz ThunderBolt GPSDO. When I disconnected this external 10 MHz signal, I was suprised to see fluctuations in the 600 Hz CW tone when running on the internal OCXO. (see graphs below).

My test signal source was a GPSDO locked signal generator with uHz resolution. I used a separate GPSDO (HP) for the 6700 so the references were not self correcting.

Can anyone explain why there is so much tone fluctuation when using the internal OCXO?

The 2nd graph was done several hours later, so the internal OCXO had warmed up and drifted.

John Stuart, KM6QX
Lafayette, CA

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Steve - N5AC, VP Engineering / CTO

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I've not looked at a graph like this for an OCXO before so I don't know what is typical. Is your concern that it deviated 10mHz over 4 minutes or that is was not more monotonic? If I had to guess, I would say that either the oven is cycling, but the crystal is not a constant temperature yet or there may be a vibration from somewhere coupling to the OCXO. If this is atypical, we can investigate.

Steve
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John Stuart KM6QX

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A communications grade OCXO should be very monotonic. In the test below, the 6700 is using a TBolt GPSDO reference, and the signal generator is using a HP GPSDO. For the last 4 minutes, I had disconnected the TBolt GPS antenna so the 10 MHz ref signal was from its cheap TCXO. The 6700's OCXO should do the same, if not better, i.e. monotonic within a mHz.

I'll let my 6700 run (OCXO stabilize) for 12 hours and then repeat my previous tests.

John, KM6QX

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John Stuart KM6QX

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I did a repeat test after my 6700 (without GPS option) had warmed up its OXCO for 10 hours. I got the same results; with an external GPSDO 10 MHz reference signal, the CW tone is extremely steady, but when using the internal OCXO the variations in the 600 Hz tone indicate the 6700's 'VFO' is not as stable as it should be.

John Stuart, KM6QX
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philip.theis, Elmer

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Nice to know the external source is automatically sought. When my network issues are solved I'll attach my Disciplined source.