External GPSDO not accepted be Flex 6500.

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  • Updated 5 years ago
I am unable to get the Flex 6500 to use an external GPSDO on the external 10 MHZ input. I am supplying a 5 dbm signal to the radio. When I check the frequency accuracy of the radio it shows -7hz on 14.1mhz which is the TCXO in the radio.

My frequency counter is using the GPSDO for its clock so if the radio uses the same signal it would have to show the exact frequency.

Do I have to Install the GPSDO option in the 6500 GPS Setup Tab for it to use an external clock?

Don
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ai6re

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Posted 5 years ago

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K6OZY, Elmer

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If i recall, the radio only checks for external GPSDO at power up.
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Ned K1NJ

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Yes, it checks for the 10 MHz input first if a GPSDO is not installed, then
defaults to the internal oscillator if it's not found.

There is some confusion concerning the 10 MHz input connector.
The on-line pdf shows it in a different place than on the actual
rear panel. So make sure the correctly labelled connector on the
panel is the one you are using. My 10 MHz source worked immediately
with no fuss. The radio just finds it and uses it.
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ai6re

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Ned, I am using the one labled on the radio as 10mhz in. It is the top left RCA on my 6500. Do you know what level signal you are providing to the input?
I did not want to overload it so I am just sending 5dbm.

I have powered off and removed the power to the radio but it still shows the TCXO in use. How do you know it is using the external signal?

Regards, Don...
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ai6re

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I just raised the GPSDO signal to 6dbm and the radio switched to the external clock and is working fine. It now shows exactly on frequency with the GPS so the solution is to update the spec for external clock signal to 6dbm for dependable external use.

This problem can be closed as an update to the specification for external clock signal level.

Thanks, Don
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ai6re

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I was mistaken. The problem remains and at 50mhz the TCXO shows -24hz so the radio is still not using the external signal. This is the same amount of error I see with both Rubidium Clock and GPSDO calibrated frequency counter.

My questions remain: Am I using the correct socket; top left RCA next to the ACC DB15)?
If others have this working then how do they verify the radio is using their external signal?
What is the signal level being used when it works?
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Ned K1NJ

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Don,
When I first tried the radio I didn't an Rb source. The radio was
"off" by 5 or 10 Hz. at 10 MHz WWV. I asked about the calibration method
to get the frequency zeroed, but was told that would probably come along
in a future release. Then I added the Rb standard. I don't know its exact
output power, but it's < 10 dBm.
I checked frequency by using two slices listening to WWV but offset by
a fixed amount e.g. 500 Hz. I use one slice USB and the other LSB so each
hears the same note and listen to the beat. This would work with any stable signal I suppose. Without the 10 MHz reference, its off several Hz. With the reference
the note sounds more like a phase shift and is well under 0.1 Hz.
The connector seems to be the same one you are using, upper row, next
to the acc'y connector near the middle of the panel.
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Graham / KE9H

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Don:

1.) The red RCA type connector at the upper left of the RCA connector cluster is the correct input port for an external 10 MHz Input. It is properly labeled on all production radios.

2.) The level must be a MINIMUM of +4 dBm for the input to function.
This is 1.0 Volt peak to peak. It will work with either sine or square wave inputs, although a sine wave input will be quieter with respect to 10 MHz harmonics.

3.) The radio will only look at the 10 MHz input for input selection while it is booting, so you will need to reboot the radio after providing the proper input signal, before it will start using it. If the signal disappears while the radio is running, it will switch back to one of the internal sources, but will not attempt to re-select the external 10 MHz input until the next time the radio is booted (turned off then back on, or power cycled.)

If this does not work for you, please contact service / support.

--- Graham / KE9H

==
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ai6re

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Graham,
You had the information I needed to get this working.
1 volt across 50 ohms is .02 watts or 13dbm. That is almost 8 times the power of 5 dbm at 50 ohms, stated on an earlier post to the community by Tim.

Once I provided the 13dbm signal to the radio it works perfectly and the counter shows exactly the same as the radio.

This problem can be closed as an update to the specification for external clock signal level to a 13dbm / 50ohm or 1 Volt peak to peak sine or square wave..

Thanks, Don
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Graham - KE9H, Employee

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Don:

I am glad to hear you are up and running.

Assuming a sine wave:

1 Volt peak-to-peak (the minimum input) is 0.357 V RMS is 0.002 Watts or 4.1 dBm into 50 Ohms.

1 Volt RMS is 2.8 Volts peak-to-peak is 0.02 Watts or +13 dBm into 50 Ohms.

To convert from peak-to-peak to RMS for a sine wave, divide by 2.8. It is the RMS value you use to calculate power in P=E^2/R or the dBm power calculations.

Either level should work. The acceptable input level is 1 Volt peak-to-peak up to 3.3 Volts peak-to-peak. Into 50 Ohms, this would be about +4 dBm to about +15 dBm.

Best regards,
--- Graham /KE9H

==
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ai6re

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Graham,
Thanks for the correction to my measurments. You are certainly correct in using the RMS value and I now understand the error in my measurements. I was just using a scope and a variable attenuator and forgot some long ago education on power.

Thanks, Don... AI6RE