I recently got a GNSS receiver along with the antenna from SparkFun, the SparkPNT GNSS Disciplined Oscillator, and the 10 MHz output on the box seems to be 20 MHz and not 10 MHz. The output sine wave also appears distorted when looking at the signal on the oscilloscope, with multiple harmonics. I am local and I’d be happy to drop off the equipment if you’d like to take a look at it.
Odd - Do you mind posting photos of the scope screen(s)?
Please also tell us how you have the 10MHz loaded. Is your ‘scope set to 50Ohm? Or ~1MOhm?
Best wishes,
Paul
This is the signal I see on the scope. This is with 1 MOhm termination. The signal also looks the same with a 50 Ohm termination attenuator.
Thank you @gabriela ,
Very strange. That looks like the sum of a 10MHz square wave with a higher frequency sine wave.
How is the GNSSDO connected to the ‘scope? Are you using the provided SMA-BNC cable?
Is anything else connected? Perhaps a signal generator?!
Please check you have GND connected.
I see FFT at the Top Right of your screenshot. Is that relevant?
Best wishes,
Paul
Hi Paul,
I have tried different SMA cables and have seen the same result. The signal is connected directly from SMA to an SMA/BNC adaptor. There is a ‘GND’ on the green pinout rail but I am not using that at all. The receiver is powered via USB connection to the computer. The FFT on the scope was just to measure the frequency components of the signal, which showed different harmonics at 10, 20, 30 MHz, etc.
Connections used:
- Antenna mounted on rooftop to the receiver with good quality TNC cable to SMA connection
- 10 MHz output SMA to frequency counter or oscilloscope via SMA/BNC adaptor
- Config mosaic USB power to computer (for power)
Thanks for the quick response!
Gabriela
Hi @gabriela ,
OK. Thank you.
Regarding the GND connection: I was wondering if you were using a 'scope probe to study the signal and had forgotten to connect the GND clip? But you are using an SMA-BNC adapter, so that doesn’t apply.
The signal should look like this:
The small amount of overshoot / ringing is due to it not being loaded perfectly.
I have my GNSSDO VCCIO switch set to 3.3V. My 'scope is set to 1MOhm input impedance and DC-coupling.
Could it be a power supply issue? The GNSSDO draws close to 500mA from USB. Slightly more if you have an SD card inserted. Perhaps your computer port can not deliver enough current? Please try powering it from the supplied wall adapter using the supplied USB-C cable. Does that improve the signal?
If that does not fix it, we’ll need to replace the unit for you. Please let me know and I will put you in touch with the support team.
Thank you,
Paul
Hi Paul,
I’ve powered the system so that it is directly powered from the wall outlet with the included USB-C cable and I am still measuring 20 MHz on the frequency counter. Below is the signal on the scope. It looks slightly less distorted but still not what you are showing.
I’m thinking we may need to send this unit back to you. My company is local so I can drop it off anytime. Let me know how you’d like me to to move forward.
Best regards,
Gabriela
Hi @gabriela ,
Thank you for the update.
It looks like you have your 'scope timebase set to 5ms per division? Can you please try again with it set to 50ns per division - like in your first screenshot.
Thank you,
Paul
Is the frequency counter a separate unit - fed by the same BNC cable as your 'scope?
What type of instrument is it? Could it be generating a sine signal?
To be safe, please ensure you have the GNSSDO 10MHz output connected directly to the 'scope, only to the 'scope, and nothing else… 
Thanks!
Paul
Hi Paul,
The frequency counter is connected separately so each time I take a measurement the signal is connected directly to either the scope or frequency counter. The frequency counter I’m using is from the SRS FS740 frequency reference, which also has a frequency counter function. I’ve double checked all of my connections and I’m still measuring 20 MHz output and a distorted signal. Below is an image from the scope with the 50 ns time division scale. It does appear that the feature has a frequency of 10 MHz, but not the square wave that you’ve shown. I’ve taken out an extra BNC adaptor so the signal looks slightly different from the images I sent earlier today.
Best,
Gabriela
Hi Gabriela (@gabriela ),
Thank you for the update.
I am concerned that you somehow have an output from the SRS FS740 Frequency Reference connected to the 10MHz output from the GNSSDO.
To my eyes, the waveform on your oscilloscope looks like the sum (addition) of a 10MHz Square Wave with a higher frequency sine wave. To me, it looks like the sum of the 10MHz square wave output from the GNSSDO, with a higher frequency from the SRS FS740.
The waveform on your oscilloscope is approximately 8.0 Volts peak to peak (4 divisions at 2.00V per division). The GNSSDO can not produce 8.0Vpp.
The GNSSDO 10MHz output is produced by a 74LVC2T45 transceiver. The 74LVC2T45 is powered by “VCCIO”. VCCIO is either 3.3V or 5.0V, depending on the position of the VCCIO slide switch. The default setting is 3.3V.
It is impossible for the 74LVC2T45 to produce signals higher than 5.0Vpp.
If I’m honest, I believe you have misunderstood your connections from the GNSSDO to the 'scope and the SRS FS740. I believe you have connected the GNSSDO 10MHz to an output from the SRS, and your 'scope is displaying the sum of the two waveforms.
Please do the following:
Please connect the GNSSDO GNSS SMA connector to the supplied GNSS antenna using only the supplied SMA-TNC cable. No other connections. No other adaptors or tees. GNSS SMA direct to the antenna via a single cable, and nowhere else.
Please connect the GNSSDO 10MHz SMA connector to your 'scope using the supplied SMA-BNC cable. No other connections. No other BNC adaptors or tees. 10MHz SMA direct to the 'scope via a single cable, and nowhere else.
Please ensure the SRS FS740 is completely disconnected. Please unplug all cables going to the FS740. Better yet, please completely disconnect it, put it on a shelf and leave it well alone.
If you can tell me that you still see the distorted waveform with a single cable from the GNSSDO to your 'scope, then I will happily arrange for your GNSSDO to be replaced. Because if the GNSSDO is generating 8.0Vpp, it is breaking the Laws of Physics… [End of Star Trek Scotty impersonations! 
]
Very best wishes,
Paul
Hi Paul,
I’ve connected the 10 MHz output directly to the scope with the SMA cable provided and I’m seeing that same signal I sent previously. The SRS FS740 has nothing to do with the oscilloscope measurements. I am only using the input on that just as a frequency counter. The antenna cable is a very thick, good quality cable and the antenna is currently on the rooftop so we cannot change that connection.
I have found that I can add a tee to the end of the BNC cable with a 50 Ohm termination on the other end and the signal looks similar to what you have shared (maybe a slightly smaller signal). So I guess this is an impedance matching issue.
I understand why you’d want us to put the FS740 on the shelf! HAHA, but apparently it still works decently well.
Best,
Gaby
Hi Gaby (@gabriela ),
I am sorry. I owe you a big apology…
I had forgotten that, on my GNSSDO, jumpers A and E are open. By default, they are closed:
If I close jumper A, I see:
If you are feeding a digital load, you will want jumper A to be closed. If you are only feeding high impedance inputs, like your 'scope, then opening jumper A avoids the overshoot and ringing.
I am sorry for doubting you - and the FS740!
I have put a big label on my GNSSDO to remind myself that the jumpers are open.
We will be releasing new firmware for the GNSSDO in a few days. It contains some improvements you may find useful. Please keep an eye on the Firmware Upgrade link. I will update that when v2.1 is released. I will also be adding a Dockerfile and instructions on how to compile the firmware locally using Docker (Desktop), just in case you want to make any firmware alterations of your own.
Please let us know if you need more help and thank you for supporting our products.
Best wishes,
Paul
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Hi Paul,
Ok that makes a lot more sense now, thanks for the clarification haha. No worries, communication through online messaging sometimes gets lost. I think the datasheet may highlight that note about the jumpers but that’s good to know for future reference.
I may leave the jumpers as they are then because I won’t be using the oscilloscope for my measurements and the equipment I will be using has an input impedance of 50 Ohms.
Best regards,
Gabriela
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