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GPSDO 10 MHz

Si5351A + u-blox M7: Clock generator tests

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With the Si5351A mastered, time for some tests with the actual application, the GPS receiver (and later GPSDO system).

Two tests were performed:

(1) Using the Si5351A clock generator with a standard, not specifically selected or particularly stable crystal. Presumably, an AT cut crystal, running at 25 MHz.

(2) With the Si5351A driven by a OCXO (HP 10811A, part of a 8662A unit – will be later replaced by a stand-alone unit using a HP 10811A with power supply and distribution amplifier, but essentially, to the same effect).

The first test, we need to set the registers of the Si5351A (check the Silicon Labs AN 618 Application Note – check it in a quiet hour, because it has a lot of heavy content…). 25 MHz clock input, 26 MHz OSC0 output – can be achieved by running the VCO at 650 MHz, and integer-only dividers.

For the calculation, I use a self-made Excel sheet, which allows me to play around with the numbers to figure out the best combination of dividers and VCO frequency.

Here, the over test setup:

That’s the Xtal used – it came with the board, and I couldn’t find any data on it. Appears to be an AT cut crystal. Starting from room temperature, frequency, will go down with increasing temperature, and up with colder temperatures.

Such temperature variation is easily introduced here in Japan. First, keeping the room at 22 degC by the aircon (A/C) unit, then switching it off overnight (cooling the room by a few degrees, maybe down to 17 degC), then, next morning, heating up again to 22 degC (and a bit more as we had a sunny day).

The detail below also shows the small thermal mass of the Si5351A, free-hanging in air. Better to enclose it or to add some thermal mass later, to avoid thermal-gradient introduced noise or instability. You can clearly see the on-off cycles of the A/C unit regulating the room temperature.

After all this study, we find out that we can use the GPS and Xtal as a quarz thermometer (such thermometers really exist!)!

Test 2, now running with a 10 MHz reference clock, and still 26 MHz output to the u-blox M7 (in all cases, the u-Blox has been modified by removing the TCXO, and feeding the clock signal directly to the GPS chip). Sure I known that I am running the Si5351A outside of the specified range – but after all the research, Si5351A Spec, Myth and Truth, I believe this is OK.

The test setup – note the dotted additions – this will be the later phase locked circuit.

The register settings – running the Si5351A VCO at 780 MHz allows the use of integer-only dividers.
Capacitance set to 4 pF (the lowest value possible for the Si5351A, but it has little effect on sensitivity, 10 pF default setting works as well, maybe 1 dB decreased sensitivity, but we are anyway feeding more then enough power to the Xtal A input of the Si5351A).

The GPS clock drift data – not very exiting – no drift at all, less than 1 ppb over a few hours. There is a slight frequency offset, because the electronic frequency control (EFC) of the 10811A OCXO set to 0 Volt, rather to the proper value for exactly 10 MHz, only for convenience and to avoid any artifacts from EFC DAC noise or drift.

The position accuracy also seems better with the stable oscillator, but may need to check this again after acquiring data for a full day or so.

All in all, more than a proof of concept. Next steps include setting up a stand-alone OCXO (I have a couple of spare 10811A OCXOs around), a distribution amplifier (nothing special, planning to use some 74HC14 with small signal transformers for isolation), and getting the PLL code into a microcontroller. For the DAC, I will use a fairly basic model, and provide a low pass filter at the output (much faster than the digitally-generated long time constant low pass of the PLL loop, but still slow compared to common standards) to reduce noise.

So far, the bill of material is very low, just a few dollars, including the GPS receiver. My goal is to stay below USD 10 total, excluding the OCXO, to achieve better than 10-9 stability, and an output with no jitter or other issues.

GPSDO 10 MHz

u-blox GPS receiver: a self-regulating clock, and a GPSDO, and all of this, for the lowest cost

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The quest for precise timing, it is a mainstay topic for all serious electronic enthusiasts, and for a good reason – it offers so much insights into receivers, oscillators, phase detectors, regulation theory. After mastering such design, the hobbyist has himself earned a masters (or at least bachelor’s) degree.

With the advent of compact and really powderful GPS receivers, like the u-blox devices, receiving GPS signals is no problem any more, and in fact, it has never been over the last 20 years, with various Motorola receivers, etc.

The u-blox devices have a feature that makes a reference frequency (derived from its internal 48 MHz clock) directly available, rather than just the 1 pps signal that is not all that easy to use for locking a 10 MHz reference to it. The u-blox signal, which can provide a jittery 10 MHz, or, preferably, integer-divided 48 MHz (e.g., 8 or 4 or 2 MHz), has been widely used as a reference frequency in the amateur world, and u-blox company and other recommend to use an external PLL to clean up the signal according to below scheme. This implies that the GPS will be running on a drifting local osciallator, and with a good amount of knowledge and software u-blox is mastering the drift prediction and corrections, and after all such effort an external oscillator, typically, an OCXO is kept in sync with the GPS true clock, by even more phase detectors and control loops. It is doable, logical, practical, but not very clever.

Sure there a better and much more expensive GPS receivers, and even special timing-related u-blox devices (about 10x more expensive than the regular receivers), which can control an internal VTCXO (voltage-controlled temperature compensated local oscillator). With such approach, the drift of the local osciallator will be small, and all in sync with the GPS frequency, but still, it is not as precise as a really good metrology grade OCXO. I am still relying on some well aged HP 10811A oscillators.

That’s the magic neo-7m device, or a Chinese copy of it, you never know – but all that really counts is good reception, and this can be easily checked.

Which secrets hide inside the metal can? Well, let’s find out. Most important part, the G7020-KT GPS processor, it is a remarkable piece of engineering, and u-blox must have a crew of the most well educated, highly paid and hard working people to come up with such devices. Also, they know that they must protect their inventions, and even the datasheet of this device is strictly confidential, although you can find it at many places. What you can’t find are some secret control codes that would allow us to use a 10 MHz clock directly as the clock source for this chip – it is running on 26 MHz by default, for whatever reason! Internally, the other frequencies are synthesized from this 26 MHz anyway.

The typical TCXO performance, it is not bad, drifting along, and we can do some further stability analysis on it. For such a small thermal mass, the performance appears quite good. Accurate to 0.5 ppm over temperature, and 1 ppm per year.

That’s the GPS with the TCXO…

With no effort, the TCXO can be removed, just by holding a soldering iron to it to heat it up.

After removing it, we just solder a thin RF cable in position. In my temporary workshop here, I don’t have better tools, so this must work for now. Ideally, you add a decoupling cap, and solder a wire to a solder post or other propper connection or contact.

We have no good information what level of power is needed at this input, ideally, a 0.8 V p-p min. signal, DC coupled, but we don’t have such signal generator here, only a HP 8662A, which has sinewave output. Using the u-center software, and experimenting, the receiver works well from about -5 dBm of coupled power at the clock input. Operating at 0 dbm, that’s enough, we don’t want to fry this chip.

Even with a small antenna, good reception, within the (wooden) Japanese house.

Now, the feature we are going to use – not the reference frequency output of the u-blox, but the UBX NAV-CLOCK message, which is no less than a phase detector and drift measurement device, of the clock signal, relative to the GPS true-software-reconstructed clock. Marvelous.

As the new 26.0000000 MHz source, we use a 8662A generator with HP 10811A reference, and an EFC (analog frequency control input, about 0.1 Hz per Volt). On top, a 35601A interface, only using the DAC portion of it to generate a tuning voltage from the host computer (connected via GPIB). It is not the most handy DAC, but the only one I have around at the moment.

First, we try without any feedback – Allen deviation. First, the plot using the original TCXO, next, the same receiver (at the same location and setting), with the 8662A (freerunning).

The 8662a – not yet fully warmed up, but already one decade better – or even more, because of the resultion of the phase detector (1 ns!).

Next, we need to do some programming – this will later be put into a microcontroller, but for now, we use a regular PC, running a C program. This program reads the NAV-CLOCK message from the u-blox receiver, does a magic calculation, and then sets the EFC voltage of the OCXO, which in turn determines the 26.000 MHz clock for the same u-blox receiver. And after not too long time, all is frequency looked.

Here, some first results (using a rather small bandwidth regulation loop, just to proof the principle without waiting for too long time).

Introduced some artificial disturbaces, and the system is reacting well.

Next – using a Silicon Labs clock generator, and a stand-alone OCXO to do the same thing, and then, the software needs to be put in a small microcontroller (currently running a rather calculation intensive floating point algorithm). Stay tuned.