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All,
In a effort to repair my NanoVNA to work above 300 mhz, please see /g/nanovna-users/topic/34518859#4791. So i order a new si5351a chip, and replaced it last night. The new chip did not help solve the issue, i though i had matched the chip to original spec but upon further inspection of the spec sheet show to max freq to be only 200 mhz. The link to what i ordered is below. Can anyone point to the correct chip? or any other tips to make my nanovna work about 300 mhz.

On Wed, Jan 8, 2020 at 04:54 AM, KE8CPD wrote:

" Can anyone point to the correct chip? or any other tips to make my nanovna work about 300 mhz. "
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The NanoVNA only works to 300 MHz on fundamentals. Operation to 900 MHz or 1500 MHz is done using 3rd or 5th harmonics of the fundamentals. Although the si5351a is only spec'd to 200 MHz, it can be used to 300 MHz without cherry picking.

- Herb

Officially the maximum output frequency of any SI5351 is 800MHz divided by 4 is 200MHz.
Many will be able to produce 300MHz but the increase of the VCO of the internal PLL to 1200MHz implies the SI5351 will become much hotter and it is being considered "out of spec"
So you can not formally buy a SI5351 that will go to 300MHz.

If you have a problem with the SI5351 on a nanoVNA not being able to reach the out of spec 300MHz there is a build in solution that will reduce the maximum frequency of the SI5351 before it switches to harmonics mode.

You need to connect to the nanoVNA in console mode using a terminal emulator (putty or any other) and use the "threshold" command to verify at which transition to harmonics mode the problems disappear. This could be any value between 200 and 300 (MHz). Once found you do a "save conf" from the console to store the max threshold value so it will be remembered even if you power off.


--
NanoVNA Wiki: /g/nanovna-users/wiki/home
NanoVNA Files: /g/nanovna-users/files
Erik, PD0EK

In addition to what Erik just said, if you ever perform a full clearconfig command via the console or install new firmware, you will need to reset the threshold level again.

Hi

SiLabs datasheets are so and so when it comes to clarity. The current datasheet says that Si5351A can go up to 200 MHz. However, the max VCO frequency is 900 MHz, and with the then "forced" divider ratio of 4 this results in a max output frequency of 225 MHz. Is this within or outside of the spec? Not really sure - one could argue that the 4 divider cannot work properly above 200 MHz, but it does. However, many VCOs can go close to 1200 MHz from where the 300 MHz comes from, i.e. 1200 MHz / 4, but, again way out of spec., and not all Si5351A will be able to do this. My experience is that a "guaranteed" fmax is 280 MHz.

Bottom line: 300 MHz is a lucky punch, i.e. may work and it may not. I would not put any money on it.

Bo

Hi,
For clarification: An SI5351 outputs square-wave signals that contain only odd multiples of the fundamental frequency (3 * f, 5 * f, ...).
To reach 1500 MHz, 7 * 214.285 MHz are therefore required as fundamental frequency.
In the analyzer according to EU1KY, however, we did not go beyond 5 times, and I reached e.g. 1450 MHz with 5 * 290 MHz. Prerequisite was a test of the maximum possible frequency of the SI3531 circuit.
Wolfgang, DH1AKF

Hi

In this old message I posted some pictures of the Si5351A output as the fundamental frequency approaches 300 MHz:
/g/nanovna-users/message/3285?p=,,,50,0,0,0::Created,,pll,50,2,0,34315096

A clarification of the clarification :-) The Si5351A does indeed outputs odd harmonics but also even ones and spurious too.

Bo

Hello, Bo,
You will probably agree with me that only odd multiples of the fundamental frequency can be used for our measurements. They have amplitudes much higher than the even ones. I beg you not to create any further confusion, especially since many beginners are reading this.
I have worked on the further development of the EU1KY analyser and know what I am talking about.
73, Wolfgang, DH1AKF

On Wed, Jan 8, 2020 at 09:46 AM, Larry Rothman wrote:

if you ever perform a full clearconfig command via the console or install new
firmware, you will need to reset the threshold level again.

Gee, it would sure be nice if some of the firmware upgrade instructions actually mentioned that you need to reset
the threshold level. :-)

No version of the Si5351 is specified to operate about 200 MHz. The
original Si5351A, B, and C were specified to 160 MHz. The later revision,
the Si5351A-B (or B-B or C-B) are specified to work up to 200 MHz, and that
is the one you will find in just about any product you buy today. (The B
suffix parts also reduce the minimum output frequency from 8 KHz to 2.5
KHz.) Popular breakout boards like the one from Adafruit still specify an
upper limit of 160 MHz, but current production contains the Si5351A-B with
its 200 MHz spec and the boards will work up to and beyond that frequency.

Nearly all ham experimentation with the Si5351 uses the A variant. That is
the least expensive version: it comes in a 10 pin MSOP package and offers
three outputs. When the upgraded B-suffix version was released, an
Si5351A-B-GM in a 20 lead QFN package was added that offers 8 outputs but
no additional features. The Si5351B and Si5351C come in a 20 pin QFN
package; they offer eight outputs and additional clocking features. (The
first generation was also offered in a 24 pin QSOP but that was
discontinued.) Versions with pre-programmed output frequencies are also
available; they will have part numbers with additional numbers following
the B suffix.

For ham projects, the specific chip you want is the Si5351A-B-GT or
Si5351A-B-GTR. The only difference is the packaging; the GT comes in a
tube, the GTR comes in tape and reel. If you're buying for prototyping, you
will get either chips poured in an antistatic bag or a cut tape. Buy
whichever your favorite distributor has in stock or the one in the
packaging you prefer; they are usually the same price.

Hams have found that the chips can be pushed up to frequencies well above
their specifications. Other specs, like power consumption, harmonic
distortion, and phase noise are not guaranteed if the chip is operated
above 200 MHz. The NanoVNA counts on being able to run it at higher
frequencies to achieve fundamental mode operation up to 300 MHz. In the
other direction, the QCX transceiver relies on operating one of the primary
PLLs well below its specified lower limit of 600 MHz to make it possible to
generate quadrature outputs on the 80 meter band.

Unlike the Si570, a popular oscillator in other ham designs that is sold in
multiple frequency grades (with the lower grades having limits that are
programmed into the chip), the Si5351A is not locked in any way by the
manufacturer. Users are free to push the chip all the way to its inherent
limits.

225 MHz output can be achieved by operating the PLL at 900 MHz with a divider of four; that's within the spec for the PLL's VCO but the chip is not guaranteed to meet all its specifications at that frequency. Power consumption may rise and it may not other specs such as output jitter. All chips seem to have have no trouble reaching that.

300 MHz output is done by operating the PLL at higher frequencies that are out of spec: 1200 MHz for 300 MHz output. Most chips seem to at least be able to get close; exactly how close you get depends on how well you did in the silicon lottery, the operating temperature, and the operating voltage you use. Higher voltage will usually allow higher frequencies, just as it does with CPUs and RAM in computers. The NanoVNA powers the Si5351A with a 3.3V LDO, but better results might be achieved by getting its power from a 5V regulator and diode drops as is done in the QCX, which results in a higher operating voltage near 3.6V.

The input frequency might also make some difference in high frequency performance, though I haven't seen any data on what effect might have. The Adafruit and Etherkit breakout boards use a 25 MHz crystal; the Etherkit board has a TCXO option. QRP Labs products (breakout board and QCX) use a 27 MHz crystal. The NanoVNA splits the difference with a 26 MHz clock.

Operation of the NanoVNA above 300 MHz uses harmonics of the Si5351A output. As the output of the Si5351A is nominally a square wave, the odd harmonics are the strongest. I say "nominally" because it's a switching circuit, but the actual output waveform is affected by the non-zero switching time of the MOSFETS and the output capacitance of the circuit. It doesn't look all that much like a square wave when you approach the upper limits of the Si5351.

I am not a fan of the Si5351. It was intended to be used for digital clock generation for processors and peripherals, not for RF signal generation where reasonable signal quality is needed. Its only attribute is it is cheap, and can do low frequency.

It is the weakest piece of the nanoVNA design. It has a lot of close in spurious outputs, terrible jitter and therefore terrible sideband noise. It is the dominate item limiting dynamic range.

The spec is almost useless and applications notes are difficult to read and understand. The app note AN614 Document change list is laughable with the amount of major errors caught in the document.

Other then that it is a great chip. A technological leap above the transmitter on the Titanic ;