si5351a issue spectral output above 90Mhz


Phil Crockford
 

I have built a number of the Si5351a synthesizer boards for use in a prog-rock and sig-gen projects and they seem to work fine below 90Mhz and the use of a suitable Low Pass filter removes the products above the wanted frequency. I have a spectrum analyser capable of display to a 10hz depth up to 1.5Gig and all seems fine below approximately 90Mhz. I see that filters exist for 2 meters and above and ordered a 220Mhz LP Filter. I programmed a Prog-Rock for 212Mhz and found that there is frequency content below the wanted frequency  that are only -30db down above and below the wanted frequency of  212Mhz and would not be removed by the use of a LF Filter without removing the wanted frequency. I have looked online at the analyser picture of the 220Mhz filer and the picture of the o/p of the Si5351a DDS, prior to any filtering, at 220Mhz does show these products above and below the wanted frequency. To comply with the rules these should be at least -43db below the fundamental frequency. Can I ask if someone with suitable spectrum analyser could confirm these findings.
Phil G8IOA. 


f1us
 

hi

i could confirm these results

SI5351 is very bad above 20mhz and very sensitive to frequency settings

it is worst if you use 2 outputs  !!

impossible to clean it !

it is due to fractionnal dividers.

73

F1US

 

 

 

 

> Message du 27/01/20 12:56
> De : "Phil Crockford" <pcc@...>
> A : QRPLabs@groups.io
> Copie à :
> Objet : [QRPLabs] si5351a issue spectral output above 90Mhz
>
> I have built a number of the Si5351a synthesizer boards for use in a prog-rock and sig-gen projects and they seem to work fine below 90Mhz and the use of a suitable Low Pass filter removes the products above the wanted frequency. I have a spectrum analyser capable of display to a 10hz depth up to 1.5Gig and all seems fine below approximately 90Mhz. I see that filters exist for 2 meters and above and ordered a 220Mhz LP Filter. I programmed a Prog-Rock for 212Mhz and found that there is frequency content below the wanted frequency  that are only -30db down above and below the wanted frequency of  212Mhz and would not be removed by the use of a LF Filter without removing the wanted frequency. I have looked online at the analyser picture of the 220Mhz filer and the picture of the o/p of the Si5351a DDS, prior to any filtering, at 220Mhz does show these products above and below the wanted frequency. To comply with the rules these should be at least -43db below the fundamental frequency. Can I ask if someone with suitable spectrum analyser could confirm these findings.
Phil G8IOA. 


Hans Summers
 

 

hi

i could confirm these results

SI5351 is very bad above 20mhz and very sensitive to frequency settings

it is worst if you use 2 outputs  !!

impossible to clean it !

it is due to fractionnal dividers.

All QRP Labs firmware uses even integer MultiSynth dividers (not fractional). 

I would not be surprised if spectral purity started to degrade as we get way up into VHF but I would expect it to be still very good at 20MHz. 

If using two outputs beware the crosstalk issues; these are reduced to a negligible level if you load the outputs lightly (i.e. use a buffer, not a low impedance load). 

73 Hans G0UPL
http://qrp-labs.com


f1us
 

thanks for infos regarding crosstalk.

 

in datasheet we coud read p11 :

 

high-frequency intermediate clock, while the second stage of synthesis uses high resolution MultiSynth fractional dividers.

 

other specs are good but spurs levels are really too high for VHF use.

up to 30mhz output, ladder crystals filters are very efficients for one frequency

73

 

 

 

 

> Message du 27/01/20 14:05
> De : "Hans Summers" <hans.summers@...>
> A : QRPLabs@groups.io
> Copie à :
> Objet : Re: [QRPLabs] si5351a issue spectral output above 90Mhz
>
>
 
>

> hi

> i could confirm these results

> SI5351 is very bad above 20mhz and very sensitive to frequency settings

> it is worst if you use 2 outputs  !!

> impossible to clean it !

> it is due to fractionnal dividers.

All QRP Labs firmware uses even integer MultiSynth dividers (not fractional). 

>
I would not be surprised if spectral purity started to degrade as we get way up into VHF but I would expect it to be still very good at 20MHz. 

>
If using two outputs beware the crosstalk issues; these are reduced to a negligible level if you load the outputs lightly (i.e. use a buffer, not a low impedance load). 

>
73 Hans G0UPL
> http://qrp-labs.com

>

>

>


Phil Crockford
 

 

Thanks for your reply. I don’t understand your statement” All QRP Labs firmware uses even integer MultiSynth dividers (not fractional).”

Can you explain Please, as I am using the Pro-Rock and Sig-Gen as provided by QRP Labs.

 

I have notice the other effects you mention.

Phil G8IOA 

 

 

From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of Hans Summers
Sent: 27 January 2020 13:05
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] si5351a issue spectral output above 90Mhz

 

 

hi

i could confirm these results

SI5351 is very bad above 20mhz and very sensitive to frequency settings

it is worst if you use 2 outputs  !!

impossible to clean it !

it is due to fractionnal dividers.

All QRP Labs firmware uses even integer MultiSynth dividers (not fractional). 

 

I would not be surprised if spectral purity started to degrade as we get way up into VHF but I would expect it to be still very good at 20MHz. 

 

If using two outputs beware the crosstalk issues; these are reduced to a negligible level if you load the outputs lightly (i.e. use a buffer, not a low impedance load). 

 

73 Hans G0UPL
http://qrp-labs.com

 


Phil Crockford
 

Hi and thanks for getting back to me so quickly. Not so much of a problem when used for a receiver at 220Mhz but most certainly a problem for TX as the unwanted signal content is, in my opinion, not acceptable for TX just using a low pass filter. I will design a Band pass filter at 220Mhz and see if I can reduce the unwanted below the wanted signal enough to make it acceptable. It will have to be very narrow, around 3Mhz wide at the most to make a difference.

This observation is not intended to be a criticism of the QRP products that work just fine at the original frequencies they were designed for.

 

Thanks again for your help.

 

Phil G8ioa.

 

From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of f1us
Sent: 27 January 2020 15:33
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] si5351a issue spectral output above 90Mhz

 

thanks for infos regarding crosstalk.

 

in datasheet we coud read p11 :

 

high-frequency intermediate clock, while the second stage of synthesis uses high resolution MultiSynth fractional dividers.

 

other specs are good but spurs levels are really too high for VHF use.

up to 30mhz output, ladder crystals filters are very efficients for one frequency

73

 

 

 

 

> Message du 27/01/20 14:05
> De : "Hans Summers" <hans.summers@...>
> A : QRPLabs@groups.io
> Copie à :
> Objet : Re: [QRPLabs] si5351a issue spectral output above 90Mhz
>
>

 
>

> hi

> i could confirm these results

> SI5351 is very bad above 20mhz and very sensitive to frequency settings

> it is worst if you use 2 outputs  !!

> impossible to clean it !

> it is due to fractionnal dividers.

All QRP Labs firmware uses even integer MultiSynth dividers (not fractional). 


>

I would not be surprised if spectral purity started to degrade as we get way up into VHF but I would expect it to be still very good at 20MHz. 


>

If using two outputs beware the crosstalk issues; these are reduced to a negligible level if you load the outputs lightly (i.e. use a buffer, not a low impedance load). 


>

73 Hans G0UPL
> http://qrp-labs.com


>


Hans Summers
 

Hi Phil, F1US

The Si5351 contains two stages of synthesis. Note that it is incorrect to call an Si5351A a DDS (Direct Digital Synthesizer) which is a completely different synthesis technique. 

The first stage is a PLL 
which multiplies the 27MHz reference frequency up. There is a VCO whose output should be in the range 600-900Hz and the loop is closed by a divider which is fractional (20-bit fractional part).

The second stage is a divider block ("MultiSynth divider") which divides the VCO frequency produced in the first stage, down to the output frequency. This divider is also fractional with a 20,bit fractional part. 

[For the sake of completeness I should also mention the third stage, which is a divider configurable as one of 7 powers of two from 1 to 128 - this is normally only used to achieve low frequency output, under about 1MHz].

Whilst both the first and second stage dividers have integer and fractional parts, they can both be operated with an integer division ratio, simply by setting the fractional part to zero. If both are integers, then it will not be possible to achieve fine resolution tuning, only widely spaced freqiencies will be available. So generally one or other of the two stages must be used with fractional configuration.

The datasheet recommendation for best low jitter (phase noise) performance is to use even integer division in the second stage (MutliSynth divider) which is what all the QRP Labs firmware does. There are some Si5351A libraries which do not do this, they fix the PLL divider at an integer value and use fractional division in the MultiSynth stage - these libraries will generally produce an inferior output in terms of phase noise and spurious products. 

As may be expected in a PLL system, spectral purity of the Si5351A outout is better at lower frequencies than high ones. 

Phil you mentioned you used the Si5351A for 212MHz output. Note that this is beyond the specified maximum 200MHz of the device. Whilst they have been found to work and be stable up to 290 or 300MHz, it is not clear what negative impact this has on performance specifications.

To give the Si5351A chip the best chance to be as clean as possible at VHF the following recommendations should be followed. Some of these apply even if you are using a complete product like ProgRock or VFO/SigGen. 

1. Use a clean power supply. A noisy supply will inflict its junk on the output to some extent.

2. Use even integer division in the MultiSynth stage - if you are writing your own code and use someone else's library, try to find a decent one that doesn't do it the other way round. 

3. Use load impedances of at least 1KOhms. Don't drive things like diode mixers directly, use buffers. This improves phase noise performance and eliminates cross talk problems between Si5351A outputs.

4. Use reference frequency crystals in the range 25-27MHz as specified in the Si5351A datasheet - frequencies outside this range have been found to degrade spectral purity considerably. 

73 Hans G0UPL 

On Mon, Jan 27, 2020, 20:52 Phil Crockford <pcc@...> wrote:

 

Thanks for your reply. I don’t understand your statement” All QRP Labs firmware uses even integer MultiSynth dividers (not fractional).”

Can you explain Please, as I am using the Pro-Rock and Sig-Gen as provided by QRP Labs.

 

I have notice the other effects you mention.

Phil G8IOA 

 

 

From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of Hans Summers
Sent: 27 January 2020 13:05
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] si5351a issue spectral output above 90Mhz

 

 

hi

i could confirm these results

SI5351 is very bad above 20mhz and very sensitive to frequency settings

it is worst if you use 2 outputs  !!

impossible to clean it !

it is due to fractionnal dividers.

All QRP Labs firmware uses even integer MultiSynth dividers (not fractional). 

 

I would not be surprised if spectral purity started to degrade as we get way up into VHF but I would expect it to be still very good at 20MHz. 

 

If using two outputs beware the crosstalk issues; these are reduced to a negligible level if you load the outputs lightly (i.e. use a buffer, not a low impedance load). 

 

73 Hans G0UPL
http://qrp-labs.com

 


Glen Leinweber
 

Hans,
Thank you for adding phase noise measurements for SI5351. Just another
value-added QRP_Labs feature for those of us who build-to-learn. Noise
floor measurements seem prone to mis-interpretation and measurement
errors. Including your noise-measurement setup progression is particularly
enlightening.

Hans didn't test fractional MultiSynth dividers for the very good reason that
QCX uses integer MultiSynth division only. Don't confuse the SI5351
MultiSynth PLL with its MultiSynth divider. I believe that QCX uses
fractional division in the PLL, but integer division in the divider. Seems very
reasonable that QCX spurious response is quite good with this
arrangement: SI5351 configuration (187 internal registers!) can be set
up many different ways, impacting spurious response.

Hans' measurements of phase noise would include spurii, but I'm wondering if all the
noise averaging would hide sharp spurious responses of jittery PLL, mushing them into
the measured noise floor of about -135dBc/Hz? I see that some phase noise is
measured not including spurs, i.e. between spurious peaks.
Perhaps the internal SI5351 fractional PLL that takes 27 MHz up to nearly 900 MHz
is mostly contributing to the -135dBc/Hz measured noise floor? All guesses, since
the SI5351 data sheet is so sketchy. Silicon Labs could learn a thing or two
about documentation from QRP-Labs ;-)


ajparent1/KB1GMX
 

The biggest issue is phase jitter is essentially noise in the frequency
domain ie: FM.  All of the papers that address this try to translate that
from to amplitude where noise is measured (usually).  Its very system
dependent.

Since the SI5351 is really a UHF PLL followed by integer or fractional
(user decides) counters its easy and hard to evaluate noise and side
bands.  Every divide lowers the phase noise of the UHF VCO and the
multiply from 27mhz to PLL VCO frequency multiplys the the phase noise.

So on a 1:1 basis at 27mhz or below its likely to be as good or only slightly
poorer than the 27mhz reference.  At higher than 90-100mhz you in the
path problem if even pushing the internal PLL higher to 1ghz you max
divisor is only 10 for 100mhz!

Using a SA to evaluate spurs be very careful to not introduce spurs
or worse from incorrect technique.

For those that need better there is he SI570 (also a pain to program).
Or go to a crystal and multipliers.

For those that want cheap and easy above 100mhz, sorry you need to
look at other parts if it isn't good enough for you.  They exist but not
cheap and likely harder to use.

As is complaining about a device that was designed as a clock source
data systems is fortuitous for us RF weenies that it works.

Allison
-------------------------------
Please reply on list so we can share.
No direct email, it goes to bit bucket due address harvesting in groups.IO


Allan Nelsson
 

I don't necessarily think that the Si5351A itself is bad above 90 MHz. I agree that there are better solutions but also at a much higher cost. My RFzero is usable at 200 MHz with spurs at -60 dBc:

http://www.rfzero.net/documentation/rf/ I don't have a spectrum analyzer myself but I trust the measurement. Compared to my U3S the RFzero is much better and the best use of a Si5351A I know.

73, Allan / OZ5XN 


Craig Johnson
 

Hi, I am newbie withis qro solid state stuff. I just competed the assembly of the wee 40
Meter rig.  It is not working .  It appears that the 27 meg Osc is not doit’s it’s thing.  Nothing on the scope.  Just a line of noise.  Xtal defective?  Defective synthesizer chi?

Tnx. W7WXQ Craig


Alan G4ZFQ
 

Craig,

For a start you need to start a new topic with a title giving the kit you refer to and the fault symptom. For example "QCX 40m Si5351 error message"
Mixing topics is not good your problem has nothing to do with si5351spectral output. A fresh topic will attract the troubleshooters.

It appears that the 27 meg Osc is not doit’s it’s thing.  Nothing on the scope.  Just a line of noise.
Was a Si5351 error message what prompted you to look?

Where did you check with the scope? Are you certain that applying the probes did not stop oscillation? Listening on a RX around 27.004MHz might be more reliable.
Xtal
defective?  Defective synthesizer chi?
Any other tests? Very careful look at joints with magnification? Voltages?

73 Alan G4ZFQ


Phil Crockford
 

Allan. It is the phase jitter that is the problem above 90Mhz and the production of what seem to be sidebands above and below the wanted signal that was the subject of my post and the subsequent pollution of the band if not removed and used on air. Others have confirmed this and given the reason. If you have an analyser and can look at the wanted 220Mhz signal you will see them close in to the 220Mhz frequency above and below. They are thought too close to be removed by normal filtering as they are too close to the wanted signal.

If you look at the QRP website at the 220Mhz filter, the author prided a spectrum analyser picture of the oscillator output prior to LPF and they are visible in the display above and below.

My thanks to Hans and others who have confirmed these findings.

 

Phil G8IOA

From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of Allan Nelsson
Sent: 28 January 2020 01:18
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] si5351a issue spectral output above 90Mhz

 

I don't necessarily think that the Si5351A itself is bad above 90 MHz. I agree that there are better solutions but also at a much higher cost. My RFzero is usable at 200 MHz with spurs at -60 dBc:

http://www.rfzero.net/documentation/rf/ I don't have a spectrum analyzer myself but I trust the measurement. Compared to my U3S the RFzero is much better and the best use of a Si5351A I know.

73, Allan / OZ5XN 


Phil Crockford
 

Allison.

It is not that it is not good enough for me, it is that others may think it is, by using it for TX above 90Mhz and causing pollution and may interfere with others on the band. By causing pollution on the bands it puts us, as HAMS, in a bad light when complaining about other users on shared bands, like VHF, when they cause interference. My post was not a complaint merely seeking conformation of my findings.

 

I thank you for your other input.

Phil G8IOA.

 

From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of ajparent1/KB1GMX
Sent: 27 January 2020 20:53
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] si5351a issue spectral output above 90Mhz

 

The biggest issue is phase jitter is essentially noise in the frequency
domain ie: FM.  All of the papers that address this try to translate that
from to amplitude where noise is measured (usually).  Its very system
dependent.

Since the SI5351 is really a UHF PLL followed by integer or fractional
(user decides) counters its easy and hard to evaluate noise and side
bands.  Every divide lowers the phase noise of the UHF VCO and the
multiply from 27mhz to PLL VCO frequency multiplys the the phase noise.

So on a 1:1 basis at 27mhz or below its likely to be as good or only slightly
poorer than the 27mhz reference.  At higher than 90-100mhz you in the
path problem if even pushing the internal PLL higher to 1ghz you max
divisor is only 10 for 100mhz!

Using a SA to evaluate spurs be very careful to not introduce spurs
or worse from incorrect technique.

For those that need better there is he SI570 (also a pain to program).
Or go to a crystal and multipliers.

For those that want cheap and easy above 100mhz, sorry you need to
look at other parts if it isn't good enough for you.  They exist but not
cheap and likely harder to use.

As is complaining about a device that was designed as a clock source
data systems is fortuitous for us RF weenies that it works.

Allison
-------------------------------
Please reply on list so we can share.
No direct email, it goes to bit bucket due address harvesting in groups.IO


Alan G4ZFQ
 

It is not that it is not good enough for me,
Phil,

Did you look at http://www.rfzero.net/documentation/rf/ ?
I see two different conclusions.
Have you any comment? Can one be good and another not?

73 Alan G4ZFQ


Alan de G1FXB
 

Hi Phil,

There is a paper on the web, just that it's somewhere!
from memory it's written by one of the well known amateur beacon builders or possibly one of the microwave guys, early on with the rapid adoption of the 5351 by amateurs.
I had a quick look at G4DDK / TNT and OZ2M sites and the particular doc doesn't leap out, sorry not much help......

Briefly it explained it was a limitation of the mathematics in how the device works, I'm sure they would have explored the fractional / interger operation
Conclusion while OK for ones own personal use on a bench, it was unlikely to be suitable on a RF rich commercial site where intermod would reign supreme.
The suggested workaround was that the Si5351 could be used similar to a reverse DDS topology, where the wanted frequency is generated by a cleaner oscillator and steered by the 5351's output.

The QRP-Labs U3S had PI4 beacon generation included by request early on, there were some beacons built certainly for 4 & 6M (maybe Hans still has an idea of who the user's were.)
I believe at least one group (central / eastern europe?) was troubled by mixing products, that only became apparent on-site.


regards Alan





On 28/01/2020 11:22, Phil Crockford wrote:

Allan. It is the phase jitter that is the problem above 90Mhz and the production of what seem to be sidebands above and below the wanted signal that was the subject of my post and the subsequent pollution of the band if not removed and used on air. Others have confirmed this and given the reason. If you have an analyser and can look at the wanted 220Mhz signal you will see them close in to the 220Mhz frequency above and below. They are thought too close to be removed by normal filtering as they are too close to the wanted signal.

If you look at the QRP website at the 220Mhz filter, the author prided a spectrum analyser picture of the oscillator output prior to LPF and they are visible in the display above and below.

My thanks to Hans and others who have confirmed these findings.

 

Phil G8IOA

From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of Allan Nelsson
Sent: 28 January 2020 01:18
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] si5351a issue spectral output above 90Mhz

 

I don't necessarily think that the Si5351A itself is bad above 90 MHz. I agree that there are better solutions but also at a much higher cost. My RFzero is usable at 200 MHz with spurs at -60 dBc:

http://www.rfzero.net/documentation/rf/ I don't have a spectrum analyzer myself but I trust the measurement. Compared to my U3S the RFzero is much better and the best use of a Si5351A I know.

73, Allan / OZ5XN 


-- 
Smell Czech corruptions are inevitable


Phil Crockford
 

Hi Alan,
Many thanks for pointing me at the document in your email.
I have not read it completely at this time but I will.

On first glance I am confused by some of this document and the clams within it.

It seems to be contradictory in itself.

I will read and digest it and get back to you.

Thanks once more for you input.

73 Phil G8ioa

-----Original Message-----
From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of Alan G4ZFQ
Sent: 28 January 2020 12:55
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] si5351a issue spectral output above 90Mhz


It is not that it is not good enough for me,
Phil,

Did you look at http://www.rfzero.net/documentation/rf/ ?
I see two different conclusions.
Have you any comment? Can one be good and another not?

73 Alan G4ZFQ


Kārlis Goba
 

A quick comment before people draw conclusions from a single datapoint. The Si5351 can often be configured in multiple ways to produce a single given frequency. That is, you can have arbitrary multipliers and dividers, even if integer dividers are used, there is a multitude of configurations you can choose. Typically the code that drives Si5351 chooses one configuration according to designer's choice and chip limitations, e.g. keeping the internal VFO in the specified range. So, for the 220 MHz problem, another set of spurs (not necessarily better) might be obtained when using different configuration of internal multipliers/dividers.

--
Karlis YL3JG


Hans Summers
 

Hello Allan and all,

PCB Layout is important. PCB layout becomes more important as the frequencies get higher and higher. The QRP Labs Synth board and Ultimate3S combination do NOT have a PCB layout which is ideal for VHF. 

At the time of the design maximum 10m band operation was envisaged. The U3S evolved from the U3 with its AD9850 DDS which was limited to 30MHz maximum. Later the Si5351A Synth kit http://qrp-labs.com/synth was designed to replace the AD9850 DDS module, the U3S was the same as the U3 except added a DC bias offset to the PA MOSFET gate to restore power output (Si5351A output 3.3V wasn't enough to drive the BS170 as hard as the 5V from the AD9850 did). 

Nobody designing for VHF frequencies would have pin header plug-in modules on multiple boards like that - each of those pins has about 6nH inductance by itself, plus the considerable trace lengths, lack of ground plane etc. 

Keep the purpose in mind. U3S was designed originally for HF. The 20MHz system crystal and 27MHz reference crystal probably leak around and mix with things too. 

There are three classes of spurious products to consider:

1) Harmonics. These are expected, by the nature of the Si5351A which generates a squarewave outout. They are easily seen on a spectrum analyzer. Generally as long as the frequency range is not wide, they are not difficult to deal with using filtering. 

2) Spurious products at unrelated frequencies and also generally easily visible on the spectrum analyzer. These can be a result of the synthesis process itself (i.e. an artifact generated by the Si5351A chip itself), conducted in from other parts of the circuit via supply lines, radiated from other parts of the board and wiring, etc. PCB layout is important here. I would not find it surprising if RF Zero is superior in this regard, to Ultimate3S etc. Because of the modular nature of the QRP Labs system, the VHF performance won't be optimum. Spurious products can be hard to filter out if they are close to the wanted carrier. 

3) Phase noise. Phase noise is very difficult to see or measure on a spectrum analyzer. I'm not convinced that even the highest performance spectrum analyzers can directly make a reasonable assessment of phase noise unless the phase noise levels are really high. Phase noise results from the phase locked loop process itself; the Si5351A has a quite low loop bandwidth and the phase noise is correspondingly pretty good for a PLL.

Phase noise is very important in some applications like transmitters and receivers because it results in reciprocal mixing of strong signals away from the wanted signal, with the phase noise of the local oscillator, producing spurious received signals in the receiver passband. 

My phase noise measurements on the Si5351A are here http://qrp-labs.com/qcx/phasenoise and indicate that loading the outputs at 50-ohms results in a 10dB phase noise degradation compared to significantly higher loads. The RF Zero page on RF Performance measurements does not present phase noise results but I'd expect them to be worse than QCX etc because their Si5351A drives a 50-ohm load impedance. 

Buffering the output would be best from a phase noise perspective but buffer amplifiers can introduce distortions of their own. So it's difficult to know what to do in a general purpose module; in a complete system one can evaluate performance trade-offs at a system level in mind of the application's required performance and cost constraints. 

At HF frequencies the Si5351A is a great performer at a very low price. At VHF, UHF and multiplied up beyond - suitability would depend on the application requirements and spectral purity won't be good enough for every application. 

73 Hans G0UPL 

On Tue, Jan 28, 2020, 04:17 Allan Nelsson <allan@...> wrote:

I don't necessarily think that the Si5351A itself is bad above 90 MHz. I agree that there are better solutions but also at a much higher cost. My RFzero is usable at 200 MHz with spurs at -60 dBc:

http://www.rfzero.net/documentation/rf/ I don't have a spectrum analyzer myself but I trust the measurement. Compared to my U3S the RFzero is much better and the best use of a Si5351A I know.

73, Allan / OZ5XN 


Craig Johnson
 

thankyou Alan


On Mon, Jan 27, 2020 at 11:47 PM Alan G4ZFQ <alan4alan@...> wrote:
Craig,

For a start you need to start a new topic with a title giving the kit
you refer to and the fault symptom. For example "QCX 40m Si5351 error
message"
Mixing topics is not good your problem has nothing to do with
si5351spectral output. A fresh topic will attract the troubleshooters.

>It appears that the 27 meg Osc is not
> doit’s it’s thing.  Nothing on the scope.  Just a line of noise.

Was a Si5351 error message what prompted you to look?

Where did you check with the scope? Are you certain that applying the
probes did not stop oscillation? Listening on a RX around 27.004MHz
might be more reliable.
  Xtal
> defective?  Defective synthesizer chi?

Any other tests? Very careful look at joints with magnification? Voltages?

73 Alan G4ZFQ