Date   

Re: QRP Monthly Challenge

neil G4BRK
 

Tried the 1300z session on 20m, as I have several times in the last few months.
Still no QSOs at all despite numerous RBN spots for my CQs all including US.
At least I heard some stations this time :-)  Antenna a 1/4 wave vertical.

Maybe 20m isn't the best band for QRP in the current radio conditions.

73,  Neil  G4BRK


Re: Bias setting 50W. PA #qcx #pa

Don, ND6T
 

Wasn't heat. It was only keyed about 15 seconds. Immediately upon failure I felt the heat sink and it was still stone cold (the lab was unheated and 10 C.).
I originally thought that there had to be some other cause. Turned out to be the MOSFETs.
Yep!  Dummy load was a 250W thick film mounted on an enormous heat sink and better than 35 dB return loss. Input  during bias setting was BNC  mount with 32 dB RL. Power supply was a well filtered and regulated analog Harrison Labs. A separate 4.5 volt supply applied PTT signal.

Looks like a little bit of DIBL to me. Not something to be expected on a big 'ol power MOSFET. But sure feels like.

The amplifier works perfectly, just exactly like it is supposed to. Up to the point of failure (which is my fault entirely) the original transistors did, too. My point in posting was that it might be prudent to perform the bias setting at the highest value of supply voltage anticipated. Can't hurt.

I would not assume that there are many amplifiers built where such large excursions of supply voltage are encountered when considering bias settings. Most of those are in linear service and not as subject to variations in threshold. This is actually the first time that I have ever blown an IRF510, come to think about it. I've used them in a lot of rigs. An incredible device and dirt cheap. Then again, this is the first time that I've pushed them this hard. My bad.


Re: 50W amplifier: power rating of TR switch

Hans Summers
 

Hi John

See the references in the theoretical section of my 50W PA manual to the document by W6JL. And his QRZ page https://www.qrz.com/db/w6jl 

Don W6JL uses a single 1N5408 diode for switching 600W RF, forward biased with 100mA. 

The requirements on Don's switch are not the same as mine in the 50W PA. And it's different again in QSX, and in QCX. Often T/R switches need to be considered at the system level. A T/R switch design can't necessarily just be transplanted from one place to everywhere else. 

Note that sufficient forward bias current is very important, as I found out in another project. All the more so at lower frequencies, where eventually the 1N4007 loses its PIN characteristics and starts to return to rectifier behaviour; then you suddenly end up with a whole bunch of unwelcome harmonics starting with the 2nd. 

Solid state T/R switching is not, in my opinion, when done properly, a reasonable way to reduce current consumption of the switch, compared to relays. But solid state does have a lot of benefits compared to relays including smaller physical size, lower cost, silent operation, very fast switching and longer lifetime. 

73 Hans G0UPL 

On Mon, Jan 27, 2020, 16:39 John Seboldt K0JD <k0jd-l@...> wrote:
Glad to see the 1N4007-based diode TR switching in the 50W amp kit. I
also have a 120W linear amp kit, long ago completed but has been waiting
for final packaging with TR switching and filtering. Do you think this
switch will work at the 100W or so level? If not, who can guide me to a
nice polished design for a switch at the 100-200W level?

John K0JD
Milwaukee, WI






Re: si5351a issue spectral output above 90Mhz

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
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Re: QRP Monthly Challenge

N3MNT
 

Two 1900hr contact both non QCX. First contact was impressed any plans to order QCX. 


Re: 50W amplifier: power rating of TR switch

ajparent1/KB1GMX
 

The 1N4007 is a 1A diode.

That is a magic intersection for power and current 1A into 50 ohms is 50W.
So for higher power you need a heftier diode with good RF switching and
they are both scarce or your forced to a RF PIN device at higher cost
as those for HF use do not go cheap.

At 120W (out I presume) that's well over 1.4A and you will need a heaftier diode.

Look here for another design for low power http://www.4sqrp.com/MagicBox.php
thats the Jim Kortge MagicBox.

That said I've run 1N4007 in parallel for a 100W amp but you need a few changes
to get that to work.  Also the reverse bias derived from the RF has to be higher so
the diodes used there also need higher voltage rating while sill rectifying at RF.
Scaling up to higher power is not trivial or easy.


Allison
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Re: QRP Monthly Challenge

Hans Summers
 

Correction... JR1BLX just after 11pm local time (2000Z)... 

On Mon, Jan 27, 2020 at 11:16 PM Hans Summers via Groups.Io <hans.summers=gmail.com@groups.io> wrote:
Hi all

Family life intervened and I only managed the last 20 minutes... I did work on 40m, 7029.5:

YU7AE Kara QRP, but not QCX
OM5VS Vlado (not QRP)

I was very pleased to be called, after the OM5VS QSO and just after 10pm local time here, by Tada JR1BLX... yes I QSO'ed Japan 9,000km with my 5W QCX 40m... very nice!

73 Hans G0UPL

On Mon, Jan 27, 2020 at 11:04 PM Brian N7BKV <cl@...> wrote:
With my QCX 40 I clearly heard KJ4BQS at about 1945z on 7030.  No joy on contact though.  I am grid CM97vu in California near Yosemite NP.  Will be back on for 0300 round.

GL

Brian N
N7BKV


Re: QRP Monthly Challenge

Hans Summers
 

Hi all

Family life intervened and I only managed the last 20 minutes... I did work on 40m, 7029.5:

YU7AE Kara QRP, but not QCX
OM5VS Vlado (not QRP)

I was very pleased to be called, after the OM5VS QSO and just after 10pm local time here, by Tada JR1BLX... yes I QSO'ed Japan 9,000km with my 5W QCX 40m... very nice!

73 Hans G0UPL

On Mon, Jan 27, 2020 at 11:04 PM Brian N7BKV <cl@...> wrote:
With my QCX 40 I clearly heard KJ4BQS at about 1945z on 7030.  No joy on contact though.  I am grid CM97vu in California near Yosemite NP.  Will be back on for 0300 round.

GL

Brian N
N7BKV


Re: Reverse Polarity Protection [Was: Experiences with 50 watt Amp]

ajparent1/KB1GMX
 

John,

If at 100W your IC725 is only pulling 15A somethings wrong.  It will be 18-20A.
Typical SSB is about 220 too 240 W input power at nominal 13.8V.  The finals
run about about 45-to 55% so for 100W out they alone require 200W DC input
and the rest of the radio another 20-40W (VFO system, a processing and
transmit driver chain).  Measured radio is about 2A RX and 3 to 4.5A TX
no signal.

Parallel devices work if done correctly.  Incorrectly they can be unstable.
Obviously RF layout is everything and most RF MOSFETs are more
efficieint at 28V and higher.   I have an amp for 6M using eight total IRF510s
in parallel push pull (4x4) for a mere 220W at 30V.  Since the device s are
current limited to get more power you have to run more voltage (and deal
with the impedance). 

The 50W amp is class C at 70% (no worse than 60%) efficiency and the power out
means power in of about 72-83W average at 20V or about 4A.
Further the IRF510 has a max current of 5.6A!

The current draw is NOT twice 3.6A, it is 3.6A average as only one is on (conducting)
at any one point in time.  Class C push pull.

So during normal ops the dissipated heat is 22 to 30W and the heat sinks are adequate.
The yabut is that the IRF510 die has a 2.5 degree C rise for every watt of heat dissipated
due to the thermal resistance of the package (die to flange) is the limiting issue.

At 30W (80w DC in for 50W RF out)  between the two devices is 15W of heat each
and the die will heat and additional 37.5C to 62.5C (room temp is 25C).  That's at
60% efficient.  If we degrade that to 50% we are at 25W each device and we are at
a whopping 87.5C die temp.  Since the heatsink is at 25C it has thermal mass
(intertia) and will not instantly heat up and will take a while to reach temp while
the die is getting warmer.  As the heatsink heats the die is hotter still.

So despite the large enough heatsink you can still heat the device to destruction before
it warm the heat sink much over 50C.  Long key down is obviously a big problem.
IT was designed for class C and CW use.

FT8 has proven to the commercial transceiver users and builders that heat is an
issue and the only solution is big heatsinks and fans.  or a corresponding reduction
in power.  This was true for the old school AM and RTTY users heating 3-500Zs
to a nice orange glow.  Now people are frying their finals never noticing the
heatsink is very HOT.

Allison
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Re: QRP Monthly Challenge

Brian N7BKV
 

With my QCX 40 I clearly heard KJ4BQS at about 1945z on 7030.  No joy on contact though.  I am grid CM97vu in California near Yosemite NP.  Will be back on for 0300 round.

GL

Brian N
N7BKV


Re: QRP Monthly Challenge

Martin DK3UW
 

heard a lot of stn but could not ge a single one despite good RBN feedback on 7 mhz
OK2BQN at the beginning but very week.

I give up for 2day
73s
Martin
DK3 UW


Re: Bias setting 50W. PA #qcx #pa

J68HZ
 

You beat me to this and it is the salient point of most in service part failures:

 

I've mentioned to people in the past (and likely forgotten) running the amp
with bias move s it from fairly efficient class C to less efficeint class AB or
AB1.  What you get for that is linearity (not required for CW) and HEAT. 
That heat is an enemy as the IRF510 has a thermal resistance from the
DIE to the flange that limits its ability to dissipate heat.   That leads to
a calculation where to keep the die under 150C you have to reduce
power for every degree C heating you have to reduce power by .29W
and we start at room temp of 25C.    The other ways to say that is for
every watt of power not put to the load is heating the device and it
increases temperature 2.5 degrees C for every watt.  So when you
run the bias up you get heat, if you key down for long periods you
get heat.  There is even with the generous heatsink there is a
finite limit.

Let’s remember that IRF510’s were designed to run in switching power supplies and as motor current switches with a duty cycle of 50% or less at  their listed full ratings.  The thermo-conductivity of the die with adequate heat transfer is what limits this part, and operating it at greater than 50% duty…. in a higher class… say AB2 or greater will cause more heat than design and possible part failure.  While this is mostly a design concern, one can surely get into trouble by cranking the on bias without understanding all of the limiting parameters involved.

 

 

Dr. William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ

 

Owner - Operator

Big Signal Ranch – K9ZC

Staunton, Illinois

 

Owner – Operator

Villa Grand Piton – J68HZ

Soufriere, St. Lucia W.I.

Rent it: www.VillaGrandPiton.com

Like us on Facebook!

 

Moderator – North American QRO Group at Groups.IO.

 

email:  bill@...

 

 

From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of ajparent1/KB1GMX
Sent: Monday, January 27, 2020 1:35 PM
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] Bias setting 50W. PA #qcx #pa

 

All was well until the current dropped and the power meter did likewise. Quite the surprise.

Don,

How long was key down?  Reason I ask is what you thought you were seeing
was likely not what you thought.

IRF510 like most Hex, Tench, VMOS, and other LDmos FETs  exhibit a bias
point shift not with voltage, but temperature.   That is with a fixed bias and
fixed drain voltage increasing die temperature will see an increasing current,
which will heat the die further and increase the current, rinse later and
repeat to failure..

What that is the Gate threshold decreases with increasing temperature.
The other reason is that with high power out the device also heats, more
power more heat.  So what appears to be bias related and if set right
its very low should not cause that.  However is set high without understanding 
how that can head to failure.

See figure 7 of datasheet page supplies not gate threshold for 175C(very hot)
and 25C (room temp).  Tj means the temperature of the junction (die).

I've mentioned to people in the past (and likely forgotten) running the amp
with bias move s it from fairly efficient class C to less efficeint class AB or
AB1.  What you get for that is linearity (not required for CW) and HEAT. 
That heat is an enemy as the IRF510 has a thermal resistance from the
DIE to the flange that limits its ability to dissipate heat.   That leads to
a calculation where to keep the die under 150C you have to reduce
power for every degree C heating you have to reduce power by .29W
and we start at room temp of 25C.    The other ways to say that is for
every watt of power not put to the load is heating the device and it
increases temperature 2.5 degrees C for every watt.  So when you
run the bias up you get heat, if you key down for long periods you
get heat.  There is even with the generous heatsink there is a
finite limit.

For those the feel the IRF510 is insufficient for a inexpensive amp a
pair of MRF137, or MRFE101 would be far more robust at more than 
60$ for a pair.  Then again they need more supporting parts as well 
so a cheap amp is out of the question.

Allison
-------------------------------
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Re: Bias setting 50W. PA #qcx #pa

ajparent1/KB1GMX
 

All was well until the current dropped and the power meter did likewise. Quite the surprise.

Don,

How long was key down?  Reason I ask is what you thought you were seeing
was likely not what you thought.

IRF510 like most Hex, Tench, VMOS, and other LDmos FETs  exhibit a bias
point shift not with voltage, but temperature.   That is with a fixed bias and
fixed drain voltage increasing die temperature will see an increasing current,
which will heat the die further and increase the current, rinse later and
repeat to failure..

What that is the Gate threshold decreases with increasing temperature.
The other reason is that with high power out the device also heats, more
power more heat.  So what appears to be bias related and if set right
its very low should not cause that.  However is set high without understanding 
how that can head to failure.

See figure 7 of datasheet page supplies not gate threshold for 175C(very hot)
and 25C (room temp).  Tj means the temperature of the junction (die).

I've mentioned to people in the past (and likely forgotten) running the amp
with bias move s it from fairly efficient class C to less efficeint class AB or
AB1.  What you get for that is linearity (not required for CW) and HEAT. 
That heat is an enemy as the IRF510 has a thermal resistance from the
DIE to the flange that limits its ability to dissipate heat.   That leads to
a calculation where to keep the die under 150C you have to reduce
power for every degree C heating you have to reduce power by .29W
and we start at room temp of 25C.    The other ways to say that is for
every watt of power not put to the load is heating the device and it
increases temperature 2.5 degrees C for every watt.  So when you
run the bias up you get heat, if you key down for long periods you
get heat.  There is even with the generous heatsink there is a
finite limit.

For those the feel the IRF510 is insufficient for a inexpensive amp a
pair of MRF137, or MRFE101 would be far more robust at more than 
60$ for a pair.  Then again they need more supporting parts as well 
so a cheap amp is out of the question.

Allison
-------------------------------
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Re: si5351a issue spectral output above 90Mhz

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 ;-)


Re: si5351a issue spectral output above 90Mhz

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

 


Re: Bias setting 50W. PA #qcx #pa

ajparent1/KB1GMX
 

An aside re: IRF510s and so called fragility.

I have four amps that are not QRPL design and all have many hours in
linear service from 10W to 55W and one using parallel device that runs a
very comfortable 220W.    To date and two of those amps are over a decade
old have not blown a device.    The 220W monster (running on 6M) is even
running the poor irf510s at 30V so its in the range of tickling the dragons tail
for explosive failure.  So far 3 contests and its performing well. 

I also have 5 QRPL amps 4 of the 10W over  a year old and running great
and now the 50W.  I see no bad behavior from them.  Should I I'll say what
and why.

Yet I see people that blast them apart at 5W all the time.  So due to poor design,
long leads and other faults some of those are legendary for being unstable
so no real surprise.  Others its a wonder on my part why or how.

Remember in most cases the IRF510 is wired in a circuit where if any reason
they go to full conduction they receive the full wrath of the power supply
(or battery).  Typically that is not milliamps its many amps if not 10s of amps.

Any who the QRPL amp is class C so the bias should be set so there is NO
drain current (less than 10ma per device).  It should not change any amount
(IE from 10 ma to 20ma from 10V to 20V).  

ALSO the devices are called MOSFETs and the gate is indeed static
sensitive.  I have killed a few in winter that way not even installed in
the project. it is possible to partially fail the gate and have it go the
rest of the way [sudden dead device] when running under power.
This also applies to BS170s, 2n7000 and other MOSFETs.

Amplifiers must always have a load, even if there is no RF input.
When testing an amp for the first time I usually put a big dummy load
at the output and a smaller on on the input as then the amp should be
stable.   I bring up the voltage and bias very carefully.   Maybe because
I treat them as if they are expensive and should not be abused.


Allison
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Re: Bias setting 50W. PA #qcx #pa

Don, ND6T
 

The gate voltage does not change. The conduction point seems to. That is what surprised me.
No, a QCX was attached and transmitting as I slowly advanced the supply voltage. Current was gradually rising. All was well until the current dropped and the power meter did likewise. Quite the surprise.
The new transistors demonstrate the change in conduction point. I had not checked the originals. By "conduction point" I mean the bias setting that just barely increases the static transmit current without input signal. Setting that back to the static current with, say, 10 volts supplied to the amp. Increase the supply 5 or 6 volts and you will see a sudden rise in current without changing the bias control. Yes, I set my bias at a 13.8 volt supply voltage but then increased the supply as an operational test. My error was that I was transmitting continually and not stopping to see if the static current changed while it was still keyed. If I had done that then I would have seen that the static current (around 82 ma.) had increased. Hindsight.

73,
Don


Re: Bias setting 50W. PA #qcx #pa

ajparent1/KB1GMX
 

IF the bias is power supply dependent you have made an error as 
the bias supply is regulated using IC1 a 78L05.  Mine changes very
little from 14 to 20V.  O went and measured mine and its a small
change and not enough to cause failure.

there is a yabut...  Running the amp without a load on the input and
output may result in oscillation and failure.  This would be true if
any of the electronic switching was less than correct as well.

I have seen neither but any error in assembly can lead to the above.

Allison
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Re: si5351a issue spectral output above 90Mhz

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


>


Re: si5351a issue spectral output above 90Mhz

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