Date   

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


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
-------------------------------
Please reply on list so we can share.
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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

 


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

Don, ND6T
 

Ron, a reasonable person would think that the bias would remain the same over various supply voltages. That's why I didn't bother to test at other than  the 13.8 volts. Replacing those transistors reminded me to always check for my own edification. Yes, the bias supply is rock solid and I have always thought that the source to gate bias was a stable thing even if there was no drain voltage. I was so wrong. It doesn't change much, just enough over that wide range to cause grief. I should have set it at 20 volts. Next time I will check it.
I took my time building it, enjoying the experience, and it worked perfectly the first time. It's very pleasant to have a new kit behave that way. This wasn't a big deal for me as I had plenty of spares, thanks to Hans' choice of commonly available parts.

Try it the next time that you have the amplifier open and with a variable supply. Please let me know if I was wrong. If I wasn't incorrect then perhaps the next version of that spectacular instruction manual migh include it.
73, Don


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

George Korper
 

In my case I was greedy and did not take the warning in the manual seriously enough to turn the

Bias anti clockwise from the setting a tad. 

On 20 that tad costs output more than on 40 from what i'm reading. But my lesson was learning to take that warning very, very seriously. 
Again no one will hear the slight difference. Don, I'm happy you noticed this because this is where my troubles started and I melted down before I blew the finals. 
The fact is that even though the bias is at a constant voltage, it is very sensitive. My advice to myself is to spend longer in the practice mode,
as I don't have replacement transistors. They are on order for sure because I see all over the web to keep a few taped to the enclosure (hi hi). 


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

Ronald Taylor
 

Don, I’m not sure how your issue actully occurred. The bias voltage is taken from a 5 volt regulator that puts out a constant 5 volts to the bias set pot whether your input voltage is 13.8 or 20 volts. So your bias should not change with input voltage. It must have been caused by something else…

Ron

On Mon, Jan 27, 2020 at 7:18 AM Don, ND6T via Groups.Io <nd6t_6=yahoo.com@groups.io> wrote:
Another lesson learned! It seems to be most prudent to set the power supply to the highest anticipated voltage when setting the bias adjustment.
I set mine at 13.8 volts when I first set mine but, while doing initial testing, I increased it. When it reached 20 volts both output transistors suddenly and quietly expired. After I replaced them I found that, indeed, the bias adjustment was supply voltage dependent.
Since I was using a current limited supply nothing else was affected. In fact, I spent a couple of hours searching for some other (non-existent) fault since there was no visual evidence of a problem. The output merely dropped from 45 watts to a couple of milliwatts in an instant. 
Great kit! Highly recommend it. I probably won't use the amplifier much but It was too nifty to pass up. Thanks Hans! 73, Don


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

John Clements
 

With a 50W amp I would consider or be on the lookout for heat sinking as it could get pretty warm/hot depending on mode and length: (50W/13.8V=3.6A*2=~7.25A),  The "2" is efficiency of the amplifier.  For example my IC-725 will run at 100W output but the draw is closer to 15A.  100W/13.8V=~7.2A/15A = 50%  1/50%=2.  So you can see the efficiency is ~50%.  You amp draw mileage may vary.  Additionally at 7.25A the mofset will have a  voltage drop of .47V (7.25A * .065ohm) and dissipate ~3.4W of power (AKA heat). Of course these are continuous so CW and SSB will be less heat dissipated.

I have used them on my IC-725 at full power (100W or about 15A of draw) and at that point the resistive losses and heat are too much even with CW and SSB (both styles).  Although the MOSFETs survived the beating (barely), I had to increase the power supply voltage to compensate for the voltage drop across the MOSFET (which of course causes more heat......).  One thing I have not tried is to parallel  them together for more current flow. Initially made to go inside QRP gear but I do like to push the limits.

72 John kc9on


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

George Korper
 

Maybe Hans could just put in the circuit two holes and I'll put in a Schottky diode ( Hi Hi).
If its 25 watts on 20 instead of 30 so what. Sunspots will cure all this rubbish. 


Re: si5351a issue spectral output above 90Mhz

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

>

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