|
Well I learned an important lesson tonight (this is why we prototype and test).? Those T50-1 cores I was using for the auto tuner are not gonna work.? They saturate at 64 watts PEP and then cause side IMD products above that power level.? I’ve moved up to the T80 cores which are good to over 120 watts.? I also found a great source for latching relays…$0.75 each in quantity 100 (cheaper in 500 and 1,000) so can keep this low power!? I built the Discriminator using an AD8302 and it works perfectly for phase and amplitude detection. ? ? This also leads me to wonder how Hans Summer can claim that his LPF’s are good to over 100 watts…? I have a few and will test that theory. ? I’m starting a blog for updates on the design and build so I don’t take up bandwidth here. ? ? 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: Like us on Facebook! ? ? email:? bill@... ?
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Hi Bill,
Where is your blog?
Also, I think Hans only claims 10W for his LPFs. Where did they safely get to?
Regards Simon VK3ELH?
|
Bill, Before you get too invested in small toroids consider that the tuner must deal with higher currents than an LPF. Saturation results from current, not power. 100 watts and high impedance ratios requires larger cores. Best to do the math before ordering. 73, ND6T
|
Not sure what brought this on but the teaching point of my post for those who would otherwise make current, voltage, saturation calculations for Inductors in RF service and build from them is to prototype first. ?My recollection in relation to Hans Summers LPF kits (nothing to do with my tuner design) is that guys are using them in amplifiers up to 100 watts... and i wonder if they have had the parts on the bench to really see the limitations of the filters. ?More clear?
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:
email:??bill@... ?
toggle quoted message
Show quoted text
On Jan 11, 2018, at 8:09 AM, Don Cantrell via Groups.Io < nd6t_6@...> wrote: Bill, Before you get too invested in small toroids consider that the tuner must deal with higher currents than an LPF. Saturation results from current, not power. 100 watts and high impedance ratios requires larger cores. Best to do the math before ordering. 73, ND6T
|
Bill, After Don's comment, I did some googling and came up with the following info. There's so much to learn about this hobby!
?
On Thursday, January 11, 2018 12:05 PM, K9HZ <bill@...> wrote:
Not sure what brought this on but the teaching point of my post for those who would otherwise make current, voltage, saturation calculations for Inductors in RF service and build from them is to prototype first. ?My recollection in relation to Hans Summers LPF kits (nothing to do with my tuner design) is that guys are using them in amplifiers up to 100 watts... and i wonder if they have had the parts on the bench to really see the limitations of the filters. ?More clear?
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:
?
On Jan 11, 2018, at 8:09 AM, Don Cantrell via Groups.Io < nd6t_6@...> wrote: Bill, Before you get too invested in small toroids consider that the tuner must deal with higher currents than an LPF. Saturation results from current, not power. 100 watts and high impedance ratios requires larger cores. Best to do the math before ordering. 73, ND6T
|
I have and use the qrp-labs LPFs I wouldn't dare go over 10W (and that's pushing it) They are rated for 5w, I don't know where or why anyone would think 100W would be acceptable.
toggle quoted message
Show quoted text
Not sure what brought this on but the teaching point of my post for those who would otherwise make current, voltage, saturation calculations for Inductors in RF service and build from them is to prototype first. ?My recollection in relation
to Hans Summers LPF kits (nothing to do with my tuner design) is that guys are using them in amplifiers up to 100 watts... and i wonder if they have had the parts on the bench to really see the limitations of the filters. ?More clear?
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
Make our villa your home for an unforgettable stay on St Lucia! Looking for an incredibly romantic spot for your honeymoon or anniversary? Or just need time away from ...
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email:??bill@...
?
On Jan 11, 2018, at 8:09 AM, Don Cantrell via Groups.Io < nd6t_6@...> wrote:
Bill, Before you get too invested in small toroids consider that the tuner must deal with higher currents than an LPF. Saturation results from current, not power. 100 watts and high impedance ratios requires larger cores. Best to do the math before ordering.
73, ND6T
|
While I acknowledge the discussion is regarding choice of toroids
for an AMU project.
QRP Labs / Hans, Points out that his LPF design is suitable UP TO 5
watts as supplied in his kits.
See bottom section of the web page
Alan
On 11/01/2018 17:05, K9HZ wrote:
toggle quoted message
Show quoted text
Not sure what brought this on but the teaching point of my post
for those who would otherwise make current, voltage, saturation
calculations for Inductors in RF service and build from them is to
prototype first. ?My recollection in relation to Hans Summers LPF
kits (nothing to do with my tuner design) is that guys are using
them in amplifiers up to 100 watts... and i wonder if they have
had the parts on the bench to really see the limitations of the
filters. ?More clear?
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:
email:??bill@...
?
On Jan 11, 2018, at 8:09 AM, Don Cantrell via Groups.Io < nd6t_6@...>
wrote:
Bill, Before you get too invested in small toroids consider
that the tuner must deal with higher currents than an LPF.
Saturation results from current, not power. 100 watts and high
impedance ratios requires larger cores. Best to do the math
before ordering. 73, ND6T
|
Perhaps you are thinking of this SWR Bridge?
It is apparently good for up to 100W, intermittent use.
toggle quoted message
Show quoted text
On Thu, Jan 11, 2018 at 09:05 am, K9HZ wrote:
My recollection in relation to Hans Summers LPF kits (nothing to do with my tuner design) is that guys are using them in amplifiers up to 100 watts...
|
This SWR birdge:
? ?
toggle quoted message
Show quoted text
On Thu, Jan 11, 2018 at 10:11 am, Jerry Gaffke wrote:
Perhaps you are thinking of this SWR Bridge?
It is apparently good for up to 100W, intermittent use.
|
My comment is in regards to discussion in his group.?
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:
email:??bill@... ?
toggle quoted message
Show quoted text
On Jan 11, 2018, at 11:31 AM, Alan de G1FXB via Groups.Io < g1fxb@...> wrote:
While I acknowledge the discussion is regarding choice of toroids
for an AMU project.
QRP Labs / Hans, Points out that his LPF design is suitable UP TO 5
watts as supplied in his kits.
See bottom section of the web page
Alan
On 11/01/2018 17:05, K9HZ wrote:
Not sure what brought this on but the teaching point of my post
for those who would otherwise make current, voltage, saturation
calculations for Inductors in RF service and build from them is to
prototype first. ?My recollection in relation to Hans Summers LPF
kits (nothing to do with my tuner design) is that guys are using
them in amplifiers up to 100 watts... and i wonder if they have
had the parts on the bench to really see the limitations of the
filters. ?More clear?
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:
email:??bill@...
?
On Jan 11, 2018, at 8:09 AM, Don Cantrell via Groups.Io < nd6t_6@...>
wrote:
Bill, Before you get too invested in small toroids consider
that the tuner must deal with higher currents than an LPF.
Saturation results from current, not power. 100 watts and high
impedance ratios requires larger cores. Best to do the math
before ordering. 73, ND6T
|
No It looks like i need to refrain from posting and just direct you-all to my website now when the kit is complete.?
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:
email:??bill@... ?
toggle quoted message
Show quoted text
On Jan 11, 2018, at 12:11 PM, Jerry Gaffke via Groups.Io < jgaffke@...> wrote: Perhaps you are thinking of this SWR Bridge? It is apparently good for up to 100W, intermittent use. On Thu, Jan 11, 2018 at 09:05 am, K9HZ wrote:
My recollection in relation to Hans Summers LPF kits (nothing to do with my tuner design) is that guys are using them in amplifiers up to 100 watts...
|
Thanks'
Have you a link to the thread?
I'm one of the ones building a PA based on the work of WA2EBY.)
Alan
On 11/01/2018 18:16, K9HZ wrote:
toggle quoted message
Show quoted text
My comment is in regards to discussion in his group.?
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:
email:??bill@...
?
On Jan 11, 2018, at 11:31 AM, Alan de G1FXB via Groups.Io < g1fxb@...>
wrote:
While I acknowledge the discussion is regarding choice of
toroids for an AMU project.
QRP Labs / Hans, Points out that his LPF design is suitable UP
TO 5 watts as supplied in his kits.
See bottom section of the web page
Alan
On 11/01/2018 17:05, K9HZ wrote:
Not sure what brought this on but the teaching point of my
post for those who would otherwise make current, voltage,
saturation calculations for Inductors in RF service and
build from them is to prototype first. ?My recollection in
relation to Hans Summers LPF kits (nothing to do with my
tuner design) is that guys are using them in amplifiers up
to 100 watts... and i wonder if they have had the parts on
the bench to really see the limitations of the filters.
?More clear?
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:
email:??bill@...
?
On Jan 11, 2018, at 8:09 AM, Don Cantrell via Groups.Io
< nd6t_6@...>
wrote:
Bill, Before you get too invested in small toroids
consider that the tuner must deal with higher currents
than an LPF. Saturation results from current, not power.
100 watts and high impedance ratios requires larger
cores. Best to do the math before ordering. 73, ND6T
|
The QRP Labs kit ordering page says 5W. In fact I believe they are fine for at least 10W and this is noted on the LPF Kit page
10W is well within the specified ratings of the cores according to the original G-QRP club website article (see for PDF of the article). 10W is also well within the voltage ratings of the NP0/C0G low-loss RF capacitors used in the LPF kits.
It may be possible to push it beyond 10W. Tolerance of SWR mismatch would be reduced (and eventually disappear). Another factor is component tolerances and winding differences resulting in variation in inductance values. If the filter starts producing attenuation at frequencies lower than it should... i.e. at the transmit frequency - then remember that attenuation means reduction in output power. The reduced power has to go *somewhere* and in fact it is likely to be largely dissipated as heat in the LPF components.?
I'm not sure how far beyond 10W would be Ok before generating smoke. Anyone who has these QRP Labs 7-element (3 inductors, 4 capacitors) filters on their tiny 1.5 x 0.5 inches board (37 x 13mm) would intuitively not think it right to put 100W through it. Even if everything measured perfectly I wouldn't want to go anywhere near 100W!
A confession... during PA development of the QRP Labs 5W HF PA kit (Class C) , I did put 30W continuous keyed through a QRP Labs LPF kit... No smoke ;-)
73 Hans G0UPL? ?
|
The auto tuner capacitors have to be rated for the highest voltage at the highest impedance. This is an extreme requirement. To illustrate, a 10 watt RF signal will put out a peak 30v across a 50 ohms load. However, an end-fed half wave will exhibit about 3000 ohms impedance. At 10 watts, this amounts to a voltahe peak of 250 v. You have to choose the capacitors for 250v rating even for a QRP power level.
On 12 Jan 2018 12:32 am, "Hans Summers" < hans.summers@...> wrote: The QRP Labs kit ordering page says 5W. In fact I believe they are fine for at least 10W and this is noted on the LPF Kit page
10W is well within the specified ratings of the cores according to the original G-QRP club website article (see for PDF of the article). 10W is also well within the voltage ratings of the NP0/C0G low-loss RF capacitors used in the LPF kits.
It may be possible to push it beyond 10W. Tolerance of SWR mismatch would be reduced (and eventually disappear). Another factor is component tolerances and winding differences resulting in variation in inductance values. If the filter starts producing attenuation at frequencies lower than it should... i.e. at the transmit frequency - then remember that attenuation means reduction in output power. The reduced power has to go *somewhere* and in fact it is likely to be largely dissipated as heat in the LPF components.?
I'm not sure how far beyond 10W would be Ok before generating smoke. Anyone who has these QRP Labs 7-element (3 inductors, 4 capacitors) filters on their tiny 1.5 x 0.5 inches board (37 x 13mm) would intuitively not think it right to put 100W through it. Even if everything measured perfectly I wouldn't want to go anywhere near 100W!
A confession... during PA development of the QRP Labs 5W HF PA kit (Class C) , I did put 30W continuous keyed through a QRP Labs LPF kit... No smoke ;-)
73 Hans G0UPL? ?
|
Han's, Thanks for a definitive statement.
A selection of filters and a nice relay switch double sided
daughter board would have being nice and I wasn't expecting to be
able to use yours for circa 40 watts.
I'll plagiarise another design and make do with vero... :-[
Alan
On 11/01/2018 19:01, Hans Summers
wrote:
toggle quoted message
Show quoted text
The QRP Labs kit ordering page says 5W. In fact I
believe they are fine for at least 10W and this is noted on the
LPF Kit page
10W is well within the specified ratings of the
cores according to the original G-QRP club website article
(see for
PDF of the article). 10W is also well within the voltage
ratings of the NP0/C0G low-loss RF capacitors used in the LPF
kits.
It may be possible to push it beyond 10W.
Tolerance of SWR mismatch would be reduced (and eventually
disappear). Another factor is component tolerances and winding
differences resulting in variation in inductance values. If
the filter starts producing attenuation at frequencies lower
than it should... i.e. at the transmit frequency - then
remember that attenuation means reduction in output power. The
reduced power has to go *somewhere* and in fact it is likely
to be largely dissipated as heat in the LPF components.?
I'm not sure how far beyond 10W would be Ok
before generating smoke. Anyone who has these QRP Labs
7-element (3 inductors, 4 capacitors) filters on their tiny
1.5 x 0.5 inches board (37 x 13mm) would intuitively not think
it right to put 100W through it. Even if everything measured
perfectly I wouldn't want to go anywhere near 100W!
A confession... during PA development of the QRP
Labs 5W HF PA kit (Class C) , I did
put 30W continuous keyed through a QRP Labs LPF kit... No
smoke ;-)
73 Hans G0UPL?
?
|
Farhan
An interesting question. In the LPF, does the capacitor see the peak-peak voltage? It is connected to ground. So, doesn't that mean it sees the peak voltage (1/2 the peak-peak)?
73 Hans G0UPL
|
You are correct, the LFP caps see half of the peak-to-peak voltage, since that RF is centered on ground. In Farhan's example of 3000 ohms at 10W: Vrms*Vrms/Ohms = Watts? ? so? ? Vrms = sqrt(Watts * Ohms) = sqrt(3000*10) = 173.2 Volts RMS across the cap The peak voltage is sqrt(2)=1.414 time the RMS voltage (assuming this is a sine wave halfway through the filter), so 173.2*1.414 = 245 Volts peak to ground. Hans, on a completely different note, you may find this interesting, It takes the three si5351 msynth register values for p1, p2, p3 as reported by Clockbuilderpro and back computes values for a, b, c Just got it to work this morning, seems right but may yet have issues. I assume a,b,c must all be integers. However, a and b could take on fractional values in increments of 1/128 and still yield integer values for p1,p2,p3. I wonder if that's legal, though not seeing that out of Clockbuilderpro. Clockbuilderpro seems to do a massive search for the lowest possible integer values of a,b,c in both PLL and output msynth's when I ask for one output of a particular frequency, often leaving both msynths in fractional mode. So low integer values are likely the primary way to reduce phase noise. This is a considerably different approach than all the C code for the si5351 I find out there on the interwebs Regardless, my guess is that the si5351 has less phase noise in all cases than the typical LC vfo.
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char** argv) {
int floorg, floorc, ax128, bx128, a, b, c, p1, p2, p3;
if (argc==4) {
p1 = atoi(argv[1]);
p2 = atoi(argv[2]);
p3 = atoi(argv[3]);
} else {
printf("Usage: Takes 3 int args for si5351 msynth reg values p1,p2,p3\n");
printf("reports int values for a,b,c where a+b/c is the divide ratio\n");
exit(1);
}
printf(" p1:%d p2:%d p3:%d \n", p1, p2, p3);
c = p3;
// Given an si5351 msynth divide of (a + b/c):
// p1 = 128*a + Floor(128*b/c) - 512
// p2 = 128*b - c*Floor(128*b/c)
// p3 = c // All from Silabs AN619
// Since b<c, Floor(128*b/c) has a value between 0 and 127 inclusive
// We compute a and b for each of those values, determine which is legal
for (floorg=0; floorg<128; floorg++) { // Guess value of Floor(128*b/c)
ax128 = p1 + 512 - floorg; // Where ax128 is a*128
bx128 = p2 + c*floorg; // and bx128 is b*128
floorc = bx128/c; // Compute value Floor(128*b/c)
a = ax128/128; b=bx128/128;
// Report result if value of Floor(128*b/c) is consistent with guess,
// and if the computed values for a and b are integers, and b<c.
if (floorc==floorg && ax128%128==0 && bx128%128==0 && b<c)
printf(" a:%d b:%d c:%d \n", a, b, c);
}
exit(0);
}
Jerry, KE7ER
On Thu, Jan 11, 2018 at 12:39 pm, Hans Summers wrote:
An interesting question. In the LPF, does the capacitor see the peak-peak voltage? It is connected to ground. So, doesn't that mean it sees the peak voltage (1/2 the peak-peak)?
?
|
Hans, My calculations were for an L network that matches high impedance loads, not for LPFs that have low impedance on both sides (like the qrp labs LPFs). Btw, folks, the qrp labs labs use very high Q capacitors. These are a must of low loss. -f
On 12 Jan 2018 2:33 am, "Jerry Gaffke via Groups.Io" <jgaffke= [email protected]> wrote: You are correct, the LFP caps see half of the peak-to-peak voltage, since that RF is centered on ground.
In Farhan's example of 3000 ohms at 10W:
Vrms*Vrms/Ohms = Watts? ? so? ? Vrms = sqrt(Watts * Ohms) = sqrt(3000*10) = 173.2 Volts RMS across the cap
The peak voltage is sqrt(2)=1.414 time the RMS voltage (assuming this is a sine wave halfway through the filter), so 173.2*1.414 = 245 Volts peak to ground.
Hans, on a completely different note, you may find this interesting,
It takes the three si5351 msynth register values for p1, p2, p3 as reported by Clockbuilderpro
and back computes values for a, b, c
Just got it to work this morning, seems right but may yet have issues.
I assume a,b,c must all be integers.
However, a and b could take on fractional values in increments of 1/128 and still yield integer values for p1,p2,p3.
I wonder if that's legal, though not seeing that out of Clockbuilderpro.
Clockbuilderpro seems to do a massive search for the lowest possible integer values of a,b,c in both PLL and output msynth's
when I ask for one output of a particular frequency, often leaving both msynths in fractional mode.
So low integer values are likely the primary way to reduce phase noise.
This is a considerably different approach than all the C code for the si5351 I find out there on the interwebs
Regardless, my guess is that the si5351 has less phase noise in all cases than the typical LC vfo.
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char** argv) {
int floorg, floorc, ax128, bx128, a, b, c, p1, p2, p3;
if (argc==4) {
p1 = atoi(argv[1]);
p2 = atoi(argv[2]);
p3 = atoi(argv[3]);
} else {
printf("Usage: Takes 3 int args for si5351 msynth reg values p1,p2,p3\n");
printf("reports int values for a,b,c where a+b/c is the divide ratio\n");
exit(1);
}
printf(" p1:%d p2:%d p3:%d \n", p1, p2, p3);
c = p3;
// Given an si5351 msynth divide of (a + b/c):
// p1 = 128*a + Floor(128*b/c) - 512
// p2 = 128*b - c*Floor(128*b/c)
// p3 = c // All from Silabs AN619
// Since b<c, Floor(128*b/c) has a value between 0 and 127 inclusive
// We compute a and b for each of those values, determine which is legal
for (floorg=0; floorg<128; floorg++) { // Guess value of Floor(128*b/c)
ax128 = p1 + 512 - floorg; // Where ax128 is a*128
bx128 = p2 + c*floorg; // and bx128 is b*128
floorc = bx128/c; // Compute value Floor(128*b/c)
a = ax128/128; b=bx128/128;
// Report result if value of Floor(128*b/c) is consistent with guess,
// and if the computed values for a and b are integers, and b<c.
if (floorc==floorg && ax128%128==0 && bx128%128==0 && b<c)
printf(" a:%d b:%d c:%d \n", a, b, c);
}
exit(0);
}
Jerry, KE7ER
On Thu, Jan 11, 2018 at 12:39 pm, Hans Summers wrote:
An interesting question. In the LPF, does the capacitor see the peak-peak voltage? It is connected to ground. So, doesn't that mean it sees the peak voltage (1/2 the peak-peak)?
?
|
If the LPF is presented a high impedance load (such as a broken antenna connector) the voltages will go up.
Though not as high as the 3000 ohm example suggested, the IRF510 cannot send 10 Watts into 3000 ohms.
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Show quoted text
On Thu, Jan 11, 2018 at 06:44 pm, Ashhar Farhan wrote:
Hans,
My calculations were for an L network that matches high impedance loads, not for LPFs that have low impedance on both sides (like the qrp labs LPFs).
Btw, folks, the qrp labs labs use very high Q capacitors. These are a must of low loss.
?
|
OK, it increases, but not by much.
I ran an LTSpice simulation of the Bitx40 final, see a 50% rise in peak voltages when the antenna goes open circuit.
Not something I had ever though much about.
toggle quoted message
Show quoted text
On Thu, Jan 11, 2018 at 06:51 pm, Jerry Gaffke wrote:
If the LPF is presented a high impedance load (such as a broken antenna connector) the voltages will go up.
Though not as high as the 3000 ohm example suggested, the IRF510 cannot send 10 Watts into 3000 ohms.
|
K9HZ
Don, ND6T
N8DAH
Alan de G1FXB