MH
Matt Huszagh
Sun, Apr 23, 2023 1:21 AM
Hi,
I built Bruce Griffith's common emitter transformer feedback amplifier
(http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
mostly identical to the first schematic in the previous link. But, I
added a second stage for increased reverse isolation. I placed 65R
across the first stage output, which gave me about the right gain in
simulations. I also added a second 2-stage buffer in parallel. I
custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
use this to get buffered/isolated front and rear 10 MHz outputs from the
internal 10811A in my HP 5065A.
Overall, the performance seems excellent. I've attached a residual phase
noise measurement, showing the combined 2 cascaded stages. These results
seem only a little worse than what John Miles measured for his single
stage (http://www.ke5fx.com/norton.htm).
I measured the reverse isolation to be 76 dB and 77 dB and the isolation
between outputs to be 49 dB or 59 dB, depending on which output I inject
the signal into. This is all at 10 MHz of course.
However, I am experiencing one issue, which is that if I add a
capacitive load to the output (such as from a coaxial cable), I get
large oscillations (10+Vrms) at the output. The oscillations are at
about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
when the output is unloaded, nor when it's loaded by the typical
50ohms. Since this only appeared when I added a capacitive load to the
output, I figured it was due to the parallel LC between the output
winding and coaxial capacitance. My output impedance came out a bit
lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
between the output winding and coaxial connector. This cut the
oscillations nearly in half (now about 6.5Vrms), but didn't kill
it. I've also tried loading down the output with a 1k parallel resistor,
but though this helps too, the oscillations are still pretty
large. Going lower helps further, but I don't want to load down the
output too much.
Any thoughts? Has anyone else seen this behavior? I can post more
details and images.
Thanks,
Matt
Hi,
I built Bruce Griffith's common emitter transformer feedback amplifier
(http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
mostly identical to the first schematic in the previous link. But, I
added a second stage for increased reverse isolation. I placed 65R
across the first stage output, which gave me about the right gain in
simulations. I also added a second 2-stage buffer in parallel. I
custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
use this to get buffered/isolated front and rear 10 MHz outputs from the
internal 10811A in my HP 5065A.
Overall, the performance seems excellent. I've attached a residual phase
noise measurement, showing the combined 2 cascaded stages. These results
seem only a little worse than what John Miles measured for his single
stage (http://www.ke5fx.com/norton.htm).
I measured the reverse isolation to be 76 dB and 77 dB and the isolation
between outputs to be 49 dB or 59 dB, depending on which output I inject
the signal into. This is all at 10 MHz of course.
However, I am experiencing one issue, which is that if I add a
capacitive load to the output (such as from a coaxial cable), I get
large oscillations (10+Vrms) at the output. The oscillations are at
about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
when the output is unloaded, nor when it's loaded by the typical
50ohms. Since this only appeared when I added a capacitive load to the
output, I figured it was due to the parallel LC between the output
winding and coaxial capacitance. My output impedance came out a bit
lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
between the output winding and coaxial connector. This cut the
oscillations nearly in half (now about 6.5Vrms), but didn't kill
it. I've also tried loading down the output with a 1k parallel resistor,
but though this helps too, the oscillations are still pretty
large. Going lower helps further, but I don't want to load down the
output too much.
Any thoughts? Has anyone else seen this behavior? I can post more
details and images.
Thanks,
Matt
T
timeok@timeok.it
Sun, Apr 23, 2023 1:11 PM
Hi Matt,
I've built a buffer amplifier just for the use you want.
The design comes from Bruce and was optimized and engineered by me and a group of friends.
You can see the solution at the addresses I enclose.
http://www.timeok.it/wp-content/uploads/2015/08/Frequency-Distribution-Design-Basic-Module-v-4.1.pdf
http://www.timeok.it/wp-content/uploads/2015/08/hp5065A-option-H10-v-2.00-2.pdf
Compared to the one you propose, it has several improvements, such as a higher input impedance and a much higher reverse isolation that exceeds 100dB.
I hope this info can be useful to you,
Luciano
Luciano P. S. Paramithiotti
timeok@timeok.it
www.timeok.it
Da "Matt Huszagh via time-nuts" time-nuts@lists.febo.com
A "Discussion of precise time and frequency measurement" time-nuts@lists.febo.com
Cc "Matt Huszagh" huszaghmatt@gmail.com
Data Sat, 22 Apr 2023 18:21:07 -0700
Oggetto [time-nuts] Bruce's common emitter transformer feedback amplifier: oscillations with capacitive load
Hi,
I built Bruce Griffith's common emitter transformer feedback amplifier
(http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
mostly identical to the first schematic in the previous link. But, I
added a second stage for increased reverse isolation. I placed 65R
across the first stage output, which gave me about the right gain in
simulations. I also added a second 2-stage buffer in parallel. I
custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
use this to get buffered/isolated front and rear 10 MHz outputs from the
internal 10811A in my HP 5065A.
Overall, the performance seems excellent. I've attached a residual phase
noise measurement, showing the combined 2 cascaded stages. These results
seem only a little worse than what John Miles measured for his single
stage (http://www.ke5fx.com/norton.htm).
I measured the reverse isolation to be 76 dB and 77 dB and the isolation
between outputs to be 49 dB or 59 dB, depending on which output I inject
the signal into. This is all at 10 MHz of course.
However, I am experiencing one issue, which is that if I add a
capacitive load to the output (such as from a coaxial cable), I get
large oscillations (10+Vrms) at the output. The oscillations are at
about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
when the output is unloaded, nor when it's loaded by the typical
50ohms. Since this only appeared when I added a capacitive load to the
output, I figured it was due to the parallel LC between the output
winding and coaxial capacitance. My output impedance came out a bit
lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
between the output winding and coaxial connector. This cut the
oscillations nearly in half (now about 6.5Vrms), but didn't kill
it. I've also tried loading down the output with a 1k parallel resistor,
but though this helps too, the oscillations are still pretty
large. Going lower helps further, but I don't want to load down the
output too much.
Any thoughts? Has anyone else seen this behavior? I can post more
details and images.
Thanks,
Matt
Hi Matt,
I've built a buffer amplifier just for the use you want.
The design comes from Bruce and was optimized and engineered by me and a group of friends.
You can see the solution at the addresses I enclose.
http://www.timeok.it/wp-content/uploads/2015/08/Frequency-Distribution-Design-Basic-Module-v-4.1.pdf
http://www.timeok.it/wp-content/uploads/2015/08/hp5065A-option-H10-v-2.00-2.pdf
Compared to the one you propose, it has several improvements, such as a higher input impedance and a much higher reverse isolation that exceeds 100dB.
I hope this info can be useful to you,
Luciano
Luciano P. S. Paramithiotti
timeok@timeok.it
www.timeok.it
Da "Matt Huszagh via time-nuts" time-nuts@lists.febo.com
A "Discussion of precise time and frequency measurement" time-nuts@lists.febo.com
Cc "Matt Huszagh" huszaghmatt@gmail.com
Data Sat, 22 Apr 2023 18:21:07 -0700
Oggetto [time-nuts] Bruce's common emitter transformer feedback amplifier: oscillations with capacitive load
Hi,
I built Bruce Griffith's common emitter transformer feedback amplifier
(http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
mostly identical to the first schematic in the previous link. But, I
added a second stage for increased reverse isolation. I placed 65R
across the first stage output, which gave me about the right gain in
simulations. I also added a second 2-stage buffer in parallel. I
custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
use this to get buffered/isolated front and rear 10 MHz outputs from the
internal 10811A in my HP 5065A.
Overall, the performance seems excellent. I've attached a residual phase
noise measurement, showing the combined 2 cascaded stages. These results
seem only a little worse than what John Miles measured for his single
stage (http://www.ke5fx.com/norton.htm).
I measured the reverse isolation to be 76 dB and 77 dB and the isolation
between outputs to be 49 dB or 59 dB, depending on which output I inject
the signal into. This is all at 10 MHz of course.
However, I am experiencing one issue, which is that if I add a
capacitive load to the output (such as from a coaxial cable), I get
large oscillations (10+Vrms) at the output. The oscillations are at
about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
when the output is unloaded, nor when it's loaded by the typical
50ohms. Since this only appeared when I added a capacitive load to the
output, I figured it was due to the parallel LC between the output
winding and coaxial capacitance. My output impedance came out a bit
lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
between the output winding and coaxial connector. This cut the
oscillations nearly in half (now about 6.5Vrms), but didn't kill
it. I've also tried loading down the output with a 1k parallel resistor,
but though this helps too, the oscillations are still pretty
large. Going lower helps further, but I don't want to load down the
output too much.
Any thoughts? Has anyone else seen this behavior? I can post more
details and images.
Thanks,
Matt
BC
Bob Camp
Sun, Apr 23, 2023 1:42 PM
Hi
A schematic would be a big help here …..
Flying blind, best guess is that your output stage has an issue. If the two output bufferers
are identical to the input stage, that’s probably the issue. A different configuration for the
buffers would be the most obvious fix.
Bob
On Apr 22, 2023, at 9:21 PM, Matt Huszagh via time-nuts time-nuts@lists.febo.com wrote:
Hi,
I built Bruce Griffith's common emitter transformer feedback amplifier
(http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
mostly identical to the first schematic in the previous link. But, I
added a second stage for increased reverse isolation. I placed 65R
across the first stage output, which gave me about the right gain in
simulations. I also added a second 2-stage buffer in parallel. I
custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
use this to get buffered/isolated front and rear 10 MHz outputs from the
internal 10811A in my HP 5065A.
Overall, the performance seems excellent. I've attached a residual phase
noise measurement, showing the combined 2 cascaded stages. These results
seem only a little worse than what John Miles measured for his single
stage (http://www.ke5fx.com/norton.htm).
I measured the reverse isolation to be 76 dB and 77 dB and the isolation
between outputs to be 49 dB or 59 dB, depending on which output I inject
the signal into. This is all at 10 MHz of course.
However, I am experiencing one issue, which is that if I add a
capacitive load to the output (such as from a coaxial cable), I get
large oscillations (10+Vrms) at the output. The oscillations are at
about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
when the output is unloaded, nor when it's loaded by the typical
50ohms. Since this only appeared when I added a capacitive load to the
output, I figured it was due to the parallel LC between the output
winding and coaxial capacitance. My output impedance came out a bit
lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
between the output winding and coaxial connector. This cut the
oscillations nearly in half (now about 6.5Vrms), but didn't kill
it. I've also tried loading down the output with a 1k parallel resistor,
but though this helps too, the oscillations are still pretty
large. Going lower helps further, but I don't want to load down the
output too much.
Any thoughts? Has anyone else seen this behavior? I can post more
details and images.
Thanks,
Matt
<bruce.small.jpg>_______________________________________________
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Hi
A schematic would be a big help here …..
Flying blind, best guess is that your output stage has an issue. If the two output bufferers
are identical to the input stage, that’s probably the issue. A different configuration for the
buffers would be the most obvious fix.
Bob
> On Apr 22, 2023, at 9:21 PM, Matt Huszagh via time-nuts <time-nuts@lists.febo.com> wrote:
>
> Hi,
>
> I built Bruce Griffith's common emitter transformer feedback amplifier
> (http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
> mostly identical to the first schematic in the previous link. But, I
> added a second stage for increased reverse isolation. I placed 65R
> across the first stage output, which gave me about the right gain in
> simulations. I also added a second 2-stage buffer in parallel. I
> custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
> use this to get buffered/isolated front and rear 10 MHz outputs from the
> internal 10811A in my HP 5065A.
>
> Overall, the performance seems excellent. I've attached a residual phase
> noise measurement, showing the combined 2 cascaded stages. These results
> seem only a little worse than what John Miles measured for his single
> stage (http://www.ke5fx.com/norton.htm).
>
> I measured the reverse isolation to be 76 dB and 77 dB and the isolation
> between outputs to be 49 dB or 59 dB, depending on which output I inject
> the signal into. This is all at 10 MHz of course.
>
> However, I am experiencing one issue, which is that if I add a
> capacitive load to the output (such as from a coaxial cable), I get
> large oscillations (10+Vrms) at the output. The oscillations are at
> about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
> when the output is unloaded, nor when it's loaded by the typical
> 50ohms. Since this only appeared when I added a capacitive load to the
> output, I figured it was due to the parallel LC between the output
> winding and coaxial capacitance. My output impedance came out a bit
> lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
> between the output winding and coaxial connector. This cut the
> oscillations nearly in half (now about 6.5Vrms), but didn't kill
> it. I've also tried loading down the output with a 1k parallel resistor,
> but though this helps too, the oscillations are still pretty
> large. Going lower helps further, but I don't want to load down the
> output too much.
>
> Any thoughts? Has anyone else seen this behavior? I can post more
> details and images.
>
> Thanks,
> Matt
>
> <bruce.small.jpg>_______________________________________________
> time-nuts mailing list -- time-nuts@lists.febo.com
> To unsubscribe send an email to time-nuts-leave@lists.febo.com
MH
Matt Huszagh
Sun, Apr 23, 2023 2:15 PM
A schematic would be a big help here …..
Flying blind, best guess is that your output stage has an issue. If the two output bufferers
are identical to the input stage, that’s probably the issue. A different configuration for the
buffers would be the most obvious fix.
Hi Bob,
I've attached the full schematic. Hope that helps.
Matt
Bob Camp via time-nuts <time-nuts@lists.febo.com> writes:
> A schematic would be a big help here …..
>
> Flying blind, best guess is that your output stage has an issue. If the two output bufferers
> are identical to the input stage, that’s probably the issue. A different configuration for the
> buffers would be the most obvious fix.
Hi Bob,
I've attached the full schematic. Hope that helps.
Matt
NP
Neal Pollack
Sun, Apr 23, 2023 4:14 PM
Luciano:
I would like to try the amplifier for my 10MHz output.
Where can I find details for the toroid transformer,
how many windings, and what gauge wire?
Thanks,
Neal
N6YFM
On Sun, Apr 23, 2023 at 6:32 AM Luciano via time-nuts <
time-nuts@lists.febo.com> wrote:
Hi Matt,
I've built a buffer amplifier just for the use you want.
The design comes from Bruce and was optimized and engineered by me and
a group of friends.
You can see the solution at the addresses I enclose.
http://www.timeok.it/wp-content/uploads/2015/08/Frequency-Distribution-Design-Basic-Module-v-4.1.pdf
http://www.timeok.it/wp-content/uploads/2015/08/hp5065A-option-H10-v-2.00-2.pdf
Compared to the one you propose, it has several improvements, such as a
higher input impedance and a much higher reverse isolation that exceeds
100dB.
I hope this info can be useful to you,
Luciano
Luciano P. S. Paramithiotti
timeok@timeok.it
www.timeok.it
Da "Matt Huszagh via time-nuts" time-nuts@lists.febo.com
A "Discussion of precise time and frequency measurement"
time-nuts@lists.febo.com
Cc "Matt Huszagh" huszaghmatt@gmail.com
Data Sat, 22 Apr 2023 18:21:07 -0700
Oggetto [time-nuts] Bruce's common emitter transformer feedback
amplifier: oscillations with capacitive load
Hi,
I built Bruce Griffith's common emitter transformer feedback amplifier
(http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
mostly identical to the first schematic in the previous link. But, I
added a second stage for increased reverse isolation. I placed 65R
across the first stage output, which gave me about the right gain in
simulations. I also added a second 2-stage buffer in parallel. I
custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
use this to get buffered/isolated front and rear 10 MHz outputs from the
internal 10811A in my HP 5065A.
Overall, the performance seems excellent. I've attached a residual phase
noise measurement, showing the combined 2 cascaded stages. These results
seem only a little worse than what John Miles measured for his single
stage (http://www.ke5fx.com/norton.htm).
I measured the reverse isolation to be 76 dB and 77 dB and the isolation
between outputs to be 49 dB or 59 dB, depending on which output I inject
the signal into. This is all at 10 MHz of course.
However, I am experiencing one issue, which is that if I add a
capacitive load to the output (such as from a coaxial cable), I get
large oscillations (10+Vrms) at the output. The oscillations are at
about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
when the output is unloaded, nor when it's loaded by the typical
50ohms. Since this only appeared when I added a capacitive load to the
output, I figured it was due to the parallel LC between the output
winding and coaxial capacitance. My output impedance came out a bit
lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
between the output winding and coaxial connector. This cut the
oscillations nearly in half (now about 6.5Vrms), but didn't kill
it. I've also tried loading down the output with a 1k parallel resistor,
but though this helps too, the oscillations are still pretty
large. Going lower helps further, but I don't want to load down the
output too much.
Any thoughts? Has anyone else seen this behavior? I can post more
details and images.
Thanks,
Matt
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Luciano:
I would like to try the amplifier for my 10MHz output.
Where can I find details for the toroid transformer,
how many windings, and what gauge wire?
Thanks,
Neal
N6YFM
On Sun, Apr 23, 2023 at 6:32 AM Luciano via time-nuts <
time-nuts@lists.febo.com> wrote:
>
> Hi Matt,
>
> I've built a buffer amplifier just for the use you want.
>
> The design comes from Bruce and was optimized and engineered by me and
> a group of friends.
>
> You can see the solution at the addresses I enclose.
>
>
> http://www.timeok.it/wp-content/uploads/2015/08/Frequency-Distribution-Design-Basic-Module-v-4.1.pdf
>
>
> http://www.timeok.it/wp-content/uploads/2015/08/hp5065A-option-H10-v-2.00-2.pdf
>
> Compared to the one you propose, it has several improvements, such as a
> higher input impedance and a much higher reverse isolation that exceeds
> 100dB.
>
> I hope this info can be useful to you,
>
> Luciano
>
> Luciano P. S. Paramithiotti
> timeok@timeok.it
> www.timeok.it
>
> Da "Matt Huszagh via time-nuts" time-nuts@lists.febo.com
> A "Discussion of precise time and frequency measurement"
> time-nuts@lists.febo.com
> Cc "Matt Huszagh" huszaghmatt@gmail.com
> Data Sat, 22 Apr 2023 18:21:07 -0700
> Oggetto [time-nuts] Bruce's common emitter transformer feedback
> amplifier: oscillations with capacitive load
> Hi,
>
> I built Bruce Griffith's common emitter transformer feedback amplifier
> (http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
> mostly identical to the first schematic in the previous link. But, I
> added a second stage for increased reverse isolation. I placed 65R
> across the first stage output, which gave me about the right gain in
> simulations. I also added a second 2-stage buffer in parallel. I
> custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
> use this to get buffered/isolated front and rear 10 MHz outputs from the
> internal 10811A in my HP 5065A.
>
> Overall, the performance seems excellent. I've attached a residual phase
> noise measurement, showing the combined 2 cascaded stages. These results
> seem only a little worse than what John Miles measured for his single
> stage (http://www.ke5fx.com/norton.htm).
>
> I measured the reverse isolation to be 76 dB and 77 dB and the isolation
> between outputs to be 49 dB or 59 dB, depending on which output I inject
> the signal into. This is all at 10 MHz of course.
>
> However, I am experiencing one issue, which is that if I add a
> capacitive load to the output (such as from a coaxial cable), I get
> large oscillations (10+Vrms) at the output. The oscillations are at
> about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
> when the output is unloaded, nor when it's loaded by the typical
> 50ohms. Since this only appeared when I added a capacitive load to the
> output, I figured it was due to the parallel LC between the output
> winding and coaxial capacitance. My output impedance came out a bit
> lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
> between the output winding and coaxial connector. This cut the
> oscillations nearly in half (now about 6.5Vrms), but didn't kill
> it. I've also tried loading down the output with a 1k parallel resistor,
> but though this helps too, the oscillations are still pretty
> large. Going lower helps further, but I don't want to load down the
> output too much.
>
> Any thoughts? Has anyone else seen this behavior? I can post more
> details and images.
>
> Thanks,
> Matt
> _______________________________________________
> time-nuts mailing list -- time-nuts@lists.febo.com
> To unsubscribe send an email to time-nuts-leave@lists.febo.com
>
G
ghf@hoffmann-hochfrequenz.de
Sun, Apr 23, 2023 10:30 PM
I would not call these things CE amplifiers at all. CE implies that the
emitter is somewhat grounded, and not that it follows the base voltage
by virtue of feedback through the transformer.
If anything, the transistor feels like a follower, even if the
voltage there is ignored. The output signal is the collector
CURRENT, and if you would use a cascode stage, that would be
clear at first sight. A cascode or even two would be a good
idea wrt backward isolation. Cascodes do not add noise
worth speaking of.
Backward isolation is not helped if you do transformer feedback
around a cascode.
Followers have the habit to oscillate when there is the slightest
capacitive load at the emitter, that is the mechanism of nearly
all microwave VCOs. The high RL impedance at the emitter is
nothing more than an invitation, and a load capacitance that
comes through the transformer just completes that.
I would not wonder if there was a negative real part of input
impedance over some frequencies.
And Norton amplifiers ( CB btw ) are not known for excellent
backward isolation; there is not much more to win than the
reverse of the forward gain.
Chris Trask has written interesting things about "augmented"
amplifiers, which seems to be Norton++, but it's well after
midnight here. Should be easy to google. No idea if it
would help.
regards, Gerhard
Am 2023-04-23 15:42, schrieb Bob Camp via time-nuts:
Hi
A schematic would be a big help here …..
Flying blind, best guess is that your output stage has an issue. If
the two output bufferers
are identical to the input stage, that’s probably the issue. A
different configuration for the
buffers would be the most obvious fix.
Bob
On Apr 22, 2023, at 9:21 PM, Matt Huszagh via time-nuts
time-nuts@lists.febo.com wrote:
Hi,
I built Bruce Griffith's common emitter transformer feedback amplifier
(http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
mostly identical to the first schematic in the previous link. But, I
added a second stage for increased reverse isolation. I placed 65R
across the first stage output, which gave me about the right gain in
simulations. I also added a second 2-stage buffer in parallel. I
custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
use this to get buffered/isolated front and rear 10 MHz outputs from
the
internal 10811A in my HP 5065A.
Overall, the performance seems excellent. I've attached a residual
phase
noise measurement, showing the combined 2 cascaded stages. These
results
seem only a little worse than what John Miles measured for his single
stage (http://www.ke5fx.com/norton.htm).
I measured the reverse isolation to be 76 dB and 77 dB and the
isolation
between outputs to be 49 dB or 59 dB, depending on which output I
inject
the signal into. This is all at 10 MHz of course.
However, I am experiencing one issue, which is that if I add a
capacitive load to the output (such as from a coaxial cable), I get
large oscillations (10+Vrms) at the output. The oscillations are at
about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
when the output is unloaded, nor when it's loaded by the typical
50ohms. Since this only appeared when I added a capacitive load to the
output, I figured it was due to the parallel LC between the output
winding and coaxial capacitance. My output impedance came out a bit
lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
between the output winding and coaxial connector. This cut the
oscillations nearly in half (now about 6.5Vrms), but didn't kill
it. I've also tried loading down the output with a 1k parallel
resistor,
but though this helps too, the oscillations are still pretty
large. Going lower helps further, but I don't want to load down the
output too much.
Any thoughts? Has anyone else seen this behavior? I can post more
details and images.
Thanks,
Matt
I would not call these things CE amplifiers at all. CE implies that the
emitter is somewhat grounded, and not that it follows the base voltage
by virtue of feedback through the transformer.
If anything, the transistor feels like a follower, even if the
voltage there is ignored. The output signal is the collector
CURRENT, and if you would use a cascode stage, that would be
clear at first sight. A cascode or even two would be a good
idea wrt backward isolation. Cascodes do not add noise
worth speaking of.
Backward isolation is not helped if you do transformer feedback
around a cascode.
Followers have the habit to oscillate when there is the slightest
capacitive load at the emitter, that is the mechanism of nearly
all microwave VCOs. The high RL impedance at the emitter is
nothing more than an invitation, and a load capacitance that
comes through the transformer just completes that.
I would not wonder if there was a negative real part of input
impedance over some frequencies.
And Norton amplifiers ( CB btw ) are not known for excellent
backward isolation; there is not much more to win than the
reverse of the forward gain.
Chris Trask has written interesting things about "augmented"
amplifiers, which seems to be Norton++, but it's well after
midnight here. Should be easy to google. No idea if it
would help.
regards, Gerhard
Am 2023-04-23 15:42, schrieb Bob Camp via time-nuts:
> Hi
>
> A schematic would be a big help here …..
>
> Flying blind, best guess is that your output stage has an issue. If
> the two output bufferers
> are identical to the input stage, that’s probably the issue. A
> different configuration for the
> buffers would be the most obvious fix.
>
> Bob
>
>> On Apr 22, 2023, at 9:21 PM, Matt Huszagh via time-nuts
>> <time-nuts@lists.febo.com> wrote:
>>
>> Hi,
>>
>> I built Bruce Griffith's common emitter transformer feedback amplifier
>> (http://www.ko4bb.com/~bruce/OCXOBufferAmplifiers.html). My build is
>> mostly identical to the first schematic in the previous link. But, I
>> added a second stage for increased reverse isolation. I placed 65R
>> across the first stage output, which gave me about the right gain in
>> simulations. I also added a second 2-stage buffer in parallel. I
>> custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
>> use this to get buffered/isolated front and rear 10 MHz outputs from
>> the
>> internal 10811A in my HP 5065A.
>>
>> Overall, the performance seems excellent. I've attached a residual
>> phase
>> noise measurement, showing the combined 2 cascaded stages. These
>> results
>> seem only a little worse than what John Miles measured for his single
>> stage (http://www.ke5fx.com/norton.htm).
>>
>> I measured the reverse isolation to be 76 dB and 77 dB and the
>> isolation
>> between outputs to be 49 dB or 59 dB, depending on which output I
>> inject
>> the signal into. This is all at 10 MHz of course.
>>
>> However, I am experiencing one issue, which is that if I add a
>> capacitive load to the output (such as from a coaxial cable), I get
>> large oscillations (10+Vrms) at the output. The oscillations are at
>> about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
>> when the output is unloaded, nor when it's loaded by the typical
>> 50ohms. Since this only appeared when I added a capacitive load to the
>> output, I figured it was due to the parallel LC between the output
>> winding and coaxial capacitance. My output impedance came out a bit
>> lower than I'd wanted (35ohms), so I tried inserting 10ohms in series
>> between the output winding and coaxial connector. This cut the
>> oscillations nearly in half (now about 6.5Vrms), but didn't kill
>> it. I've also tried loading down the output with a 1k parallel
>> resistor,
>> but though this helps too, the oscillations are still pretty
>> large. Going lower helps further, but I don't want to load down the
>> output too much.
>>
>> Any thoughts? Has anyone else seen this behavior? I can post more
>> details and images.
>>
>> Thanks,
>> Matt
BG
Bruce Griffiths
Tue, Apr 25, 2023 4:37 AM
Excessive leakage inductance can be an issue with some instability evident if the interstage transformer isn't terminated in 50 ohms. A transformer coupling factor greater than 0.95 is desirable for the nominal design winding inductances. A capacitive load is a poor approximation to the impedance presented by a length of unterminated coax at RF frequencies. The impedance can range from open to short circuit depending on the cable length. Saturation and/or cutoff can occur if the input signal is too large for the supply voltage or bias current.
Bruce
On 24/04/2023 02:15 NZST Matt Huszagh via time-nuts time-nuts@lists.febo.com wrote:
Bob Camp via time-nuts time-nuts@lists.febo.com writes:
A schematic would be a big help here …..
Flying blind, best guess is that your output stage has an issue. If the two output bufferers
are identical to the input stage, that’s probably the issue. A different configuration for the
buffers would be the most obvious fix.
Hi Bob,
I've attached the full schematic. Hope that helps.
Matt
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Excessive leakage inductance can be an issue with some instability evident if the interstage transformer isn't terminated in 50 ohms. A transformer coupling factor greater than 0.95 is desirable for the nominal design winding inductances. A capacitive load is a poor approximation to the impedance presented by a length of unterminated coax at RF frequencies. The impedance can range from open to short circuit depending on the cable length. Saturation and/or cutoff can occur if the input signal is too large for the supply voltage or bias current.
Bruce
> On 24/04/2023 02:15 NZST Matt Huszagh via time-nuts <time-nuts@lists.febo.com> wrote:
>
>
> Bob Camp via time-nuts <time-nuts@lists.febo.com> writes:
>
> > A schematic would be a big help here …..
> >
> > Flying blind, best guess is that your output stage has an issue. If the two output bufferers
> > are identical to the input stage, that’s probably the issue. A different configuration for the
> > buffers would be the most obvious fix.
>
> Hi Bob,
>
> I've attached the full schematic. Hope that helps.
>
> Matt
>
> _______________________________________________
> time-nuts mailing list -- time-nuts@lists.febo.com
> To unsubscribe send an email to time-nuts-leave@lists.febo.com
R(
Richard (Rick) Karlquist
Sat, Apr 29, 2023 9:11 PM
On 4/22/2023 6:21 PM, Matt Huszagh via time-nuts wrote:
custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
use this to get buffered/isolated front and rear 10 MHz outputs from the
large oscillations (10+Vrms) at the output. The oscillations are at
about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
when the output is unloaded, nor when it's loaded by the typical
50ohms. Since this only appeared when I added a capacitive load to the
output, I figured it was due to the parallel LC between the output
winding and coaxial capacitance. My output impedance came out a bit
Matt
Your 67 matl has rather low permeability and high Q. IMHO, not a good
choice for a 10 MHz transformer. I am wondering if it is contributing
to your oscillation.
Rick N6RK
On 4/22/2023 6:21 PM, Matt Huszagh via time-nuts wrote:
> custom-wound the transformer on a Fair-Rite 67 toroid. I'm planning to
> use this to get buffered/isolated front and rear 10 MHz outputs from the
> large oscillations (10+Vrms) at the output. The oscillations are at
> about 20MHz for a 1m long RG400 cable (94pF/m). These are not present
> when the output is unloaded, nor when it's loaded by the typical
> 50ohms. Since this only appeared when I added a capacitive load to the
> output, I figured it was due to the parallel LC between the output
> winding and coaxial capacitance. My output impedance came out a bit
>
> Matt
>
Your 67 matl has rather low permeability and high Q. IMHO, not a good
choice for a 10 MHz transformer. I am wondering if it is contributing
to your oscillation.
Rick N6RK
