I am currently experimenting with construction of my first Dual-Gate
Mosfet Amp.

Below is an attempt to explain the circuit typology as best I can

Freq = 148 MHz to 152 MHz

Power supply is 8V, Bias on G2 is 4V. Source is grounded

Drain has Variable inductor to Vcc and Cap to ground to form first stage
of a double-tuned circuit. This then couples through a small cap to the
second half of the DTC.

RF input to Single tuned circuit with tapped capacitors attached to G1
of BF998.

C1 to ground is 56pF, C2 to G1 is 15pF, L from G1 to ground is variable
with nominal inductance of 85nH (tunes 75nH to 110nH), Q approx. 100

The Drain has the same type of inductor to Vcc (decoupled Vcc end).
C3 is 18pF from drain to ground (This forms part of the double-tuned
circuit on the output).
Coupling to second half of DTC is 1pF. Second half of DTC C4 = 18pF to
ground and L is same as previous inductors (85nH nom.).

This will later go to G1 of a Dual-Gate Mosfet mixer but for testing I
replaced the 15pF on second DTC with series 18pF and 56pF to form a
capacitive divider down to 50 Ohms for my spectrum analyzer input.

With this configuration the circuit will oscillate on its own when the
input is removed. If I replace the Capo from drain to ground with a trim
cap I can adjust it so the self-oscillation stops but as soon as I try
to change the tuning of the inductor from drain to Vcc, it starts to
oscillate on its own.

If I remove the cap from drain to ground altogether, the oscillations
stop but I almost the ability to tune the Inductor, it becomes extremely
broad tuning.(The input inductor and output DTC inductor tune fine though).

I have tried a series 33R between the Drain of the mosfet and the
Inductor/Capacitor and this did not help.

The measured gain with the capacitor removed was 17dB. Drain current was
4.8mA. 3dB bandwidth was 3.2MHz.

The circuit is laid out "dead-bug" style on a flat bare PCB. I'm not
sure if it is a layout problem or typology issue.
I have seen damping used across tuned circuits in output stages of
rf amps and may try this next. But, I would like to know what is
happening because if I manage to stop the oscillation, I am not sure if
it is marginal and will reappear with changes in temperature or drive
level etc.

Any help much appreciated.

Regards

David

On Fri, 30 Jun 2006 01:20:52 GMT, David
wrote:

I am currently experimenting with construction of my first Dual-Gate
Mosfet Amp.

Below is an attempt to explain the circuit typology as best I can

Freq = 148 MHz to 152 MHz

Power supply is 8V, Bias on G2 is 4V. Source is grounded

Drain has Variable inductor to Vcc and Cap to ground to form first stage
of a double-tuned circuit. This then couples through a small cap to the
second half of the DTC.

RF input to Single tuned circuit with tapped capacitors attached to G1
of BF998.

C1 to ground is 56pF, C2 to G1 is 15pF, L from G1 to ground is variable
with nominal inductance of 85nH (tunes 75nH to 110nH), Q approx. 100

The Drain has the same type of inductor to Vcc (decoupled Vcc end).
C3 is 18pF from drain to ground (This forms part of the double-tuned
circuit on the output).
Coupling to second half of DTC is 1pF. Second half of DTC C4 = 18pF to
ground and L is same as previous inductors (85nH nom.).

This will later go to G1 of a Dual-Gate Mosfet mixer but for testing I
replaced the 15pF on second DTC with series 18pF and 56pF to form a
capacitive divider down to 50 Ohms for my spectrum analyzer input.

With this configuration the circuit will oscillate on its own when the
input is removed. If I replace the Capo from drain to ground with a trim
cap I can adjust it so the self-oscillation stops but as soon as I try
to change the tuning of the inductor from drain to Vcc, it starts to
oscillate on its own.

If I remove the cap from drain to ground altogether, the oscillations
stop but I almost the ability to tune the Inductor, it becomes extremely
broad tuning.(The input inductor and output DTC inductor tune fine though).

I have tried a series 33R between the Drain of the mosfet and the
Inductor/Capacitor and this did not help.

The measured gain with the capacitor removed was 17dB. Drain current was
4.8mA. 3dB bandwidth was 3.2MHz.

The circuit is laid out "dead-bug" style on a flat bare PCB. I'm not
sure if it is a layout problem or typology issue.
I have seen damping used across tuned circuits in output stages of
rf amps and may try this next. But, I would like to know what is
happening because if I manage to stop the oscillation, I am not sure if
it is marginal and will reappear with changes in temperature or drive
level etc.

Any help much appreciated.

Regards

David

Usually DGFET amps are stable. But they are also very high
gain.

The likely culprit is the input can see the output (incidental
coupling). I always build this with a shield partition right across
the device and also a ferrite bead in the G2 lead near the deivce
as possible (or for SMD in the Source leg bypass cap). These
devices will take off at uhf due to gain and less then careful
bypassing. That includes making sure the G2 bias resistor
(Gate to DC source) does not couple RF back. As part of this
also insure the input and output leads to the connector are short
as possible or even coax.


Allison

Your bypasses from G2 to ground and from the top of the drain inductor
to ground should be physically small, have very nearly zero lead length,
and connect directly to the drain or the bottom of the drain resistor if
used.

Roy Lewallen, W7EL

Thanks for the feedback (no pun intended I appreciate it.

I will add a shield between input and output circuit. I do remember
seeing this in other designs. In my prototype construction I do have the
input and output coils fairly close and in the same orientation so I
imagine there very well could be coupling between them.

the fact that the circuit oscillates very close to the tuned frequency
is probably a clue that positive feedback between input and output is
happening. The source is grounded so Drain - Gate seems feasible.

I will also take the advice of adding a ferrite bead between G2 pin and
the voltage divider for G2. (Currently 100k+100k with 1n decoupling).

Can anyone suggest a reasonably simple method to determine noise figure
of this stage ? I do not have a distortion meter. I have directional
couplers, sig gens, sepctrum analyzer and digital power meter. Can I use
these somehow ?

Thanks in advance again


Best regards

David

wrote:
On Fri, 30 Jun 2006 01:20:52 GMT, David
wrote:

I am currently experimenting with construction of my first Dual-Gate
Mosfet Amp.

Below is an attempt to explain the circuit typology as best I can

Freq = 148 MHz to 152 MHz

Power supply is 8V, Bias on G2 is 4V. Source is grounded

Drain has Variable inductor to Vcc and Cap to ground to form first stage
of a double-tuned circuit. This then couples through a small cap to the
second half of the DTC.

RF input to Single tuned circuit with tapped capacitors attached to G1
of BF998.

C1 to ground is 56pF, C2 to G1 is 15pF, L from G1 to ground is variable
with nominal inductance of 85nH (tunes 75nH to 110nH), Q approx. 100

The Drain has the same type of inductor to Vcc (decoupled Vcc end).
C3 is 18pF from drain to ground (This forms part of the double-tuned
circuit on the output).
Coupling to second half of DTC is 1pF. Second half of DTC C4 = 18pF to
ground and L is same as previous inductors (85nH nom.).

This will later go to G1 of a Dual-Gate Mosfet mixer but for testing I
replaced the 15pF on second DTC with series 18pF and 56pF to form a
capacitive divider down to 50 Ohms for my spectrum analyzer input.

With this configuration the circuit will oscillate on its own when the
input is removed. If I replace the Capo from drain to ground with a trim
cap I can adjust it so the self-oscillation stops but as soon as I try
to change the tuning of the inductor from drain to Vcc, it starts to
oscillate on its own.

If I remove the cap from drain to ground altogether, the oscillations
stop but I almost the ability to tune the Inductor, it becomes extremely
broad tuning.(The input inductor and output DTC inductor tune fine though).

I have tried a series 33R between the Drain of the mosfet and the
Inductor/Capacitor and this did not help.

The measured gain with the capacitor removed was 17dB. Drain current was
4.8mA. 3dB bandwidth was 3.2MHz.

The circuit is laid out "dead-bug" style on a flat bare PCB. I'm not
sure if it is a layout problem or typology issue.
I have seen damping used across tuned circuits in output stages of
rf amps and may try this next. But, I would like to know what is
happening because if I manage to stop the oscillation, I am not sure if
it is marginal and will reappear with changes in temperature or drive
level etc.

Any help much appreciated.

Regards

David

Usually DGFET amps are stable. But they are also very high
gain.

The likely culprit is the input can see the output (incidental
coupling). I always build this with a shield partition right across
the device and also a ferrite bead in the G2 lead near the deivce
as possible (or for SMD in the Source leg bypass cap). These
devices will take off at uhf due to gain and less then careful
bypassing. That includes making sure the G2 bias resistor
(Gate to DC source) does not couple RF back. As part of this
also insure the input and output leads to the connector are short
as possible or even coax.


Allison

I have applied the suggestions and still can't manage to "tame the beast".

As long as the cap to ground from drain is in circuit the amp becomes
unstable at oscillates close to the centre frequency.

If I remove the cap the amp settles down but I loose tuning of the drain
circuit and the gain drops down.

I have a picture of the layout and schematic if anyone would be prepared
to take a look and see if there is something obvious.

The shield is placed right across the top of the BF998 to separate input
from output.
All component leads are cut as close to the component body as possible.

I also added another shield between G2 and the input circuit so the
input now is totally isolated from the rest of the ckt

Regards

David

Roy Lewallen wrote:
Your bypasses from G2 to ground and from the top of the drain inductor
to ground should be physically small, have very nearly zero lead length,
and connect directly to the drain or the bottom of the drain resistor if
used.

Roy Lewallen, W7EL

I have conducted further investigation and discovered the following

1. Coupling between tuned circuits in DTC was a little high (reduce this
from 1pF to 0.5pF)
2. The Cap from Drain to ground (1st tuned ckt in DTC) was a little
large. Reduced this to 10pF from 15pF.

These changes enable smooth tuning of each inductor without any
surprises. Gain has shot up to around 24dB and 3dB bandwidth is around
3.5MHz at 151 MHz Fo.

What I do notice is that when I remove the input cable from the signal
generator, the circuit starts to oscillate. If I terminate the floating
cable with say a 6dB attenuator, the oscillation stops.

Should I have something in front of the Single tuned input circuit such
as a RFC, 3dB resistive pad or LPF to define input impedance ? Or is it
maybe just the input floating as the Tapped capacitor divider has
nothing to ground on the input side (FET side of course has the inductor
to ground).

Thanks

Regards

David

David wrote:
I have applied the suggestions and still can't manage to "tame the beast".

As long as the cap to ground from drain is in circuit the amp becomes
unstable at oscillates close to the centre frequency.

If I remove the cap the amp settles down but I loose tuning of the drain
circuit and the gain drops down.

I have a picture of the layout and schematic if anyone would be prepared
to take a look and see if there is something obvious.

The shield is placed right across the top of the BF998 to separate input
from output.
All component leads are cut as close to the component body as possible.

I also added another shield between G2 and the input circuit so the
input now is totally isolated from the rest of the ckt

Regards

David

Roy Lewallen wrote:
Your bypasses from G2 to ground and from the top of the drain inductor
to ground should be physically small, have very nearly zero lead
length, and connect directly to the drain or the bottom of the drain
resistor if used.

Roy Lewallen, W7EL

On Fri, 30 Jun 2006 03:36:28 +0000, David wrote:

Thanks for the feedback (no pun intended I appreciate it.

I will add a shield between input and output circuit. I do remember
seeing this in other designs. In my prototype construction I do have the
input and output coils fairly close and in the same orientation so I
imagine there very well could be coupling between them.


I had a similar problem with a 2M RF amp. I cured it by
reversing the winding direction of one of the inductors.


Can anyone suggest a reasonably simple method to determine noise figure
of this stage ?

I'm a bit reluctant to suggest a method that I haven't tried
mysef, but you should be able to calculate NF by measuring the
receiver noise output with a hot and cold resistor connected
to the input. The usual method involves boiling water and
melting ice. http://tinyurl.com/psdfo

73, Ed. EI9GQ.

--
Remove 'X' to reply by e-mail.
Linux 2.6.17

"Eamon Skelton" wrote in message
...
On Fri, 30 Jun 2006 03:36:28 +0000, David wrote:

Thanks for the feedback (no pun intended I appreciate it.

I will add a shield between input and output circuit. I do remember
seeing this in other designs. In my prototype construction I do have the
input and output coils fairly close and in the same orientation so I
imagine there very well could be coupling between them.


I had a similar problem with a 2M RF amp. I cured it by
reversing the winding direction of one of the inductors.


Can anyone suggest a reasonably simple method to determine noise figure
of this stage ?

I'm a bit reluctant to suggest a method that I haven't tried
mysef, but you should be able to calculate NF by measuring the
receiver noise output with a hot and cold resistor connected
to the input. The usual method involves boiling water and
melting ice. http://tinyurl.com/psdfo

73, Ed. EI9GQ.


David,
I don't know your technical level, but here goes. I thought the dual
gate is a type of cascode arrangement and should be stable owing to the
isolation of the "top" device... Anyway...

You are very correct about trying to "band-aid" it with de-Q'ing resistors.
You need to find the root cause, not hope a patch works. You are well aware
of and trying to solve a possible proximity feedback issue, so I won't go
there.

It is not clear what your term "sprogging" may mean. There was a term
"Squeeging" which referred to oscillation or what is also called
regen(regeneration) which is typically used for dirty (sideband-type),
rather than clean (single frequency) oscillation.

Is the oscillation a clean, oscillator signal (pure carrier), or does it
have sidebands?
If sidebands, are they clear and distinct or is it more of a wideband
grunge/garbage?
Are the sidebands mirrored on both sides or are they lopsided?
These are all clues.

If you have regular sidebands or grunge, you have a low frequency
oscillation component and need to also look at bypassing. 33-200 ohms or a
ferrite bead in series with a bypass can help diagnose it.
If the sidebands are unequal, there are most likely both AM and FM
components to the oscillation. Putting a regular scope on the supply or
other places which are supposed to be bypassed might see the signal, helping
to ID the cause.

Be aware that two bypasses in parallel, of different size, can become a
tuned circuit and therefore a high impedance at frequencies where the larger
one is above self resonance and appears inductive.

Pay extreemly close attention to the ground currents for the input and
output circuitry. You want to have an absolute minimum of common path for
these. I didn't see anything in your circuit description about the FET
source. If it is self biased with a resistor & cap combination or what, but
from the source you should have two distinct paths in the ground you have to
the respectice components tith no length of this "ground current path" in
common - where both currents share a common path. you can even cut the PCB
so there are two distinct ground planes which only meet at the FET source or
its resistor/bypass cap.
Watch the power supply side pretty much the same. Where does the Drain
bypass connect to the plane?

Hope this helps.
73, Steve, K9DCI

Thanks all for your input into this issue. I appreciate all the feedback
I have received.

I do like to discover and understand the mechanism for this issue so I
can avoid it next time.

"Sprogging" was probably not the best term to use afterall.

The unwanted response is a clean carrier with no sidebands.
This carrier appears within the tuning band of the amp tuned circuits
and moves as either the input tuning
or Drain tuning is adjusted.

The Source is connected to ground plane.

The circuit now only just starts to produce a very low level signal
(maybe around 60 dB down) when I unplug the input cable from
the signal generator. If I terminate the floating cable with a 50 Ohm
attenuator, the oscillation stops.

The most improvement has been achieved by addition of shielding between
Input circuit and drain circuit.

I suspect that I am getting radiated signal fed back. I also suppose
that as long as the path loss between drain and input is greater than
the gain of the amp then the circuit should not be able to oscillate.

One other issue I were considering was that even though the circuit is
built with very short leads directly on a continuous ground plane (PCB),
Some "Pads" have been made by cutting away copper. But because the PCB
is double sided, I wonder if the interaction between these pads make the
lower ground plane and pads act like a capacitor (ie. energy coupled
from the pads into the lower ground plane and up into other pads through
capacitive coupling).

Maybe if it was single sided board or I add heaps of track pins from top
to bottom layer would prove if this was the case.

Another thing is the coils do not have their own shields so they
probably act like antennas. I think it would be prudent to use only
shielded cans as I cannot always ensure large physical distance between
them.

Regards

David

Steve N. wrote:
"Eamon Skelton" wrote in message
...
On Fri, 30 Jun 2006 03:36:28 +0000, David wrote:

Thanks for the feedback (no pun intended I appreciate it.

I will add a shield between input and output circuit. I do remember
seeing this in other designs. In my prototype construction I do have the
input and output coils fairly close and in the same orientation so I
imagine there very well could be coupling between them.

I had a similar problem with a 2M RF amp. I cured it by
reversing the winding direction of one of the inductors.


Can anyone suggest a reasonably simple method to determine noise figure
of this stage ?
I'm a bit reluctant to suggest a method that I haven't tried
mysef, but you should be able to calculate NF by measuring the
receiver noise output with a hot and cold resistor connected
to the input. The usual method involves boiling water and
melting ice. http://tinyurl.com/psdfo

73, Ed. EI9GQ.


David,
I don't know your technical level, but here goes. I thought the dual
gate is a type of cascode arrangement and should be stable owing to the
isolation of the "top" device... Anyway...

You are very correct about trying to "band-aid" it with de-Q'ing resistors.
You need to find the root cause, not hope a patch works. You are well aware
of and trying to solve a possible proximity feedback issue, so I won't go
there.

It is not clear what your term "sprogging" may mean. There was a term
"Squeeging" which referred to oscillation or what is also called
regen(regeneration) which is typically used for dirty (sideband-type),
rather than clean (single frequency) oscillation.

Is the oscillation a clean, oscillator signal (pure carrier), or does it
have sidebands?
If sidebands, are they clear and distinct or is it more of a wideband
grunge/garbage?
Are the sidebands mirrored on both sides or are they lopsided?
These are all clues.

If you have regular sidebands or grunge, you have a low frequency
oscillation component and need to also look at bypassing. 33-200 ohms or a
ferrite bead in series with a bypass can help diagnose it.
If the sidebands are unequal, there are most likely both AM and FM
components to the oscillation. Putting a regular scope on the supply or
other places which are supposed to be bypassed might see the signal, helping
to ID the cause.

Be aware that two bypasses in parallel, of different size, can become a
tuned circuit and therefore a high impedance at frequencies where the larger
one is above self resonance and appears inductive.

Pay extreemly close attention to the ground currents for the input and
output circuitry. You want to have an absolute minimum of common path for
these. I didn't see anything in your circuit description about the FET
source. If it is self biased with a resistor & cap combination or what, but
from the source you should have two distinct paths in the ground you have to
the respectice components tith no length of this "ground current path" in
common - where both currents share a common path. you can even cut the PCB
so there are two distinct ground planes which only meet at the FET source or
its resistor/bypass cap.
Watch the power supply side pretty much the same. Where does the Drain
bypass connect to the plane?

Hope this helps.
73, Steve, K9DCI

On Fri, 30 Jun 2006 23:46:14 GMT, David
wrote:

Thanks all for your input into this issue. I appreciate all the feedback
I have received.

I do like to discover and understand the mechanism for this issue so I
can avoid it next time.

"Sprogging" was probably not the best term to use afterall.

The unwanted response is a clean carrier with no sidebands.
This carrier appears within the tuning band of the amp tuned circuits
and moves as either the input tuning
or Drain tuning is adjusted.

The Source is connected to ground plane.

The circuit now only just starts to produce a very low level signal
(maybe around 60 dB down) when I unplug the input cable from
the signal generator. If I terminate the floating cable with a 50 Ohm
attenuator, the oscillation stops.

The most improvement has been achieved by addition of shielding between
Input circuit and drain circuit.

I suspect that I am getting radiated signal fed back. I also suppose
that as long as the path loss between drain and input is greater than
the gain of the amp then the circuit should not be able to oscillate.

You may try changing the input coil (less L and more C) and see if
that helps.


One other issue I were considering was that even though the circuit is
built with very short leads directly on a continuous ground plane (PCB),
Some "Pads" have been made by cutting away copper. But because the PCB
is double sided, I wonder if the interaction between these pads make the
lower ground plane and pads act like a capacitor (ie. energy coupled
from the pads into the lower ground plane and up into other pads through
capacitive coupling).

Grount the top to the bottom of two sided boards. I use foil or braid
to wrap the edges.
Also add a few holes and put wires from top to bottom near the source
lead.

Next time rather than cutting pads in the board try this, Cut tiny
islands of PCB material and super glue them down. Then you have
isolated pads for SMT parts. For devices like BF998 in SMT I've
used board "chips" that are 0.1x0.2 inch and that works fine.

FYI when I used that device the source has a 100 ohm resistor to
ground and the bypass cap is .001(for 6m) with a ferrite bead
on the non grounded lead. That is a "stock" circuit for DG fets
and it seems well behaved to me.

Maybe if it was single sided board or I add heaps of track pins from top
to bottom layer would prove if this was the case.

Do the latter. If you wrap the edges and put pins through near
grounded compnents that's enough (likely only a few needed).

Another thing is the coils do not have their own shields so they
probably act like antennas. I think it would be prudent to use only
shielded cans as I cannot always ensure large physical distance between
them.

That helps.

Also make sure the output side has a valid load on it too.

Do not load or resistive pad the input as that degrades the NF and
gain.

Allison


Regards

David

Steve N. wrote:
"Eamon Skelton" wrote in message
...
On Fri, 30 Jun 2006 03:36:28 +0000, David wrote:

Thanks for the feedback (no pun intended I appreciate it.

I will add a shield between input and output circuit. I do remember
seeing this in other designs. In my prototype construction I do have the
input and output coils fairly close and in the same orientation so I
imagine there very well could be coupling between them.

I had a similar problem with a 2M RF amp. I cured it by
reversing the winding direction of one of the inductors.


Can anyone suggest a reasonably simple method to determine noise figure
of this stage ?
I'm a bit reluctant to suggest a method that I haven't tried
mysef, but you should be able to calculate NF by measuring the
receiver noise output with a hot and cold resistor connected
to the input. The usual method involves boiling water and
melting ice. http://tinyurl.com/psdfo

73, Ed. EI9GQ.


David,
I don't know your technical level, but here goes. I thought the dual
gate is a type of cascode arrangement and should be stable owing to the
isolation of the "top" device... Anyway...

You are very correct about trying to "band-aid" it with de-Q'ing resistors.
You need to find the root cause, not hope a patch works. You are well aware
of and trying to solve a possible proximity feedback issue, so I won't go
there.

It is not clear what your term "sprogging" may mean. There was a term
"Squeeging" which referred to oscillation or what is also called
regen(regeneration) which is typically used for dirty (sideband-type),
rather than clean (single frequency) oscillation.

Is the oscillation a clean, oscillator signal (pure carrier), or does it
have sidebands?
If sidebands, are they clear and distinct or is it more of a wideband
grunge/garbage?
Are the sidebands mirrored on both sides or are they lopsided?
These are all clues.

If you have regular sidebands or grunge, you have a low frequency
oscillation component and need to also look at bypassing. 33-200 ohms or a
ferrite bead in series with a bypass can help diagnose it.
If the sidebands are unequal, there are most likely both AM and FM
components to the oscillation. Putting a regular scope on the supply or
other places which are supposed to be bypassed might see the signal, helping
to ID the cause.

Be aware that two bypasses in parallel, of different size, can become a
tuned circuit and therefore a high impedance at frequencies where the larger
one is above self resonance and appears inductive.

Pay extreemly close attention to the ground currents for the input and
output circuitry. You want to have an absolute minimum of common path for
these. I didn't see anything in your circuit description about the FET
source. If it is self biased with a resistor & cap combination or what, but
from the source you should have two distinct paths in the ground you have to
the respectice components tith no length of this "ground current path" in
common - where both currents share a common path. you can even cut the PCB
so there are two distinct ground planes which only meet at the FET source or
its resistor/bypass cap.
Watch the power supply side pretty much the same. Where does the Drain
bypass connect to the plane?

Hope this helps.
73, Steve, K9DCI