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.
==============================
With only having double sided PCB material I drill various holes and
bond the 2 planes at different locations by soldering wires on both planes.
For VHF/UHF circuits (in addition to the above) I line the circumference
of the material by folding 6mm wide thin (abt 0.2mm thick)copper strip
onto both planes and solder. I found a 'half a mile long ' roll of this
material at a flea market .

Frank GM0CSZ / KN6WH

Thanks for your input everyone.

I re-constructed the layout using SMD components on a bare copper PCB.
This time I changed the typology slightly and added 100R with 1n in the
source of the BF998. I placed a 2k7 across the drain coil to set correct
Rp for the DTC bandwidth.

I also changed the tuning coils for fixed coils and tuned with variable
caps instead.

The circuit seems stable now. The Amp has total gain of 23dB (includes
loss of the input filter and double tuned circuit).
All caps are tuning properly with no unexpected behaviour.

I have not yet added any shielding or connected vias from top to bottom
plane of the PCB.

The tuning range is almost as calculated (6MHz Bandwidth at Fo = 150 MHz).

Does anyone know what an reasonable 1dB compression point would be for
a the BF998 at 150 Mhz ? The drain current seems a bit low (about 3mA)
and I think this may need increasing to get a reasonable dynamic range ?

The gate 2 bias is currently 4V. This should not be pinching off the
drain current at all. I wonder if the 100 Ohm in the source should be
made smaller ? Any ideas.

Thanks

Regards

David



Highland Ham wrote:
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.
==============================
With only having double sided PCB material I drill various holes and
bond the 2 planes at different locations by soldering wires on both planes.
For VHF/UHF circuits (in addition to the above) I line the circumference
of the material by folding 6mm wide thin (abt 0.2mm thick)copper strip
onto both planes and solder. I found a 'half a mile long ' roll of this
material at a flea market .

Frank GM0CSZ / KN6WH

On Sat, 01 Jul 2006 13:43:29 GMT, David
wrote:

Thanks for your input everyone.

I re-constructed the layout using SMD components on a bare copper PCB.
This time I changed the typology slightly and added 100R with 1n in the
source of the BF998. I placed a 2k7 across the drain coil to set correct
Rp for the DTC bandwidth.

I also changed the tuning coils for fixed coils and tuned with variable
caps instead.

The circuit seems stable now. The Amp has total gain of 23dB (includes
loss of the input filter and double tuned circuit).
All caps are tuning properly with no unexpected behaviour.

I have not yet added any shielding or connected vias from top to bottom
plane of the PCB.

The tuning range is almost as calculated (6MHz Bandwidth at Fo = 150 MHz).

Does anyone know what an reasonable 1dB compression point would be for
a the BF998 at 150 Mhz ? The drain current seems a bit low (about 3mA)
and I think this may need increasing to get a reasonable dynamic range ?

Before you get too concerned over the amps DR you need to consider
that the RX following will be typically be degraded in DR by the
amount of gain ahead of it. That would be true for most cases unless
the RX has a IP if +10DBm or better.

I haven't meaured many of those amps for DR but 1db compression
is around a few DBm ( one was at +2dbm and the other was +5dbm
output) by then most recievers are grossly overloaded.

The gate 2 bias is currently 4V. This should not be pinching off the
drain current at all. I wonder if the 100 Ohm in the source should be
made smaller ? Any ideas.

FETs have good dynamic range for their current. You can lower the
100r but the increase in DR is small. You can also play with the
bias to gate 2 (go for the high current side of peak gain).

Allison


Thanks

Regards

David



Highland Ham wrote:
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.
==============================
With only having double sided PCB material I drill various holes and
bond the 2 planes at different locations by soldering wires on both planes.
For VHF/UHF circuits (in addition to the above) I line the circumference
of the material by folding 6mm wide thin (abt 0.2mm thick)copper strip
onto both planes and solder. I found a 'half a mile long ' roll of this
material at a flea market .

Frank GM0CSZ / KN6WH