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