21-02-2021, 12:31 PM
Reference the 'scope probe capacitance...
Following on from 'scope probe capacitive loading, my own activities are at the point where a low-capacitance probe would be rather useful.
As they cost stupid money - and are rather easily damaged - I had a go at rolling my own yesterday.
Choice was, junction FET or valve. FET would be easier, and might even be slightly lower capacitance. On the other hand, valve would be more robust, and transients on the input would be shrugged off.
I used a 1j24b as cathode follower, circuit is below. It's operating as a pentode, so g2 has to be bypassed to the cathode. With both the cathode and screen-grid following the voltage on the grid, inter-electrode capacitances are bootstrapped so input capacitance is as low as it could be.
The filament supply is an AAA alkaline cell, down to 1.3V which is OK for the 1.2V of the 1j24b.
Performance? The probe works to 5MHz OK. Input capacitance? Adding 180kΩ in series with the input reduces amplitude as seen on the 'scope from 0.6V to 0.2V. A quick calculation then shows that the grid input shunt capacitance is around 2pF.
Is it worth it? Well, standard x10 passive probe has input capacitance around 14pF, so this gives about 7x reduction. On the other hand, the input swing, particularly at higher frequencies, is limited (not fully assessed as yet). The floating battery probably does no favours as it will have a significant capacitance of its own to 0V: possibly a small button cell might be better (the valve's filament consumes only 12mA). Layout right now is something of a birds-nest. Probably a peaking choke between the output, and the 'scope lead may help.
Comments invited!
Following on from 'scope probe capacitive loading, my own activities are at the point where a low-capacitance probe would be rather useful.
As they cost stupid money - and are rather easily damaged - I had a go at rolling my own yesterday.
Choice was, junction FET or valve. FET would be easier, and might even be slightly lower capacitance. On the other hand, valve would be more robust, and transients on the input would be shrugged off.
I used a 1j24b as cathode follower, circuit is below. It's operating as a pentode, so g2 has to be bypassed to the cathode. With both the cathode and screen-grid following the voltage on the grid, inter-electrode capacitances are bootstrapped so input capacitance is as low as it could be.
The filament supply is an AAA alkaline cell, down to 1.3V which is OK for the 1.2V of the 1j24b.
Performance? The probe works to 5MHz OK. Input capacitance? Adding 180kΩ in series with the input reduces amplitude as seen on the 'scope from 0.6V to 0.2V. A quick calculation then shows that the grid input shunt capacitance is around 2pF.
Is it worth it? Well, standard x10 passive probe has input capacitance around 14pF, so this gives about 7x reduction. On the other hand, the input swing, particularly at higher frequencies, is limited (not fully assessed as yet). The floating battery probably does no favours as it will have a significant capacitance of its own to 0V: possibly a small button cell might be better (the valve's filament consumes only 12mA). Layout right now is something of a birds-nest. Probably a peaking choke between the output, and the 'scope lead may help.
Comments invited!
