21-02-2021, 06:12 PM
Kalee20 Wrote: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.
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I used a 1j24b as cathode follower
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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.
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Is it worth it? Well, standard x10 passive probe has input capacitance around 14pF, so this gives about 7x reduction.
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Comments invited!
Hi Peter.
That seems to be a worthwhile project, it may become a necessity for me, it hasn't just been a nuisance to have to just bear in mind the extra capacitance in a circuit, its having to alter the circuit to make it work while being measured in some cases. Its easy then to forget its been done and then get into a right muddle. And this all happening at low RF.
Would results more qualitative than quantitative, regards measurements? It should at least show resonance of a tuned circuit pretty near where it should be if capacitive loading could cause a shift.
My oscillator project has morphed into one with amplitude regulation, the output being sampled, rectified, then the DC subtracted from a reference and used to bias the grid of the valve. It still remains quite simple, though the additional diodes are sand not hollow, as the regulation was a bit soggy when I tried with an EB91 in place of the two 1N4148's.... The output is 18volts PP....regulation settles at 1% across the tuning range as against 10% without regulation. The technique is to trim the anode load resistor to give as level a response as possible before putting the regulation in place....
The circuit as posted works well, despite being built from bench sweepings.
One point that I've found important to note here about those axial chokes is this, they don't seem to be wound with any regards to hand, I can't really express easily, but some seem to be "left hand thread" others "right hand thread"
basically where phasing is important, you can't be certain that they are correct just from orientation..... The two I've used in this circuit are an example of this.Thanks.
Amie.
