How 'Fast' is this coil?

[Martin G7MRV]

Ive set up the damping on my Surf PI 10" coil. This is the trace on my old 20MHz scope



Im set to 20uS per division for the timebase, now to me, it looks like my coil response is around 10uS! But this is a very basic 'buched' wound coil, nothing special to reduce self capacitance, other than its made with the wires from very expensive professional multicore network cables!

I was expecting a reading nearer 20uS for a coil of this type?

Now, have I inadvertently made a nice fast coil, or have I mucked up reading the scope?

[Qiaozhi]

To measure the delay accurately, you need to trigger the scope from the output of the TX oscillator (channel 1), and monitor the preamp output using channel 2.

[Martin G7MRV]

The 'scope is being triggered off the tab of the MOSFET

Either with the external trigger or the scopes own timebase, I have the time/div set at 20uS and V/div at 1v. SWP VAR set to CAL position.

Ive also set the time/div at 10uS and I make it 8uS!

[Qiaozhi]

The 'scope is being triggered off the tab of the MOSFET

Either with the external trigger or the scopes own timebase, I have the time/div set at 20uS and V/div at 1v. SWP VAR set to CAL position.

Ive also set the time/div at 10uS and I make it 8uS!

What is the inductance of your coil, and the number of turns?
Do you have the coil connected to the PCB via a coax cable?

[Martin G7MRV]

Inductance is 270uH, 1.4 ohm. I forgot to count the number of turns but its around 20. 10 inch coil. Bundled but then very tightly wrapped in 3M PVC tape

Its currently positioned in 'free space' (ie as far as I can from other metal in my workshop) on a fibreglass rod. Its connection to the PCB is a pair of 26swg enameled wires parallel about 10mm apart and some 1m long (I guess pretty low capacitance!) The damping resistance is at the coil.

Im still trying to source a suitable low capacitance coax

[Qiaozhi]

Inductance is 270uH, 1.4 ohm. I forgot to count the number of turns but its around 20. 10 inch coil. Bundled but then very tightly wrapped in 3M PVC tape

Its currently positioned in 'free space' (ie as far as I can from other metal in my workshop) on a fibreglass rod. Its connection to the PCB is a pair of 26swg enameled wires parallel about 10mm apart and some 1m long (I guess pretty low capacitance!) The damping resistance is at the coil.

Im still trying to source a suitable low capacitance coax

In that case, I would think an 8 to 10us sample delay is quite possible. Your coil is under 300uH, without shielding, no coax cable connected, and presumably no coil connector. However, I'm a little surprised that the Surf-PI preamp is allowing you to sample that early.

[green]

Ive set up the damping on my Surf PI 10" coil. This is the trace on my old 20MHz scope

[ATTACH]32692[/ATTACH]

Im set to 20uS per division for the timebase, now to me, it looks like my coil response is around 10uS! But this is a very basic 'buched' wound coil, nothing special to reduce self capacitance, other than its made with the wires from very expensive professional multicore network cables!

I was expecting a reading nearer 20uS for a coil of this type?

Now, have I inadvertently made a nice fast coil, or have I mucked up reading the scope?

Not familiar with the Surf PI. I would expect the signal to be near zero volts the same as it is after decay before turning the coil on. Don't see it, maybe do to scope triggering. Is 55 usec. correct for coil on time? Are you triggering on fet turn on or off?

[Martin G7MRV]

Green - You cant see the trace just before the step for the pulse, this is due to the limitations of this old Hitachi scope, the trace has had to be shifted in position on the CRT to get a photo

Qiaozhi - Thanks, that makes sense. The direct coonections and the wide spaced parallel feed, which is quite short, offer very low capacitance. I expect then that the higher capacitance when I swap to the proper feed setup will slow the coil.

This is my first foray into detector building (Im an RF engineer) so not 100% au fay with the way they work. It seems the info of exactly whats going on with a Surf PI is spread out all over the place, at least, Ive never found a single document explaining all the specs in one place!

Im going to look at shielding the coil this evening

[green]

[Green - You cant see the trace just before the step for the pulse, this is due to the limitations of this old Hitachi scope, the trace has had to be shifted in position on the CRT to get a photo]

Thanks for the reply