Antenna ferrite loopsticks verses air core?

I increased the input to 5Vpp this gave me about 1.236Vpp on the Drain and 1.25Vpp on the Source. For a difference of 14mVpp.

Mikek

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Reply to
amdx
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The net gain's a result of the *divider* formed by the series coupling cap and (the FET input capacitance + strays), multiplied by the follower gain. I asked Mike to measure the follower gain so we could then calculate the input capacitance.

Bootstrapping should reduce the FET input capacitance by a factor of

1/(1-G), where G = the gain of the bootstrap.

That's true. Mike could use a shielded connector and cable if he didn't mind putting his series cap at the probe.

Down to the strays, if we want, or even less. See above. This circuit should be under 1/2 pF, by my estimate.

Cheers, James Arthur

+12V +12V -+- -+- | | | [22k] R5 Q1 \| | 8.0v BC547B |---+-------. 7.4v .
Reply to
dagmargoodboat

That's good technique, but the result is a little puzzling--with bootstrapping gain this close to unity we'd expect better cancellation of the FET capacitances.

The FET-follower into a current-sink load has a gain of 0.98 (measured). That should cancel 98% of Cgs, the largest FET capacitance (5pF).

The drain-follower circuit also has a gain of roughly (250mV-4mV)/250mV = .98, which, following the .98 gain of the FET stage, produces a drain bootstrap gain of 0.96. That should cancel 96% of the BF256C's Cdg (1.2pF), the remaining input-related capacitance.

Either there's close to 1pF of stray capacitance to ground at the input node, or ... I've missed something. Like, what's your test frequency? I've been assuming 1MHz (crystal-radio band).

Cheers, James Arthur

Reply to
dagmargoodboat

Yes, of course, but the lost voltage is across the input cap. The voltage follower gain will be nearly 1 in all variations. The capacitance of the amp input is a function of it's intrinsic capacitance and the gain. Closer to 1 the gain is the smaller the bootstrap makes the effective input capacitance.

That's not what I asked. What is important in determining the circuit gain is the amp's effective capacitance value compared to the input cap. To get the gain of the input amp with input cap close to 1, the effective input capacitance of the amp would need to be ball park 10 times lower than the input cap.

If the gain of the input amp were made more than 1, (to compensate for the lost gain in the cap divider) the circuit could easily become unstable. A follower won't have gain of 1 or more, only approach 1.

I don't know how close you can get to a gain of 1 for the amp and input cap, but I think it will be hard to get the amp effective input capacitance anywhere near 0.04 pf. If you can do that, you really don't need the input cap. I believe it's purpose was to mitigate the input capacitance of the amp so it didn't impact the circuit being measured. If you can get the amp effective input capacitance below 0.4 pF, you can do away with the input cap and it's voltage division.

Remember what the input cap is for. That's why I suggested it might be useful to put it in the probe end. Using a coax with the shield as guard, there should be little problem with noise. Heck, you are measuring antennas/coils. Kleijer found a radio station in his measurements because the coil was picking it up.

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Rick C
Reply to
rickman

Yes, 1 MHZ. I've checked my circuit layout many times, But still, any thing that could be wrong to cause this? I'll go check resistor values while waiting for a response, maybe I got two of them reversed, I don't expect it though. Thanks, Mikek

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Reply to
amdx

If the resistor values were swapped the DC levels would be off. That's what they are there for, to set the bias points. What's the DC level at point (C) just before the output cap?

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Rick C
Reply to
rickman

I think you might want to re-read the thread. With a low Cin(eff), net gain should be close to unity, *including* the coupling cap.

Cheers, James Arthur

Reply to
dagmargoodboat

From the d.c. voltages you posted, the resistors seem fine. Maybe a photo of the layout would clear things up? We can take this to email rather than clutter s.e.d. if you'd like--my email address is good.

Rick's right about the shield-driving scheme--it'll never fly as drawn.

Solutions: move the 0.4pF to the probe tip, or, at some point, you could use a much-larger series cap., e.g. 10pF.

Kleijer's 0.3pF input cap was to reduce his FET's 5pF loading to 0.3pF.

But if we ever get our FET down to 0.3pF, the series cap's absolute value becomes less important. 10pF coupling in series with 0.3pF at the FET, for example, is still 0.3pF loading at the input, but would make the shield drive ~= Vg(T1) ~= Vin, allowing us to cancel the coax capacitance.

---- 10pF Vg(T1) Vin >---------||---. --+- | (input C, post-bootstrapping) | --- 0.3pF Ceff | --- | | | === | '----< shield drive

Cheers, James Arthur

Reply to
dagmargoodboat

The plan is to permanently wire the coax end to the tuning cap, and then swap different coils to it. I was about to say that a series cap at the probe not a good plan, but I could put some type of permanent connection on my tuning cap to hold the

0.4pf cap and the connect the coax to that. I suspect I'd be adding capacitance with that though.
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Reply to
amdx

I never said that wasn't true. My point is that to get close to unity including the input cap, the effective input capacitance of the amplifier has to be *much* lower than the input cap. At that point, why

*have* the input cap? Is there DC to be blocked? In fact, at that point the input cap could be replaced with a much larger value since its value no longer determines the circuit input capacitance.

Wasn't that the reason the input cap is in the circuit, to reduce the input capacitance of the circuit exactly like the 10x dividing resistor in an oscilloscope probe raises the input resistance of the scope?

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Rick C
Reply to
rickman

If you add the 0.4 pF cap with the coax on the amplifier side of the cap, you will be adding 0.4 pF capacitance maximum. If you add the coax without the cap or put the cap on the amplifier end of the coax without the amplifier driving the shield, you will likely be adding a lot more than 0.4 pF.

Why do you want to add the coax permanently?

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Rick C
Reply to
rickman

That's what I was saying. There *is* a page 11, page 10 just doesn't have a link to it. Go back to the index.

He gets good Q with FOAM PVC. Totally different from solid PVC, much less of it and a lot more air. The point is foam anything else worked even better.

How do you accomplish that without an amplifier? You did notice the site is all about *crystal* radios, right?

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Rick C
Reply to
rickman

Yes, that's why I posted the DC values, and I said I didn't expect it though. But we seem to have problem so need to check everything possible. The DC level at (C) is 2.7V. Mikek

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Reply to
amdx

Of course. Point (C) is tied to point (A) through the BE junction of Q2. Duh...

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Rick C
Reply to
rickman

I don't care if it is coax or a pair of wires. I want a battery operated fixed testing setup. The 50 ohm output will after a peak detector or demodulator and then a meter, may need a log amp. With it I could compare the output of different ferrite antennas. I could drive an antenna with my sig/gen and measure Q using the 3 db method, etc.

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Reply to
amdx

Ok, I understand now. I wasn't getting that this is a test fixture. It will never be operated without the amp. Where you put the captive coax doesn't matter then.

Since your series cap can be made pretty consistently, I would suggest adding it to the fixture before the coax. Then the entire coax and amplifier capacitance impacts to the test fixture will be minimized regardless of how well you can mitigate the capacitance through the bootstrap circuit.

If you need another cap for another type of test, you can hole punch another fairly easily and it should have pretty much the same capacitance. One advantage of having a significantly larger capacitance at the amp input is that it will have less impact on the end to end gain if it varies.

How long a cable will you use? By that I mean how many pF?

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Rick C
Reply to
rickman

I have no understanding of how putting the 0.4pf cap at the end of the coax makes it easier to cancel. I probably need to go back and read the thread.

Re, how many pf of coax, I was thinking maybe 6 inches of RG58, maybe

13pf. This a question of component layout to minimize losses. However, it look to me like I could just set the tuning capacitor on top of the amplifier housing, Probably even have physical contact between the stator frame and the amp housing. That would only leave a 2-1/2" wire to connect the amp input pin the the rotor tab on the capacitor. Here is a picture of my first thought.

Ignore the added trim cap, it was needed for a test. Although I will probably include the trim cap, but I will mount it forward about an inch. The trim cap was too big and I had to remove six plates, I'll probably remove at least one more, as it was still hard to peak some of my coils. Mikek

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Reply to
amdx

What I meant was, the point of adding the 0.4 pF cap in line is to limit the capacitance seen by the device being measured. If you leave off the cap the device being tested sees the entire capacitance of the measuring device. If you add the cap in front of all the added capacitance the device being measured only sees the combination which will always be less than the 0.4 pF cap (series capacitors combine like parallel resistances, 1/Ctot = 1/C + 1/C2).

If you put the cap at the end of the coax right next to the unit you are testing, then all capacitance after that point is isolated so the unit doesn't "see" it. That's the advantage of putting it there.

This does nothing about the impact of the added capacitance of the coax and the input capacitance of the amp in terms of affecting the amplitude of the signal. The larger this capacitance, the more the signal is attenuated at the input of the amp. So it will help the gain of the overall amp circuit to reduce the input capacitance of the amp.

That said... Since you can add gain after the input stage, why exactly is it important to reduce the impact of the amplifier input capacitance to nearly nothing? The original amp had a voltage loss of 17:1 from the capacitance voltage divider, if I am reading it all correctly. This is now reduced to 4:1. When you add the capacitance of a coax to the circuit the voltage divider ratio will go back up significantly.

I would try to minimize the length of the coax to minimize the capacitance. If you can improve the bootstrapping of the circuit that will be great. But when you reach the limit of what it will do, it will be easy enough to set the gain of the following stages to make the overall gain what you want, which is 1. I think it will be more important to construct this so the gain is stable rather than trying to get the gain of the first stage to be as close to 1 as possible.

I would say the mounting looks like a good idea. I'm not sure I would bother with a coax. I'd use a stiffer wire so it is less likely to bend, maybe 14 gauge. As it moves around it changes the capacitance which changes the gain. If you do use coax, I would use a big, fat, stiff coax with a low capacitance and low loss. I know it wouldn't be much loss in a few inches of cable, but I was amazed at the small things that would affect Kleijer's circuits including bushings on the variable capacitors. So I can only imagine the coax materials will make a difference.

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Rick C
Reply to
rickman

Great, now I see the reasoning.

The only importance would be, you would not need to retune after adding the amp to an LC. Doesn't affect a fixed instrument, unless you have calibrated tuning cap. Also, if the amps input capacitance was lossy like having a cap with poor bushing connection, this would lower Q. And, it would be neat to have an instrument that when connected has no measurable effect on the circuit under test.

The original amp had a voltage loss of 17:1 from the

If I do mount the cap and amp like that, I would make an aluminum plate similar to the one holding the trim cap, but reaching both screws near the bottom of the cap. The plate would be large enough to cover part of the amp so I could put a couple of screws into the amp enclosure. For the fixed instrument, I see no problem putting the 0.4pf cap at the end of the coax or wire, I would add a tie point on the tuning cap and put my 0.4pf from the stator to the tie point and then the tie point to the amp input. For a portable amp it would be better inside the enclosure with about

6" of coax. I was think an isolated bnc, but I measured one and it has 2 pf of capacitance, so maybe just coax all the way into the enclosure and the 0.4pf tacked on the center conductor.

A question for anyone, what are these connectors called?

Thanks, Mikek

If you do use coax, I would use a big, fat,

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Reply to
amdx

Indeed. I change the number in the URL by hand by starting with page 11 he change the URL structure. Well, I am not complaining, my web site design skills aren't that great either.

Look at page 7. Very nice Q values except for the dirty pipe yet this is all on regular PVC pipe.

You need an amplifier for that. If you want to you can make an energy-harvesting one, sans battery. I mentioned this method purely as a hint, in case anyone ever needs high Q without making fancy large coils and babying them.

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Regards, Joerg 

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Reply to
Joerg

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