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Guru
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6MHz 20W +/-100V RF Generator Design Required for Ion Trap

02/25/2023 5:59 AM

Hello Friends,

6MHz 20W +/-100V RF Generator Design Required for Ion Trap.

I am planning a MOSFET based switching amplifier to generate 200V Peak to Peak voltage and then to use LC filter network to get cleaner 6MHz RF into 5pF to 10pF load.

Has anyone tried such design?

There is 13.56MHz reference design link here 13.56MHz 1kW RF Source Design

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Guru
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#1

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

02/25/2023 9:35 PM

You're going to have a purely capacitive load? You will get a lot of reflection. I don't see how your specification is possible. RG 58 cable is nominally 28.8 pF per foot.

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Guru

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#2
In reply to #1

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

02/26/2023 2:35 AM

There are a lot better choices for loading than RG58, RG213, Microwave cable.

See link.

https://www.digikey.com.au/en/product-highlight/a/amphenol-rf-division/cable-connectors

Is there a problem with using a Ham radio transmitter tuned to 6MHz?

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Guru
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#3
In reply to #2

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

02/26/2023 11:24 AM

My cable selection was not meant as an optimum choice. It was meant to highlight that the "load" for this could not be the proffered purely reactive value of 5 to 10 pF. RG213 cable has a nominal capacitance of 30.8 pF per foot.

Your suggestion that a Ham radio transmitter does bring the idea that antenna design will be critical. That is precisely my point. What type of antenna design will this amplifier drives will present a completely different load than just a small capacitor. Unfortunately the specified 6 MHz signal puts this inside a broadcast band restricted by FCC regulations. This can be done inside a Faraday cage (RF dummy load, resonant cavity) without interfering with authorized communications but that information is unknown.

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#7
In reply to #3

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

03/07/2023 1:46 AM

As the correspondent is in India, maybe the FCC rules and band allocation do not apply in that country for the frequency with which he wishes to experiment.

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Guru
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#8
In reply to #7

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

03/07/2023 11:56 AM

He's also showing US, Canada, and Great Britain flags. Why can't he tell us himself or answer any questions?

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Guru
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#11
In reply to #7

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

06/25/2023 10:45 AM

Device can be shielded. No transmission antenna.

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#10
In reply to #2

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

06/25/2023 10:43 AM

AM Power amplifier or ultrasonic power amplifier may work.

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#4

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

02/26/2023 2:43 PM

"6MHz 20W +/-100V"

This makes no sense. Does the Ion Trap require power or just voltage? I recommend a PI network for the output. Then the 50-10pF would be part of the network. Most Ham radios would already have it, but 6MHz might require tuning it beyond its range. If you need a coax then a 50 or 75 ohm load would be required as a termination to avoid reflection of capacitance or inductance back to the generator/transmitter. How clean of a signal do you need?"

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#12
In reply to #4

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

06/25/2023 10:48 AM

Just voltages at small charging current for pF range capacitance which gets formed inside the ion flow path.

Short low pF cable can be used. Device can be near the trap.

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#5

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

02/26/2023 2:48 PM

Would it be possible to use a handheld VNA and show us the impedance of the input, while in operation? Will the impedance stay constant in operation?

Does the 6 MHz input need to be shaped, like a gate circuit? Sorry, not familiar with ion traps.

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#13
In reply to #5

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

06/25/2023 10:52 AM

Ion traps are like parallel quad electrodes inside a shielded tube and ions of select mass get trapped in the oscillatory field.

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#14
In reply to #5

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

06/25/2023 10:53 AM

It is purely a capacitive load.

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#6

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

02/27/2023 4:21 AM

<...6MHz 20W...> and <... 13.56MHz 1kW...> are readily available commercially. Google "short wave radio transmitters".

Local jurisdiction may require a application for a radio broadcasting licence and it is in the user's domain to apply for such a thing through the proper channels.

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#9

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

03/30/2023 11:12 AM

Your spec. of +/- 200V peak and 20 Watts, [I assume r.m.s. or heating power] and a sine wave (single frequency) comes out to 1000 ohms resistive load.

Radio frequency Amplifiers are usually designed for a resistive load of 50 ohms. Inductance or capacitive loads do not absorb power in theory. In the absence of a resistive load, power will be reflected back into the FETs causing them to overheat or exceed voltage/current ratings & fail.

Using a switching FET to make a square wave causes a mass of harmonics to leak out and interfere with other apparatus. Meeting R.F. emission regulations is a serious problem, even though the amplifier you referenced is an AB [linear] one, not a "switching amplifier" which would use pulse-width control & a filter to get a variable sine or speech/music wave. Permitted emissions on the cables to an equipment @ 6 MHZ are 0.02 μWatt in 10 kHz bandwidth receiver.

If you just want volts into a capacitance @ 6 MHZ, a tuned transformer may serve with little watts needed.

I would suggest you find a licenced radio amateur transmitter locally who could modify a second-hand 7 MHz transmitter to your 6 MHz requirement. Military surplus may tune to 6MHz anyhow. There is probably a club near you, often called "Ham Radio" publically, which can cause offence.

Many amateur transmitters are 150W & robust enough to survive capacitance loads at lower powers or with deliberate resistive loads added. Basically, 6 MHz may come down to adding capacitance to 7MHz filters & matching networks. These transmitters have to be matched to antennas of odd lengths (to fit site dimensions) but operate on bands from 1.8 to 30 MHz - appearing as inductance or capacitance to the feeder with small radiation resistance on some bands - requiring adjustable matching networks.

Amateurs will be familiar with the RF interference problems & needed shielding enclosures/cables at 6-7 MHz & will have receivers which can tune to unwanted emissions. You must be licenced to radiate any more than what is allowed for a television or computer or other electrical equipment by national EMC standards.

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Guru
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#15
In reply to #9

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

06/25/2023 10:57 AM

It is possible to resistively terminate signal to reduce load impedance and to waste some power. This can avoid reflections too.

Inductive termination requires tuning.

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Guru

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#18
In reply to #15

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

06/28/2023 5:09 PM

I did point out in first sentence of penultimate paragraph of post #9 use of some resistive load to avoid too reactive a load.

You did not give any frequency range, just "6 MHz". A resonant circuit with the usual Q or magnification factor of 10 for a transmitter "tank" circuit would have negligible variation over a 60 kHz bandwidth at 6 MHz.

This may help... http://hfsignals.blogspot.com/p/25-watt-linear-for-40-and-20.html

A 7 MHz 25 watt amplifier [more with supply pushed to 30VDC as it suggests]. It is from an Indian source.

You would not need all the relay control/switching or 20m/14 MHz filter, so, for your purpose, circuit is a lot simpler than it looks. The filter is low-pass, so there is nothing to adjust/tune.

From answers you gave to others, you need a balanced output - so you will need a BALUN [balanced to unbalanced transformer - connect backwards for unbalanced to balanced]. Simplest way would be a two winding transformer with step-up and symmetrical capacitors to earth from each end of secondary.

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Guru

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#19
In reply to #18

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

07/04/2023 1:59 PM

Apologies for not giving Web reference for post #18.

http://hfsignals.blogspot.com/p/25-watt-linear-for-40-and-20.html

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Power-User

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#16

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

06/25/2023 1:37 PM

Are the electrodes connected in parallel and make one terminal, and the other terminal the shield?

How are the electrodes connected? How are the electrodes fed with one signal?

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Guru
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#17
In reply to #16

Re: 6MHz 20W +/-100V RF Generator Design Required for Ion Trap

06/27/2023 3:45 AM

NO. Two separate electrodes as you bias a piezoelectric device on two sides and then shield the device from external fields.

There will be some capacitive link to ground from electrodes.

Electrodes are positive and negative AC with respect to the shield which forms a capacitance from electrodes to the cylindrical shield. Among electrodes the capacitance is far more smaller.

Ions have very small current in pA to nA range or one can say these are number of ions that usually flow thru and get trapped in the Ion Trap and oscillate in the trap to form their larger density at one place.

Device is ion filter by trapping ions of particular mass. Other ions simply go thru the tube in the field.

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