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Author Topic: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp  (Read 1380 times)

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OK just looked at your Amidon calculator link, I should have looked at that previously, I now realize I have type 2 red, not the type 6 yellow, and red looks only good for 30Mhz, I will look again on ebay and re order.

OK just ordered a ciuple of Amidon T44-6 Yellow, these are in the UK so should be fairly fast
   

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I added 2 more turns to the coil to see how it performed, it now only oscillates at twice 45.250Mhz and the tuning on the pi last cap is sharp to get the peak resonance, so it would appear I need to take a turn off and try again.

EDIT
OK that didnt work out, in the test above i was sniffing the signal with the scope probe earth connected to the coil only, it appears the scope is only able to sniff twice the resonant frequency, i cut 1 turn at a timne off the coil and sniffed and it would always pick up the double frequency, if i connected my 560 Ohm and cap and went across the coil i would pick up 45.250Mhz not twice the frequency, strangley the last cap in the pifilter always had to be fully engaged to get max pk-pk at 45.250Mhz, something strange going on, probably because my pi coil and first variable cap are incorrect, i will move to calculate these values and construct the final coil using calculators now.
In the picture attached with the pi filter the second cap is half adjusted and this was to get maxmium amplitude for the 90Mhz signal seen on the sniffer probe.

« Last Edit: 2019-02-08, 17:58:38 by Peterae »
   

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It's not as complicated as it may seem...
Sorry for my ignorance Peter, ultimately what are you trying to accomplish/create?


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I am using a DDS to feed a 1.5 watt pre amplifier and this will then feed a home built 16 watt fet amplifier that will then feed a tuned LC.

I have a quartz glass tube filled with pure iron powder that will be heated above it's curie temperature, the tuned output inductor is wound over the quartz tube containing the hot iron, there is also a strong static magnetic field that aligns the irons atoms spin 90 degrees from the axis of the coil and tube.

I need to transfer as much power as possible into the iron powder at 45.25Mhz as possible.

I'm really waiting on a couple of cores at the moment to build the inductors in a couple of pi networks to impedance match the preamp output to the fet amp stage input and then another pi network to match the impedance of the output amp to the tuned LC.

because the scope probe has been loading the LC i thought i would try sniffing with the scope probe but this did not work.
In the above post the idea was to see what coil I could wind with the maximum turns I could use with the pi network to obtain the best resonance.


The device is a cross between Meyer/Mace and Gillespie/Coleman, Meyer used copper and iron and Coleman used Colbalt, both devices supposedly generate energy from the iron/copper/cobalt fuel by destabilizing the atom to release energy in the form of beta radiation, Meyers device was reported to generate 30 times the input power, I am heating the iron above it's curie temperature to help with skin depth and increase signal penetration.

I started out trying to build a xtal oscillator I bought on ebay which was 45.666Mhz and was hoping we could drop that down, but at the moment I have given up on that and reverted to my DDS generator.
   
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I think if you use a 2 or 3 turn air core coil (OD say 2 cm) for sniffing and hook up such coil directly to the scope probe, then you could pick up a more stable sample from the coil to be 'sniffed' (mutual coupling).  Move the sniffing coil close to the other coil and when you start seeing a waveform then adjust distance and tune the other coil circuit, in steps.  Aim for as loose coupling  as possible.
Consider that your series 560 Ohm may still not give enough isolation when you use it... 

Gyula
   

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It's not as complicated as it may seem...
What are you using for the 1.5W preamp?


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It's not as complicated as it may seem...
A question for gyula...

Are RF transistors manufactured to be optimized for a certain frequency band? For eg. could a transistor that is supposedly optimized for 136MHz-174MHz operate just as well in a circuit designed for 45MHz?

I have access to a number of VHF (~ 155MHz) RF Power BJT transistors and could send them to Peter.


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The pre amp was an ebay buy 20-500MHz 1.5 Watt

unfortunately I have no other information on it.
Here's a picture
   
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I advise against buying amps that are not properly described, it usually means that the seller is not competent in the field and sells amps as he could sell lettuces or sofas.
This one seems very acceptable, but perhaps not the same price.
Note that if this is not explicitly specified, in general the amp is not linear, i.e. the output power will not be exactly proportional to the input power. It's the case of this one in the link. It's only annoying for amplitude modulation, but here we work at a constant level.


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A question for gyula...

Are RF transistors manufactured to be optimized for a certain frequency band? For eg. could a transistor that is supposedly optimized for 136MHz-174MHz operate just as well in a circuit designed for 45MHz?

I have access to a number of VHF (~ 155MHz) RF Power BJT transistors and could send them to Peter.

Well, my personal opinion is they can operate if the matching networks are redesigned or simply newly designed directly for the lower frequency (in this case for around 45 MHz). The gain of the transistor will be higher so the feedback network also needs special attention to tame it, avoiding unwanted oscillations. 
Manufacturers specify their devices for certain frequency bands that suits for certain industry or military etc applications For those frequency bands the data sheet normally includes working conditions, input and output complex RF impedances, S parameters etc but outside that band the parameters are not always given,  so one needs to extrapolate data. 

The problem to be solved in Peter's case is to match an inductive load (a coil having iron core in powder form) to the output of a power amplifier at 45 MHz while the output power expected to be fed into the coil would need to be in the order of 15-20 W at least.

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The pre amp was an ebay buy 20-500MHz 1.5 Watt

unfortunately I have no other information on it.
Here's a picture

Hi Peter,

By the look of the PCB, it has two stages, the input drives a low power wideband integrated circuit IC1 (like say a MAR-6 or ERA-6 for instance) and this drives the other amplifier, IC2,  see attachment.  These two integrated circuits are called MMIC (monolithic microwave integrated circuits) and usually they are designed for having nearly 50 to 70 Ohm input and output impedances for a specified wide frequency range.  Data sheet for Mar-6 is here  https://ww2.minicircuits.com/pdfs/MAR-6+.pdf   The other IC can also have various manufacturers, providing the 1.5W output across a 50 Ohm load, there are two SMD coupling capacitors from the IC pin towards the SMA power output pin.  You may be able to see the label on the two MMIC chips by a magnifier, unless they are wiped away for good.  IC2 has its own bias adjust circuit with the trimmer potmeter under IC2.  IC1 drives IC2 via two series coupling capacitors.

The switch mode power supply circuit surely receives the DC input voltage range and provides a regulated supply voltage for the two amplifier ICs.  In the picture below (taken from ebay) the IC type is LM2576HV if I see it correctly,  data sheet: http://www.ti.com/lit/ds/symlink/lm2576.pdf 
This amplifier is still available at ebay:
https://www.ebay.com/itm/352486108062 

What F6FLT is referring to is okay too for a good preamplifier (with 3W output) and here is the same amplifier for much cheaper if someone wishes to buy such:
https://www.ebay.com/itm/352486108062 

Gyula
« Last Edit: 2019-02-09, 14:15:07 by gyula »
   

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OK some updates.

The iron/resin core has been made, specs are 1.228kg 56% iron 44% Resin Approx. sprayed matt black, see picture, just printing the magnet holder.

Also the yellow 44-6 cores arrived, i put 4 turns on one and measured it, meter says 120nH calcs say 78nH, i made the new pi network, i had a 20Ohm load resistor and recalculated the values for the pi, i used 2 x 120pf variable caps, C1 was in parralel with a 220pf cap and C2 in parralel with a 330pf, it appears to give a good output across the 20 Ohm load at 5.8V RMS giving me a dissipated power of 1.682Watts.

One magnet is epoxied in (the easy bit) keeper is drying in foreground.
« Last Edit: 2019-02-09, 20:46:34 by Peterae »
   

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What do you make of this, I have mentioned this some time ago else where, it's also buried in one of Arie Degues patents.
https://www.electronicdesign.com/analog/resonant-circuit-generates-high-frequency-magnetic-field
   

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It's not as complicated as it may seem...
It looks interesting Peter. Just hope the relatively high Q doesn't make it difficult to realize.


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What do you make of this, I have mentioned this some time ago else where, it's also buried in one of Arie Degues patents.
https://www.electronicdesign.com/analog/resonant-circuit-generates-high-frequency-magnetic-field
It is a good idea,
However, I run into a problem with it in pulsed NMR, because when the Tx LC network rings at the target frequency, then it is impossible to distinguish the LC oscillation from the NMR signal later, if an RF blanking is not used (e.g. a MOSFET which shorts or opens the LC circuit so it stops ringing immediately or a PIN diode which purposely detunes the LC circuit).

Also to tune the Tx LC network well, I had to sense the generated RF magnetic field, ...so I used another coil as a sensor. 
Unfortunately, the sensor coil had its own self-resonances and a non-constant frequency characteristic, that confused my measurements and tuning process.  The solution turned out to be a GMR head from a broken hard drive (the only difficulty was learning how to power its integrated head amplifier). Otherwise, it is a very good linear RF magnetic field sensor.
« Last Edit: 2019-02-10, 16:03:47 by verpies »
   

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Hi verpies
I don't believe I need to sense the NMR response, I will just scan around 45.525Mhz and try to detect beta radiation emission.
Ah, but you should sense the response.
This is because without sensing you will not know the width of the 90º pulse.  The width of this pulse depends on the amplitude of the RF field penetrating the sample.  The higher the amplitude the narrower the pulse.
If your pulse is too wide, then the spin axes will flip more than 90º, for example 110º   ...or 180º ...or 360º which will amount to nothing.

If you want to scan blindly, you'd need to scan BOTH the frequency and the RF amplitude or RF burst width.  That is a lot of work...and while you are at it, you are really blind to what's happening inside.
   
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Hi Peter,

Glad you have achieved that nice sine wave across the 20 Ohm resistor, this will drive the linear MOSFET power amplifier. Of course, you will need to remove C2, 330 pF and its trimmer from the filter when you attach the Pi filter output (that includes the 20 Ohm resistor) to the MOSFET gate-source input. The trimmer cap across C1 (220 pF) can remain in place should any retuning due to the MOSFET is needed or not.

Regarding your coil calculation, the data for 0.281 uH from the formulas could likely be okay for the inductance but if you place the iron powder as the 'core' for this air core coil, then the inductance will very likely be increased, no?  Or you started out with the 0.281 uH inductance value (which resonates with 22 pF at the needed higher frequency 63.996 MHz) that already has the iron powder core?
Here I assume you calculated this coil for doing the excitation for the iron powder.

Gyula
   

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verpies
OK i see, interesting that Meyer and Coleman did not have a DC bias field or spin detector, but instead both used the conduction of the iron for power output, i wonder what happens to current flow in the iron when the atoms flip, what i am saying is that i wonder if the flip manifests itself linearly through the iron as a resistance change or a change in the current flowing through the iron.

gyula
Thanks, indeed we are ready to connect the amplifier input.

Ah i forgot about the iron, well i maybe able to compare the inductance measurement before and after inserting the iron to get an idea whats happening.


Interestingly the above article states that parralel LC is not best for circulating current,

'The coil current is generally small in parallel resonance'

and suggests that normally series would be used

'Up to now, the most practical and efficient way to drive high current through a magnetic coil is to use a series resonant circuit'

I think it would be good if I drop a low ohm resistor in series with the inductor and see if I can measure the current in the LC
   

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verpies
OK i see, interesting that Meyer and Coleman did not have a DC bias field or spin detector,
It is not surprising, because the magnetic domains in iron provide their own 33T DC bias field.
It helps to align the domains in one direction, but even without such forced domain alignment, the ferromagnetic remanence keeps their directions from being totally randomized and from cancelling out.

what i am saying is that i wonder if the flip manifests itself linearly through the iron as a resistance change or a change in the current flowing through the iron.
There could be some relationship.  I did not check it yet.

I think it would be good if I drop a low ohm resistor in series with the inductor and see if I can measure the current in the LC
Instead if detuning the LC circuit with such measurement, use a GMR head from a broken hard drive to sense the magnitude of the MMF generated by the coil, which is directly proportional to the current flowing in it.
This is minimally invasive and reliable.
   

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I got the second magnet mounted, I thought I was going to have problems due to the North / South pulling at each other, but it turned out the pull to the iron core was greater :)
Not yet sure what sort of magnetic field strength I have, I do have some Ratio metric Hall sensors but these only go to 900G, if anyone knows of one that goes up to the required 6000 gauss let me know please.

I did connect up the preamp, input pi filter, amp and tried a LC using calculations for 45.250Mhz, I placed 4*1 Ohm resistors in parallel to give 0.25 Ohm in series with the final inductor so I could monitor current. I did not manage to capture waveforms, by scoping across the final LC I did manage to get 191V pk-pk nice sinewave at 45.250Mhz but the fet overheated and burnt.


Things I need to do and solve, the little brown 120pf variable caps on my preamp pi filter stage fell apart, not sure if it melted but I need to find better variable caps or maybe switch to the 45pf ceramic caps I have, i need to wind the 2 inductors on the amplifier on toroids, i presume i will need bigger cores for this due to saturation currents.
I may need to add a pot to attenuate the drive signal to the amp.


The heatsink on my fet is way too small to dissipate the heat, i will rebuild next time with larger heatsink.




   
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Hi Peter,

Please provide:  what was the DC supply voltage to the MOSFET?  What is the exact type of the MOSFET?
                         
The MOSFET is supposed to operate in Class A mode (maybe Class AB) and an idle drain current should be adjusted first with the gate bias potmeter. It may be around not more than perhaps 50-100 mA DC when there is no any RF drive (of course the 20 Ohm should be there). 

The coil in the drain circuit should be a choke type, maybe you can scavenge such from mains input of PC supplies or other appliances.

There should a kind of matching circuit between the drain output and the RF coil that would include the iron powder, especially if you wish to drive it with the series resonance method you referred to, to use the double current advantage via the coil. The impedance of such network will be in the some Ohm or less range, this network is almost a short circuit load for the drain output, this is why a matching circuit would be needed. Perhaps, just perhaps the output Pi filter could do this matching, this involves a second coil for the Pi filter.  Probably other matching circuits could be devised.

The smaller type trimmer caps do not 'bare' too much RF current or voltage, unfortunately.

Gyula
   

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Hi Gyula

Yep I have some chokes somewhere.

The fet as per the circuit IRF610 which has a VDS of 200V, I wonder if my drain was seeing the pk-pk voltage i was seeing across the LC causing it to blow.

The circuit stated 50V supply at 0.5A, i did try 50V and adjusted the pot to bring the current up, i cannot remember the settings when it blew, dont get too hung up, i was jumping the gun somewhat, i built this amp board a couple of years ago when i was following a similar path, i will rebuild it from the things i have learnt so far, bigger heatsink and better quality caps.


Now what i really need is a setup process, i think i am going to need to set the input amplitude and i think a small pot will not be high enough wattage, so i will probably use 2 fixed resistors to form a divider chain using metal film resistors I can always change the values to get the drive I need, where is the best place for the resistor divider, does it go straight after the pi filter but before the 10nf cap feeding the fet gate or after the 10nf cap feeding the fet gate.?

Also to prove i have perfect drive i need a way to power the amp up and scope the drain, preferably without a pi or load to start, will it damage the fet if i have nothing connected to the drain or should i connect a resistor as a load maybe.?

Once i know i have good fet drive i then connect the output pi filter and drive the 50Ohm dummy load resistor

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Hi Peter,

Indeed the 50V supply voltage must have been too high as a start for the amplifier if there was no any previous matching / tuning attempt at a lower supply voltage.   

Regarding a setup process, here is a possibility.  This text is going to be long, sorry,  I think it is needed. 

See a modified input circuit for the amplifier attached,  I added a resistive divider ("a resistor Pi") ahead of the input Pi filter. The 470 Ohm potmeter can be any normal one, not a wire wound type, to be able to reduce the > 1 W RF power from the output of your preamp to 0.2-0.3 W level, not to overdrive the linear amplifier.

Next, remove the MOSFET, put a 360 pF across the 22 Ohm resistor and drive the input of the resistive diivider from the preamp. Adjust C1 of the Pi filter for max sine wave amplitude around 45 MHz, measure across the 22 Ohm resistor with the scope probe. Check by turning the 470 Ohm potmeter between what minimum and maximum range it can vary the RF voltage amplitude across the 22 Ohm.  Leave the wiper at say 2-3 V peak to peak across the 22 Ohm, if this low amplitude is not possible (i.e. at the full 470 Ohm wiper position the voltage is still higher than that), then place  say a 100 or 150 Ohm resistor in series with the potmeter and repeat this adjustment.

Now remove the preamplifier, i.e. no RF drive, and also remove the 360 pF cap which so far substituted the gate-source capacitance and put the MOSFET back to the circuit.  Also, remove the choke from the drain and put a 50 Ohm dummy load instead,  capable of the expected 15-20 W dissipation.  It is ok you wish to see and check the signal at the drain but you have to have a load there too for feeding the MOSFET with the supply voltage. 

You may wish to start with much lower than 50 V like say 10-12 V DC input only.  First the DC operating point is to be adjusted with the Set Bias potmeter: you need to measure the DC drain current and set it first say to 200 mA only at the 10-12V supply voltage and no any RF drive yet. The wiper of the bias potmeter should be set in advance to start from about 2V because the minimum threshold voltage for this MOSFET is 2V.  But your MOSFET may have say 3.2V threshold voltage from where drain current can start flowing at all, so slowly turn the bias pot up from the 2 V and watch the drain current rising to about 200 mA.

Now if you increase the supply voltage towards 40 or 50 V,  still monitor the DC drain current, it will increase above the previous 200 mA of course (without touching the already set bias potmeter) and the heat dissipation at 50 V will increase beyond 10 - 12 W, a big heat sink is needed for the MOSFET of course. Naturally the same drain current will dissipate heat in the dummy load too, it should be able to dissipate it.

I suggest to run the MOSFET at this operating point for at least several minutes to check temperature and drain current stability (with still no RF drive input yet). 

If all seem ok, then increase drain current up to 400-450 mA, supply voltage is at 50 V and see how heat sink temperature increases, hopefully a small ventilator would not be needed to cool it.  Run the setup for several minutes at least.

Then, if you think, apply the smallest RF drive the 470 Ohm pot let through (as previously set above) and check the RF amplitude across the dummy load or across the drain and the negative rail, ideally they have the same voltage across them.  Then you could increase RF drive by turning the 470 Ohm pot and monitor the drain current and the RF amplitude at the drain or the dummy load. 

It is possible you need to retune a little the C1 cap in the input Pi filter when the supply voltage for the MOSFET is in the 40-50 V range because the gate-source capacitance changes to a lower value and this may affect input Pi filter matching.

It is possible the RF input drive to the MOSFET (depending on where the 470 Ohm pot is set) will increase drain current beyond the DC current previously set by the bias pot.  Well, a small overdrive is not yet a problem but you can see this as a starting clipping (limiting) at the peaks of the drain-source voltage wave form.  It is also possible you adjust DC drain current to say 600 mA to increase output power, this would allow an increased input drive too to achieve higher output, only the MOSFET dissipation is the limit.

If all seems ok, then so far the 0.5 A drain current at 50V supply voltage will provide roughly 12.5 W RF power in the 50 Ohm dummy load (the other 12.5 W is dissipated in the MOSFET). Such is the case for an ideal Class A power amplifier. 

Next step is to agree on how you try to match the coil with the iron powder core to the drain of the MOSFET.  The Pi filter at the output first sounds good but your coil will represent either a much higher than 50 Ohm impedance load when tuned to be a paralell resonant LC tank at 45 MHz  or will represent a much lower than 50 Ohm load when tuned to be a series resonant LC circuit,  so the output Pi filter should be thought over.

Gyula
   

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OMG Gyula!, you have a really good understanding of RF amplifiers!
   

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Gyula
Thank you very much for the time this has taken you, fantastic and will keep me on track with a good chance of succeeding. ;)

I have been thinking about how to carry forward, before i complete the amp, i need to study Parralel LC, series LC and the C-LC article above, monitor current flowing in the L component and try to work out the impedance of all 3 methods , then add the iron tube and heat to 600 Deg C and try again the 3 methods above.

Lots to do but it should be interesting.


   
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