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Author Topic: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp  (Read 1997 times)

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I've been trying out the L-LC tuned circuit from that article, driving from my 1.5 Watt pre amp through the pi network, feeding a capacitor and then feeding the parallel LC, I used the circuit from figure 11 and 1 ohm resistors to monitor current, when i tune the LC current for max level and then disconnect and use a GDO to drive it loosely coupled, max current appears at about 33Mhz which is indeed 0.7071 of C-LC driven circuit at 45.250Mhz, this appears to prove the math of the article.

https://www.accelinstruments.com/Applications/WaveformAmp/Magnetic-Field-Generator.html

I am have trouble with current measurements across my 1 Ohm resistor see waveform at 4.95V RMS, which would mean i am dissipating 4.95 Watts, which is pretty clever seen as i am driving it with 1.6Watts, and as the resistor is not getting really hot (31 Deg C in 15 Deg C ambiant) i must assume these resistors do indeed have inductance.

So i will try next a film resistor to see if that cures my problem, until i get an accurate voltage i cannot work out the current or the magnetic field strength, i will say that there is a strong magnetic field around the coil because it's the first time i can easily sniff 45.250Mhz sine around the area with my scope probe with a couple of turns (35V pk-pk) or even with the earth clip on the probe end.

The impedance of the C-LC circuit is 4R or 4Ohm in the above case
« Last Edit: 2019-02-15, 20:46:37 by Peterae »
   
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I am have trouble with current measurements across my 1 Ohm resistor see waveform at 4.95V RMS, which would mean i am dissipating 4.95 Watts, which is pretty clever seen as i am driving it with 1.6Watts, and as the resistor is not getting really hot (31 Deg C in 15 Deg C ambiant) i must assume these resistors do indeed have inductance.
...

It is only recently that I realized how difficult it is to measure high frequency signals with a scope.
The end of the probes is not coaxial, it acts as an antenna or is capacitively coupled. Then the ground wire creates a loop, in which we have induced currents.
The probes must be eliminated and the scope connected directly to the resistance by a coaxial cable designed for high frequencies (very well shielded).
In addition, the cable length must be small in comparison to the shortest wavelength of the signal, otherwise there are phase shifts or resonant line effects, because the input impedance of the scope is unrelated to the output impedance of the setup.



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Certainly would explain the problems i am having, i am now thinking that the easiest way to do power measurements is to measure the heat produced by the resistance load. :(
   
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With a series circuit composed of a germanium or schottky diode and a capacitor, in parallel on the resistance, the peak HF voltage can be measured as a DC across the capacitor, provided that the signal exceeds the diode threshold (about 0.3v, to be added to the measured voltage). It's a basic method but I find it much more accurate than the scope.



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I found a pro's build of my amplifier for sale, it's interesting seeing what he has done.

https://www.ebay.co.uk/itm/151274234687

From today, I tried a parallel LC, for some reason I can only sniff 2nd harmonic with a coil.
I tried C-LC and sniffing this is comparable in amplitude to the sniffing I did with series LC.


I think for ease, Series LC is the way to go, but how to deal with a very low impedance that the amp will see, maybe a series resistor could be used, we know the current the inductor see's is common for the resistor, capacitor and inductor.


So as follows

Inductor is 12 Turns of 1.5mm wire wound to a length of 50mm inductance calculates to 559nH


for series resonance with a 559nH inductor I need a capacitance value of 22.13pF


if we use a 5 Ohm Series resistor and we manage a 30pk-pk drive voltage then we get


Xl = 2pifl = 158.93Ohms Inductive Reactance.


Xc= 1/2pifc = 158.93Ohms Capacitive Reactance.


Circuit Impedance Z = sqr(R2+(Xl-Xc)2) = 5 Ohm R is effectively the load resistance.


I = Vs / Z = 30/5 = 6 amps

Vr = I * R = 30 Volts

Vl = I * Xl = 953.58 Volts


Vc = I * Xc = 953.58 Volts


Magnetic field flux = 18.0956 Gauss


« Last Edit: 2019-02-17, 17:52:24 by Peterae »
   
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I think for ease, Series LC is the way to go, but how to deal with a very low impedance that the amp will see, maybe a series resistor could be used, we know the current the inductor see's is common for the resistor, capacitor and inductor.
...

Hi Peter,

No need for using a series resistor if you meant it for easing the matching of the very low impedance, it can be transformed up by a  matching network.

I attached the schema again with the addition of a possible matching network to transform a small (2-3 Ohm at resonance, indicated in the blue block) real impedance to the drain output circuit of the MOSFET. It is called an L matching network and needs a coil (L) and a capacitor (C) only. The C capacitor should include the drain-source output capacitance of the MOSFET plus a trimmer to make up for the needed value. For the coil a toroidal winding would be the best with ample core cross section but an air core one could also be used. See this link on calculating the L and C values:  https://home.sandiego.edu/~ekim/e194rfs01/jwmatcher/matcher2.html

Source resistance 50 Ohm (this would be the up-transformed impedance the drain circuit will see as a load)
Load resistance    2 Ohm (this is either your "current amplified resonant" or a simple series LC resonant circuit)
For the Source and Load Reactances use zero
For the Desired Q use say 5
For Frequency use 45.25e6

The L and C values you need will appear under the LOWPASS Hi-Low MATCHING NETWORK circuit schematic (first row on the right), I received L=34.46 nH and C=344.6 pF. 

You can use a normal series LC resonant circuit within the blue block too, for simplicity, if you wish, instead of the current amplified resonant circuit.

The tuning procedure (to use first a normal series resonant LC as the load for the power amplifier output) would be advisable like this (fully separate the coil mentioned below from the circuit) :

1) fill in the iron powder for the coil assigned for exciting it. (you have already that plastic coil holder for this right?)
2) try to measure the inductance of this coil with the L meter. perhaps first without the iron powder, then with it.
3) then try to find a capacitor which when connected in parallel with this coil (that filled with the iron powder) gives a parallel resonance around 45 MHz, checked with your grid dip meter. IF the capacitor needed for this comes to be a very small value like under 10 pF, then reduce the number of turns of the coil. Beware: if the iron powder can move inside the coil, as you handle the coil holder with your hand, its inductance will surely change!
4) aim for a capacitor of at least 15-20 pF to give resonance around 45 MHz with the coil.
5) when done, you now have a simple series resonant LC circuit if you connect the capacitor in series with the coil, giving a very low value real impedance (any value like 1-3 Ohm) which will be stepped up towards 50 Ohm by the LC matching network.

Before you build and switch on this total circuit, I advise to go through the procedure first with the 50 Ohm dummy load connected into the drain circuit as I wrote in the previous post.

When that is done and seems ok, there may come placing the choke coil into the drain with the matching LC circuit as calculated from the link and the L coil is wound and the C cap value is considered with the output cap of the MOSFET (CDSS=55 pF at 25 V supply), and also the pretuned output series resonant circuit could be connected. Start with 12 V again first, only then raise the supply voltage higher and higher to 50 V. The main tuning may involve the trimmer cap adjustment across the drain-source for voltage maximum indicated by the scope probe with a sniffer coil. The L coil may also be tuned by pulling away the turns or pushing the turns closer slowly and carefully, it can greatly influence the up-transformed impedance towards the drain, hence the output power too. Notice if you increase this coil value, then the C cap would need to be decreased to keep the network at resonance (but the loaded Q is around 5 only, so bandwidth will surely be wide).

Gyula

ADDITION:  here is  shorter link to the ebay offer,  https://www.ebay.co.uk/itm/151274234687  you may wish to use it in your above post, so the horizontal width of the page will be restored to normal  (for those who do not use wide screen).

By the way, the same FM Linear Amplifier is available as a DIY Kit here:
https://dutchrfshop.nl/en/diy-kits-pcb-s/589-diy-kit-15-watt-amplifier-87-108mhz-rd01mus-rd15hvf1.html     

The schematics, the Bill of Materials are also included to see.  All the L and C values used for matching and in the filters are valid for the FM band which is roughly twice as high as the 45 MHz needed here.  Also, the output impedance is surely matched to a 50 Ohm load, as usual.
The kit does not include the coils but the wire for winding them...    :D   By mentioning this kit, I do not hint to buy it... you decide.   :)
« Last Edit: 2019-02-16, 23:08:57 by gyula »
   

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Hi Gyula

Thats a nice amp kit, very tempting, i will give the home build a go first ;)

I'm going to need some high voltage caps, looking at 10Kv 22pf and adjust the inductor length for fine tune.


I have enough to start the build now.

thankyou very much for your help, very much appreciated, you are an incredible engineer.  ;)


Just ordered a radiation detector https://www.ebay.co.uk/itm/143006383989

and 2 of these caps
https://www.ebay.co.uk/itm/273236302508
« Last Edit: 2019-02-17, 19:19:30 by Peterae »
   

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I am still working on this in the background, still plenty to do.

I made the framework for the fuel rod out of 3d printed hollow sections and poured resin into it, this is to make it more durable from the harsh conditions it will operate under and elevated temperatures which plastic alone is not good at handling.

The fuel rod was made of 2 round printed formers filled with 1200 Deg C compound to glue the quartz tube in place, each end has a copper wire coiled inside the tube to make electrical contact with the iron powder filling.

At first the iron did not conduct very well over 10 MOhm, but after heating with a blow torch it now measure about 1 Ohm and heats nicely with a dc supply connected, I will need chokes at each end to isolate the circuit from the 45MHz oscillations.

2 pictures below have the magnet assembly fitted as well.

I forgot to add the 45mhz coil onto the quartz tube before gluing the end caps on, OOPS the coil is small diameter than the end caps, so i will need to devise a way to wind the coil over the quartz tube without breaking anything, i have some kilm paper which i will wrapp around the quartz tube to insulate it and stop the heat affecting the Inductor coil.
   
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Hi Peter,

Nice progress and build of the magnet assembly. Is the quartz tube fully packed with iron powder filling? (so that the powder should not move freely within the tube?)

Well, on the 45 MHz coil winding: probably only a few turns is needed to have the max some hundred nH inductance, so making that around the quartz tube may not cause much problem versus say making 20 turns.
The iron powder will certainly increase inductance and this is why I think of a few turns (4-5 maximum) only. This seems to be a limitation because the process or effect may perhaps happen more readily when the number of turns is higher excitation_wise, while a higher number of turns gives higher inductance at 45 MHz which then involves a higher inductive impedance hence less current for excitation,  unless the excitation power is increased and increased. But the supply voltage to the 45 MHz power amplifier can be increased if needed, maybe at a price of using a better MOSFET.

Gyula
   

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Hi Gyula

Thanks

Yes the iron powder is packed in the tube, I first glued one end and let that dry, I then heated the quartz tube slightly with a blow torch to get any possible moisture out and once cooled slightly I place the wire in the end and then added a little more iron powder to burry the electrode and then packed glue into the quartz tube end, once that was dry I added the printed end shell and packed that with high temp glue.


I believe it maybe possible to use 2 fets in parallel to double the power, I will see how many turns the inductor requires with the iron in place, although the objective will be to heat the iron above it's curie point after initial tests

Peter
   

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Well, on the 45 MHz coil winding: probably only a few turns is needed to have the max some hundred nH inductance,
...yes and you really should use fine Litz wire for all HF windings.
   
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