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OverUnity Research > Benches > Peterae Public Bench (Moderator: Peterae) > Towards a 45.525MHz 16 Watt Amp
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Author Topic: Towards a 45.525MHz 16 Watt Amp  (Read 22967 times)
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Towards a 45.525MHz 16 Watt Amp « on: 2019-01-20, 18:21:35 »

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OK so I have decided to try and build a Voltage trimmed 45.525 Mhz crystal oscillator & 16 Watt amplifier to drive a coil for NMR investigation.

I have ordered a cheap second hand crystal stamped 45.666MC and am hoping I can trim that with capacitance down to the required frequency.

First though I wanted to get the 16 Watt final stage built see attached circuit.

I already have a DDS and preamp from times past.

Today I wanted to try building the first step of the Amplifier, the input PI filter or CLC filter.

I only have a few variable capacitors that are adjustable up to about 80pf and want to find the required cap values to replace these with fixed values.
The input capacitor i had to parralel the variable cap with a 220pf capacitor (Little red Wima)
There are photos of the DDS preamp and the PI filter setup.
Also there are scope shots showing attenuation at different frequencies.

I was not sure how you adjust the 2 vriable caps, i was getting distortion on the incomming (Yellow trace) waveform at lower frequencies, i assume this is ok if the output waveform is good, i switched fft on so i could minimize the 2nd harmonic.

EDIT looks like i am getting just over 2 Watts in my 50 Ohm load
« Last Edit: 2019-04-28, 18:37:38 by Peterae »
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #1 on: 2019-01-20, 22:10:52 »

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I will need to heat the iron filings at some point so have just ordered 1 of these, so I can pack the tube with iron and seal the ends with fire clay with a wire protruding from each end, I hope to use the irons resistance as the heater.

100mm Long
10mm External Dia
2.0mm Wall 6mm Internal Dia approx
Heat resistant to Over 1000 C
Heat Shock Proof

https://www.ebay.co.uk/itm/331525223850
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #2 on: 2019-01-20, 22:28:11 »
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Hi Peter,

If I understand correctly, your DDS drives a preamplifier which produces about 2W in a 50 Ohm load attached to the output of the input PI filter.

Well, if you wish to use the IRF610 power MOSFET for the 16W amplifier, then you need to include roughly 140 pF gate-source capacitance (Cgs) across the 50 Ohm load because this capacitance will be present when the MOSFET is included in the circuit.
So the input PI filter needs to be examined also if you simply add a 150 pF capacitor in parallel with the 50 Ohm, to represent the Cgs. 140 or 150 pF, not critical because it will change eventually in the function of the drain voltage anyway.  Data sheet says 140 pF typical value at 25V DC drain-source voltage for an IRF610.

Regarding your 45.666 MHz crystal,  you will probably have difficulties in pulling it down (roughly 141 kHz) with capacitance only, a series L inductance would most likely be needed. But it can be done for sure.

Here is an example for a good oscillator in which two 14050 kHz crystals (connected in parallel) are pulled between 14000 and 14065 kHz, with good stability.    https://amqrp.org/projects/NB6M%20contestvxos/VXO%20Experiments.html 

Because the 45.525 MHz is roughly 3 times as high as 14 MHz, the 4.7 uH series coils could be linearly reduced by 1/3 i.e. use 4.7/3 uH for each but of course a single coil could be used there, the real uH value greatly depends on the crystals. 
As you can see the 2nd schematic includes a single 2 uH coil only for the 2 paralleled 14060 kHz crystals.  You may wish to have at least two 45.666 MHz crystals, it helps pullability (but should work with a single one too).

Notice the varicap tuning possibility at the bottom of the series coil(s), this you can use, so either a varicap or instead simply put there a good quality variable air capacitor of at least 40-50 pF with a turning knob.

Gyula
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #3 on: 2019-01-20, 22:56:11 »

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Quote from: Peterae on 2019-01-20, 22:10:52
I will need to heat the iron filings at some point so have just ordered 1 of these, so I can pack the tube with iron and seal the ends with fire clay with a wire protruding from each end, I hope to use the irons resistance as the heater.
That tube is pretty thin! I expected something over 10mm ID.
Anyway, I hope your iron filings are really a fine powder because large filings will have a problem with the skin effect just like solid iron.

Also, two common N40 NdFeB magnets without a yoke will provide only 500mT. See:
https://youtu.be/B015P0XFl9g?t=76

BTW: Heating the iron powder in an inert atmosphere for 2 days at 400C improves the iron's NMR line sharpness 3 times.
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #4 on: 2019-01-21, 19:48:20 »

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Hi verpies

The tube is thin but are we concerned about electrons exiting the iron, I am going to use the magnets as detectors by wiring them to a very sensitive charge detection amplifier, so if we do get beta particle collision to the magnets we will see a change in charge and know for sure the device is working.


The iron filings are not that small and I'm not even sure of their purity, I am looking for a source of better quality, I know that if I dissolve iron electrolytically in copper sulphate (pure fine copper displaces) and then dissolve zinc electrolytically I will get pure fine iron particles displaced but am not yet sure how easy it would be to wash, clean and dry them without oxidation (inert hydrogen atmosphere is needed really and sounds a bit dangerous to me), we are going to heat the iron filings to a temperature above their curie point so why would we get the skin effect??

we would be annealing the iron anyway as part of the heating cooling process.
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #5 on: 2019-01-21, 20:08:42 »

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Hi Gyula

Very interesting about the series inductor, I will have a play when it arrives.

and interesting about the capacitance of the fet this will be in series with the 10nF decoupling capacitor so I am looking at about 138pf, maybe I am going to have to leave a variable cap in the second cap position for fine tuning when the fet stage is fitted.


I was not sure if i should have a 50 ohm resistance across the input of the pi filter to terminate the output of the preamp, 2 watts is way more than needed to drive the irf610 says only 0.3 watts needed, i might be able to do that using the oscillator stage and the do away with the preamp altogether.


I have had success before using standard diodes as varicaps in a previous 45mhz oscillator i built and may try these again, it's a very old crystal, possibly second world war, i am hoping it's age may have lowered the cut frequency down a bit due to ageing.

I am wondering if It maybe a good idea to buy a dummy load and SWR meter to test the 16W amp stage before going on to work out the final load coil construction.
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #6 on: 2019-01-22, 22:27:52 »
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Hi Peter,

For the series coil an RF powdered iron toroidal core would be good like Amidon T-56-6 (yellow) and make say 30 turns to have a high Q 3.6 uH coil and make at least 1 tap on it, wire OD could be 0.3mm. Here is a calculator: https://www.changpuak.ch/electronics/amidon_toroid_calculator.php  Yesterday I quoted 2 uH series coil from the 2nd schematic but sorry it is 12 uH for the 14 MHz crystals so 1/3 of that for the 45MHz one will be close and with the tap you will have one more choice without unwinding.

Regarding the input capacitance of the MOSFET, it changes from within the same type MOSFET batch so do not care about the effect of the series 10 nF, it is really negligible. If you can leave a variable cap in the 2nd cap position, then do so to help vary the tuning/matching in any case.

If the output impedance of the preamp is far from 50 Ohm, then do you have any info what impedance is involved there? any preamp specification or maybe schematic?  Not utterly important though (see below why), so no problem if you have no info.

I suggest leaving the preamp in the setup to protect the output of the DDS from any MOSFET "backfire" event, should such occur. If the 2W proves to be too high, then you can still use a passive resistor divider at the preamp input or output to reduce the RF level, the presence of such dividers can help matching between the stages to a certain degree too.

The varicap type shown in the link I gave yesterday is 1SV149 Toshiba. Works between 1V and 8V tuning voltages and has roughly 480 pF and 30 pF at those voltages, respectively. So quite a high capacitance ratio, it establishes the tuning range also for the crystal oscillator. OF course you can connect several varicaps or maybe standard diodes in parallel to increase their capacitance, the higher the capacitance change you can achieve, the wider the tuning range possibility for the crystal will be.

Yes a series inductance is needed when the frequency of a crystal is to be pulled downwards from its natural series resonance frequency. What is more, a tuneable coil would be needed but such is difficult to make to observe high Q in a solenoid form. This is why a variable capacitor (or varicap) is connected in series with a high Q coil, the idea is that by changing the capacitive reactance, the overall reactive impedance (series LC) changes and ideally the resulting reactance should remain mainly an inductive one to pull downwords the crystal.

A dummy load for 45-50 MHz could still be made from carbon resistors I believe, it maybe cheaper than an off the self one?  Regarding SWR meter, well maybe it is not really needed: the load eventually will be an inductive coil, not a pure resistor. But it is up to you, to see matching at the output Pi filter first into a dummy load may be useful. 

Gyula
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #7 on: 2019-01-23, 19:09:55 »

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No info on the preamp, seems to use a small chip as the main amp, looks like there's 2 series caps on the output with no termination resistor fitted.
thanks for your help gyula, we will see how it goes at the weekend ;)
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #8 on: 2019-01-25, 17:22:11 »

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I just had a go at building the Xtal oscillator.

I tried the circuit below, seemed easier to start with, I couldn't remember where I found it but I thought it was 50MHz but maybe not :(


I did not have 150pf caps for the base divider and used 220pf each cap instead, going to work out the impact of that tonight.

I used a BC337-16 instead of the 2n3904.
I soldered the crystal case to GND.

The sinewave is not perfect but a bit out of shape, the frequency is a third of the required frequency 15.21396MHz at 0volts and 15.2154 at 30V tuning voltage.

I used a hand made 830nH inductor.

When I put the output cap divider in place it stopped oscillating (22pf in series with 10pf) maybe the value are wrong for the required frequency? I think these need to be bigger values.


EDIT maybe the 220pf caps should be smaller for the correct frequency. I will try 47pf and 33pf caps tomorrow.
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #9 on: 2019-01-25, 19:19:24 »

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@Peterae

Is your planned RF amp capable of supporting a sequence of 45.525MHz bursts in case the more primitive CW NMR technique yields an output signal amplitude that is too low?
e.g.: if you put a quick on/off RF switch between the oscillator and the amp...

P.S.
Did you get my private message about the "other source" of an RF amp ?
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #10 on: 2019-01-25, 19:29:04 »

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Hi verpies
I am sure I can find a way to turn the oscillator on and off, maybe using a switchable buffer stage.

Yes sorry I meant to reply to your email, no need yet, let me try at 16 Watt and get the system built ;)

EDIT Looks like i have a 3rd Overtone Crystal
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #11 on: 2019-01-25, 19:45:13 »

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Ah this looks good for what I need
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #12 on: 2019-01-25, 20:09:47 »
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Quote from: Peterae on 2019-01-25, 19:45:13
Ah this looks good for what I need
If you don't want to bother :  :)
https://www.ebay.com/itm/Vertex-Standard-VX4000L-6-Meter-Low-Band-37-50Mhz-Transceiver-70-watt-Yaesu/273667720755?hash=item3fb7de0633:g:wUkAAOSw5EpcFsyH

Cheap in the USA but almost doubled due to shipping to Europe  >:(


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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #13 on: 2019-01-25, 20:15:22 »

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Very Tempting F6FLT
Although I am enjoying myself so far and trying to learn at the same time.

I think I can put a tuned LC in the emitter to force 3rd harmonic, I will try tomorrow. 200nH and 63pf
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #14 on: 2019-01-25, 23:22:35 »
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Hi Peter,

You can see some more info on overtone oscillators here
http://www.fishpool.org.uk/overtone_oscillator.htm   

The 'trick' is to tune the coil in the collector to the needed overtone frequency with the trimmer cap (7-45pF), i.e. for 45.525 MHz.  This way the transistor can amplify at that frequency only because at 45.5 MHZ the collector will have a higher impedance than what is in its emitter (220 Ohm||33 pF), so oscillation will be maintained. 
There is the base-emitter capacitor missing here what is included in your finding in the above schema but you can test which is better.
It often helps if you connect a few pF (4.7 to 10pF) capacitor between the collector and emitter in the schema I show. The cap across the emitter resistor (33pF) can also be varied while watching the output on the scope.

Of course you need to connect the inductance I referred to earlier to pull the frequency downwards in series with the xtal and also with a variable cap or varicap just as shown in the schema in your reply #8 or in the lower right corner in the schema in your reply #11. 

Gyula
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #15 on: 2019-01-26, 15:45:15 »
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Quote from: Peterae on 2019-01-25, 20:15:22
...
I think I can put a tuned LC in the emitter to force 3rd harmonic, I will try tomorrow. 200nH and 63pf

There is a 100 pF capacitor between the emitter and the ground, it is a low enough impedance for the third harmonics, I don't think this LC circuit is necessary. Moreover it could disturb the oscillator.
It is on the collector side that this tuned circuit is necessary, and it is there, it is the 250nH / 10->50pF, to be tuned on the harmonic, not on the fundamental.  The other 250nH / 10->50pF connected to the output performs filtering and impedance matching, and the capacity that couples the two LC circuits is only 1pF to improve filtering. Increasing it may allow a better transfer of power, but we will have a little more unwanted products.

One of the advantages of this Clapp oscillator is that you can do almost anything you want on the collector side without disturbing the oscillator at the emitter.



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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #16 on: 2019-01-26, 17:59:58 »

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Hi F6FLT
Only just seem your recommendations, thanks ;)
I think in that circuit, he is running a xtal at it's fundamental frequency and filtering the 3rd harmonic, I now think his circuit is no good for what I wanted hence your suggestion on modifications needed ;)
I now wonder if the circuit I built yesterday would have indeed worked properly if I had sorted the diode capacitance out as below.

I re-built the circuit as per gyula suggestion, see below circuit with an inductor and diode in the gnd side of the xtal.


Took a while to work out why I had a horrible waveform at the fundamental frequency, turned out my diode did not have enough capacitance for the circuit to work properly, ran out of time eventually.

Been scratching my head and am still scratching my head, I have 46Mhz in the scope shot, the crystal is 45.666Mhz, I seem to have gone up in frequency, I have seen it working at 45.657Mhz
« Last Edit: 2019-01-26, 18:33:48 by Peterae »
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #17 on: 2019-01-26, 19:03:23 »
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Hi Peter,

I would suggest:
1) leave out the big coil in series with the xtal and also the varicap diode, and connect the xtal pin to the gnd directly, and see where it oscillates. (write down the frequency)
2) now lift up the xtal pin from the gnd and insert the coil in series,  the coil's other leg goes to gnd, and see the oscillating frequency  (write down the frequency)
3) lift up the series coil's leg and insert a variable cap of at least 40 pF or higher and see how now the frequency changes (write down the frequency range)

If your scope probe is directly tied to the collector shown as TP1 (and the gnd clip to the common negative rail), then the roughly 10-14 pF probe capacitance is built into the circuit.   A better place is via output 5, via the series 560 Ohm + 10nF but the best would be an active buffer stage, probably your preamplifier would be good for this buffer stage?  Hopefully it has not got a too low input impedance to load down the oscillator output too much to kill oscillation.  Nevertheless, couple the oscillator output via the 560 Ohm + 10 nF to the input of the preamp and see the amplitudes at the input and the output of the preamp (and measure frequency at the output of the preamp).

Gyula
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #18 on: 2019-01-26, 21:41:04 »

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Thanks gyula

We will see how we get on tomorrow ;)
Hopefully if I can go up that much I can I can go down in frequency as well, seems very stable at 46Mhz.
I only had a 50-120pf variable cap for the collector and had to wind a small inductor to reach third order, I tuned it using my GDO and scoping across it, so is probably set a little high due to probe capacitance.I have been using a 100nF to feed the scope probe from the collector to stop the probe interfering with measurements.


I did play with the large inductor coil spacing and pulling the coil turns apart a little it dropped to 42.5MHz but seemed unstable at this frequency, so something strange going on at the moment, your suggestions above should shed some light i think.
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #19 on: 2019-01-26, 22:14:49 »
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Well, the 100 nF cap to feed the scope probe from the collector does not stop the probe interfering with the measurements, unfortunately.  The best would be to use the preamp as I suggested above.  If you do not yet wish to use it,  then use either the 560 Ohm series resistor as shown in schema or even a higher, 1-2 KOhm series resistor and the 100 nF in series if you wish, The cap does only DC blocking, otherwise a short circuit at 45 MHz. 

When you had the 42.5 MHz oscillations at the output, it was already an LC oscillator and the xtal did not work.

One more thing if I may: What may need to be changed (not to fight so much with the oscillator) is the big solenoid coil you wound for the xtal to pull it down.  If you happen to have any Amidon or other powdered iron toroidal core for that coil (I mentioned the yellow one above with tap), that would be good because this big coil can couple easily to the collector coil and affect operation in an unwanted way.
It is okay that the axles of the two coils are perpendicular to each other but the big air core coil may have too big stray field around it, reaching the collector coil. This is why a confinement into a toroidal core would be useful.
IF you do not have such, try to order some, together with some trimmer capacitors.  But I understand if you do not wish to invest too much into this project what is absolutly neccessary.

Gyula
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #20 on: 2019-01-27, 10:50:25 »
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I agree with Gyula. The setup must be adjusted and tested without varicap and large coil. Once everything is correct, the LCvaricap can be added (with a smaller size and a more confined field, for the reasons given by Gyula).
However, I don't really see the point of not having a quartz directly at the desired frequency. Quartzes operating at 45 MHz are commercially available, they operate directly at this frequency even if they are cut for 1/3, 1/5 or 1/7 of the frequency (I even have some at more than 100 MHz). This results in a stronger signal directly at the right frequency.


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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #21 on: 2019-01-27, 14:39:35 »
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Well, the frequency for the xtal needed is 45.525 MHz, the 45.000 MHz may not be good for the Meyer-Mace effect (at least as per the math formula). Such off the shelf xtals for 45.525 MHz are not readily available components, only in the relatively close vicinity like 45.666 MHz Peter found and tries to pull downwards.

I searched for off the shelf 45.525 MHz xtal (or close to this) but have not found.

Here is cheap 15.2 MHz xtal (roughly 1/3 of the target)
https://www.mouser.co.uk/ProductDetail/IQD/LFXTAL017300Bulk?qs=sGAEpiMZZMsBj6bBr9Q9aR%2fuGiDjvlISt9vCKfK%2fVG%252bPoJarV0hR0Q==   Using 2 or 3 pieces of such xtals in parallel the 45.525 MHz target can be achived with series LC tuning. This is for the case if the present 45.666 MHz cannot be pulled down enough. 

Regarding RF toroid cores, here is a source but there surely are more of course:
http://powermagnetics.co.uk/pace-components/micrometals-iron-powder-Miscellaneous/rf-applications/t-44-17-micrometals-iron-powder-toroid

(I mentioned the T-56-6 already, it is also useful though recommended up to 40 MHz but still much, much better than an air core coil, the 40 MHz is not a rigorouos limit of course). 

Gyula
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #22 on: 2019-01-27, 16:18:07 »

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First I disconnected the long inductor and grounded the xtal end, see attached waveform seems to consist of different amplitude waveforms of the same frequency that are DC offset.

It would not oscillate with just the long inductor connected to ground and could not seem to tempt it to oscillate.

Anyway before going further I will change my tuned collector to a toroid and a lower value trimmer and order some iron cores for the inductors.

I've just topped up with polyester, ceramic dipped caps and a trimmer kit.

https://www.ebay.co.uk/itm/700pcs-24-Values-Mylar-Polyester-Film-Capacitor-Assortment-Pack-Kit-DC-100V-F1E3/113149175254?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

https://www.ebay.co.uk/itm/1X-1200-Pcs-24-Values-Ceramic-Capacitor-DIP-Monolithic-Multilayer-Ceramic-C-O1R8/283265583025?epid=20023293182&hash=item41f3f1c7b1:g:l2gAAOSw3LVb8Onk:rk:1:pf:0

https://www.ebay.co.uk/itm/80x-6mm-Plastic-Cover-Trimmer-Variable-Capacitance-Capacitor-Kit-for-Arduino/401615234996?hash=item5d822227b4:g:wZ8AAOSwB19b8Eou:rk:4:pf:0

I need to get a 50Ohm load for my amp 25Watt.

Question as we are going to try and drive the output coil at resonance I am going to need a tuning cap to be paralleled up to the final coil, I imagine this is going to need to be a high voltage vane type, what sort of voltage would be a good choice to order? I know this is going to be low pf as we want to get as many turns on the coil as possible.



 
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #23 on: 2019-01-27, 17:04:13 »
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Well, at least 500V rated variable would be needed as a minimum value, have not studied the choices.  Unfortunately, the final coil's iron core will have a certain and yet unknown permeability at 45.5 MHz, so the maximum number of turns somehow will be limited just because it may let you using only a few pF (say less than 10 pF) tuning capacitor. Putting this otherwise: you are limited to use only as many turns as would let use a max 10 pF variable because the coil self capacitance and the stray capacitances will possibly be in the 5-7 pF range (a rough estimate). 

Just curious: the scope probe was at the open end of the 560 Ohm+10nF when you took the scope shot, right?  8) 
It is good you will have toroid cores, make sure to provide a tap or two for easing tuning.  Until a clean 45.6 MHz output sinewave is achieved, by trimming the emitter capacitor too etc. do not use any series coil with the xtal. 

Gyula
   
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Re: Towards a 45.525 MHz Crystal Oscillator & 16 Watt Amp « Reply #24 on: 2019-01-27, 17:34:31 »

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I may have used a 100n in series with a 560 Ohm, but yes this was in place and scoping at the end.

You say don't use a series inductor with the xtal, you mean only while we get a good 45.6MHz but it will be needed to lower the operating frequency?
   
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