Towards a 45.525MHz 16 Watt Amp

[gyula]

Yes I do.

Re on the probe at the 560 Ohm + 10 or 100 nF end:  for such probe loading the output, the output is virtually unloaded, so the LC circuit tuned to around 45.5 MHz should have a nice maximum voltage value as the trimmer cap is tuned.   
Try to drive the input of your preamp via the 560 Ohm + 10 or 100 nF cap and see the amplitude at the input of the preamp. this would give an indication of the input impedance of the preamp.  Try to tune the trimmer in the collector to maximum if needed.

Gyula

[Peterae]

verpies
The on off bursts, do you have a time period for the on and for the off, just trying to plan ahead

PS just ordered 500 grams of high purity 100 mesh iron powder.

Things to work out and do.


Get oscillator working at 45.525Mhz and be variable +/- 100Khz.

Build switchable buffer stage.

Complete Amp stage.

Find best coil diameter/ length for resonance at required frequencies.

Method of heating fine iron powder in quartz tube, preferably by passing a DC current through the iron powder.

Build very sensitive charge detector and connect to magnets to detect Beta/ electron impact.

Monitor current through iron tube as this maybe indicative that we have a working device.

static bias magnetic field, is now clear that a steel U shape is needed with magnets to concentrate field for the required strength of 600mT , experimentation required, I will probably use 100mm long N50 magnets.


Method of operation

Heat Iron above Curie Temperature and stabilized

Turn off Heating

Set test frequency

Turn on Frequency Bursts for a set time period.

Test for charge particles, according to Coleman/Gilespie device will run for a while.

Change to next test frequency.

Go back and start heating cycle.

[gyula]

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

Hi Peter,

You could consider such RF resistors   https://www.ebay.co.uk/itm/153312707527 

You could mount it on a heat sink and fasten a BNC or SMA female socket to the heat sink. 
Or use just a 30-40 cm long piece of coax cable and bolt its braid to the flange of the RF resistor (which is mounted by two bolts onto a heatsink) and solder the center conductor of the coax to the small tip of the resistor (there is 50 Ohm between this tip and the flange).

Otherwise, here is construction of dummy loads using such RF resistors:
https://pa0fri.home.xs4all.nl/Diversen/Dummyl/dummyloadeng.htm   

An off the shelf 25W dummay load:  https://www.ebay.com/itm/263706066179   

Gyula

[Peterae]

Thanks Gyula just ordered the resistor, I have a sma lead and cpu heatsink

Some of this stuff is going to take a while to arrive

[gyula]

Okay, and you would need some thermal grease to insure good heat conduction between the flange and the heat sink.
https://www.ebay.co.uk/itm/382244421173 

I found kinda professional mounting method, just for information to see, and read also the PDF file attached.

Gyula

[Verpies]

The on off bursts, do you have a time period for the on and for the off, just trying to plan ahead

Microseconds for Pulsed NMR.  I cannot give you a concrete interval because it depends on the strength of the RF field.  The stronger - the shorter.
For CW NMR there are no bursts.

static bias magnetic field, is now clear that a steel U shape is needed with magnets to concentrate field for the required strength, experimentation required, I will probably use 100mm long N50 magnets.

It helps but it is not absolutely necessary for iron and other ferromagnetic materials.  Strong uniform DC bias field is necessary for paramagnetic materials, though.

Perpendicular Tx and Rx coils help a lot with isolation and sensitivity.
Impedance matching them is a bitch.

Using an RF switch to disconnect the Tx coil after a burst so the Tx amplifier's impedance does not load down the nuclear oscillations goes along way to improve the amplitudes at the Rx coil.

[Peterae]

Microseconds for Pulsed NMR.  I cannot give you a concrete interval because it depends on the strength of the RF field.  The stronger - the shorter.
For CW NMR there are no bursts.

So is it ok to have an extra long break between pulses so that we make sure we are long enough say we were to guess worst case RF field strength and set time period to turn off the generator.

It helps but it is not absolutely necessary for iron and other ferromagnetic materials.  Strong uniform DC bias field is necessary for paramagnetic materials, though.

What was the field strength you indicated that i am trying to obtain from the magnets, i know you posted a figure somewhere but cannot find it.

Perpendicular Tx and Rx coils help a lot with isolation and sensitivity.
Impedance matching them is a bitch.

Do i need a Rx coil?  according to Meyer and Coleman my output power should appear longitudinally through the iron in half wave bursts.

Using an RF switch to disconnect the Tx coil after a burst so the Tx amplifier's impedance does not load down the nuclear oscillations goes along way to improve the amplitudes at the Rx coil.

The Tx coil is tuned, if we remember from McFreey maybe the tuned coil LC may pick up energy from the beta emissions and back feed or even self destruct if the feedback energy is too great,disconnecting the LC from the amplifier may make thismore likely.?

[Verpies]

So is it OK to have an extra long break between pulses so that we make sure we are long enough say we were to guess worst case RF field strength and set time period to turn off the generator.

Long breaks between RF bursts are OK.

What was the field strength you indicated that i am trying to obtain from the magnets, i know you posted a figure somewhere but cannot find it.

For iron 750mT max.  For other materials (paramagnetic) at least 1.2T which is easily achievable with two N52 Neodymium magnets.

Do i need a Rx coil?  according to Meyer and Coleman my output power should appear longitudinally through the iron in half wave bursts.

No, but it really helps if you want to make baby steps and get a good feel for the NMR alone.

The Tx coil is tuned, if we remember from McFreey maybe the tuned coil LC may pick up energy from the beta emissions and back feed or even self destruct if the feedback energy is too great,disconnecting the LC from the amplifier may make thismore likely.?

IMO - less likely.

[Peterae]

OK thanks for the figures

you say 750mT max whats the middle ground figure, I need something to aim for.

Been trying to work out how to seal the ends of my quartz tube and just found the answer.
1200 Deg C silicon glue

HIGH TEMPERATURE 1200'C ADHESIVE GLUE FOR EXHAUST FIREPLACE OVENS COLLECTORS 70 ml new

just need to work out how to apply it after forming a vacuum on the quartz tube, maybe I don't need a vacuum if I heat the iron hot enough and due to expansion I may get a vacuum or at least drive of any available oxygen when hot and hopefully the glue will go off fairly fast at an elevated temperature.

[Verpies]

you say 750mT max whats the middle ground figure, I need something to aim for.

Shoot for 600mT

Been trying to work out how to seal the ends of my quartz tube and just found the answer.
1200 Deg C silicon glue - HIGH TEMPERATURE 1200'C ADHESIVE GLUE FOR EXHAUST FIREPLACE OVENS COLLECTORS 70 ml new

Does it make a hermetic seal so air does not get in over time ?

just need to work out how to apply it after forming a vacuum on the quartz tube, maybe I don't need a vacuum

I find it much easier to push out the air with welding Argon than pulling a vacuum.

[Peterae]

Does it make a hermetic seal so air does not get in over time ?

I wont know for sure until it arrives but

high adhesion

This a fireproof silicate adhesive designed for assembly and repairs of exhaust systems (collectors and exhaust silencers with holes in them), ovens, fireplaces, and damaged ceramic and metallic ducts – carrying away very hot combustion gas or fluid at a temperature over 1000°C. Resistant to fumes, dilute acids, water, chemicals and low temperature. It has very high adhesion and fast bonding time.

[Verpies]

Here is a nice video about magnets in a yoke.
https://youtu.be/U8mnrO-gM8s

For good NMR experiments at least 20mm gap between 100mm O.D. disk magnets is convenient, so the magnetic yoke needs to be much bigger than the one shown in the video.
Also, the gap cannot be surrounded by aluminum spacers...or anything else that is conductive, because of RF eddy currents.  Plastic in or near the gap is tolerable, but too much of it in the gap (or near it) limits the dexterous maneuverability and versatility of the magnet assembly.





The diagram above shows how a stator from a big old industrial motor can be converted to a magnetic yoke.  The middle version (circular) and right version (semicircular) will work.
My local junk yard has many big old industrial motors with cracked casings, bent rotors ...but intact stators.  They sell them by weight according to their scrap value.

[Peterae]

3D print a hollow former and fill with iron resin mix, can be any size and not limited to the stator that needs to be found

[Peterae]

I got some decent 10-40pf variable caps.
Still waiting on the cores but found some in my box that gave appropriate uH when a few turns were wound onto them.


I changed the fixed capacitor in the emitter to a 10-40pf variable and this cured the horrible waveform I had last time, seems this value is quiet critical for correct operation, there is a picture of the new setup with 2 variable caps, 1 in emitter and 1 in collector in parallel with the new 200uH inductor.


Scope shot 2 is with the xtal grounded one side
Scope shot 3 is with a 1uH inductor wound on a toroidal core in series with the xtal to ground
Scope shot 4 is with a 47pf cap in the ground leg of the xtals inductor leg.

[F6FLT]

Clean signal!
Now on the way to amplification?

[gyula]

Hi Peter,

Nice results.

You wrote 200 uH inductor, I guess you meant 200 nH? if that referred to the toroidal core shown with 2 turns?
For a start the core is good with its 2 turns but 200 nH would need at least 61 pF tuning capacitor for 45.5 MHz, so
maybe the 200 nH measurement is to be checked?   The goal is to have 15-20 turns on a good core, not 2 or 3.
I suggest using at least a 600 nH coil and the 40 pF trimmer set to half way (20-21 pF) will give resonance at 45.5 MHz.

When this is done, a second coil of say 1 to 10 uH seems to be needed in series with the xtal in the ground leg. Maybe this xtal is not so active for pulling it down, I do not know, the 1 uH in series with it caused roughly 3 kHz less frequency only.

Gyula

[Peterae]

Thanks Gyula
Yes sorry I made a mistake on the inductor value

I don't seem to be able to get many more turns on that unknown core, if the inductance goes up it drops to fundamental frequency, I will now wait for the new cores before proceeding, maybe the xtal cannot be pulled down enough, we will then revert to the DDS board which is ok but I wish I had better control of it as the tft processor that came with it is a bit mickey mouse.

Bit worried about the iron powder, when I picked it up from the post office I faced questions of what it was, I hope there's no illegal uses GULP, and to make it worse I only ordered 500 grams which was plenty but now I am going to build a large core I estimate I will need another 1kg bottle of it, see pictures below of the former for the core.

I ordered 2 N52 60x20x10mm magnets to fit the core.

[Verpies]

Bit worried about the iron powder, when I picked it up from the post office I faced questions of what it was, I hope there's no illegal uses GULP,

Of course there is, but does that make you a criminal ?
There are criminal uses for dihydrogen monoxide, too...

BTW: If you mix iron powder with epoxy, its grains might become too far apart for some reactions and the epoxy itself will resonate, too....albeit at different frequency.

[Peterae]

I've just been looking at the output of my dds and 1.5 Watt preamp, I tried scoping with no load and with a 50.11 Ohm load, although I am to believe this resistor has no inductance I can measure about 50nH on my meter.

Now I am not sure how valid this is with the present waveforms but I understand that I could find the output impedance of my preamp by subtracting the voltages when loaded and unloaded multiplying this by the load resistor and dividing by the loaded voltage.

The ffrequency was set to 45.250Mhz

So we have Zo = 50.11 * (23.6 - 9.99) / 9.99 = 68.26 Ohms

See scope shot 1 & 2

then I tried it with the 560 Ohm in series with a 0.1uF cap and connected the scope probe to the cap to try and isolate the probe.


see scope shot 3 & 4


So this time we have Zo = 50.11 * ( 6.48 - 3.67) / 3.67 = 38.36

Edit something strange going on, just going to recheck the scope readings again and will update.
OK the values seem to check out, amazed how I am getting such large pk-pk unloaded.

[Peterae]

So I am just trying to get a feel for things for now.

I wound a coil which seemed to have a self resonant frequency of about 70Mhz in the hope that I can add a little capacitance to LC it at 45.25Mhz.
I bought some caps.

First test was to hook up my PI filter to the 50.11 Ohm load, see scope shot 1.

Second test was to hook my PI filter up and put the LC across the output, tune the pi and LC for maximum Amplitude

The piston cap made virtually no difference to the LC amplitude from fully wound in and fully wound out, value is 0.8pf to 10pf, see scope shot 2

Next i disconnected the Cap altogether leaving just the inductor see scope shope 3

Next I hooked a larger variable cap up across the inductor this time 10pf to 45pf this time I could definitely adjust the LC for a peak, see scope shot 5

I dont know how acurate this LCR meter is, certainly moves around a bit trying to measure nH inductance and just measured the tuned cap value in 5 and it measures 53.45pf (cap is 10-45pf but old stock) inductor measured 0.37uH

when I tap those LC values into a LC calculator I get 35.8Mhz plus I had my scope probe across the LC so something is out somewhere.

[Peterae]

I think from the above tests I can add more turns to the inductor until I only need about 5-10pf piston cap for resonance.

[gyula]

Hi Peter,

First let me ask what kind of LC (or LCR) meter you have?  I refer to the end of your first sentence where you wrote you measured 50 nH on your meter for your load resistor (in your reply #43).

At frequencies as high as 45.25 MHz a 10 pF capacitor (your probe probably has as a minimum?) has 351 Ohm reactance, this shunts your load and may greatly influence the result.  What is also not good indication: the waveform changes as you change the measuring methods.

As a simple solution, try to use higher value resistors in place of the 560 Ohm to isolate better the probe. Say use 3 or 4  560 or even 1 or 2 kOhm metal film resistors in series, exact values are not important because the amplitudes the scope shows in such measurements are only comparative, need not be exact. Do not use a single resistor say 10 kOhm, just build it up from some series ones, to reduce their self capacitance.  The individual values in the series resistor chain are not critical. 

Will return to your other posts above tomorrow.

Gyula

[Peterae]

The LCR meter is a TH2821a looks like the range is 0.1pf-999.9uF and 0.1uH-999.9H

So it looks like anything under 100nH is out of range, although capacitance should be ok, I must admit it's been ages since I looked the manual over but there is a series and parallel measurement mode and I have not been setting that and I just looked and it's been in series mode, maybe that should be parallel mode? I've put the link up for the manual, I also use it while plugged into the DC adapter as the battery has never been up to much, and the manual seems to indicate that sometimes this can interfere with accuracy, this was not a cheap meter but must be 5+ years old now, I will give the different modes a try next weekend.

https://d17bck4wpaw2mg.cloudfront.net/att/a/2/6/c/a26cq3/th2821a-manual.pdf

Maybe I should loosely couple the scope probe to the coil and then re do the unloaded voltage and loaded voltage and see it this works without actually the scope probe even being connected.

I just plugged  my inductor dimensions into a calculator and it calculates out at 364nH which is quiet amazing as I measured 370nH.


It looks like my capacitance measurement is way off and I bet this is because I did it in series mode.

Hang on though I just realized the last variable cap of the pi filter is in parallel with the LC and appears to be set to maximum capacitance which if I remember was about 80pf although it could be higher, if that's correct I am running with 80pf+53.45 or 133.45pf now if I plug that into a LC calculator I get a resonant frequency of 24.2Mhz so how can I be resonating at 45.25Mhz.
Sorry so many things I find very confusing, give me logic any day

Maybe we need to keep things simple we could spend all year working these things through, I need a simple efficient way to tune the LC with my hot iron tube in place and get maximum energy transfer to the inductor.

I think I really need to build another PI network, use toroids for the inductance, use 1 pi network after my pre-amp feeding my 16 Watt amplifier and then the main PI filter feeding the LC coil, in fact because the last capacitor in the PI filter is across the final inductor, I would imagine I can do away with any parallel smaller value cap across the final inductor.

[gyula]

Hi Peter,

I agree to keep things simple. Working around 45 MHz is not easy as you find.

So to step forward, terminate the output of your Pi filter with a 22 Ohm, at least 1/2 W rated resistor.  A normal metal film type is ok because its parasitics will be included in the Pi filter. The idea is to transform this 22 Ohm up to say around 60 Ohm as seen from the input of the Pi filter where the preamp output is connected, ok? So the preamp will drive the 22 Ohm via the Pi filter.  There are two reasons for doing this, one is the needed tuning capacitor value across the filter output will be roughly in the 350-360 pF range (this helps utilize the gate-source capacitance of the MOSFET you indicated to use), and the other thing is the 22 Ohm is already a low enough value to keep input (gate-source) impedance of the MOSFET amplifier also low, to avoid self oscillations a linear or nearly linear amplifier is prone to. I know the transformed 60 Ohm is not 50 Ohm but the output of the preamp will equally be happy with it for sure.
See this link and use 60 Ohm for RL and 22 Ohm for RA, loaded Q=4 and frequency=45.5 MHz, you will get around 233 pF for C1 and 363 pF for C2. The coil would be about 78 nH, rather low, either use the formula you already used for the mechanical sizes or wait for the small RF toroidal cores.
http://people.physics.anu.edu.au/~dxt103/calculators/pi_tank.php

With this I also mean do not bother with testing what the output impedance of the preamp may be, it will certainly be good for it to "see" roughly the 60 Ohm transformed impedance. (Of course if you wish, you can still test it as I wrote above earlier.)  And the input of the preamp will see the output impedance of the DDS what I assume has around 50 Ohm impedance, right? Do you know any spec on the DDS? 

You have a nice LCR meter, thanks for the link to the manual. I think it measures one way only and the processor calculates from the measurement either the a series values or the equivalent parallel values, see page 21-22. (what it measures may also depend on the test frequency and on the lowest and the highest limits of the measurement ranges)  For measuring 100 nH for instance I think the test frequency should be at 10 kHz. There is a calibration procedure to be performed as is written, page 20-21.

I understand your quest on driving a coil from the Pi filter and tune to about 45.5 MHz, this surely gives a feeling on such things.  (The high peak to peak amplitudes come from the LC resonance as you know.) 
When you have the iron powder in place with the coil around it and all this is embedded into the iron 'jaws', then you will see what inductance is going to come about and tuning method can be chosen only then, I think. 

Gyula

[Peterae]

Hi gyula
I must thank you for your patience and very important help

I did power up the amp stage last weekend and it appears to work but didn't do much as the next step is to built another pi network to feed it, which I will do as per your above post, to make such large pf values up I will parallel a fixed value cap to each variable cap.

Now when i glanced over the dds board it does not appear to be 50 Ohm terminated, there appears to a multiple stage maybe pi network on the dds output and there is a unsoldered surface mount resistor with the resistor missing on the output of this network,so it would appear a resistor could be soldered across if needed, i  do have 50 ohm smd resistor, the preamp seems to have a agc because when I adjust the output level on the DDS the output of the preamp is constant unless the level is virtually off.

The other bit of news is that there is a SMA input on the DDS board as well and i have a feeling this can be driven with an external signal to turn the output on and off,i will look into this at some point.

The cores have arrived, i could not find the sizes you stated on ebay, I ordered Amidon T37-2, i am now wondering if they would be too small, maybe not for the xtal and the preamp pi filter but maybe i should get bigger for the output pi filter stage that will feed the LC.