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Author Topic: 5 watt crystal oscillator transmitter  (Read 14202 times)
makeRF
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« on: July 28, 2014, 0401 UTC »

I was playing around with amplifying a basic crystal oscillator transmitter.  I was able to get it up to 5 watts with an IRF510.  As I mention in my write up, this isn't a design you want to put on the air.  IMHO it's far too dangerous to be amplifying square waves to 5 watts -- the RF pollution will be all over the place.  But, it's a fun little circuit to experiment with into a dummy load, and burn your fingers (the IRF510 gets HOT!).

More Fun With Crystal Oscillators: Amplifying RF:
http://makerf.com/posts/more-fun-with-crystal-oscillators-amplifying-rf
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OZNRH
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« Reply #1 on: July 30, 2014, 1023 UTC »

Hi makeRF  Smiley.

If I was you, so put a 100K resistor direct from Gate to ground. I do not like the gate hangs and 'float'  Undecided..

Best 73 from Ole  Smiley
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makeRF
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« Reply #2 on: July 30, 2014, 1535 UTC »

If I was you, so put a 100K resistor direct from Gate to ground. I do not like the gate hangs and 'float'  Undecided..
That's a good point.  I did originally have a 100K resistor in there for biasing, but was having problems with it distorting the waveform.  But, I also changed things from the original circuit I threw on the breadboard, so I'll try adding it in again and see what happens.
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ka1iic
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« Reply #3 on: July 30, 2014, 1601 UTC »

I used a VX-8141 1MHZ oscillator for a small part 15 transmitter I built and the signal is clean... no square wave that I noticed and the only thing I had to work out was the normal frequency harmonics ie: 2, 3 ,4 MHZ.  It did toss out a lot of harmonics that way but that was a simple problem to work out with filtering.  But the RF final I used was a common BJT 2N2222...
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OZNRH
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« Reply #4 on: July 30, 2014, 1709 UTC »

@makeRF:

You talk about distortion, everything from this construction is distortion, with or without 100K.. You can only talk about the symmetry on the square wave and I'm sure it's not a duty cycle on 50% you see with a scope on the FET-Drain.
I also wondered about the 1000 uF. in the antenna, 0.1uF. should be able to do it with 1 to 2 MHz ..
I would strongly recommend a LP filter in the end, a simple C-L-C filter will do wonders. Just my opinion.

Best 73 Ole Smiley..
« Last Edit: July 30, 2014, 1737 UTC by OZNRH » Logged

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makeRF
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« Reply #5 on: July 31, 2014, 0108 UTC »

OZNRH, thanks for the input!  I added two 100K ohm resistors as a voltage divider on the IRF510 gate for biasing.  This seems to have done the trick.  I also tried the 0.1 uF cap and this too worked just as well as the 1000uF capacitor.  I now measure 4 watts RF going into a dummy load.

I'm more pleased with this circuit, because it's less of a hack.  I was scratching my head when originally playing with this wondering why I couldn't bias it and make it work.  I must have had something plugged in wrong at the time.
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ff
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« Reply #6 on: July 31, 2014, 1354 UTC »

Hi MakeRF.  Let me join the dogpile here.  One thing I would add is a 0.1 uF cap to ground on the V+ side of your 3.3 uH choke.  That will work with the choke to help keep RF out of your voltage feed.  Also, using two 100K as a divider sets your gate bias point at 6V.  In this situation there's no reason to make the IRF amplify linearly.  The higher the bias point, the hotter the IRF will run because it will be conducting for longer portions of the cycle.  A single gate resistor to ground will set the gate impedance.  I normally use one between 1K-10K, but that's just my fetish Smiley.  And Ole makes great points - use a 0.1 uF coupling cap in the drain circuit followed by a C-L-C LPF.  This will improve your output waveform by reducing the harmonic energy. I love your site!
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makeRF
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« Reply #7 on: July 31, 2014, 1422 UTC »

Hey ff, thanks for the comments, and I appreciate the kind words about my site!  I'm planning on getting an improved version of the article out with these suggestions (with credits to this forum!).

One thing I would add is a 0.1 uF cap to ground on the V+ side of your 3.3 uH choke.  That will work with the choke to help keep RF out of your voltage feed.
I added a 100uF capacitor at the RFC as a voltage stabilizer.  Would the 0.1uF be enough capacitance?  I guess it depends on the stability of the power supply.  I've usually used a higher value electrolytic as voltage stabilizers.  One structure I've seen in similar circuits, is a 100uF capacitor and a 0.1uF in series at the RFC.  I assume there is a reason for this, but it just looks like a 99.9uF capacitor to me.

Also, using two 100K as a divider sets your gate bias point at 6V.  In this situation there's no reason to make the IRF amplify linearly.  The higher the bias point, the hotter the IRF will run because it will be conducting for longer portions of the cycle.  A single gate resistor to ground will set the gate impedance.  I normally use one between 1K-10K, but that's just my fetish Smiley.
For some reason my IRF510 stops working if I just add a single resistor bias on the gate.  If I lower the voltage with the divider it starts working.  I'm really not sure why this is, as I believe it's suppose to operate with a 12v input.  I might edit the divider though to bring the voltage up to +9 or +10v and see if it still operates.  Maybe I have some knock off FET's, although I bought them from a reputable source.

And Ole makes great points - use a 0.1 uF coupling cap in the drain circuit followed by a C-L-C LPF.  This will improve your output waveform by reducing the harmonic energy.
Yes, for sure if I was to use this in any serious way.  I've plugged it into the altoids low pass filter I've previously created just to see how it cleans things up, although that filter is for 40m, so it's not really ideal.  It is improving the waveform on the harmonics in that range though.

At the moment I'm just using the canned crystal oscillator as a simple RF source to amplify.  I don't plan to keep it in a final design, but I do find it interesting to be able to max the power output of these things.  I usually build things out in stages, and then combine into something more useful -- It's all a learning experience for me.  One project I have in queue, is to better understand some standard crystal oscillator designs, and once I'm happy with a configuration, combine the two into a QRP transmitter.  Baby steps  Smiley
« Last Edit: July 31, 2014, 1427 UTC by makeRF » Logged
ff
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« Reply #8 on: July 31, 2014, 1536 UTC »

For some reason my IRF510 stops working if I just add a single resistor bias on the gate.  If I lower the voltage with the divider it starts working.  I'm really not sure why this is, as I believe it's suppose to operate with a 12v input.  I might edit the divider though to bring the voltage up to +9 or +10v and see if it still operates.

Yes, my bad.  Your oscillator probably doesn't have enough signal voltage output to drive the IRF into conduction.  Lacking a driver stage, the divider is probably the easiest way to go.  The 510 should be driven into saturation with about 8V on the gate (they do vary a bit) so you should keep the bias point south of there.  Personally I'd try about 2V initially (like Dave Martin's Corsair) and increase it gradually to determine what tradeoff of output vs waste heat you are willing to have.  And yes, your 100uF is probably enough filtering.  However good construction practice calls for another cap on the high side, and physically close to the choke, with a timing constant more suitable for RF, like 0.1uF.  That one is your call, but redundancy is good policy when playing with RF circuits.  And I always take baby steps when playing with my circuits too... 73!
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OZNRH
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« Reply #9 on: July 31, 2014, 1933 UTC »

Hi again here Smiley.

Here is a diagram more after my head Cheesy.. As 'ff' write, the signal from the oscillator is too 'small' to driving the FET. Right now it work nearly as a kind of class C with a 'to small' input, so yes a 'biasing' will probably help. Take a look on my diagram as inspiration.

Ole Smiley..


* Crystal2.jpg (57.82 KB, 900x310 - viewed 588 times.)
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makeRF
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« Reply #10 on: August 04, 2014, 1635 UTC »

@OZNRH Thanks for taking the time to sketch up the schematic improvements!  I would have replied sooner, but I was away for a few days.  I'll be trying all the discussed improvements soon.
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Antennae
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« Reply #11 on: September 23, 2014, 0231 UTC »

Whoa, 5W, now you're talking! Its a lot more than your other transmitter.  The NS-40 is a CW transmitter that uses the IRF510 transistor. The instructions say their circuit is 95% efficient and the transistor doesn't get hot. 

Here's the web page with links at the bottom for the diagram:
http://www.4sqrp.com/NS40.php
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makeRF
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« Reply #12 on: October 31, 2014, 0110 UTC »

I finally got around to improving this circuit, thanks to the suggestions posted on this thread:
http://makerf.com/posts/a-better-amplified-crystal-oscillator-transmitter

As I mention in my writeup, depending on the crystal oscillator used, I get anywhere from 1 watt to 20 watts out of this!  Some of those metal can oscillators can generate some serious power that the IRF510 amplifies greatly.  I'll be working on another oscillator stage for this circuit soon and dump the crystal oscillator for obvious reasons.
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