Amplifying 27Mhz XTAL Oscillator

[stcoso]

Hi, I'm trying bulding a small (4-5W) RF gen from a XTAL oscillator.

I've used the following schematic to get the XTAL oscillating properly (just added a 0.7uH inductor accross C3 to get the xtal oscillating at the 3rd harmonics)  :


Obviously the output is very small (100mVpp). So i used another 2n3904 (in common emitter config, with a cap bypassing the emitter resistance). I get a nice sine wave, 1 Vrms (with a 50Ohm resistor as a load) but no real power at all.

I need something around 4W to drive a linear CB amplifier that i already have.

Searching through the internet i found a lot of schematics for small amplifier like the one i need but they use  some quiet old RF transistor wich are not so easy to find in local shops.

So..  i'm looking for some ideas that possibly involves parts easy to find.


Thank you

[vk3yedotcom]

You will likely need several transistors to get it up to 5w.  Say a couple 2N2222A, then a 2N3866 or 2N4427 (to get you up to about 0.5 - 1w) then an MRF475 or similar for 5w. 

Unfortunately RF transistors are rare and are unlikely to be available from local shops. Maybe you have a junked 27 MHz CB you could pillage?

[stcoso]

I just found local shop with IRF510 available.. so i'm looking for a schematic that could use these FETs instead of transistor.

[T3sl4co1l]

IRF510 is quite poor at this frequency.

You're better off using ten 2N3904's in parallel!

Tim

[snarkysparky]

First to get 5 watts into a 50 ohm load you will need a sinewave of magnitude.

5 =  V^2/50    or   V  =  15.8 V

So your final stage will need to be run from 25-30 Volts.

And the current magnitude will be

I^2*R = 5 W      or  I = sqrt(0.1)   =  0.316 A

I agree with Tim use several 2n3904 in parallel with individual emitter resistors.

[Z80]

So your final stage will need to be run from 25-30 Volts.

It doesn't work like that, you can get any power from any voltage as long as you can supply the current and get your output stage impedance low enough.  Once you have generated your power you then just transform it to whatever impedance your next stage is via a transformer or low pass filter.  Output stage resistance needed in terms of voltage and power out is approx V^2 / 2*output power.
As vk3yedotcom has stated, you will likely need 3 stages, a pre amp, 1W of drive and 5W final.  The IRF510 is a poor choice at 27MHz, It could possibly be tortured into working, maybe with a resonant output or perhaps 2 in push pull but it's performance really plummets above 15MHz.  Why not buy an old FM CB of Fleabay and use that as a plug and play signal source, or at least hack your oscillator into it's PA stage?

[CJay]

Buying an old CB would be the simplest way to do this, doesn't need to be pretty or have lots of features.

But, if I had to do this myself, I'd have used the test circuit like the one in the Mitsubishi 2SC1969 datasheet or even pulled the schematic from the Rotel 220 service manual or a comparable CB.

Cheap transistors like the BD139 make excellent PA transistors at 27MHz, capable of 2-3 watts.

If I wanted to go down the cheap FET route, FQP13N10 seems to be the current MOSFET of choice to abuse at 27MHz and they're cheap, really cheap, well under a Euro each, pressing for a 'proper' RF MOSFET, the RD06HHF1 is not expensive and is easy to find as well.

Plenty of heatsinking using an old CPU cooler would be a good idea too.

As far as 27MHz goes, the linear amplifier will almost definitely still produce power at 13.56MHz, most CB amplifiers have little/no input or output filtering.

Chances are they will also use cheap MOSFETs like the FQP13N10 as well.

 

[Martin.M]

mostly used are japanese RF Power Transistors. The bay is full of them, 2SC1307 and like that will do the job perfect

greetings
Martin

[CJay]

Finding a 'genuine' 2SC1307 or any of the Mitsubishi bipolar RF transistors is very difficult, the vast majority for sale on eBay are fakes and not even good ones.

Better options are 2SC2078 as they're still in use in new designs but even there you need to be careful as there are many fakes for sale.

[DimitriP]

IRF510 is quite poor at this frequency.

You're better off using ten 2N3904's in parallel!

Tim

Good thing this guy didn't know this: http://frenning.dk/OZ1PIF_HOMEPAGE/50MHz_IRF510.htm

[CJay]

Good thing this guy didn't know this: http://frenning.dk/OZ1PIF_HOMEPAGE/50MHz_IRF510.htm

Shhh, don't tell him or it'll stop working.

Remarkably similar PCB to one I designed for use on a lower HF band and very similar power aims though I'd question his /P statement at 12.5Kg 

[T3sl4co1l]

It helps that it's narrowband.

Good thing "this guy" didn't know either,



But as you can see, it takes quite some drive:



(It's also rather inefficient, but class A and AB is desirable for a low distortion wideband amplifier.)

This is why I suggest using something else.  They're not as easy to use: low gain, lots of capacitance, lots of lead inductance.

Not that a gazillion 2N3904s in parallel is any better, but you should hope for something intermediate that's, say, the size of a TIP29 for instance, but still has the speed of a bunch of 2N3904 in parallel.

The suggestion of BD139 is quite good, if you can find one that's actually specified for fT and such.  Sadly, the base spec is more basic than a 2N2222! *scoff*

CRT video output transistors may be of use.  I used a pair in the above amplifier.  Audio amp VAS (voltage amplifier stage) transistors would be good too, if they're fast enough (again, common choices like MJE340 don't happen to be rated for AC performance, what a ripoff).

(Ed: oh yeah, that's the one: I have a couple 2SC2690A's that are rated quite favorably.  Superb for VAS duty, or for a modest SW RF amp!)

Commodity MOSFETs are pot luck.  Even the humble 2N7000 has quite a low impedance: a current capability of 100s mA, and 30pF+ of capacitances.  It rolls off slowly above 20 or 50MHz, and needs ever lower supply voltage and source/load impedance.  It's been said it can switch as fast as fast as you can drive it; but therein lies the problem, you can't drive it faster than maybe 200MHz, at any reasonable sort of impedance (it needs as little as a few ohms up there), and still expect overall gain.

And not that a stack of 2N7000s is any better than a stack of 2N3904s, but the point is it's worse: lower impedance and gain.  And this behavior will be typical of most commodity MOSFETs.

The irony is, if you go out of your way and buy a proper RF MOSFET, they're orders of magnitude faster, like two of them.  You could put together a single stage 27MHz amplifier that does the job (well, maybe not single, but two stages for sure), but now you have to worry about parasitic oscillations over 1GHz!

Tim

[stcoso]

Well i decided to experiment with IRF510.. i used two of them (with the first driving the second) to get somenthing like 1W , without optimizing the gate biasing resistors (maybe some advice in this would be appreciated xD)

[CJay]

Some handy notes on IRF510 biasing here:

http://www.normanfield.com/radioirf510.htm

They also apply to the other switching FETs mentioned.

[vk6zgo]

If you want to amplify 27MHz,building a wideband amplifier is a very poor way to do it.
Tuned RF amplifiers have been used for nearly a century where serious gain is needed at a single frequency.
Have a look at the ARRL Handbook or RSGB Manual to learn more about such amplifier stages.

[stcoso]

I know a very little about RF PA and mosfet driving circuit.. but i found interesting the idea of the resonant gate driving techniques.. so i take the datasheet of IRF510, looked for Ciss and Coss for my VDS, calculated the series inductance for getting the resonance at the frequency of my XTAL..  i didn't have variable inductors (nor LCR meters) so it's not properly tuned but i think it worked nice..

[Martin.M]

Hi, I'm trying bulding a small (4-5W) RF gen from a XTAL oscillator.

I've used the following schematic to get the XTAL oscillating properly (just added a 0.7uH inductor accross C3 to get the xtal oscillating at the 3rd harmonics)

There is not one coil, so I think this will deliver you the complete family of harmonics.

what`s about a push-pull-oscillator what can deliver the power directly self?

greetings
Martin

[stcoso]

The 5th harmonic (56 MHz) at the output stages is 25dB down before the low pass filter.

[stcoso]

So, i managed to find a CB RTX (model pony cb-78) for 8$.  It works fine (it puts out something like 2W on a 50Ohm load) but i've got no control over the output power and so .. i need to modify..
Unfortunately no schematics are available..

How bad could be just cutting the trace to the collector of the output stage transistor ( a 2sc1239) and connecting it to a variable power supply?

[CJay]

So, i managed to find a CB RTX (model pony cb-78) for 8$.  It works fine (it puts out something like 2W on a 50Ohm load) but i've got no control over the output power and so .. i need to modify..
Unfortunately no schematics are available..

How bad could be just cutting the trace to the collector of the output stage transistor ( a 2sc1239) and connecting it to a variable power supply?

Similar?

http://www.cbtricks.com/radios/kanda_tsushin_kogyo/pony_cb72a/graphics/pony_cb72a_sch.jpg

however, you're right, varying the collector voltage to the driver and final will work, just try to trace it back a little and at least find the RF choke so you're not turning your supply wires into antennae

[stcoso]

Yeah, almost the same (mine has 23 channel) . The power stages seems identical.. 2sc775, 2sc1239 and many other components (including the output filter)


So.. in order to make it variable (and eliminating the modulation) where is the best place to cut  ?



I'm really thankful 

[Paul Rose]

So.. in order to make it variable (and eliminating the modulation) where is the best place to cut  ?

Break right at TP1.  Possibly there is even a jumper here that you can cut?  Feed your supply into the upper point of TP1 on the schematic.

That way you feed the final TR11 through choke L6 and still get the benefit of bypass cap C47

[stcoso]

Actually the jumper at TP1 it's not even soldered.. it's a piece of wire with connectors




EDIT: it worked fine, now i can regulate the output power 

thank you all 

[stcoso]

I have a strange (for me) problem. I've used an LM317T (on a PCB as here http://xtronic.org/circuit/regulated-power-supply-lm317/) with a 20V 200VA transformer. Below 10V it's all fine.. when i go just a little bit higher i get a strange (20Hz 2Vpp) ripple on the output that powers the collector (measured with my scope)..
I'm italian so mains=50Hz and i'm obviously using a full bridge rectifier


Ok, solved... impedance mismatch and reflected power.. i'm gonna get the proper impedance matching stuff


Again, thanks all