What is an RF Choke? Am I using the right component

Hi all I have a 1 transistor bjt oscillator operating at 54Mhz. I designed it using s paramaters for the BFG97 Vce10v Ic 70mA. The Value of Rc is pretty low in the 10 to 100ohm region. I presume I need an RF choke above the Rc connecting to the Vcc 12volt line. Currently I'm using a axial choke rated at 200microHenry. Am I doing this right. My circuit seems to behave as if the power supply is shorted after a few mins of operation. I'm not sure whats causing it.

Can you give me some helpful advice please? Regards, Kevin.

Perhaps you should do some more reading on the various oscillator configurations. There is a lot of instructional info on this subject around, such as

The most common types of L-C oscillator are the Hartley and Colpitts.

In basic terms, you will note in the description of the BJT HARLEY OSCILLATOR that the purpose of the RFC and its associated C3 perform the function of "decoupling" the ac side of the oscillator from the dc supply. The RFC is effectively a low dc resistance as far as the voltage supply to the bjt collector yet it has a high impedance to the signal frequency. Therefore the RFC prevents the dc supply (battery) from loading down, or short circuiting, the oscillations. C3 is low impedance at the oscillator frequency and it by-passes the signal around the dc supply. Without both components your oscillator will have difficulty in operating efficiently.

Hi Guys, Thanks for the great response there. I have the circuit designed on the back of single sided PCB using copper adhesive tape so it has a ground plane to help stability etc.

I had a hunch that the Ft of the BFG97 might be too high.

I will swap in a lower Ft transistor I have the mps5179 to hand Ft900Mhz and I'll see how this helps things

There is a circuit diagram of my design above in the hyper link.

Cheers, Kevin.

line.

Kevin, some points :

1. The BFG97 has an FT of 5GHz and is a very hot transistor for a 54 MHz application. It will almost certainly oscillate at 54 MHz and at least a couple of other frequencies at the same time ! I suggest a part with a 200 to 500 MHz FT - and that will include parts typically used in audio applications ! This is a general principle - don't pick a hotter transistor than you need. Particularly if you don't have a 5+ GHz spectrum analyser, you will have no idea what the hot transistor is doing.

1. Your S-parameter design is a worthy piece of work, but at 54 MHz, just about anything will work. The lower FT transistor you choose will probably not have S-parameters published for it. You may be able to convert its datasheet info to S parameters, or use a simple hybrid pi model. I suggest you just replace the BFG97 with your new transistor and turn on !

2. We need to see your schematic before we can assess the choke. 200uH is almost certainly way too big and its winding capacitance will dominate at 54 MHz, making it look like a capacitor ! But aside from the value of the choke, we need to see what the circuit requires. Many oscillators need a low impedance to ground at the collector, whether provided by a low load impedance or a low value resistor to a bypassed supply, because the collector has to pump those pulses of current which keep the tuned circuit going.

1. The 70 mA of collector current will need to work into a low impedance load and a low impedance tap on your tuned circuit, otherwise the transistor will saturate on waveform peaks, heavily damp the tuned circuit and generally misbehave. The small signal S parameters only tell that your oscillator will start -- nothing about the output waveform. So you will want a reduced collector current and/or a low impedance circuit.

If you have a library handy, you may be able to find the RSGB or ARRL handbook or other books to give you inspiration.

Can you draw your circuit with ASCII symbols and let us see it ?

Roger

Kevin, The 200 uH choke is probably self resonant way below 54 MHz, and will look like a capacitor. As a ballpark figure, the old Ohmite Z50 RF chokes were intended to be used at 50 - 54 MHz, and had an inductance of 7 uH. I have also used the small molded 5.6 uH chokes at 50 MHz.

Tam

Your configuration can handle a nice high impedance choke in the collector, because the load R5 helps give a defined collector impedance. Your 50 ohms load gets transformed by C3 to about 100 ohms at the collector, so several hundred ohms reactance would be nice for U2 Say at least 600 ohms at 54 MHz = 1.7 uH . Looking at some typical small choke specs, I see that I can easily go up to 10uH before self resonance, so somewhere about there.

Your 900 MHz Ft part is the upper limit of what I'd be comfortable with. In your favour for stability is the the modest amount of shunt feedback. My attitude is, I want things to work - I don't want an adventure every time I want an oscillator.

If you chose 70mA because that is what the S-parameters were measured at, then it is time to work out what you yourself require for your application. How many milliwatts do you want to deliver into your 50 Ohm load ? In rough terms for a class C amp (oscillator) Power out = (Vcc * Vcc) / (2 * Rload) where Rload is resistance due to load and other losses at collector - it assumes resistance not reactance exists at collector, but it puts you in the ballpark. Wes Haywards rule of thumb for oscillator peak AC collector swing is Vc = 2 * Ie * Rc (Ie is designed dc bias value of emitter current without oscillation, Rc is AC resistance seen at collector). Your peak swing must be well under your DC collector voltage , otherwise the collector will swing down to ground and saturate. So you adjust collector current and collector load resistance to get the power you want wihout saturation.

From the bias components, I'm guessing 100+ mA will flow. At that current, the MP5179 will not be much good. I suggest R1 = 100R and R2 = 47K for about

I haven't done any maths on your tuned circuit network, but it looks willing.

Roger