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

[Kevin Doyle]

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.

 

[Ross Herbert]

On Wed, 22 Jun 2005 02:36:43 +0100, "Kevin Doyle"
<irishdude79@REMOVEyahoo.ie> wrote:

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

http://www.tpub.com/content/neets/14181/css/14181_81.htm
http://www.radioelectronicschool.com/reading/reading27.pdf

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.

 

[truegridtz]

"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsg3ue.1439$R5.470@news.indigo.ie...

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.

Kevin, This is what I suspect is the case with your project.

(1) if you are using hand-made circuitry it most probably won't operate
properly at 54MHz. There is too much stray L and C that couples the
components perniciously and this causes violent oscillations This is
probably why your PS is working so hard.. You probably need to use surface
mount. and a properly designed circuit board.

(2) an RF choke is an inductor that is made for RF frequency. I don't know
a lot about radio. A "tank" circuit is an LRC circuit that usually has
component values within a certain range. An RF choke has the inductance for
such a hi-freq circuit.

Hope this helps, and hopefully I am not wrong about this. Mark

 

[Roger Lascelles]

"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsg3ue.1439$R5.470@news.indigo.ie...

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.

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.

2. 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 !

3. 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.

4. 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 Doyle]

http://homepage.eircom.net/~doylekevin/54MhzOsc.JPG

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.

"Roger Lascelles" <despam_rklasl@aanet.com.au> wrote in message
news:1119424619.d4d10153492ce67e9951455abb87f3f0@teranews...

"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsg3ue.1439$R5.470@news.indigo.ie...
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.

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.

2. 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 !

3. 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.

4. 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 Doyle]

http://homepage.eircom.net/~doylekevin/54MhzCircuit.jpg

Hi again,
I'm also put a picture of the circuit at the above link. 120Kb size 1024x768
dimensions
Cheers
Kevin.
"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
news:Mreue.1466$R5.418@news.indigo.ie...

http://homepage.eircom.net/~doylekevin/54MhzOsc.JPG

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.

"Roger Lascelles" <despam_rklasl@aanet.com.au> wrote in message
news:1119424619.d4d10153492ce67e9951455abb87f3f0@teranews...
"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsg3ue.1439$R5.470@news.indigo.ie...
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.

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.

2. 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 !

3. 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.

4. 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

 

[Tam/WB2TT]

"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsg3ue.1439$R5.470@news.indigo.ie...

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.

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

 

[Roger Lascelles]

"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
newsGeue.1467$R5.464@news.indigo.ie...

http://homepage.eircom.net/~doylekevin/54MhzCircuit.jpg

Hi again,
I'm also put a picture of the circuit at the above link. 120Kb size
1024x768
dimensions
Cheers
Kevin.
"Kevin Doyle" <irishdude79@REMOVEyahoo.ie> wrote in message
news:Mreue.1466$R5.418@news.indigo.ie...
http://homepage.eircom.net/~doylekevin/54MhzOsc.JPG

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.

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
20 mA. That is about as hard as you can work that transistor.

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

Roger