A chock coil

[walid1]

In many FM Txs they use a coil connected from the o/p of a high frequency stage to the power supply, this coil is of fairly larg value (for example 100uH). The purose of using such coils is to chock (prevent) high frequency sigals from going to other stage through the power supply.
For example in 100 MHz FM Tx, they use 100 uH coil which has impedance of (2*pi*f*L) about 63 Kohm.
My question: Is it better to use 1 mH (1000uH) which has 630 Kohm ot taking the size into account?
Or is this true:"the more L the more chock effect"? What are the limits?
thanks.

 

[audioguru2]

Hi Walid.
It is called a choke, and if its value is too high then the capacitance between the layers of windings might be so high that it parallel resonates at a lower frequency than you want. Above its resonance it behaves like a capacitor instead of an inductor.

 

[Staigen1]

Also, isn't a 100uH choke a little bit high at 100 Mhz?

//Staigen

 

[audioguru2]

We think its value doesn't need to be so high but this author of an FM transmitter likes to use it as a collector load impedance:

View attachment 37584

 

[Staigen1]

Hehe
Isn't that a typo, should be nH instead? ???

//Staigen

 

[audioguru2]

The circuit's author likes big values. Look at the huge value for the emitter bypass cap. It would still work very well at a frequency 200 times lower.

 

[walid1]

Hi audioguru and Staigen

How you think calculate this value, this what i need to know
gesign gesign gesign gesign gesign gesign gesign gesign gesign

 

[Staigen1]

Oops! isn't that also too big, i belive that a cap that big at this freq lives a life of its own, and dont decouple the emitter resistor very well! Something else curious with this schematic? Of course, no emphasis! Maybee thats why the coupling cap between the mic and the input is so small! Moore?

//Staigen

 

[audioguru2]

With a small coupling cap so it doesn't have any bass, and no pre-emphasis so it doesn't have any treble, then it would sound much worse than an old telephone. 8)

 

[Staigen1]

Hi Walid

How you think calculate this value, this what i need to know

Thumb of rule: Not moore than 1 nF at 100 Mhz for decoupling! This goes for toobz, but i think it is relevant for transistors too, it dont have to do with if it is a transistor or a tube.

gesign gesign gesign gesign gesign gesign gesign gesign gesign

What is "gesign"

To Audioguru: Hehe

//Staigen

 

[audioguru2]

Until you are experienced in circuit "gesign" it is advised that you build a "bototype". (He, he.) ;D
Then you can tweak parts values in the prototype to make it work as well as it can.
In the prototype you will learn that 100MHz is a very high frequency.
At such a high frequency, you will learn that 100nH works well, but 100uH works like a capacitor.
At such a high frequency, you will learn that a 1nF ceramic cap with short leads works very well, but 100nF is an inductor. ;D

 

[walid1]

Until you are experienced in circuit "gesign" it is advised that you build a "bototype". (He, he.) ;D
Then you can tweak parts values in the prototype to make it work as well as it can.
In the prototype you will learn that 100MHz is a very high frequency.
At such a high frequency, you will learn that 100nH works well, but 100uH works like a capacitor.
At such a high frequency, you will learn that a 1nF ceramic cap with short leads works very well, but 100nF is an inductor. ;D

I have not oscilloscope or other devices to monitor the change so i ask

 

[audioguru2]

Without proper test equipment you will be working blindly.
Aren't you the guy who wants to be an expert circuit designer? I have never seen a blind expert circuit designer.

It is fine for you to ask about the details of design, then you will learn from us. ;D

 

[walid1]

No problem I talk with a designer from USA and he told me that he rarely use the oscilloscope, the understanding more important than instrument or come first.
yours
walid