Help about 5 Watt FM PCB design

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contau

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Hi all, I have a question about PCB. Pls help me.
How to integrating 5 Watt FM cỉrcuit (100Mhz) with 0.5 Watt driver in circuit board?
 

The 5W output power of your FM transmitter is much higher than the minimum power where you need a registered crystal controlled circuit and a broadcast licence to operate it in most countries.
It will probably cause interference for many people listening to a licenced radio station and might even cause interference to aircraft communications.
Its harmonics will probably cause interference to many other communications like police, ambulance and fire department. it will probably also cause TV interference.

My FM transmitter circuit is also illegal and has an output power of about 0.077W into a 75 ohm antenna and its range is more than 2km to a very sensitive home stereo or car stereo over a large river valley. Its range is across the street to a cheap FM radio.
 

Thanks your answer. The harmonics of transmitter can rejected by filter circuits, so it is not problem. The interference between high frequency current of 5 Watt circuit with driver stage make noise at the audio input and unwanted oscilation. I try to shielding and guarding, but it hardly radiate all power to antenna. pls help me
 

My question about PCB not circuit principle. If seperating power and driver stage on two different board, it will operate very good.
 

Without seeing your schematic I do not know if your audio input is very sensitive for a microphone, or if it has low gain for a line level input. It would be simple to filter out 100MHz from the input but allow 15kHz audio to pass. Maybe your ground and/or positive supply traces are too long or too thin.
 

So is it possible to integrating high power with its driver stage in one circuit copper board?
 

So how to revent interference between electro-magnetic field of high power part with driver stage?
 

Audioguru said:
Thick high current traces and parts close together making the traces very short.
Did you add a filter to the audio input?
Click to expand...
The audio noise maybe come from power supply of vco.
Due to high power, the high frequency loop current is very large. It makes electro-magnetic field that interference with driver stage. how to reject it?
 

5W out from a 500mW driver stage is **NOTHING**, it is just 10dB of gain.
With good layout, a ground plane and some attention to impedance matching and decoupling (Ferrite beads and 100pF caps are your friends) this is trivial to do.

I routinely pull off 40dB gain on a single two stage board, (50mW in, 500W out @ 144MHz, modern LDMOS rocks), and would not expect to have any issues putting an exciter on the same board (I prefer not to, manly for thermal management reasons, but would not expect additional stability issues).

Layout is everything at these frequencies, and remember that decoupling like it was audio does not work up in the VHF.
Two layer boards with solid ground planes are good, 4 layer are better.

If the layout is sane then audio noise is probably rf either getting into the audio input node or is coupling via the power supply (Strangest one I had was squegging due to RF on the power rail causing the PLL to loose lock, which shutdown the power stage until the loop reaquired lock, which put the rf back on the power rails...., caused some hair tearing that did).

Regards, Dan.
 
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    contau

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Would you pls explain the red line? The decoupling capacitor value in low frequency is very larger than rf but it also stay at power line of each part.
 

At VHF and up capacitor parasitic inductance becomes a major issue.

For example most electrolytics appear as inductors above a few MHz, and are completely ineffective by the time you hit 100MHz.
Physically small MLCCs are the way to go for fast signals, and a few nF of MLCC will be very much more effective then humdreds of microfarads of elco (VERY short leads are a must).

Worse the parasitic inductance can resonate with the capacitance, causing all sorts of weirdness, a common one is fo the power amp to go off generating AM due to a filter capacitor resonating with the drain load choke, this can be damped by placing a resistor of a few ohms in series with a electrolytic across the supply (It provides a resistive dissipative element in the audio and unltrasonic region, this can alternatively be placed across a second choke in the drain power supply, same effect).


100pF caps, short leads, make sure your power stage is actually stable, limit bandwidth of audio stages (JFET opamps are usually better behaved around RF then bipolar ones), ferrite beads, it all helps.

I am guessing RD06HVF1 or similar in class C as a final?

Rgards, Dan.
 


To work at radio frequency, i must build a simple L,C meter at operating frequency to calculate inductance, capacitance and resonant value.
Sometime, i test 120pico capacitor and know that it is inductor at the working frequency but that is depended on the manufactor and often be right with capacitor having lagre value.
 

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