Critique requested for circuit design

[Kevin Doyle]

Hi all,
Here I have a circuit diagram of an FM 106Mhz transmitter I built.
http://homepage.eircom.net/~doylekevin/fm-tx.jpg
I dont think I'm maximising the potential of the bfg 235 transistors. If you
could take a look at it and suggest any modifications I would be greatful.
Cheers,
Kevin.

 

[Joerg]

Hello Kevin,

A BFG235 in a 100MHz circuit is like using a Porsche or Ferrari to drive
to the grocery in town.

Anyway, are you sure it is legal to operate that circuit in Ireland?

Regards, Joerg

 

[Frank Raffaeli]

Hi all,
Here I have a circuit diagram of an FM 106Mhz transmitter I built.
http://homepage.eircom.net/~doylekevin/fm-tx.jpg
I dont think I'm maximising the potential of the bfg 235 transistors. If you
could take a look at it and suggest any modifications I would be greatful.
Cheers,
Kevin.

I'll try to be polite. First, thanks for posting your schematic. It is
easier to give feedback.

My guess from your earlier posts is that your intent is to couple this
output to an antenna. If the RF output were 170 mW of RF (it won't be),
your range would be closer to 1 mile, not 30 feet as you had
"intended", assuming a moderately efficient antenna.

below is a link to a schematic of an oscillator. The output is -3 dBm -
less than 1 milliwatt. Try that instead. BFG 235's are not for such a
low frequency. The input cannot absorb RF power at 106 MHz well. Also,
if you tried to build a low-loss matching network for a BFG 235 at this
frequency, it would be unstable, and the transistor would oscillate,
probably at 2-4 GHz.

http://www.aomwireless.com/resources/circuits/vco_110a/vco_110a.gif

Since you didn't know where Pooh Bear lives .... you're on your own.

Frank Raffaeli
http://www.aomwireless.com/

 

[Kevin Doyle]

I'll try to be polite. First, thanks for posting your schematic. It is
easier to give feedback.

My guess from your earlier posts is that your intent is to couple this
output to an antenna. If the RF output were 170 mW of RF (it won't be),
your range would be closer to 1 mile, not 30 feet as you had
"intended", assuming a moderately efficient antenna.

below is a link to a schematic of an oscillator. The output is -3 dBm -
less than 1 milliwatt. Try that instead. BFG 235's are not for such a
low frequency. The input cannot absorb RF power at 106 MHz well. Also,
if you tried to build a low-loss matching network for a BFG 235 at this
frequency, it would be unstable, and the transistor would oscillate,
probably at 2-4 GHz.

http://www.aomwireless.com/resources/circuits/vco_110a/vco_110a.gif

Since you didn't know where Pooh Bear lives .... you're on your own.

Frank Raffaeli
http://www.aomwireless.com/

Thanks Frank for your reply!
I though the BFG235 would be Ok as the s parameters for VHF seemed ok. But I
think I'll jump onto the farnell site and order a proper VHF transistor.
Thanks for the Schematic of the VCO it looks good. There is very little
noise from my own design. I don't have gear to measure it but listening to
the CW on the scanner it souds just as quite as commercial repeaters with CW
in the upper VHF band where I live.

Its a pitty about Pooh.

Kevin.

 

[Andrew Holme]

Hi all,
Here I have a circuit diagram of an FM 106Mhz transmitter I built.
http://homepage.eircom.net/~doylekevin/fm-tx.jpg
I dont think I'm maximising the potential of the bfg 235 transistors. If you
could take a look at it and suggest any modifications I would be greatful.
Cheers,
Kevin.

You need some sort of matching network at the output. You can't just
hang a 50 ohm load off the end of C14.

How did you arrive at the figure for output power?

R12 looks a bit on the large side for the job it's doing.

You don't show any power supply decoupling in the diagram.

That 3dB pad is only a 3dB attenuator when it's input and output are
both terminated in 50 ohms.

The other inter-stage coupling networks are unusual. I presume you are
controlling the drive level by adjusting the value of U2 and U3?

You seem to have a lot of gain in the driver stage. I wonder if it
would be safer to give some of it to the "buffer" stage.

 

[Mark]

you also should look up the design and purpose of a low pass filter and
use one at the output before your antenna.

I personally don't care what you radiate at 106 MHz but you should care
about 212 MHz and 318 MHz etc. You might get some people pretty pissed
off at you if you don't.

Mark

 

[Kevin Doyle]

Thanks I'll have a re-think about the whole circuit.
Looking at it now it looks like crap!!

Kevin.