Critique requested for circuit design

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.

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

Frank Raffaeli

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

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.

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.

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

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