How to connect a Colpitts oscillator to a low pass filter, and then to a log RF power detector

Thread Starter

talikarng

Joined May 27, 2017
18
How can you connect a Colpitts oscillator to an low pass filter (which will have a variable capacitor), and the output of the low pass filter to a log RF detector (e.g. AD8307)?
(I want the variable capacitor in the low pass filter to alter the attenuation of the rf signal, and the resulting power to be detected by the AD8307)

The schematic is for the Colpitts oscillator. When I add an low pass filter to "Freq_out" the oscillations die out very rapidly, I assume that adding a low pass filter affects the feedback to the tank circuit of the oscillator.
 

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dendad

Joined Feb 20, 2016
3,029
You always want to add a buffer amplifier after an oscillator for isolation to enhance stability.
So try with a loosely coupled buffer amp, then the LPF....
 

dendad

Joined Feb 20, 2016
3,029
Your circuit configuration looks a bit odd to me.
If you are relying on L1 and C2/C3 as the resonant component, it will be dependant of the power supply decoupling for circuit completion. Not a good idea for stability. The LC tank needs to be isolated from the outside world.
It is better to have a parallel tank circuit on the base I think.
Here are a couple of links that may help.
http://www.qrparci.org/wa0itp/chap10.pdf
http://www.robkalmeijer.nl/techniek/electronica/radiotechniek/hambladen/cq/1991/06/page16/index.html

I just installed LTSpice on my Mac but found I will have to view some tutorials as it is almost impossible to work without that. So later when I get some time.
 
Last edited:

dendad

Joined Feb 20, 2016
3,029
You may be. I'm still trying to get to grips with LTSpice.
It is sort of going on my mac but is pretty terrible to use. It may be the Windows version is easier. When I get some spare time I may try it.
I'm sure there are others on this forum better qualified to help you. But I'll do what I can.
 

Thread Starter

talikarng

Joined May 27, 2017
18
Thanks for this. (I got your idea about a diode after I posted my last schematic)
BTW: what do you use for simulations on the Mac?
 

dendad

Joined Feb 20, 2016
3,029
LTSpiceScreen.jpg

This is the LTSpice screen.
It keeps asking me to update but remains the same after the update.
It is ..
LTspice IV for OS X
Build Oct 3 2016, 14:49:11 US Pacific.
Still, it sort of works.
 
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