Hello,

I am currently playing abound with oscillators and since I am not very experienced with doing so I would like to ask some questions on circuits implementation issues.
The Hartley oscillator I am using is shown in the figure. This circuit should work well till frequencies of about 150 MHz, but at the moment I have it oscillate at about 5.5 MHz as I am using a solderless bread-board. Eventually I will solder it and put is in a shielded box to reduce noise in the circuit. The capacitor C2 is a pressure sensor which will change the frequency of oscillation when excited.
The main problem is that the frequency of oscillation experiences only a minor change as the capacitance of the sensor will only change by about 0.1pF. In an unexcited state the “center” capacitance is about 68pF. Thus about a 4kHz deviation in frequency only.

1) I was wondering whether there is a way to “amplify” the change in frequency with the circuit shown in the figure.

I would like to do some data acquisition on the output of the oscillator and have a simple acquisition board for that. It uses a 12-bit A/D converter which can sample up to 3MHz. As the signal is currently at about 5.5MHz I would need to do under-sampling (let’s say 1MHz) to obtain the waveform. Before feeding the signal into the A/D converter I need to buffer it and also would like to band-pass filter the signal to reduce the overall aliased noise when under-sampling.

2) What would be a good circuit of doing this?  I was thinking about a simple LC bandpass followed by a wide bandwidth amplifier to buffer?

Any tips and suggestions are appreciated.

Thanks for the response mikki33. Yes that would be a solution. I was thinking about down mixing the FM signal to get the center frequency down, which should be the same thing if I understood you correctly. Since the modulation is quite small I was thinking about under-sampling the FM output and thus “bringing out” the frequency change. In theory this should work as well, but might cause some problems with noise being aliased.

Thanks