Does my FM scanner circuit look reasonable?

[Charles Paul]

I've planned out an FM scanner that I'm going to build, but I'd like to have someone eyeball it first to make sure I'm not going to waste a lot of time on something silly. I'm more of a digital fellow, and analog is relatively new to me.

Anyway, how I intend this circuit to work is I have a voltage from a DAC to signify the frequency to tune to. This goes to a LTC6905 acting as a VCO (I copied this out of the docs for it), which then goes to a JFET mixer. The other input to the mixer is from the antenna, followed by a high pass filter and a
JFET amp. Then, I filter to get the audio component, and then I branch that off.

One side goes to a simple inverting amplifier with a push-pull output then headphone jack, and the other goes to an active rectifier, which then gets smoothed with a cap, which goes to an ADC for the digital bits to be a measure of signal amplitude.

Does this seem reasonable? I'm fairly confident in the opamp bits, and the LTC6905 (since it is straight from the app notes), but JFET's seem rather unpredictable to me.

Thank you for your help.

 

[FvM]

Which receiver concept do you try to implement here? Zero IF? What kind of output do you expect for AM and FM modulated signals?

Besides general conceptual problems, the present design will suffer from these:
- no RF is passing the "Low pass filter 3"
- the VCO has a square wave output, so if it's output would be mixed with the antenna input, you are receiving all odd harmonics of the oscillator frequency, not only the fundmental.

 

[Charles Paul]

Thanks for your help. I think I made a typo in my calculator for the low pass filter. In the Low pass filter 3, my intention was to have the low pass filter attenuate the higher harmonics from the square wave. They should have been 110 ohms and 10pF, for a roll-off of ~144MHz.

Right, I intend it to be zero IF. I'm not sure what you mean by expected output for AM and FM modulated signals. The goal is to have it being able to output normal FM broadcast band audio (~100MHz in USA).

 

[FvM]

I'm not sure what you mean by expected output for AM and FM modulated signals.

I won't expect anything similar to FM audio baseband.

 

[betwixt]

I'm very sceptical too. Unless using almost zero IF and attempting some kind of pulse counting detecting in the mixer, it won't work to resolve FM at all. You might get something out of it but it would be very inefficient. At the least you should add a filter after the mixer and a proper FM discriminator.

Brian.