FM demodulator in a ratio detector

Status
Not open for further replies.
betwixt said:
http://www.tscm.com/NEETS-v12-Modulation.pdf page 3-15 will give you more information.
Click to expand...
Thank you for the study materials.



Ok, so I want to design ratio detector that would work for such parameters: carrier frequency 98MHz, modulating signal 100kHz and Index Modulation 25 (I have read that most often the modulation index takes values from 10 to 100, of course it can be 1, but is not recommended). My question: how to choose parameters of ratio detector FM demodulator?

The LTspice schema in post # 19 was modeled on this schema:

Oh i forgot to add capacitor c3. My fault!
 

Sorry for spam, but I think I am close solving problem. This is corrected scheme:






All I have left is to change the ideal diodes to real elements. And further improve the circuit. But now I'm sure the circuit demodulates the fm signal
 

I still can't work out what the graphs represent. V(vin) needs to change frequency around the resonant point and V(vout) should rise then fall as the resonant point is passed, your signals seem to be at the same frequency and it is far too low. The resonance assuming L2 and L3 are on the same former will be at 14.711 MHz but your signal is at about 15KHz, out by a factor of 1,000 !

Brian.
 

I changed input voltage (V1) parameter and it's like this:


And this is what I receive in AC analysis:


Vout is the voltage of output between R1 and R2. Is it correct?
 

Attachments

  • 1611506851906.png
    43.6 KB · Views: 148
  • 1611507003159.png
    13.2 KB · Views: 126
Last edited:

I have no idea what is producing the curves you show but it certainly isn't what a ratio detector should give.
A ratio detector works at one specific frequency, it is decided by the capacitors C1 and C2 and L1, L2 and L3 using the formula f=1/(2 *pi *Sqrt(L*C)) and that makes the C1/L1 combination resonate at about 10.9MHz and C2/L2+L3 combination resonate at about 14.7MHz, it therefore doesn't make any sense to show the plot at much lower frequencies. The circuits should resonate at the same frequency, be coupled together and the frequency sweep should be from about 1MHz below resonance to 1MHz above it.

Brian.
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…