# Linear Frequency to Linear Voltage circuit

Joined Sep 28, 2021
266

Dear all, the circuit above is quoted from here, and I wish to try to produce similar graph in LT Spice but I am unsure of it. The LT Spice schematic is correct since I tested it with specific 10V input AC, and it produces linear voltage on its output. However, I want to do an AC sweep on this thing but I only get this weird bode plot? May I ask how to reproduce the triangle wave which indicates linear voltage vs frequency in LT Spice ? Below attached my LT Spice file

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#### Alec_t

Joined Sep 17, 2013
14,377
May I ask how to reproduce the triangle wave which indicates linear voltage vs frequency in LT Spice ?
Try this:

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#### Audioguru again

Joined Oct 21, 2019
6,758
Your simple circuit does not work!

The 5pF capacitor passes only VHF and UHF signals as shown by the measurement of -210dB at 50kHz.
-120dB is 1/millionth then -240dB is 1/billionth.
The 5pF capacitor also feeds the output 10k resistor and the 100pF capacitor forming a voltage divider, cutting the levels of your selected frequencies even more.

LTspice does not know the specs of a diode with a part number of D, try using a 1N4148.
The forward voltage of the diode plus its internal resistance messes up your peak detector circuit.
Here is a real peak detector that uses an opamp to work properly:

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#### crutschow

Joined Mar 14, 2008
34,689
However, I want to do an AC sweep on this thing but I only get this weird bode plot
AC analysis uses a linear representation of all the components, and since a diode is not linear, you can't do AC analysis for your circuit, you must use transient analysis.

#### Audioguru again

Joined Oct 21, 2019
6,758
Add the opamp like I showed to linearize the diode to get the peak detector to show levels at different frequencies.
Are you trying to re-invent an FM detector? Use a common Foster-Seeley or common Ratio Detector.