Analyzing a frequency-mixer circuit in LTSpice

[minhha04]

Summary:: I'm trying to analyze a frequency-mixer circuit using LTSpice but i keep bumping into problems. Am I understanding something wrong?

so I'm trying to design a frequency-mixer circuit. (the images are all shown below) in LTSpice, and there's a task asking me to perform a transient analysis at 100 microseconds, and find the time it takes the voltage at IFF to reach steady-state value. however, when i plotted the graph, the line (the red line in the picture) seems to be a straight line from start to finish, instead of reaching a point where the values will cease to change. is there something wrong with my schematic? or did i understand something wrong?

all the pictures in order:

1. the original schematic

2. my schematic

3. the task in question

4. my graph ( V IFF is the red line)

 

[Baluncore]

Welcome to PF.
An LTspice schematic.asc file is an ASCII text file. There can be things in your schematic.asc file that are invisible on the screen image.
If you copy and rename the file as schematic.asc.txt you can then attach it to your post and I can run it here.

 

[Baluncore]

LTspice requires 4 MHz to be written as 4MEG, 4meg or 4e6.
Where you use a capital M, I expect you are getting millihertz, not Megahertz.
3.5M = 3.5 millihertz. Try 3.5MEG or 3meg5

 

[minhha04]

LTspice requires 4 MHz to be written as 4MEG, 4meg or 4e6.
Where you use a capital M, I expect you are getting millihertz, not Megahertz.
3.5M = 3.5 millihertz. Try 3.5MEG or 3meg5

you are right! I changed it to MEG and it worked, thank you so much!

 

[Baluncore]

I changed it to MEG and it worked, thank you so much!

That is all part of the service we offer. You did the right thing asking here. The SPICE learning curve is steep, and there are some overhangs. There are also some shortcuts you will find out about later. The M or MEG gets everyone sometime.

Avoid including units in component values. The trap I like best is the one farad capacitor, 1F, that seems not to be connected.
The F is interpreted not as a farad, but as femto = 1e-15.

Search LTspice Help for MEG to find the SI prefixes. They were hidden at the end of the “General Structure and Conventions” page.

 

[jsgruszynski]

Be aware that a frequency mixer is a Large Signal "device".

Small Signal analysis in SPICE (.AC) will NOT give you the right answer. Specifically diode mixers (or any other type) will not "mix" without large signal, NONLINEAR excursions of the diodes. This is central to how any mixer works - it has to "switch" in order to achieve the nonlinearity required. So this implied you must use .TRAN or large signal transient simulation.

The mixing action comes from higher order polynomial terms of the mixing device that give you minimially square law (2nd order) or higher terms which then give you "cos RF cos LO = ..." trig identity terms which results in terms that look like cos (RF+LO) + cos (RF-LO).

 

[DaveE]

LTspice requires 4 MHz to be written as 4MEG, 4meg or 4e6.
Where you use a capital M, I expect you are getting millihertz, not Megahertz.
3.5M = 3.5 millihertz. Try 3.5MEG or 3meg5

That is sooo annoying. I know it and still regularly screw it up.