It would be nice if the tiny spectrum analyzer would also have a tracking generator.
It turns out to be rather easy to add such functionality.
As a prototype I removed the 434MHz filter between the mixer and the receiver SI4432 and started the arduino monitor to so I could set the SI4432 manually.
These are the command I entered where # precedes a comment line
#select the LO SI4432
v1
#set the LO SI4432 to a fixed frequency (435MHz) for testing
o435000000
#set the LO SI4432 to TX mode by writing 0x0B to register 0x07
x07 0B
#and set to max output by writing 0x1F to register 0x6B
x6B 1F
#select the receiver SI4432
v0
#set the receiver SI4432 to a fixed frequency of 434000000
o434000000
#set the receiver SI4432 also to TX mode by writing 0x0B to register 0x07
x07 0B
#and set to lowest output by writing 0x18 to register 0x6B to avoid intermodulation products
x6B 18
and voila, a nice sinus output of 1MHz comes from the IF port of the mixer. I did not measure IMD but on a scope a looks OK.
When I now sweep the LO, the output of the mixer nicely tracks the LO - 434MHz which is exactly the tune frequency of the spectrum analyzer.
Some additional remarks:
- Best would be to use a good 400MHz low pass filter after the mixer IF port output to remove the high LO frequency leakage (at least -20dBm)
- The SI4432 synthesizer can only step in steps of about 260Hz so keep in mind your requested output frequency will be rounded to the 260Hz grid.
- At the lowest output level you get about -10dBm output from the mixer. You can set the receiver SI4432 also to highest output level and get more than 0dBm? but the output will no longer be a sinus, more like a triangular wave
- To act as a real tracking generator you would need a second mixer and a SMA Tee to to split the LO signal to drive the two mixers
Even when you do not have only two SI4432 modules, having a signal generator able of outputting a sine wave between 0Hz and 400HMz is nice to have at no extra cost.
