All this technology for fifty bucks?
toggle quoted message
Show quoted text
On Wednesday, October 2, 2019, 11:36:26 PM PDT, <erik@...> wrote:
You have the test signal from the SI5351 send to the bridge but also to the "reference" mixer, in the reference mixer it is mixed with the output of the Local Oscillator (LO) which also comes from the SI5351.
The unbalance of the bridge goes to the "reflection" mixer where it is mixed with the same LO.
The LO is at 5kHz offset from the test signal so the output of both mixers is a IF signal at 5kHz.
Both IF's are then read into the controller using the ADC and in a DSP algorithm in the controller the IF's? are going to DSP mixers where they are mixed with two 5kHz signals 90 degrees apart (I/Q mixer) to create a DC signal.
The DC signals (I and Q) are the real and imaginary part of the complex downconverted reference and reflection input to the HW mixers. As you do not know the actual angle of amplitude of the test signal the complex test signal is divided by the complex reference signal to eliminate the phase and amplitude uncertainty.
The outcome of this division is the unbalance of the bridge scaled relative to the reference signal, both in magnitude and angle.
Attached a small pdf that contains the block diagram of a VNA using the same mechanism
