The I-V relationship of a diode is,
: voltage across diode
: current through diode
: reverse saturation current of the diode
: diode ideality factor; 1.2 for schottky diodes; 2 for silicon diodes
= 26mV @ RT
The I-V characteristic of a diode is shown in Figure 1.
For small increments() around operating point(), diode current is given by Taylor series expansion as,
In Eq.(2), second order terms contribute to mixing operation, while other terms and higher order terms produce spurious components or spurs.
Single Ended Diode Mixer
The schematic of single ended diode mixer is shown in Figure 2.
Diplexer combines the input RF and LO signals by superimposing them to drive the diode.
Diode is biased at DC voltage (), decoupled from RF and LO signal paths through a DC blocking capacitor(). RF choke blocks the RF/LO signals entering into bias source. High frequency components produced due to diode nonlinearity are filtered by IF filter, allowing only IF component to appear at the output.
The equivalent circuit of a single ended diode mixer is shown in Figure 3.
Let and . Then .
The diode current() due to voltage across it at operating point is,
If and are closely spaced, then from Eq.(3), we can see that except the IF component() due to second order term, all other terms are at much high frequency than IF, and can be filtered out by IF filter.
The output current is
Conversion gain is