The rise and fall times of the detected output
voltage from a diode detector are dependant upon the simple RC
time constant of the circuit equivlent resistance and the cpacitance of the video load. The schematic of the equivalent
circuit is shown below:
The diode in the circuit can be replaced by a voltage source and a resistor representing the resistance of the diode:
The circuit equivalent resistance is the parallel resistance of the diode (Rdiode) and the video load resistance RL.
When RF power is applied to the detector, the diode resistance is low, and current flows through the diode. When RF
power is removed the diode is reverse biased by its previously detected output voltage, and the diode resistance
is high. The fall time of the output voltage is longer, because the charge on the load capacitor CL must drain out through
RL alone, as no current flows through the high resistance of the reverse biased diode.
The rise and fall times of the measurement system can be reduced by reducing the load capacitance CL, or reducing
the load resistance RL, or both. The problem with reducing the load resistance too much is that the output voltage VOUT
decreases because the source voltage is divided by the diode resistance and the load resistance. A 50 ohm video load
will provide very low rise and fall times, but sufficient RF power must be present at the detector to allow a measurable
pulse voltage at the 50 ohm load.
Another way to reduce rise and fall times is to reduce the diode resistance. For this reason, we recommend using a
harmonic mixer as a detector for fast pulse measurements. The harmonic mixer diode has lower resistance than a
zero bias detector diode, but without bias will not be able to detect very low power RF pulses.
Another way to further reduce rise and fall times is to use a DC bias on a harmonic mixer as shown in the circuit below:
The addition of a bias current greatly reduces the dynamic resistance of the diode, and results in lower rise and fall times.
Typical values for Vbias are 12-15 volts and Rbias = 100 K ohms.
If an oscilloscope is used to measure the pulses, a typical 10 X probe will have CL = 12 pf and RL = 10 Megohms.
The circuit below will yield 63.2% risetimes of about 10 nS when used with a typical 10 X oscilloscope probe:
The application of DC bias to the harmonic mixer results in a DC offset voltage being present at the output when no RF
power is applied. A standard (grounded cathode) harmonic mixer will have a positive offset voltage and a negative going
pulse, as shown below:
time constant of the circuit equivlent resistance and the cpacitance of the video load. The schematic of the equivalent
circuit is shown below:
The diode in the circuit can be replaced by a voltage source and a resistor representing the resistance of the diode:
The circuit equivalent resistance is the parallel resistance of the diode (Rdiode) and the video load resistance RL.
When RF power is applied to the detector, the diode resistance is low, and current flows through the diode. When RF
power is removed the diode is reverse biased by its previously detected output voltage, and the diode resistance
is high. The fall time of the output voltage is longer, because the charge on the load capacitor CL must drain out through
RL alone, as no current flows through the high resistance of the reverse biased diode.
The rise and fall times of the measurement system can be reduced by reducing the load capacitance CL, or reducing
the load resistance RL, or both. The problem with reducing the load resistance too much is that the output voltage VOUT
decreases because the source voltage is divided by the diode resistance and the load resistance. A 50 ohm video load
will provide very low rise and fall times, but sufficient RF power must be present at the detector to allow a measurable
pulse voltage at the 50 ohm load.
Another way to reduce rise and fall times is to reduce the diode resistance. For this reason, we recommend using a
harmonic mixer as a detector for fast pulse measurements. The harmonic mixer diode has lower resistance than a
zero bias detector diode, but without bias will not be able to detect very low power RF pulses.
Another way to further reduce rise and fall times is to use a DC bias on a harmonic mixer as shown in the circuit below:
The addition of a bias current greatly reduces the dynamic resistance of the diode, and results in lower rise and fall times.
Typical values for Vbias are 12-15 volts and Rbias = 100 K ohms.
If an oscilloscope is used to measure the pulses, a typical 10 X probe will have CL = 12 pf and RL = 10 Megohms.
The circuit below will yield 63.2% risetimes of about 10 nS when used with a typical 10 X oscilloscope probe:
The application of DC bias to the harmonic mixer results in a DC offset voltage being present at the output when no RF
power is applied. A standard (grounded cathode) harmonic mixer will have a positive offset voltage and a negative going
pulse, as shown below:
