The Sampling Phase Detector (SPD) is composed of a Step Recovery Diode (SRD), two capacitors and two Schottky Diodes. It is designed to be used in phase locking circuits for microwave oscillators.

• Test board layout and test circuit.

Device Operation:

The following description of the SPD refers to Figure 1. The reference signal forward biases the SRD (D1). When the polarity of the reference signal reverses, the charge stored is removed and the the SRD's voltage drop goes through a fast transition. The capacitors (C1 and C2) differentiate the edge into an impulse and apply this impulse to the Schottky diodes (D2 and D3). The impulse turns the Schottky diodes on which causes them to sample the microwave signal. When the reference signal's frequency and the microwave signal's frequency are harmonically related, the same voltage point on the microwave signal is sampled and produces a steady DC output or offset voltage. When the two signals are not related, a different point on the microwave signal is sampled each time which produces a sinewave output. The frequency of the sinewave is equal to the difference between the microwave signal's frequency and the harmonic of the reference signal that is closest to the microwave signal's frequency.

Circuit Design:  Lowpass - Highpass and convert to 0Hz:

The following information refers to Figure 2. The transformer (T1) matches the reference source to the SRD and converts the signal from single ended to balanced. The SRD requires +17 to +27dBm to work properly. The impedance of the SRD is about 50ohms at +17dBm and decreases as the power is increased. R1 and R2 dampen reflections. L1 and L2 are DC returns for the IF signal and should have high reactances (> 150ohms) at the lowest microwave frequency the circuit will handle. The lowpass and highpass filters separate the microwave and IF signals. The cut off frequency of the lowpass filter should be set so the reference signal and it's harmonics are rejected. Example: if the reference frequency is 100MHz, then the IF filter should have a cut off frequency below 100MHz.

Circuit Design:  Quarterwave section and convert to 0Hz:

The following information refers to Figure 3. Instead of using a highpass filter to apply the microwave signal to the Schottky diodes, a power splitter and quarter wavelength section can be used. The idea is to apply signals that are 90 degress out of phase to opposite ends of the Schottk.

Circuit Design:  Balanced and convert to 0Hz:

The following information refers to Figure 4. This version has a balanced output. The capacitor (C1) isolates the microwave source from the IF output (highpass filter). The resistor (R3) and capacitor (C2) form a lowpass filter for rejecting the microwave and reference signals. The resistors (R4, R5, R6 and R7) provide the reference point for the IF output. R6 and R7 can be replaced by a potentiometer so that the DC offset can be nulled. The values of R4 through R7 should be 1Kohm or greater.

Convert to an IF frequency other than 0 Hz:

The SPD can also be used as a sampling mixer or sub harmonic mixer . The only change in the design is to change the IF filter so it passes the desired frequency. The conversion loss is very high ( typically 40dB) due to the large difference between the reference frequency and the microwave frequency

General Notes:

Use microwave layout rules for the physical layout around the Schottky diodes. When checking the diodes with a multimeter, be sure the maximum voltage is limited to 2V. Observe antistatic precaution.