Table of Contents
Advertisement
Block Diagram
YIG-Tuned-Filter (YTF) and GaAs FET Amplifier Driver Assembly
diode to be used to prevent the MOS FET gate from going negative, without
overloading the op-amp output.
The phase shift produced by the pole formed by R51 and the gate of MOS
FET (Q7) is compensated for by adding a zero in the integrator's response at
15 kHz. This zero also sets the loop gain to –20 dB to prevent loop peaking
due to the 333 kHz pole.
The fine tune DAC output is amplified and level shifted to produce an output
range of –10 V to +10 V. The output is then summed into the source of MOS
FET (Q7) through a 2.15 k resistor (R16). The fine tune DAC will add or
subtract up to 4.65 mA into the voltage-to-current converter output.
The voltage-to-current converter output is further filtered at its output by the
two 100 μF capacitors (C35 & C36) placed in parallel with the YTF tuning
coil. The combination of the 200 μF in parallel with the 1 H of inductance in
the tuning coil produces a pole at 15 Hz, which greatly reduces noise and
spurs. A rectifier diode (CR7) is also placed in parallel with the capacitors to
eliminate the possibility of reverse biasing.
The balun (T1) placed between the capacitors C35 & C36, and the YTF
tuning coil greatly reduces spurious pickup from the 40 kHz switching
power supply. T1 is a bifiler wound inductor which acts as a balanced
isolator with 8 mH of inductance. It allows the YTF tuning coil to float with
respect to the YTF driver assembly, reducing common mode induced spurs,
while remaining a low impedance with respect to C35 & C36. A 10 μF
capacitor is placed directly across the YTF coil to eliminate any induced 40
kHz currents.
The +50 V supply is inter-locked through connectors J4 & J5. This was
added as a protection feature for the voltage-to- current converter. If the
sense resistor (J5), or the YTF tuning coil (J4) are disconnected, the +50 V
supply is also disconnected to prevent damage to the MOS FET (Q7).
Unfortunately, the protection diode VR1 may not survive removal of the
sense resistor connector J5 while power is on.
YTF Heater Current Limiter
The YTF filter employs a thermistor-type ceramic heater to maintain
constant YIG sphere temperature over the operating temperature range and
over the range of power being dissipated by the tuning coil. This minimizes
thermal frequency drift.
The heater material resistance is designed to be low at low temperature, and
high resistance when hot. The result is a simple heater/thermostat. The
operating voltage is 28 V nominal, with a 32 V maximum. The operating
current varies between 10 mA and 175 mA. Current must be limited to 2 A
at turn-on to prevent damage.
13-70 HP 70427A/HP 70428A User's Guide
Advertisement
Table of Contents
