Main Vco; 400 Mhz Vco - Aeroflex NAV 2000R Operation Manual

The resistor network composed of R104, R105, R106, R107, R108, and R91 combine the selected RF
path with other RF paths. It is important to note that only one RF path is selected during any one time.
The RF of interest, at a level of approximately 0 dBm, is passed through a 500 MHz LPF to the leveling
circuit.
The leveling loop is composed of voltage controlled attenuator AT1, amplifier U20, detector circuits and
an error amplifier. Resistor R115, which is tapped off the output of amplifier U20, feeds the detector
circuit. Transistors Q22 and Q23 form a fixed current source which biases the hot carrier diode CR11.
CR10, the same type diode as CR11, has equal current flowing through it. In this configuration, the
voltage variations due to temperature are canceled by the matching diodes with equal currents. The
output of this detector circuit, found at the collector of Q22, is buffered by Op Amp U18B and applied to
error amp U18A. U18A is configured as an integrating amp. The output of the error amp is feedback to
the attenuator, closing the loop. Pot R137 adjusts the output level and is set for an output level of -20
dBm.
Comparators in U17 monitor the control voltage. U17 pin 7 goes low if the control voltage goes below its
lower limit. U17 pin 1 goes low if the control voltage exceeds its higher limit.

4.7.3 MAIN VCO

The MAIN VCO board covers a frequency range of 225 to 456.25 MHz. The MAIN VCO consists of four
separate selectable VCOs and a common output buffer amplifier. Each VCO covers a portion of this
tuning range. The tuning range of each VCO is as follows; VCO1 - 380 to 456.25 MHz, VCO2 - 320 to
380 MHz, VCO3 - 275 to 320 MHz, VCO4 - 225 to 275 MHz. Each VCO is of similar design. The active
element is a RF JFET transistor. This transistor is configured as a grounded gate amplifier with a
resonant structure on its drain and the output signal coupled from its source. The resonant structure is a
microline inductor paralleled with a capacitor and varactor diode. A capacitor produces feedback from
the source to the drain. The power supply is isolated by an appropriate inductor and applied to the drain.
The tuning line is isolated by a appropriate inductor and common to all four VCOs. The tuning voltage
ranges from 4 and 20 VDC with the highest voltage producing the highest frequency. Each VCO has a
bipolar switch to apply or remove bias from the JFET transistor's source. The capacitor tapped output is
combined by means of 39 ohm resistors to the input of the buffer amplifier. The buffer amplifier is a class
A transistor amplifier with the collector biased by a wide band transformer and base biased by two
resistors. The output is taken from a center tap of this transformer. The output is split by a resistor
network which pads the output to the divider buffers from the main output.

4.7.4 400 MHz VCO

The 400 MHz VCO board as a single VCO similar in design to the VCOs of the MAIN VCO board. The
output as likewise buffered by a transistor amplifier of similar design. As was explained previously, this
VCO runs only during low band operation (150 kHz - 56.25 MHz). It is always operated at 400 MHz. Its
tuning voltage should be approximately 5 VDC.
NAV 2000R SIGNAL GENERATOR - REV. 0 – JULY 26, 2007 - PG 4-27
Aeroflex Operation Manual