Programmable Step Attenuator Control; Rf Modualtor Board - Aeroflex NAV 2000R Operation Manual

4.5.14 PROGRAMMABLE STEP ATTENUATOR CONTROL

The Programmable Step Attenuator Control consists of a latchable relay driver U10 and decoder gating
for the strobe input. Each high bit of the latched attenuation word activates one discrete attenuator
section. These discrete attenuators consist of 1 dB, 2 dB, 4 dB, 8 dB, 16 dB, 32 dB, and 64 dB pads. The
64 dB attenuation step is actually two 32 dB attenuator sections that are selected together. Otherwise,
separate drive lines control each section.
To achieve a given level of attenuation between 0 dB and 127 dB, a combination of the above attenuation
values is used. The series resistors R33 through R40 provide the necessary voltage drop from +15 V to
+12 V as required by the individual attenuator relay coils. CR1 through CR8 provide a discharge path for
the negative current spike when the relay coils of the individual attenuators are de-energized.
To provide 1/10th dBm control, the voltage reference circuit to the modulator is varied using the digital to
analog (DAC) device U8 to slew the reference voltage.
4.5.15 RF MODULATOR BOARD
The RF Modulator Board amplitude modulates (AM) a CW RF carrier from an external RF source and
provides a leveled RF output. The audio modulation signal that modulates the applied RF carrier is
generated by the DSP or an external source module. The audio modulation signal is conditioned by the
audio board to achieve RF modulation. The leveling loop ensures that the output of the RF Modulator
Board is always at a fixed RF level. Temperature compensation circuitry maintains stable performance
over the operating temperature range. The schematic drawing number for the RF Modulator Board is
02-5951-00.
4.5.16 MODULATING SIGNAL TEMPERATURE COMPENSATION NETWORK
The op amp U5A is included in the temperature compensation network for the modulation signal. It is
necessary due to the temperature gain dependence of the VGA (Variable Gain Amplifier) gain control
input. The slope of the gain control voltage vs. gain changes with increasing temperature: with higher
temperature the slope increases.
Suppose 50% modulation using a single tone is achieved at room temperature. After operating the NAV
2000R for several hours in a cabinet, the temperature increases by 25 C.
compensation, the modulation percentage would become higher than 50%. To compensate for this
gain/temperature dependence, a thermistor controls the gain of a non-inverting op amp that is in the
signal path of the modulating audio to the VGA gain control input. U5A uses RT1, R28, and R29 to form
gain given by
G(T) = [1 + [ R29/[RT1(T)+R28] ]
where RT1(T) is a function of temperature. If the VGA gain is a function of temperature and input control
voltage and is written,
Gvga(T,Vagc)
then, it follows that the resultant gain be temperature independent if adequate compensation is achieved,
or
Gcomp(Vagc) = Gvga(T,Vagc)*G(T) .
NAV 2000R SIGNAL GENERATOR - REV. 0 – JULY 26, 2007 - PG 4-20
Aeroflex Operation Manual
Without temperature