Agilent Technologies 8564EC Service Manual page 274

General Troubleshooting
Block Diagram Description
Fine frequency control originates from a preselector peak DAC located
on the A3 interface assembly. Values for the preselector peak DAC are
interpolated approximately every 17 MHz based upon data taken
during the frequency response (flatness) adjustment. The preselector
bandwidth varies from greater than 30 MHz, at 2.75 GHz, to greater
than 60 MHz, at 26.5 GHz.
The mixer is ac coupled. It uses the first, second, and fourth harmonics,
mixed with the 1st LOcal oscillator, to cover the frequency range. A PIN
diode switch in A7 LOMA directs the 1st LO to the appropriate mixer.
The A14 frequency control assembly provides PIN diode bias.
A11 YTO
A11 is a YTO (YIG-tuned oscillator). YIG (yttrium-iron-garnet) is a
ferro-magnetic material which is polished into a small sphere and
precisely oriented in a magnetic field. Changes in this magnetic field
alter the frequency generated by the YTO. Current control of the
magnetic field surrounding the YIG sphere tunes the oscillator to the
desired frequency.
A12 Switched Barium-Tuned Mixer (SBTX)
The switched barium-tuned mixer (SBTX) is a combination of an RF
switch, tracking preselector, millimeter (26.5 GHz to 50 GHz) mixer,
and low-noise amplifier. The PIN diode switch directs the RF input to
the appropriate mixer in the A12 SBTX or to A10 RYTHM, where it
may be again switched to the A8 low band mixer.
The tracking preselector is a barium-tuned filter. It functions as a
tunable bandpass filter for millimeter signals. Coarse frequency control
originates from slope and offset DACs located on the A14 frequency
control assembly. (Slope and offset DAC values are loaded into
EEROM.)
Fine frequency control originates from a preselector peak DAC located
on the A3 interface assembly. Values for the preselector peak DAC are
interpolated approximately every 17 MHz based upon data taken
during the frequency response adjustment ('front-end cal'). The
preselector 3 dB bandwidth varies from approximately 200 MHz at
26.5 GHz to approximately 300 MHz at 50 GHz.
The millimeter mixer IF (3.9107 GHz) is fed through a low-noise
amplifier and one input of a PIN diode switch before being sent to the
A13 second converter. The other switch input accepts the IF output of
the A8 low band mixer.
Chapter 7 355