Theory of Operation
2715 Spectrum Analyzer Service Manual 3-77
There are ten videogating control registers on the Digital Option module. All of
these control registers arewrite only registers, with the exception of misc. gating
control register (address0x210). The other nine registers contain line count and
pixel count information used to controlthe IF gate and Display Storage
acquisition.
This system performs gating of theIF signal path at a point immediately before
the VR filter. It is composed of two components, the IF Gate itself and a control
signal conditioner.
This section performs the actualgating of 10 MHz IF signals. The desired action
is not to produce abrupt ON/OFF transitions like a switch, however, but rather to
perform controlled, rapid“fades”between ON and OFF. The intent is to
minimize the frequencydomain sidelobes arising from the gating of any signals
present in the wide passband preceding t he resolution filters. In the CATVtest
environment the dominant signal presentwould most commonly be the visual
carrier of the channelunder test, and we want to prevent the gating sidebands
from spoiling our ability to see weakdistortion components near the visual
carrier frequency. The gate’s characteristicscombined with those of the control
signal conditioner (see Control Signal Conditioner,next) define the transition
times and shapes.
In operation, 10 MHz IF signals enteringJ2 are converted to differential currents
at the collectors of Q3 and Q4. Note that the asymmetric arrangementof the
network in the emitters of Q3 andQ4 allows use of resistors instead of current
sources with little degradationof balance, taking advantage of the fact that the
base of Q4 is not driven with signal. The overalldifferential topology of the IF
gate is chosen to minimize evenorder distortion.
If IF_GATE_ENis held HIGH, Q7A and Q7D steer virtually all the differential
current from Q3 and Q4 throughto the emitters of Q2 and Q13. Q2 and Q13
serve as common base stages to keep the largevoltage swings at the input of the
transformer T1 offthe collectors of Q7A and Q7D, which have large, nonlinear,
capacitances. This is to reducedistortion (particularly odd order terms) and to
improve the output circuit bandwidth.
If IF_GATE_ENis held LOW, the differential signal currentis steered into Q1
and Q14 via Q7B and Q7C. The outputs of Q1 and Q14 are dampedinto
resistors R1 and R2.
During transitions, which are governedby the linear ramp waveform applied to
the bases of Q7B and Q7C, the differentialsignal current steered to the output is
modulated according to the transfer function o f a bipolar transistor differential
amplifier having no emitter degeneration.The resulting soft envelope rise and
fall shapes provide a rapidfall off of sidebands with frequency offset from the
visual carrier.
Video Gating ControlRegisterIF Gate SystemIF Gate Proper