Tektronix 2715 Control System Interface Bandwidth Control Identification Bit Gain Stage Routing Switches, Diagnostics

Theory of Operation

2715 Spectrum Analyzer Service Manual 3-37

The board provides the resolutionbandwidths used to fill in between filter values

installed within the VariableResolution module. The bandwidth range is 1 MHz

to 1 kHz in decade steps, and 300 Hz. Each filter path consists of a switchable

amplifier and bandpass filter combination.

The control of the RF Options by the microprocessor is through one 8 bit shift

The variable bandwidth section consists of a switching tree to selectone of five

filters. Availablebandwidth filters, in addition to those already in place in the

VariableResolution module, are 1 MHz, 100 kHz, 10 kHz, 1 kHz, and 300 Hz.

A resistor is installed between either pin 12 (d6) or pin 13 (d5) of shift register

U390 and the base of Q396 to completea path for an identification bit for the

Microprocessor.This allows the Microprocessor to determine that the 300 Hz

filter is installed on the RF Options board.

The gain stage used is a transformer feedbackstage that provides approximately

11.5 dB of gain. In the caseof the 100 kHz and 300 Hz filters, two gain stages

are used. The impedance match at one port of the Gain Stage is highly sensitive

to a proper termination beingpresented to the other port.

The routing switches are usedto route RF signals to the selected bandwidth

filter.They are composed of a transistor in heavy saturation that has relatively

large chargestorage characteristics. The topology used is a shunt series

configuration. These switches areused on both the input and output of each

filter.That is, the filters are switched in and out at both the input and the output

so that when a filter is not being usedit is effectively out of the circuit.

Each bandwidth filter is composedof a double tuned circuit (a two pole

Butterworth filter section), anamplifier to provide gain correction and isolation,

and another double tuned circuit. The intermodulation performanceof the

amplifier is less critical becaus e the amplifier is placed after the filter. The noise

power generated by thestage is reduced by placing a filter of equal bandwidth

after the gain stage.

The identification bit can provideuseful information on the proper configuration

of the instrument.

Control System InterfaceBandwidth ControlIdentification BitGain StageRouting SwitchesDiagnostics