Electro-Metrics EM-2115, Instruction Manual

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EM-2115 APPENDIX C FUNCTION AND DESCRIPTION
(EM2115C-10)
A BFO (Beat Frequency Oscillator) circuit is part of each discriminator circuit. is, When
a logic command from the Microprocessor Board (A1A3) activates this circuit, a crystal oscilla-
tor circuit is turned on. This produces a low frequency signal that is mixed with the limited IF
signal before it is applied to the discriminator. At the front panel Audio Output Connector
(A1J2), a tone will be heard in the earphones. As the receiver is tuned above and below an input
signal, the tone will increase in frequency and will disappear when the signal is tuned in.
C-10 A10 Tune Control Board
(Refer to Figure H.18)
The Tune Control Board sets the Manual, Continuous, or Single tuning modes for the re-
ceiver. Logic controlled multiplexers determine circuitry functions.
In the Manual Mode, a -8 to +8 V voltage from the front panel Main Tune Control is ap-
plied to the Manual Tune circuit. The output from this circuit is applied to an output amplifi-
er/buffer circuit whose output, 0 to 8 V, is applied to the Preamplifier/Tuning Regulator Board
(A2A16A2). In addition, the circuit produces a 0 to 1.5 V output for use by an X-Y plotter and
external oscilloscope.
In the Single Mode, a 0 to 8 V sweep voltage from the Sweep Board (A11) is applied to
the output amplifier/buffer circuit.
In the Remote (computer-controlled) Mode of receiver operation, an external -8 to +8 V
from the Microprocessor Board is applied to the Manual Tune circuit to control the frequency
tuning of the receiver.
C-11 A11 Sweep Board
(Refer to Figure H.19)
The Sweep Board circuitry controls the various sweep functions of the receiver.
A sweep generator circuit produces the 0-8 V linear ramp voltage required for sweeping
across the frequency range of the receiver. Multiplexers select the mode of circuitry operation
(Manual, Continuous, or Single) and vary the sweep rate of the circuit depending on the IF
bandwidth selected. The 0-8 V sweep voltage is applied to the Tune Control Board A10.
Sweep comparators determine the following:
a. Single Mode:
1) Sweep start--pen drop.