BASIC COUNTER
Overall operation is controlled by the Microprocessor Assembly A105. This assembly contains an eight bit
microprocessor, its control logic, and the system memory. It communicates with all other assemblies in
the instrument by means of
a
triple bus system: the data, address, and control bus. On each assembly there
is
a
Peripheral Interface Adaptor (PIA) which provides the interface between the bus system and the instru-
ment hardware.
Frequency measurements are performed by comparing an unknown signal to
a
reference frequency, namely
the time base. A 10 MHz crystal oscillator is used
as
the internal reference and
is
a
part of the Gate Gener-
ator Assembly
A107. For increased accuracy and stability, ovenized oscillator options are available, or the
user may select an external 10 MHz reference.
A frequency measurement
is
made by generating a time interval (Gate Time) consisting of
a
number
of cycles of the reference. This Gate Time is then used
as
an interval during which the input signal is count-
ed by the Count Chain Assembly
A106.
Initially, the microprocessor selects one of several available inputs to the Count Chain Assembly and the
appropriate Gate Time based on user input information; band selection, resolution, etc. The microprocessor
then initiates the measurement cycle by resetting the Count Chain to zero and allowing
a
gate to
be
gener-
ated. During the gate interval, the Count Chain accumulates the number of cycles of the input signal. At
the end of the gate time, the microprocessor reads the stored information in the Count Chain and performs
any required
calculations necessary to convert the measurement into a direct reading of the unknown
frequency. The front panel display is then updated with the new measurement results. Figure
4-1
shows
a
simplified block diagram of the counter.
BAND
2
CONVERTER
An input signal is applied to the mixer along with an appropriate local oscillator (L.O.) to generate an
IF frequency in the range of 10 MHz to 190 MHz. This signal is filtered and amplified to
a
level suitable
for direct measurement by the Count Chain.
The L.O. frequency is generated by the Voltage Controlled Oscillator
(VCO) of the Band
3
Converter.
This frequency
is
phase locked to the counter's time base and controlled by the microprocessor. A VCO
multiplier serves to either pass along the signal directly or double it. It can also turn off the signal and
pass only a DC bias to the mixer.
Two detectors provide outputs proportional to the amplitudes of both the applied
RF signal and the
resulting IF signal. These outputs are compared in the Signal Comparator, which provides a digital output
when the IF amplitude exceeds the RF amplitude.
Scans by ArtekMedia © 2007
To Next Page
Loading...