Theory of Operation
2715 Spectrum Analyzer Service Manual 3-5
The Display Storage board is capableof storing four waveforms of 512 bytes
each, four pages of text of 512 bytes each, and up t o two markers. Any combina-
tion of waveforms and text may be display ed on the CRT.If any waveform s are
being displayed, the text portion of the displayis limited to approximately 100
characters in order to avoid flicker.
Only one page of text may be displayed at any one time. The readout page is
limited to approximately 100 characters.
The waveforms and text displ ays are accessed through memory reads and writes.
When (DISPLAY) A, B, C, or D is selected, the stored waveforms in those
registers are displayed. When A, B, C, and D are deselected, th edi splay reverts
to the real time (nondigitized)video waveform, and the readout is updated during
the readout cycle.
Center Frequency Control
The Center Frequency Control system provides1st Local Oscillator (1st LO)
signal to the 1st Converter at the centerfrequency and span selected by the user.
In general, this is accomplished by a com bination of setting, counting, and
resetting (as required)of the frequencies of various oscillators.
The system is expected to control the frequency within a few Hz out of a total
operating range of approximately2 GHz.
The principal elements of the frequencycontrol system are as follows:
This is a YIG tuned oscillator that coversthe range of 2.11 to 3.91 GHz or 2.11
to 4.26 GHz for Option 50 and Option 75 (YIG = Yttrium IronGarnet.) Ideally,
its operating frequencyis exactly proportional to the strength of an internal DC
magnetic field. This field is the combinedfield of two coils, one large (Main)
and one small (FM). In practice,the frequency is uncertain by up to a few MHz
due to problems such as temperaturedrift, slight nonlinearity, and magnetic
hysteresis. Also, the oscillator has some residual FM and phase noise sidebands
due to internal causes and external noise and d rift from the driver circuit. The
remainder of the frequencycontrol system maintains the oscillator (abbreviated
YIG) at the right frequency.Usually, it is sufficient to set the YIG, then
periodically check its frequencyand make small adjustments as required.
However, in very narrowspans the YIG must be phase locked to a stable source
in order to maintain sufficientstability.
The 1st LO Interface provides drive signals forthe main coil and F M coil, a
control signal for the main coil, and power for the gain stage.

1st LO