Model 8640B
TM 9-4935-601-14-7&P
1 Hz bandwidth for various offsets from a (256 and 512
MHz) carrier. The low close-in noise characteristic is
ideally suited for the stringent adjacent channel tests
that are commonly made on a wide variety of
communication receivers.
1-23. Figure 1-3 gives a plot of the guaranteed SSB
noise performance for a 20 kHz offset from the carrier
for the 256 512 MHz band. From 230 to 450 MHz, noise
is >130 dB/Hz below the carrier level and rises to 122
dB/Hz at 550 MHz. This signal-to-noise ratio decreases
by approximately 6 dB for each division of the output
frequency down to the broadband noise floor of better
than 140 dB/Hz. This exceptional noise performance is
also preserved during FM* and in the phase lock mode.
Figure 1-3. Specified Signal-to-Phase Noise Ratio at 20
klHz Offset us Carrier Frequency (MHz).
(Stated in a 1 Hz Bandwidth.) For lower frequencies
phase-noise decreases approximately 6 dB per
frequency division downl to the broadband noise floor.
1-24. Frequency Counter 1-25. The Signal Generator
has a built-in 550 MHz frequency counter and phase
lock synchronizer. The 6-digit LED display gives a
normal resolution of 10 kHz at 500 MHz and 10 Hz at
500 kllz. The resolution can be increased using the X10
or X100 EXPAND buttons near the display. In the X100
EXPAND mode, the resolution is 100 Hz at 500 MHz
and 0.1 Hz at 500 kHz.
1-26. This resolution, combined with the high stability of
the generator, provides precision frequency selection
and meaningful measurements on high performance
receiver systems. When selecting the external doubler
band, the counter displays the doubled output frequency
directly.
1-27. When using the expand modes, it is possible for
significant digits or the decimal point to be shifted off the
display. When this occurs, an OVERFLOW light
reminds the operator that the display is not showing the
complete output frequency.
1-28. The built-in counter can also be used to count
external input signals from 1 Hz to 550 MHz and
eliminates the need for a separate frequency counter in
many measurement systems. Input sensitivity is <100
mV into 502. Using the EXPAND buttons, it is possible
to achieve a resolution of 1 Hz in the 0-10 vMHz
EXTERNAL count mode or 100 Hz in the 0-550 MHz
mode.
1-29. Phase-Lock Mode
1-30. Also included in the Signal Generator is a built-in
phase lock synchronizer that locks the RF output
frequency to the crystal time base used in the counter.
In this locked mode, output stability is better than 5 x 10-
8 /h and the spectral purity and FM capability (down to
50 Hz rates) of the unlocked mode are preserved. For
higher stability, it is possible to lock to an externally
applied 5 MHz standard. Two Model 8640B's can also
be locked together for various two-tone
measurements.
1-31. Phase locking the generator is simple just push
the front panel LOCK button. The generator is then
locked to the frequency shown on the LED display. If
lock is broken (for exampled by tuning to a new output
frequency or during warmup), there is an immediate
indication: the LED display flashes. The generator can
be relocked by releasing the LOCK button and then
relocking:
1-32. Lock can be achieve in the normal mode of the
counter or in the X10 EXPAND mode if the OVERFLOW
light is not on. It is not possible to lock in the X100
EXPAND mode or when counting external inputs.
Maximum resolution in the locked mode is I kHz at 500
MHz, increasing to 1 Hz at 500 kllz.
1-3
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