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Model 8902A
Operation
RF Frequency Tuning
FUNCTIONS
TRACK MODE, AUTO TUNING, DISPLAY FREQ INCR, kHz
$
and kHz
6
keys
DESCR I PTlON
The Measuring Receiver is considered tuned to an input signal when the frequency of the Local
Oscillator (LO) produces a n I F signal centered in the I F passband. Normally the LO frequency is
tuned either 1.5 MHz or 455 kHz above the input signal, depending o n which I F is selected. When
the inpiit signal is less than 2.5 MHz, the LO is tuned to 101.5 MHz and the input passes directly
into the IF.
T h e Measuring Receiver can be automatically o r manually tuned to the input signal:
In automatic tuning. the instrument searches for the presence of a signal over the entire input
spectrum. Once found, the LO is tuned
so
that the signal is centered properly in the IF.
In manual tuning, the desired input frequency is entered via the numeric keyboard o r kHz
ft
and kHz
4
keys, and the LO is tuned so that a signal of that frequency would be centered
properlv
la
the IF.
For both of these tuning modes, the LO can be configured t o lock itself to an internal crystal oscillator
(for low-noise measurements) o r to track the input signal as it drifts (keeping the down-converted
signal centered in the IF).
The four tuning modes are listed below:
0
Automatic Tuning; Low-noise LO
Automatic Tuning; Track Mode,
Manual Tuning; Low-noise LO, and
Manual Taning; Track Mode.
Automatic Tuning-Low-Noise LO.
If not already tuned, the Measuring Receiver searches throughout its frequency range for an input
signal. If two input signals have similar power levels, the higher frequency signal is usually selected.
The R F input signal's second and third harmonic levels must be at least 10 d B below the level of
the fundamental, and all other signals at the input must be at least 30 dB below the level of the
dcsired signal. Once tuned, the LO is locked t o an internal, voltage-controlled, crystal oscillator for
highly-stable, low-noise measurements. Tuning is maintained as long as a detectable signal is present.
If the input signal disappears (drops below -20 to -25 dBm). the Measuring Receiver initiates a
signal search. This tuning mode is selected whenever Automatic Operation is pressed and when the
instrument powers-up.
Ailtomatic Tuning-Track Mode.
The instrument searches for the inpiit signal i n the same way as described in the previous paragraph:
however, it does not lock to the internal reference oscillator. Instead, the LO is locked to the input
signal itself and tracks it as it drifts. This fbnction is useful for checking modulation or level as a
function of carrier frequency. If the input signal disappears (for example, when the LO switches
bands) the Measuring Receiver searches for, and re-acquires the input signal. Track Mode tuning is
not allowed with the 455 kHz I F o r with input signals below 10 MHz. Track Mode tuning somewhat
attenuates low-rate FM on the input signal: thus, F M measurements should only be made in track
3-141
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