3-89. Low Frequency Limit. To utilize the full dynamic
range of the instrument at low frequencies, the lowest frequency to be resolved must be at least 5 times the selected BANDWIDTH. This low frequency limit is due to the zero response described in the following paragraphs. 3-90. As the 3581 frequency is tuned toward OHz, the VTO frequency approaches the 100kHz IF. Although the VTO signal is suppressed by the use of a double balanced mixer, part of the VTO signal feeds through the 100 kHz IF Filter and appears on the meter. The response produced by the VTO signal peaks at 0 Hz and is appropriately called the "zero response". As with any other CW signal, the zero response is an amplitude vs. frequency plot of the IF Filter (Figure 3-20). The wider the bandwidth, the wider the zero response.

3-91. The amplitude and bandwidth of the zero response determines the lowest frequency that can be resolved. For any BANDWIDTH setting, the peak amplitude of the zero response is more than 30 dB below the full scale reference set by the INPUT SENSITIVITY and amplitude VERNIER controls (AMPLITUDE REF LEVEL switch in NORMAL position). With the zero response more than 30 dB below full scale and a dynamic display range of 80 dB, the maximum difference between the peak of the zero response and any measureable input signal is between 40 dB and 50 dB. Table 3-4 indicates that the maximum resolution between two signals whose relative amplitude is between 40 dB and 50 dB is 5 times the BANDWIDTH setting.

3-92. Response Time. Generally, when making swept fre-
quency measurements, it is desirable to have good resolu- tion and, at the same time, sweep as rapidly as possible. This involves a definite trade off since the narrower bandwidths provide the greatest resolution but require slower sweep rates. As the bandwidth is narrowed, the IF Filter takes longer to respond to electrical changes taking place at its input. Consequently, the sweep rate must be slow so that the signal remains in the passband long enough for the filter to fully respond. Optimum sweep rate is discussed in Paragraph 3-122. 3-93. Noise Rejection. The maximum sensitivity of the analyzer is limited by its own internally generated noise. Asoutlined in Paragraph 3-42, internal noise is a function of bandwidth, input resistance and tuned frequency. The narrower bandwidths provide the greatest noise rejection. 3-94. Frequency Setting..
3-95. The front panel FREQUENCY control tunes the frequency of the analyzer over the 0 Hz to 50 kHz range. The control can be used to manually tune the analyzer to a specific signal component or to set the start frequency of a sweep. The tuned frequency is indicated on the digital frequency display.
3-96. The FREQUENCY control has two selectable drive ratios to permit coarse or fine tuning. Coarse tuning is selected by pushing the knob toward the front panel; fine tuning is selected by pulling the knob outward. In the coarse position, one revolution of the knob changes the frequency by approximately 2.7 kHz. In the fine position, one revolution changes the frequency by approximately 73 Hz.3-97. Frequency Display. The 3581 has a built-in fre quency counter which provides a digital reading of the tuned frequency. The frequency display ranges from 0 Hz to approximately 51,000 Hz when the internal L.O. is used or from 0 Hz to 60,000 Hz when an external L.O. is used (Paragraph 3-148). Display resolution is 1 Hz. When the analyzer is tuned below 0 Hz, the frequency digits are blanked and the decimal points light to indicate an out of range condition.
3-98. The specified accuracy of the frequency display is ± 3 Hz. This means that when the analyzer is tuned to a given signal using AFC or by manually tuning for a peak amplitude reading, the frequency reading will be within ± 3 Hz of the signal frequency.
3-99. Automatic Frequency Control.
3-100. The purposes of the Automatic Frequency Control (AFC) are:

a. To simplify manual tuning by automatically fine tuning the analyzer to the signal to be measured.
b. To lock the analyzer's tuning to the signal compo nent so that measurements are not affected by frequency drift and phase noise in the signal source (or 3581).

3-101. Pull-In Range. The "pull-in range" (sometimes called "capture range") is the frequency range over which the AFC can acquire lock. In order for the AFC to pull-in and lock to a signal, the 3581 must first be manually tuned to within the pull-in range above or below that signal. For example, if the pull-in range is ± 100 Hz and the signal to be measured is 1 kHz, the 3581 must be manually tuned between 900 Hz and 1100 Hz (1 kHz ±100 Hz). The

pull-in range for the 3581 is determined by the BAND WIDTH setting, the SCALE setting and the signal ampli tude. Typical pull-in ranges are listed in Table 3-5.

3-102. Hold-In Range. The "hold-in range" is the fre- quency range over which the AFC can maintain lock. The specified hold-in range for the 3581 is ± 800 Hz. This means that once the AFC is locked to a signal, the frequency of that signal can drift up to ± 800 Hz and the AFC will remain locked. Note, however, that the drift rate of the signal must be slow enough for the AFC to track properly. The maximum rate at which the AFC can track a signal is determined by the BANDWIDTH setting. As the bandwidth is narrowed, the AFC loop becomes slower and the maximum tracking rate decreases. Table 3-6 lists the approximate maximum frequency drift rate for each BANDWIDTH setting.

3-103. Lock Frequency. The AFC Lock Frequency speci-
fication indicates that when the AFC is locked the input signal is less than ± 1 Hz away from the center of the passband. Due to component aging and environmental factors, the lock frequency may drift out of tolerance. This can be corrected by performing the Reference Oscillator Frequency Adjustment outlined in Section V. 3-104. Using the AFC. To use the AFC simply tune the analyzer to within the pull-in range of the signal to be measured and press the AFC button. The AFC UNLOCK annunciator will light, the frequency reading will change and the meter reading will increase as the analyzer is automatically tuned toward the signal frequency. When the analyzer is properly tuned or "locked" to the signal, the AFC UNLOCK annunicator will go out. Anytime the AFC is locked to a signal, the frequency reading will be within ± 3 Hz of the signal frequency.

3-105. The AFC circuit is designed to allow the analyzer to be manually tuned while the AFC is turned on. However, when the analyzer is not tuned near a signal component, the AFC circuit may respond to noise signals. This makes the frequency unstable and causes the last digit of the frequency display to rack. Also, when the Volts scale is selected, the frequency tends to drift slowly in one direction. For these reasons, it is generally more convenient to leave the AFC off while manually tuning the analyzer. The AFC should always be off when the analyzer is sweeping.
3-106. Frequency Span Setting.
3-107. For electronic and manual sweeps, the FRE- QUENCY control is used to set the start frequency and the FREQ SPAN control is used to set the spectrum width or "end frequency". Excluding the 0 Hz position, there are ten frequency span settings ranging from 50 Hz to 50 kHz. 

3-108. 0 Hz Span. With the FREQ SPAN switch set to the 0 Hz position, the instrument remains at the frequency indicated on the frequency display. The sweep generator, however, remains operative and an X—Y recorder or storage scope connected to the Recorder outputs can be swept at the rate selected by the SWEEP TIME control. The result is a graphical display of amplitude vs. time.
3-109. The amplitude vs. time feature allows the 3581 to be used as an AM detector for observing the amplitude variations of a signal that occur over relatively long periods of time. For example, the amplitude of the 10 kHz sine wave shown in Figure 3-21A appears stable on a conven tional oscilloscope but is actually varying at a very slow rate. Figure 3-21B shows an amplitude vs. time display of that signal for a 2,000 second period. The amplitude vs. time display shows that the 10 kHz signal is amplitude modulated by a triangular shaped signal whose frequency is 0.00166 Hz.

3-110.  Because of its narrow bandwidth, the 3581 cannot respond to rapid changes in amplitude.  When it is used as an