Hewlett Packard HP

Summary:

The HP 3581A wave analyzer tunes the frequency range of 15Hz to 50kHz, either manually or in swept mode. The sensitivity of the unit is 0.1µV to 100V rms. In the logarithmic mode, the dynamic range is 80dB. The 3581 C is pretty much the the same save with functions specifically designed for the telecommunications industry. The 3581C uses a WECO 310 jack and has selectable inputs for Unbalanced, Balanced Bridged or Balanced Terminated Input. The tracking generator of the 3581C also utilizes a transformer coupled balanced output, and has an audio output.

The analyzer also incorporates a tracking generator, accessible at the rear of the instrument, which allows the user to perform network analysis. (The tracking generator is accurate within 1 Hz of the receiver frequency).

Bandwidth is selectable via front panel control in increments of 3,10,30,100 and 300Hz. The unit can be swept for time periods of 0.1 to 2000 seconds, swept single-shot or set to a single frequency. In the swept-mode, the 3581 will tell you whether the period is too short!

The HP3581A, with the help of an auxiliary oscilloscope or an X-Y plotter may also be used as a spectrum analyzer. X and Y analog outputs of 0 to 5 volts are provided at the rear of the instrument. Pen Lift and External Trigger are also available at the rear. The external trigger is inhibited at -12.0 to +1.4VDC, and is activated for +4.5 to 20.0 VDC. The sweep trigger must be activated for at least 1.0 µS. Note also that after the sweep, the 3581’s circuitry incorporates a delay of 0.2 to 2.0 seconds in order for the IF to settle.

Local Oscillator

The user can also sweep the 3581 through a range of 1.0 to 1.5 MHz through the Local Oscillator input at the rear of the unit. When the external local oscillator is chosen, the frequency of measurement is given as follows:

F_t = (F_c/10) -100 kHz

The input to the local oscillator must be in the range of 0.1 to 1.0 VRMS. The impedance of the oscillator input is 220 ohms. The output of the local oscillator can also be fed into a frequency counter, utilizing the above equation, if the operator desires more accurate representation of frequency versus output.

The major controls of the 3581A/C are illustrated in the snippets from the Service Manual appearing below. For a larger image double-click on each of the images (which may need to be enlarged in your browser.)

3-6. CONTROLS, CONNECTORS AND INDICATORS.

3-7. Figures 3-1 and 3-2 illustrate and describe the function of all front and rear panel controls, connectors and indicators. Items requiring additional description are referenced to paragraphs in the General Operating Information section

3-8. GENERAL OPERATING INFORMATION.

3-9. Input Connections (3581A only).

3581C: Refer to Paragraph 3-156.

3-10. The 3581A has two INPUT terminals. The upper (red rimmed) terminal is the signal input and the lower (black rimmed) terminal is case ground. The plastic caps on the terminals unscrew to permit wire connections and the terminals are spaced so that they will accept a dual banana-plug mating connector. The input signal can be applied to the 3581A through a twisted pair, a shielded cable equipped with banana-plug connectors (-hp- 11000A Cable Assy.) or a 10:1 Voltage Divider Probe (-hp10004B). Input leads should be kept as short as possible to minimize extraneous pickup. If a 10:1 Voltage Divider Probe is to be used, connect it to the INPUT using a BNC to banana-plug adapter (-hp- Part No. 1251-2277). Before using the probe, perform the Input Probe Compensation procedure outlined in Paragraph 3-176.

3-11. Input Impedance.

3-12. The 3581A has a single-ended input which provides an input impedance of 1 megohm shunted by < 30 pF (28 pF nominal). The 3581C has three selectable input configurations: Unbalanced, Balanced Bridged and Balanced Terminated. The Unbalanced configuration provides an input impedance of 1 megohm shunted by 40 pF

(nominal). The Balanced Bridged input impedance is approximately 15 kilohms and the Balanced Terminated input impedance is 600 ohms or 900 ohms. The terminated input impedance is selected by the front panel Calibration switch (Item ae , Figure 3-1 ). Refer to Paragraph 3-158 for further information concerning the 3581C input configurations.

3-13. Figure 3-3 shows the equivalent circuit, for the3581A single-ended input. The resistor, R_in represents the1 megohm input resistance and the capacitor, Cs, represents the 28 pF shunt capacitance. Figure 3-4 is a graph showing the input impedance, Zt, as a function of frequency. At low frequencies the reactance of Cs is very high making Zt nearly equal to Rin. As frequency increases, the decreasing reactance of Cs becomes more and more significant, causing Zt to decrease. At 50 kHz, Zt is approximately 100 kilohms.

3.14 Input Constraints

Caution

The information given in Paragraphs 3-15 and 3-16 applies to the 3581C only when it is operated with the UNBALANCED or BALANCED BRIDGED input configuration. Refer to Paragraph 3-161 for special information concerning the Balanced - Terminated input configuration.

3-15. The maximum ac voltage that can be safely applied to the 3581A INPUT is determined by the INPUT SENSITIVITY switch setting (Paragraph 3-38). The 3581 input circuits are well protected and can withstand momentary (< 5 second) overloads up to 100 V rms on all input ranges. The instrument can withstand continuous overloads up to 100 V rms on the + 30 dB through - 10 dB ranges and overloads up to 50 V rms on the - 20 dB through - 70 dB ranges. Overloads greater than this may damage the instrument.

Caution

Input levels exceeding 100 V RMS on the +30 dB through - 10dB ranges, 50 V RMS on the - 20dB through -70 db Range or ± 100 V dc may damage the instrument.

3-16. DC Isolation. The 3581A input and the 3581C unbalanced and bridged inputs are capacitatively coupled to provide dc isolation. The maximum dc voltage that can be safely applied to the INPUT is ±100 V dc. Voltage levels exceeding this limit can cause breakdown of the coupling capacitor resulting in damage to the input circuitry.

3-17. The 3581A cannot be operated in a floating condition. All input and output commons are connected directly to outer-chassis (frame) ground which connects to earth ground through the offset pin of the power-cord connector. The 3581C balanced inputs and balanced tracking oscillator output are isolated from outer-chassis ground.

3-18. Grounding.

3-19. To protect operating personnel, the 358lA/C chassis must be grounded. The 3581A/C is equipped with a three-conductor power cord which, when plugged into an appropriate receptacle, grounds the instrument. The offset pin on the power plug is the ground connection.

3-20. To preserve the protection feature when operating the instrument from a two-contact outlet, use a three-prong to two-prong adapter and connect the lead on the adapter to earth ground.
3-21. For battery powered instruments (Option 001), the common binding post (or 3581C Sleeve) of the INPUT connector should be connected to earth ground or to an appropriate system ground. I/a system ground is used, be sure it is at earth ground potential and is not a voltage source.

3-22. Ground Loops.
3-23. In the design of the 3581, extra care has been taken to control internal ground currents that could produce undesirable responses or degrade the accuracy of low level measurements. Due to its wide dynamic range and high sensitivity, however, the 3581 can be affected by external ground currents or "ground ioops" which are normally caused by poor grounding. The following paragraphs briefly describe the common power-line ground loop and outline the steps that can be taken to minimize ground loop problems.
3-24. Figure 3-5A shows the input arrangement for a simple grounded measurement. R_s represents the source being measured along with any noise associated with it and is generally called the "normal-mode source". Rs represents the source resistance and the resistance of the high lead; Rg represents the resistance of the ground
lead. Current from Ein (normal-mode current) flows through Rs, Z1 and Rg and the instrument responds to the drop across Z1. As long as the grounds on both sides of Rg are identical, extraneous currents cannot circulate between the source ground and the instrument ground. If, however, the grounds are different due to voltage drops in the ground lead or currents induced into it, a new source is developed and the measurement appears as shown in Figure 3-SB. The new source, Ecm (the difference between grounds), is called the "common-mode source"
because it is common to both the high and ground lines. Common-mode current can flow through R9 or through Rs and Z1. Since Z1 is usually much larger than R5 and since they are both in parallel with Rg, most of the voltage across R will appear across Z1 causing an error in the amplitude reading.

3-25. To minimize power-line ground loops, the following guidelines should be observed:
a. Keep input leads as short as possible.
b. Provide good ground connections to minimize Rg.
c. Connect the signal source and the 3581 to the same power bus.
d. If a removable ground strap is provided on the signal source, float the source to break the common-mode current path.
e. Option 001: Battery operate the 3581; connect a separate ground lead between the common terminal of the 3581 INPUT connector and the ground terminal of the signal source.
f. 3581C: Use balanced inputs.

3-26. Measurement Configurations.
3-27. The 3581 can be used in either of two measurement configurations: open loop or closed loop. These configurations are illustrated in Figure 3-6.

3-28. Open Loop. In the open loop configuration, the 3581 functions as a signal analyzer or "selective voltmeter" which divides the input signal into its various frequency components. The amplitudes and frequencies of these components can be measured by manually tuning the analyzer to specific frequencies or by sweeping the analyzer over a given range. For swept measurements, an X-Y recorder or variable persistence (storage) scope can be connected to the rear panel Recorder outputs to provide an amplitude vs. frequency display. The amplitude vs. frequency display shows how energy is distributed as a function of frequency and, in effect, is the Fourier spectrum of the input signal (Figure 3-7). Some of the more common measurements that can be made using the open loop configuration include harmonic distortion, Intermodulation distortion, spurious, square-wave symmetry and noise.

3-29. Closed Loop. In the closed-loop configuration, the 3581 functions as a network analyzer for characterizing two-port devices such as amplifiers, attenuators and filters. For closed-loop measurements, the network to be tested is inserted between the rear panel Tracking Oscillator Output and the front panel Input. The Tracking Oscillator Output supplies a fixed level, 5 Hz to 50 kHz signal which tracks the tuned frequency of the instrument. This signal serves as stimulus for the network under test. As the frequency is manually tuned or swept over a given range, the amplitude of the signal at the output of the network varies according to the response characteristics of the network. These amplitude variations are measured by the 3581 and, when displayed in graphical form, yield an amplitude vs. frequency plot of the network (Figure 3-8). 3-30.

One method for making closed-loop measurements is to manually vary the frequency and plot a response curve point-by-point on graph paper. This method, however, is tedious, time consuming and often inaccurate since it is easy to miss important points. A faster, more accurate method is to sweep the frequency over the band of interest and display the response curve using a scope or X-Y recorder. Swept measurements provide a continual updating or "refreshing" of information. This makes it possible to adjust the network while observing the results of the adjustment.
3-31. Absolute/Relative Measurements.
3-32. Absolute Measurements. Absolute measurements are used to determine the actual amplitudes of tuned signals. The 3581A can be calibrated for absolute measurements in rms volts, dBV or dBm/600 ohms. The 3581C can be calibrated for absolute measurements in rms volts, dBm/900 ohms or dBm/600 ohms. Control settings, termination requirements and other details concerning these measurements are given in Table 3-2. For all absolute measurements, the front panel amplitude VERNIER control must be set to the CAL position and the instrument must be calibrated as outlined in Paragraph 3-173.

3-33. Relative Measurements. In signal analysis, relative measurements are used for comparing the amplitudes of two or more frequency components of a signal. In network analysis, relative measurements are used for comparing the amplitude variations of a response curve at two or more frequencies. Relative measurements do not require a calibrated scale; that is, using the amplitude VERNIER and other amplitude controls, the gain of the analyzer can be adjusted so that any input level within the range of 100 V rms to 0.1 pV rms will produce full-scale meter deflection. This arbitrary full-scale input level then serves as a reference for measuring signals that are lower in amplitude. When the linear scale is used, relative measurements are expressed in92 93percent of
full scale94. When the log scales are used, relative measurements are expressed in dB below a 0 dB reference level.
3-34. Uncal. Indicator.
3-35. As previously stated, the front panel amplitude VERNIER control must be in the CAL position for all
absolute measurements. When the VERNIER is not in the CAL position, the front panel UNCAL indicator lights to indicate that the meter scales are no longer calibrated in rms volts, dBV or dBm.
3-36. Overload Indicator.
3-3 7. Figure 3-9 is a simplified block diagram showing the 3581 Input Section. The INPUT SENSITIVITY switch and its associated VERNIER potentiometer controls the input attenuation and gain of the Input circuits to maintain the proper signal level at the input of the Mixer. This is an important function since signals that overdrive the Mixer can produce harmonic and spurious mixing products which result in erroneous meter readings. The Overload Detector at the input of the Mixer senses when the signal level exceeds the design limits and, in turn, lights the front panel OVERLOAD indicator. As previously indicated, the 3581 input circuits are well protected and can withstand momentary overloads up to 100 V rms on all ranges. In most cases, an OVERLOAD indication simply means that the input signal is overdriving the Input Circuits or the Mixer and harmonic and spurious responses may be present. Generally, any time the OVERLOAD light is off the instrument induced distortion and spurious is more than 80 dB below the full-scale reference level.

3-38. Maximum Input Level.
3-39. The maximum input level is the maximum level that can be applied to the INPUT without overloading the instrument. The maximum input level is determined only by the INPUT SENSITIVITY and amplitude VERNIER settings and is not affected by the AMPLITUDE REF LEVEL setting. With the amplitude VERNIER in the CAL position, the maximum input level is indicated by a black panel index adjoining the INPUT SENSITIVITY switch dial and the OVERLOAD indicator (Figure 3-10). For the Log scale settings, the maximum input level is defined by the black (dB) markings on the INPUT SENSITIVITY switch dial. For the 358 lA, these markings represent dBV or dBm/600 ohms. For the 358 iC, the markings represent dBm/900 ohms or dBm/600 ohms. The maximum input level for the Volts scale setting is indicated by the blue