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    HP 8903A vs 8903B

    There are three models of the HP 8903 Audio Analyzer; 8903A, 8903B, and 8903E. This page covers the A and B models. The 8903E is the distortion analyzer only -- no signal source. Its distortion analyzer is electrically identical to that of an 8903B.

    Each model has various options, the most common one being OPT001 (Rear Connectors). For the 'normal' 8903, the connectors for the signal source and the distortion analyzer are placed on the front panel. Adding OPT001 moves these connectors to the rear panel. Aside from the location of the connectors an 8903 with OPT001 is electrically identical to one without OPT001. For home labs, it is slightly more convenient to have the connectors located on the front panel. However, OPT001 tends to be more prevalent on the used market. Just remember that an aluminum angle bracket, four bulkhead BNC-BNC adaptors, and four BNC patch cables would move the connectors to near the front panel should one end up with an OPT001. It's really not the end of the world.

    HP8903A versus HP8903B

    As shown above, the 8903 comes with a pair of pull-out reference cards. They're mostly handy as a quick reference for the special functions and to decode HPIB error messages. Aside from that, they don't offer that much value in my opinion. In many cases, these cards are missing on used units. Again... No biggie. The information can easily be copied from the operating manual.

    Electrically, the 8903A and 8903B are nearly identical. However, there are two important distinctions:

    • The 8903A has a "pseudo-differential" input; the 8903B has a true differential input (see more below).
    • The 8903B has the analyzer input filters configured as modules, hence, the filters can be changed.

      There are also a few improvements in the electronics in the 8903B to lower the noise floor by a few dB. Nothing earth shaking.

      The connections for an 8903A signal source and analyzer inputs are shown below (grabbed directly from the HP 8903A service manual).

      HP8903A I/O block diagram

      On the front panel of the 8903, you'll find a small switch labeled "FLOAT". You'll find one at the input and one by the signal source output. These FLOAT switches 'float' the inputs and outputs. Floating refers to that the signal source or analyzer inputs float above ground -- i.e. are connected to ground through a high impedance -- when the FLOAT switch is opened. In other words, when the FLOAT switches are opened, the inputs/outputs become differential inputs/outputs. This allows the user to break ground loops caused by a measurement system grounded in several places.

      You will also notice that the inputs and outputs are labeled "LOW" and "HIGH". Think of the "HIGH" input as the (+) terminal of a voltmeter. The "LOW" input would then be the (-) terminal. Same for the signal source. The "HIGH" output is the signal out, the "LOW" is the signal return. In addition, there's a metal banana jack on the front panel which allows connection to chassis ground (needed for differential measurements).

      The operation of the differential inputs is where the main difference between an 8903A and an 8903B lies. In the "pseudo-differential" input of an HP 8903A, the "LOW" input MUST remain within +/- 15 V of the ground potential. The HP 8903B has a true differential input.

      The 8903B allows the "LOW" terminal potential to be anywhere within the 300 V range of the instrument -- as long as the voltage on either terminal does not exceed 425 V with respect to chassis ground.

      The 8903is shipped with the following filters:

      • 400 Hz High-Pass
      • PSOPH - Psophometric (CCITT Recommendation P53)
      • 20 kHz Low-Pass
      • 80 kHz Low-Pass

        In my 8903A, the PSOPH filter has actually been modified to be a CCIR filter – one of the options for the 8903B.

        The 8903B has the following filter options:

        • 400 Hz High-Pass
        • CCITT filter
        • CCIR filter
        • C-MESSAGE filter
        • CCIR/ARM filter
        • "A" filter

          In addition, it is shipped with the 20 kHz and 80 kHz Low-Pass filters. Those are not optional.

          The "A" filter may be handy as it would allow measurements in dB(A), which I believe uses the sensitivity characteristics of the human ear as a reference. Outside audiology, I'm not sure it has much relevance. The other filters mostly pertain to measurements of various communication systems and radio receivers. For the audio enthusiast, the 400 Hz HP, 20 kHz LP, and 80 kHz LP filters are the most commonly used. If you are buying an HP 8903B, definitely get the 400 Hz High-Pass filter. This filter is very handy for elimination of mains hum in the measurements.

          Spec Highlights

          HP 8903A:

          • Frequency range: 20 Hz ~ 100 kHz
          • Signal generator output voltage range: 0.6 mV ~ 6 V RMS (unloaded)
          • Signal generator output impedance: 600 Ω
          • Analyzer input impedance 100 kΩ || 300 pF (330 pF for the OPT001).
          • Noise & distortion floor, 20 Hz ~ 1 kHz (80 kHz BW): The greater of -80 dB or 30 µV
          • Noise & distortion floor, 20 Hz ~ 50 kHz (500 kHz BW): The greater of -70 dB or 95 µV
          • Noise & distortion floor, 50 kHz ~ 100 kHz (500 kHz BW): The greater of -65 dB or 169 µV

            The noise specs on the 8903are as follows:

            • Noise & distortion floor, 20 Hz ~ 1 kHz (80 kHz BW): The greater of -80 dB or 15 µV
            • Noise & distortion floor, 20 Hz ~ 50 kHz (500 kHz BW): The greater of -70 dB or 45 µV
            • Noise & distortion floor, 50 kHz ~ 100 kHz (500 kHz BW): The greater of -65 dB or 45 µV

              The dB numbers for the THD+N floor are referenced to the chosen measurement range. Basically, this translates directly to dynamic range. This, in turn, impacts the lowest THD+N that can be measured. I.e. per the specifications, the lowest THD+N that can be measured is 0.01 %.

              In practice, many 8903's perform better than this. I've had three or four units through my lab and they have all been capable of measuring distortion below 0.005 % at 1 kHz, 80 kHz bandwidth. But there are no guarantees, of course…

              Basic Measurements

              My intent with this section is to provide a brief Getting Started Guide covering the most fundamental functions of the HP 8903. For the key press sequences below, square brackets are used to denote keys/buttons, for example: [AMPTD].

              The keys used to enter measurement units have more than one unit on one key. They are indicated like this: [kHz/V]. While data entry is done with keys as well, I - unlike the HP manuals - have chosen to omit the square brackets on data. I.e. 1.03 rather than [1][.][0][3]. It's easier to read and much easier to type.

              Note that all voltages -- both the settings on the signal generator and the readout on the analyzer display -- are true RMS (Root Mean Square).

              Signal Source

              On power-up, the signal source defaults to 1 kHz, 0 V output. Hence, to get any output, at least the amplitude must be set. This is done with the [AMPTD] key.

              Setting the amplitude to 1 V: [AMPTD] 1 [kHz/V]

              Setting the amplitude to 23 mV: [AMPTD] 23 [Hz/mV]

              The frequency is changed with the [FREQ] key.

              Changing the frequency to 1.23 kHz: [FREQ] 1.23 [kHz/V]

              And 25 Hz: [FREQ] 25 [Hz/mV]

              The [AMPTD] and [FREQ] keys are 'sticky', i.e. once the [AMPTD] or [FREQ] keys have been pressed, the analyzer will assume that subsequent data entries are of the same type (amplitude or frequency). Hence, if a test is to be performed at 1 kHz, 2 kHz, and 5 kHz, the frequency can be set like this:

              [FREQ] 1 [kHz/V] 2 [kHz/V] 5 [kHz/V], 

              with the results of the test noted between frequency changes.

              Similarly for amplitude, using the sequence 10 mV, 100 mV, 1 V as an example:

              [AMPTD] 10 [Hz/mV] 100 [Hz/mV] 1 [kHz/V]

              And... To turn the signal source output off, set the amplitude to zero:

              [AMPTD] 0 [V/kHz].

              The zero may be omitted. I'm not sure if this is a bug or an undocumented feature, but nevertheless, [AMPTD] [V/kHz] provides a convenient shortcut for turning the source output off.

              The signal generator can be programmed to frequencies within the range of 20 Hz to 100 kHz and amplitudes from 0.6 mV to 6 V. If a value outside this range is entered, an error message (Error 20) will be shown in the display. Simply type in a valid value to clear the error.

              Note that the source is tuned in steps, both for amplitude and frequency. Hence, the output frequency will not be exactly the value entered. For the HP 8903 to meet its specifications, the actual output frequency must be within ±0.3 % of the entered value. Hence, if programmed to 1000 Hz, the actual frequency may fall in the range of 997 ~ 1003 Hz.

              The amplitude is accurate to within ±2 % (60 mV ~ 6 V, 20 Hz ~ 50 kHz), ±3 % (6 mV ~ 6 V, 20 Hz ~ 100 kHz), and ±5 % (0.6 mV ~ 6 V, 20 Hz ~ 100 kHz) respectively.

              The signal source is capable of performing an automatic frequency or amplitude sweep. However, unless an X-Y plotter is attached to the analyzer, I find this feature to be of little value. But one part of the sweep feature is handy; the ability to perform a manual sweep. This is particularly handy for a quick verification of -3 dB frequency or THD vs amplitude.

              To set the amplitude sweep step size to 100 mV, enter:

              [AMPTD INCR] 100 [mV/Hz]

              To change the amplitude in the programmed steps use:

              [AMPTD] [ArrowUp] and [AMPTD] [ArrowDown]

              Similarly, for frequency steps:

              [FREQ INCR] 2 [V/kHz]

              sets the frequency step size to 2 kHz.

              [FREQ] [ArrowUp]

              increases the frequency by one step size;

              [FREQ] [ArrowDown]

              decreases the frequency by one step size. The [×10] and [÷10] keys may be used for a crude, one step per decade, logarithmic sweep as they multiply/divide the frequency/amplitude by a factor of 10. Analyzer

              The analyzer part of the HP 8903 is capable of measuring various kinds of signal-to-noise ratios, THD+N, and AC and DC voltage. For the hobby audio enthusiast, the most interesting are: AC, DC voltage, THD+N. In addition, a special function may be used to calculate the power dissipated in a specified load resistance from the measured AC voltage.

              To measure the RMS AC voltage, simply press [AC LEVEL]. The analyzer will indicate the frequency of the most dominant frequency component of the signal in the left, "frequency", display and the total RMS signal amplitude on the right, "measurement", display. If there is no dominant frequency in the signal, the "frequency" display will flicker between various frequencies. For example, if measuring low levels of mains hum, the "frequency" display will often read somewhere between 50/60 Hz and 150/180 Hz.

              Measuring the DC voltage can be handy for measuring offset voltages on amplifiers. This function is activated by pressing the "shift" key [S] followed by [DC LEVEL] (same physical key as [AC LEVEL]). The DC voltmeter will measure voltages down to 1 mV with 4-digit readout.

              Distortion - or, specifically, THD+N - is measured by pressing the [DISTN] key. The THD+N is indicated in percent on the "measurement" display.

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