Tektronix AF501 Bandpass Filter

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Samples of e-Bay Prices
Orig CostLow BidHigh Bidaverage
$525 (1977)
$840 (1982)
$62.01 8/24/01 #1628569048 bbg
$58.00 11/2/01 #1654097093 dolemite77
$61.00 3/16/02 #1712298054 worldmike
$102.50 1/12/03 #3104780810 88ford (tested-power light bad) (11 bids $10 start + reserve)
$80.75 7/21/04 #3827907792 jtsele (8 bids $10 start) (working-guaranteed)
$103.50 8/7/04 #3831609978 uncle.billy (5 bids $50 start) (tested-guaranteed)
$100.51 (+ship) 9/27/07 #110172482467 antique-retriever,llc (14 bids $9.99 start) (powers up-returnable)
$90.78 (+$12 ship) 1/6/08 #280189273159 surplusauctions (12 bids $4.99 start) (untested)

  • Tunable Bandpass Filtering to 35 kHz
  • Signal Amplification to 50 kHz
  • Sine-Wave Generation to 35 kHz
  • Strobe Trigger Synced to Oscillator or Filter Output
  • Dial Readings in Hz or Cycles per Minute


    The AF 501 Bandpass Filter/Amplifier is an active filter whose center frequency is one-knob tunable over the entire audio range from 3 Hz to 35 kHz. Broad (Q = 5) and narrow (Q = 15) bandwidths are switch selectable. Flat amplification to 50 kHz is also available. In both filter and flat modes, amplification from 1 to 500 may be selected in a 1-2-5 sequence. The AF 501 also features an oscillator mode where the output frequency is the same as the center-tuned frequency in the filter modes. The AF 501 selectivity permits individual analysis of the fundamental and harmonic components of complex waveforms, such as signals from vibration transducers.

    The AF 501 was developed primarily for the mechanical measurement domain. The AF 501 can be used as a manual-sweep spectrum analyzer for complex sound and vibration signals. Single-frequency tuning facilitates isolation of 1X rpm signals in dynamic balancing, or viewing higher order disturbances on a crt monitor. An output pulse, synced to the filter or oscillator output signal, is available for triggering a stoboscope or oscilloscope and for frequency counting. And, of course, the AF 501 can be used in any application calling for a conventional sine-wave generator, ac coupled amplifier, or bandpass filter.

    A single knob with a frequency range from 3 hertz to 35 kilohertz is used for tuning the bandpass filter or oscillator. The dial readout, in Hz and CPM (cycles per minute), ad a range from 3 to 40 Hz and 180 to 2400 CPM. Frequency multiplication of X1, X10, X100 and X1K is provided by front-panel, self-cancelling pushbuttons.


    CharacteristicsPerformance RequirementsSupplemental Information
    Frequency Range 3 Hz to 35 Hz in 4 decade steps
    Single knob tuning
    Frequency Dial Error<5% dial between 3 - 20
    <10% dial between 20 - 30
    Frequency Multiplier X1, X10, X100, X1K
    Phase Shift <10° at tuned frequency
    Below 5 kHz
    Dial Readout Hz, and cycles per minute (cpm)
    Dial Range 3 to 40 Hz, 180 - 2400 cpm
    Dial Rotation 360°, no stops
    Knob Rotation ≈6 turns per one dial turn
    Max. Filter Attenuation >70 dB
    Filter Selectivity Q ≈ 5 (BROAD)
    Q ≈ 15 (NARROW)
    Filter Roll-Off See Figure in Manual
    Gain Range 1 - 500, 1, 2, 4 Sequence
    Gain Accuracy±3 dB (BROAD)
    ±5 dB (NARROW)
    Input Impedance ≈ 1 MΩ paralleled by ≈ 47 pF
    Max. Non-Destruct AC Input Voltage 130 volts rms
    Max, Non-Destruct DC Input Voltage ±100 volts
    Output Voltage20 V p-p (product of output amplitude in volts and frequency in kHz not to exceed 400) 
    Output Current 20 mA p-p max. (at 20 V p-p)
    Output Impedance <1Ω (with output voltage and current within limits of graph shown in manual)
    Single Ended AC coupled
    Gain 1 to 500; 1, 2, 5 sequence
    Gain Accurace±3% 
    Bandwidth<0.5 Hz to >50 kHz (at 3 dB point) 
    Input Impedance 1 MΩ paralleled by ≈47 pF
    Noise <25 mV rms (referred to Output)
    Max. Non-Destruct ac Input Voltage 130 volts rms
    Max. Non-Destruct dc Input Voltage ±100 Volts
    Output Voltage20 V p-p (product of output amplitude in volts and frequency in kHz not to exceed 400) 
    Output Current 20 mA p-p max. (at 20 V p-p)
    Output Impedance <1Ω (with output voltage and current within limits of graph shown in manual)
    Sine-Wave Output Range 3 Hz to 35 kHz
    Dial Readout Hz and cpm
    Dial Range 3 to 40 Hz, 180 - 2400 cpm
    Dial Rotation 360°, no stops
    Knob Rotation ≈6 turns per one dial turn
    Output Amplitude 1, 2, or 5 V p-p ±20%. Depending on gain position.
    Waveform Distortion <3%
    Output Current Max. 50 mA p-p
    Output Impedance <1 Ω (within 50 mA output current limit).
    Trigger Out Positive pulse, triggered when positive slope of output signal goes through 0 (used for counter, strobe-light, etc.).
    Pulse Amplitude>10 volts 
    Pulse Duration10 ±5 μs 
    Minimum Signal Out Required To Set Trigger 500 mV, p-p
    Rise and Fall Time <1 μs
    Output Impedance ≈50Ω

    above excerpts from 1977 TM 500 brochure and AF 501 BANDPASS FILTER Instruction Manual


    Controls and Connectors

    FREQUENCY Hz, CPM Dial -- Variable from 3 to 40 in Hz range; 180 to 2400 in CPM (cycles per minute) range. Selects bandpass filter frequency or oscillator output frequency.

    FREQ MULT Pushbutton -- Self-cancelling pushbuttons. Multiply the FREQUENCY Hz, CPM dial setting by the number shown on the depressed button to obtain bandpass filter tuning frequency or oscillator output frequency.

    GAIN Switch -- Selects GAIN in AMPLIFIER (BROAD and NARROW) from 1 to 500 in a 1, 2, 5 sequence. Determines the output voltage in the OSCILLATOR mode (1, 2, or 5 volts only).

    MODE Switch -- Selects AMPLIFIER, BROAD or NARROW BANDPASS FILTER, or OSCILLATOR operating mode.

    INPUT Connector -- BNC connector for amplifier or bandpass filter signal input.

    OUTPUT Connector -- BNC connector for amplifier, bandpass filter and oscillator output signal.

    TRIG OUT Connector -- BNC connector for output pulse coincident with the frequency of the output signal. The TRIG OUT pulse in BANDPASS FILTER or OSCILLATOR mode can be used to accurately measure the frequency of a repetitive input signal with a counter.

    POWER Indicator -- When lit, indicates power is applied.



    When the MODE switch is set to AMPLIFIER, the AF 501 functions as an ac-coupled, single-ended amplifier, with a bandwidth ranging from 0.5 hertz to 50 kilohertz, maximum 20 volts peak-to-peak output and X500 maximum amplification. The gain from 1 to 500 is controlled by the GAIN switch in a 1, 2, 5 sequence.


    The AF 501 may be used with a 10X voltage probe. Slew rate at the probe tip must not exceed 2.5 volts per microsevond. This is especially true when using a pulse for compensating the probe. If the slew rate limitation is exceeded, the input capacitance of the AF 501 is no longer constant making compensation impossible.

    The amplifier (and bandpass filter) output signal of maximum 20 milliamperes peak-to-peak at 20 volts peak-to-peak, and 50 milliamperes peak-to-peak at 10 volts peak-to-peak, can be used to drive the majority of galvanometers or provide amplification of low-level signals for other subsequent instrumentation. By adding a resistor in series with the output, lower current limits can be obtained when needed.

    Bandpass Filter, Narrow and Broad

    When the MODE switch is set to one of the BANDPASS FILTER positions, the AF 501 functions as a tunable bandpass filter amplifier. The tuning range is from 3 hertz to 35 kilohertz. There are two bandpass filter switch positions. The effective Q of the filter is approximately 15 in the NARROW position or approximately 5 in the BROAD position. In the BROAD setting, tuning is not as critical, but the signal will not be cleaned-up as well as in the NARROW setting.

    The FREQUENCY knob can be adjusted to tune the AF 501 to a single frequency of the input signal, which can be read from the FREQUENCY dial readout.


    The AF 501 operates as an oscillator ranging from 3 hertz to 35 kilohertz when the MODE switch is set to that position. Output voltage of 1, 2, or 5 volts peak-to-peak sine-wave, controlled by the GAIN switch is available at the OUTPUT connector. The leading digit marking the GAIN switch position indicates the voltage output. In other words, switch positions 1, 10 and 100 all provide a 1-volt output; 2, 20 and 200 produce a 2-volt output; and switch positions 5, 50, and 500 provide a 5-volt output.

    above excerpt from AF 501 BANDPASS FILTER Instruction Manual

    1-3366-1007-00Knob : Assy w/ White Dot (Frequency - Inner)
    1-5354-0437-04Ring : Knob Skirt (Frequency - Outer)
    1-2366-1207-00Knob : Gray (Mode - Inner small knob)
    1-4366-1107-01Knob ; Gray 4 Sided (Gain - Outer larger)
    1-8366-1489-72Pushbutton : Gray 'X1'
    1-9366-1257-87Pushbutton : Gray 'X10'
    1-10366-1402-41Pushbutton : Gray 'X100'
    1-11366-1402-48Pushbutton : Gray 'X1K'
    1-20 (R170A)311-1752-0Res.,Var,Nonwir, 2 X 10K Ohm, 3%, 2.75W (Spectrol 100-1313) (frequency)