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Automatic Modulation Meter


  • Frequency range 1.5 MHz to 2 GHz
  • Useful indications to 4 GHz
  • LED indication of lock, level set and function
  • Analogue display
  • Incorporates three bandpass filters, a psophometric filter, and a deemphasis network
  • Small, lightweight unit ideal for field use if battery version ordered
  • GPIB version for systems use

The Automatic Modulation Meter model 257 has been designed to simplify the task of measuring amplitude and frequency modulation on signals in the range 1.5MHz to 2GHz. By eliminating the need to tune a local oscillator and make level adjustments the 257 speeds up measurements and reduces errors. Patented circuitry ensures that the meter always locks to the highest signal available ignoring spurious signals and harmonics.

AM and FM measurements can be made at full accuracy over the frequency range l.5MHz to 2GHz and with reduced accuracy and sensitivity to 4GHz. On FM, five ranges are provided with fullscale readings of 1kHz, 3kHz, 10kHz, 30kHz and 100kHz deviation. Peak positive, peak negative or mean deviation can be displayed. On AM five ranges give full scale readings of 1%, 3%, 10%, 30% and 100%. Peak, trough or mean of percentage modulation can be displayed.

For ease of measurement, three band pass filters with minimal upper cut-off frequencies of 60kHz, 15kHz and 3.5kHz, a psophcmetric filter or a 750s de-emphasis network can be selected to filter the recovered modulation.

The different ranges and functions are selected by front panel push buttons with LED's providing positive indication of function.

The unit is small and lightweight, so making it ideal for field work, especially with the battery option installed, which will give at least 8 hours of continuous use before needing a recharge.

For automatic test facilities a GPIB option is available which provides remote control and interrogation of all functions. Note that the GPIB interface is not available if the battery option is fitted.


RF Input

Frequency Range 1.5MHz to 2.0GHz with a useful response, with reduced sensitivity, to at least 4GHz.
Tuning Automatic tuning selects the largest available signal. Correct operation requires spurious signals to be >10dB below the wanted signal.
Acquisition Time Typically less than 100ms to locate and lock to the carrier signal. Settling time for the demodulation and AF circuits is additional and is typically 1 second for a reading in the two thirds of the meter scale.
Lock and Level Indicator A LED indicates that the instrument is locked to an input signal and that the level is within range.
Input Impedance 50 ohms nominal.
Input Level The unit will lock and measure modulation for input carrier levels in the range 2mV to 1V. Full specification for noise, accuracy etc only applies over the input range 10mV to 1V.
Maximum Safe Input Level 0.5W continuous.
Local Oscillator Feed-out Typically -60dBm.

F.M. Measurement (Over Full Frequency Range)

F.M. Ranges Five ranges with full scale deviations of 1kHz, 3kHz, 10kHz, 30kHz and 100kHz.
Accuracy (+5to +45C) 2% of full scale, 1% of reading with 1kHz tone. See Audio filter specification for additional error due to AF response. Residual FM is additional.
Residual F.M. <20Hz at 100MHz
<100Hz at 500MHz
<200Hz at 1000MHz
Distortion <1% at 100kHz deviation with a 1kHz tone.
F.M. Measurement Modes Peak +ve. Peak -ve and Mean deviation.

A.M. Measurement (Over Full Frequency Range)

A.M. Ranges Five ranges with full scale indications of 1%, 3%, 10%, 30% and 100%.
Accuracy <+5to +45C) 2% of full scale, 2% of reading with a 1kHz tone. See Audio Filter specification for additional error due to audio response. Residual AM is additional.
Residual A.M. Less than 0.5% with a 15kHz bandwidth selected.
Distortion <1% for 80% A.M. with a 1kHz tone.
A.M. Measurement Modes Peak, Trough and Mean of peak and trough.

Audio Filters

60kHz Bandpass Filter 250Hz - 60kHz 0.5dB, typically 12Hz - 72kHz 3dB with HF roll off at 80dB/decade.
15kHz Bandpass Filter 250Hz - 15kHz 0.5dB, typically 12Hz -19.5kHz 3dB with HF roll off at 60dB/decade.
3.5kHz Bandpass Filter 250Hz - 3.5kHz 0.5dB, typically 12Hz - 4.0kHz 3dB with HF roll off at 100dB/decade.
Psophometric Filter Complies with CCITT Volume V P53.
De-emphasis network 750s de-emphasis, 3dB bandwidth typically 12Hz to 212Hz. HF roll off 20dB/decade.

AF Output

Demodulated A.F. Output Front Panel BNC
A.F. Output Level '0'dBm corresponding to full scale deflection.
A.F. Output Impedance 600 ohms.

Power Requirements

Mains 240V or 120V 15%, internally selectable.
Frequency 48 to 60Hz.
Power Approx 6VA.
Fuse Rear panel 100mA quick blow fuse.


Size H 105mm, W 215mm, D 305mm.
Weight Approx 1.7Kg
Approx 2.6Kg with battery option.


Operating Temperature Range 0 to 55C. Full specification over the range to 45C.
Storage Temperature Range -20C to 55C.
Humidity Max 95% RH at 30C.

GPIB Interface (Optional)

Remote control and interrogation of all functions via a GPIB interface. This interface complies to the ANSI/IEEE 488-1978 standard with message formats complient with ANSI/IEEE 728-1982 standard.

GPIB address 0 to 15 selectable by rear panel DIL switch.
Subsets Implemented SH1, AH1, T6, TE0, L4, LE0, SR1, RL1, PP0, DC1, DT0, C0.

Battery Facility (Optional)

Discharge Time >8hrs, typically 10hrs for a fully charged battery.
Recharge Time 14hrs.
Battery Test Pressing the 'Bat Chk switch displays the battery condition on the meter. A reading between 8 and 10 is required for operation.
Battery Protection Rear panel 1A slow blow fuse.

Order Codes
257 Automatic Modulation Meter with Analogue Display and Mains Operation.   09325701
257 Automatic Modulation Meter with Analogue Display and Mains/Battery Operation.   09325703
257 Automatic Modulation Meter with Analogue Display, Mains operation and GPIB.   09325705

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