1. 2305 is a high performance, microprocessor controlled, modulation meter with a comprehensive specification covering the carrier frequency range 50 kHz to 2.3 GHz. 2. Operation may be either local or remote. Local control is obtained via functionally grouped key switches which form the front panel controls and which direct all the instrument functions except those of SUPPLY ON and LF LEVEL. 3. Indication of measured quantities is provided by two l.c.d. 7-segment digital displays : frequency - either of the carrier or of the modulating tone - appears in the 8~digit left-hand display, modulation - in % a.m., kHz or radians deviation — appears in the 4 digit right-hand display. 4. The microprocessor allows maximum flexibility in arranging and controlling the circuit functions and permits remote operation via the GPIB* when the instrument is fitted with the GPIB interface optional accessory. The 2305 is thus as well suited for stand-alone manual operation as for incorporation in automatic systems with programmed operation and makes few demands on operator skill. 5. In it* simplest method of operation, the 2305 provides both automatic tuning and control of the ranging circuits, although tuning may be controlled directly by keypad entry of numerical data, and individual ranges may be retained by the use of keyboard second functions"1’ if necessary. A HOLD ON/OFF key permits all ranges and functions to be locked in their present states. 6. In both forms of tuning, the local oscillator frequency retains the same sense relationship with signal frequency needed to ensure that positive frequency and phase excursions are always presented as P+, 7. In the CARRIER ERROR mode the instrument will measure small frequency drifts or offsets by continuously subtracting a stored reference value from instantaneous measurements of the carrier. The reference value can be either a numerical entry made via the keypad or a particular instantaneous measurement transferred to store by operation of the CARRIER ERROR key. Two other frequency measurement modes provide for the display of either carrier or modulating frequency. 8. When necessary, an external 10 MHz standard may be used in place of the internal reference which forms the basis for local oscillator frequency synthesis and for frequency measurement. It is also possible to provide for direct substitution of the local oscillator signal by a source of external origin. The changeover in both cases being effected by second function keying. 9. Modulation is normally measured as peak excursions of either the amplitude, frequency or phase of the carrier signal and a choice of detector modes enables the peaks of each polarity either to be measured separately or for their average to be calculated and presented as Two additional detector modes offer (1) an averaging detector, and (2) a PEAK HOLD function. The averaging mode (NOISE AVG) is useful when noise is to be measured, and the quantity displayed when in this mode is of the peak value of a sine wave having the same average value. In PEAK HOLD, successive peak samples are examined and store and display are up-dated each' time the previous largest sample is exceeded. The PEAK HOLD mode is useful for logging transients and other aspects of modulator system performance under operational conditions. In the a.m. mode, peaks are always measured as P+ and troughs as P- . 10. The 2305 will tune and measure automatically without loss of accuracy on signal levels be tween - 18 dBm and +30 dBm. The sensitivity increases considerably towards the bottom end of the frequency range and a substantial further increase can be obtained by reverting to manual tuning. Input protection is provided above the permitted maximum of +30 dBm. Deep amplitude modulation may reduce the signal level during troughs to a point that is insufficient to ensure proper functioning of the internal frequency counter and auto-tuning may be adversely affected. The effect is only likely to occur with a.m. greater than 90% and, if it happens, the a.m. depth should be temporarily reduced to allow auto-tuning to complete. 11. Frequency deviation can be measured up to a maximum of 500 kHz for modulation frequencies up to 275 kHz. The accuracy attainable varies with modulation frequency and approaches ±0.5% at 1 kHz. 12. Phase deviation can be measured to a maximum of 500 radians up to ] kHz modulation frequency. Above this frequency the maximum measurable deviation decreases at 6 dB per octave. 13. Amplitude modulation up to 95% can be measured with accuracies approaching 1%, depending on modulation depth and frequency, up to a maximum modulation frequency of 50 kHz. Useful indications are given for depths up to 99.9%. 14. Measurements of all three types of modulation can be related to any reference level and expressed as dB. The reference quantity may be entered numerically via the keypad or may be a measurement selected from a series and transferred to store by a single key stroke. 15. The post detector bandwidth may be shaped by any one of five built-in filters which cover the varied requirements cf mobile radio, broadcasting, telemetry etc. There is also a choice of 3 de-emphasis time constants which may be introduced into the audio output signal in f.m. mode. They may also be introduced into the measurement system by a keyboard second function. External filters may be introduced into the measurement circuit in addition to and independently of any internal selection. 16. Standards of amplitude and frequency modulation are generated within the instrument for calibration purposes. The 2305 automatically runs a selfcheck routine against these standards after each switch-on and displays pass/ fail data. 2305 measurements can be recalibrated against these standards at any time by operation of the CAT key. 17. For systems use or production tests, up to 10 sets of control settings may be stored in non-volatile memory for subsequent recall and use. 18. In its POWER function, the 2305 will normally display the power entering the input terminal in dBm or watts. The measurement is based on peak voltage detection and will indicate peak power on amplitude modulated signals. The range may be extended upward by added external attenuation and a flexible system of power calibration with optional display in watts or dBm can be introduced by second functions.