INTRODUCTION 1 The Racal-Dana frequency counters Models 1998 and 1999 are microprocessor-controlled instruments offering high-accuracy measurements with a comprehensive range of facilities. MEASURED FUNCTIONS Frequency A Function 2 The Frequency A function is used to measure the frequency of the signal applied to the channel A input. A resolution of nine digits is available with a one-second gate time. Frequency B Function 3 The Frequency B function is used to measure the frequency of the signal applied to the channel B input. A resolution of nine digits is available with a one-second gate time. Period A Function 4 The Period A function is used to measure the period of the waveform applied to the channel A input. A number of periods, depending upon the resolution (and therefore the gate time) selected, are measured, and the average value is displayed. Ratio A/D Function 5 The Ratio A/D function is used to measure the ratio of the frequency applied to the channel A input to that applied to the channel D input. Ratio B/A Function (1998 Only) 6 The Ratio B/A function is used to measure the ratio of the frequency applied to the channel B input to that applied to the channel A input. CHECK FUNCTION 7 With the Check function selected a number of functional tests of the instrument’s circuits can be made without the use of additional test equipment. Although these tests do not check the instrument's performance to its published specification, they can be used to verify that the equipment is operating correctly. A suitable functional check procedure is given in Section 3. SIGNAL INPUT CHANNELS 8 Signal input channels A and B are fully independent. 9 Channel A is provided with controls to permit the selection of: (1) 1 ΜΩ or 50 Ω input impedance. (2) Input attenuation continuously variable from 0 dB to approximately 58 dB using the ATTN (X20) and SENSITIVITY controls. (3) 50 kHz low-pass filter. (4) Pulse offset (trigger level) to cater for signals of various duty cycles. 10 Channel B has a 50 Ω input impedance. On the 1998 the input is fuse- protected for signals above 7 V. On the 1999 the input is protected by an internal attenuator for signals above 4 V. ERROR INDICATION 11 Certain errors in the operation of the instrument will result in the generation of error codes, which will be displayed. Details are given in Section 4 of this manual. EXTERNAL ARMING 12 External arming of the start circuit for the measurement interval can be carried out by means of signals connected to a BNC connector mounted on the rear panel. DISPLAY 13 A 10-digit numeric display with units annunciators is used. Indicators are provided to show when overflow of the most significant digit occurs, and to show the measurement interval (gate time). HOLD FEATURE 14 The hold feature allows readings to be held indefinitely. A new measurement cycle is initiated using the RESET key. RESOLUTION AND GATE TIME 15 The gate time is determined by the display resolution selected. Details of the relationship between gate time and display resolution for each measurement mode are given in Section 4 of this manual. GATE OUTPUT 15 The internally-generated gate signal is available at a pin on the rear panel. The gate output will be delayed relative to the internal measurement gate by 10 nsec typically (15 nsec maximum). 17 When used in the single-shot mode, the gate waveform comprises a short trigger pulse, followed by a hold-off time of about 1 ms followed by the measurement gate waveform proper; this is to allow the external circuit output to stabilise after triggering. EXTERNAL FREQUENCY STANDARD INPUT 18 The instrument may be operated using an external frequency standard. The instrument will operate from the external standard, in preference to the internal standard, whenever the signal at the EXT STD INPUT socket is of sufficient amplitude. It will revert to operation from the internal standard automatically if the input from the external standard is removed. STANDBY MODE 19 When the instrument is switched to standby, the internal frequency standard continues to operate and the instrument status is maintained but the measuring circuits are switched off. If the battery pack option is fitted and an external power supply is connected, the battery is charged at the full rate. NULL FUNCTION 20 With the NULL function active the instrument displays the difference between the measured value and the value held in the internal NULL store. INITIALIZATION 21 When the instrument is first switched on, or when it is initialised via the GPIB, it is set to the following conditions: Measurement Function FREQ A Display Resolution 8 digits (0.1 second gate time) Channel A Input 1 ΜΩ input impedance No trigger offset ( ) LF filter disabled XI attenuation Null Function Disabled Null Store 0 External Arming Disabled Hold Function Disabled OPTIONS AVAILABLE Frequency Standards (04X Options) 22 A wide range of internal frequency standard options is available. The technical specifications are given in Section 1 of this manual. The frequency standard can be changed, if required, by the customer: instructions are given in Section 3. Reference Frequency Multiplier (Option 10) 23 The reference frequency multiplier is an internally-mounted, phase- locked multiplier, which permits the use of external frequency standard signals at 1 MHz, 2 MHz, 5 MHz or 10 MHz. The multiplier can be fitted by the customer: instructions are given in Section 3. GPIB Interface (Option 55) 24 An internally mounted interface to the IEEE-488-GPIB is available. This permits remote control of all the instrument's functions except the power ON/OFF switching, the standby switching, and the channel A sensitivity potentiometer setting. The interface can be fitted by the customer: instructions are given in Section 3. The GPIB interface cannot be fitted to an instrument already fitted with the battery pack option. An adapter, Racal-Dana part number 23-3254, to convert the connector to the I EC 625-1 standard is available as an accessory. Battery Pack (Option 07) 25 Fitting the internal Battery Pack Assembly permits the instrument to be used in locations where no suitable AC supply is available. The option also allows operation from an external DC supply. 26 The battery is trickle-charged whenever the instrument is operated from an AC supply and the internal/external switch is at INTERNAL BATTERIES. Charging at the full rate is carried out when the instrument is switched to the standby mode and connected to an external AC or DC supply. A full charge requires approximately 14 hours. 27 The instrument will operate continuously from a fully-charged battery for approximately 4.5 hours (1998) and 3.75 hours (1999). It will switch off automatically when the battery reaches the discharged condition. The STBY/CHRG indicator starts to flash approximately 15 minutes before this occurs. The battery life can be extended by use of the Battery-Save facility. 28 The battery pack can be fitted by the customer. Instructions are given in Section 3. When using the GPIB interface option the battery pack cannot be fitted. Rack Mounting Kits 29 The following kits permitting the instrument to be mounted in a standard 19-inch rack are available: (1) Single instrument, fixed-mount kit (Option 60A). (Racal-Dana part number 11-1648). The mounted instrument occupies half the rack width and is two rack units (3.5 inches) in height. The instrument is mounted offset in the rack and may be at either side. (2) Double instrument, fixed-mount kit (Option 60B). (Racal-Dana part number 11-1649). The panel of the mounting kit occupies the full rack width and is two rack units (3.5 inches) in height. Two instruments can be mounted side-by-side. 30 All the kits can be fitted by the customer. Instructions are given in Section 3.