SD-31 Frequency Synthesizer-Detector

The Model SD-31 Synthesizer-Detector is a high output crystal signal generator of precisely known frequency combined with a sensitive, selective detctor for RF bridge measurements of AM antenna impedance. Packaged in a single light-weight battery-powered unit, the SD031 complements bridges such as the General Radio 1606, 916, and the Delta OIB-1.

A frequency synthesizer determines the generator frequency, which can be adjusted in 0.5 KHz steps by means of a front-panel switch from 100.0 KHz to 1999.5 KHz. Frequency accuracy is the same as that of the internal reference oscillator. A front panel fine-frequency control varies the frequency up to ± .01 percent. The generator can drive a wide range ofload impedance at levels up to 20 volts RMS. It also has a variable low-level output suitable for driving a counter or for receiver frequency calibration.

The SD-31 uses for detection a coherent detector which rejects interfering signals picked up by the antenna. In this arrangement, the generator is mudulated at a low frequency, and the SD-31 detector circuit responds only to a signal having that particular modulation. A sensitive and selective receiver connected to the bridge detector output is required; this can be the Model RX-31 Receiver, available as an option with the SD-31, or an external receiver such as the Potomac Model FIM-21 Field Strength Meter. The optional RX-31 Receiver is designed to work witht eh SD-31 and is automatically tuned to the generator frequency. The SD-31 is powered by a rechargeable battery adequate for 4-8 hours of use between charges. Battery recharging is possible while the unit is operating.

1. OUTPUTS: VAR Output and Control – varies output level at VAR connector for low level applications
   LO Z – hight level output for connection to bridge for Z less than 180 ohms.
   HI Z – high level output for connection to bridge for Z greater than 180 ohms.
2. METER FUNCTION SWITCH:
   BATT – indicates charge level of batter supply
   LOCK – provides positive indication that output frequency is locked to crystal reference oscillator frequency
   OUTPUT – indicates relative output of power amplifier
   NULL SLOW – provides highly damped indication of coherent detector output.
   NULL FAST – provides undamped indication of coherent detector output.
3. AM SWITCH: Selects internal (INT) or external (EXT) Amplitude Modulation of RF output
4. DET GAIN: Adjusts gain (sensitivity) of coherent detector
5. FREQ 100.0 – 1999.5 KHz: Sets synthesizer output frequency within indicated range with separate thumb sheel switches for each digit
6. F CONTROL: Enables operator to vary synthesizer output frequency slightly above or slightly below the nominal value