Products Support Catalogs & Pricing DTV Warranty About Contact Home                                

SD-31 Frequency Synthesizer
and Coherent Detector
PDF Brochure


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.

SD-31 Block Diagram

Click here for a full view of the Block Diagram.

SD-31 Frequency Synthesizer and Coherent Detector
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.

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.

AM SWITCH: Selects internal (INT) or external (EXT) Amplitude Modulation of RF output

DET GAIN: Adjusts gain (sensitivity) of coherent detector

FREQ 100.0 - 1999.5 KHz: Sets synthesizer output frequency within indicated range with separate thumb sheel switches for each digit

F CONTROL: Enables operator to vary synthesizer output frequency slightly above or slightly below the nominal value


RX-31 The RX-31 is designed specifically as an RF interface between an impedance bridge and the SD-31 Coherent Detector.

Click here for details >>

Frequency 100.00 - 1999.5 kHz switch selected, 500 Hz steps
Frequency Accuracy ±005. +200°F to +1000°F
Frequency Vernier (AF)
Adjustment Range
±.01% of frequency approximately
Output Levels HIZ: 20 V RMS with 1000 ohmload (use for Z>180 ohms)
LOZ: 8 V RMS with50 ohm load (use for Z<180 ohms)
(HIZ & LOZ: minimum levels with fully charged battery)
VAR: 100 mV RMS approximately with 50 ohmload
AM Modulation Internal: 40 Hz (approximately) square wave
External: 1.0 mV max. required to produce 50% modulation, 300 Hz - 3 kHz.
Detector Input Requirement
(from external receiver)
1 V RMS approximately across 100 K ohms for 100% modulation of receiver RF input at 40 Hz.
Metering (Switch Selected) Battery voltage
Phase loop locking
Output monitor
Detector null, fast or slow response
AC Power Unit 105-130 VAC, 50-60 Hz, 15 VA (when charging)
Dimensions Height - 9 1/2 inches
Width - 11 1/2 inches
Depth - 6 1/2 inches
Weight 12 lbs.
Battery Operation Time 8 hours approximately after full charge depending on frequency and load.
Battery Operation Time 16 hours (unit not in operation)

Specifications subject to change without notice.

Page Top

SD-31  Frequency Synthesizer & Coherent Detector
enlarged view

  • Designed for Antenna impedance measurements with RF bridges in the presence of strong interference
  • High-level oscillator compatible with General Radio 1606 Series, 916 Series, and Delta OIB-1 Impedance Bridges
  • Frequency crystal controlled, variable in 500 Hz steps from 100.0 kHz to 1999.5 kHz
  • Versatile - can be used as an RF signal generator for trouble-shooting antenna systems; as a variable frequency oscillator for antenna site survey; or other applications requiring a precise frequency source
  • Special Coherent Detector circuit rejects interfering signals experienced during antenna measurements
  • Receiver for Detector can be external or optional built0in RX-31 Receiver
  • Powered by rechargeable batteries
  • Self-contained portable package