# Stanford Research Systems FS725 Rubidium Frequency Standard (Pre-Owned)

# Stanford Research Systems FS725 Rubidium Frequency Standard (Pre-Owned)

## Key Features At A Glance

-   Two 10 MHz and one 5 MHz 50 Ω sine outputs plus a 1 pps output, all with ultra-low phase noise
-   Phase noise <−130 dBc/Hz at 10 Hz offset; one-second Allan variance <2 × 10⁻¹¹
-   20-year aging of 5 × 10⁻⁹ (typ.) with rubidium oscillator MTBF over 200,000 hours
-   1 pps input and output for GPS synchronization, delivering Stratum 1 performance
-   RS-232 computer interface with provided Windows RbMon software for monitoring and control
-   Built-in distribution amplifiers expandable to 22 outputs via up to three option boards
-   Two SPDT status alarm relays for rubidium lock state and 1 pps synchronization
-   Compact half-width 2U chassis integrating the rubidium oscillator and low-noise AC supply

The Stanford Research Systems FS725 is a benchtop rubidium frequency standard that integrates a rubidium oscillator (SRS model PRS10), a low-noise AC power supply, and distribution amplifiers in a compact, half-width 2U chassis. It is built for stable, reliable long-term operation, with an estimated 20-year aging of less than 5 × 10⁻⁹ and a demonstrated rubidium oscillator MTBF of over 200,000 hours.

The FS725 is an ideal instrument for calibration and R&D laboratories, or any application requiring a precision frequency standard. It can be phase-locked to an external 1 pps reference such as GPS, providing Stratum 1 performance, making it suited to timing and synchronization tasks that demand a disciplined, traceable frequency source.

Every FS725 we offer is held in ValueTronics' own 20,000 square-foot secure warehouse at 1675 Cambridge Drive in Elgin, Illinois, where we stock and own the inventory we sell. Each pre-owned unit is inspected and functionally verified in-house by our technicians before it ships, while new units leave factory-sealed exactly as received from the manufacturer.

## Accessories Supplied

-   Windows RbMon control/monitor software

## Product Core & Specifications

Specification

Value

Output

Output frequencies

10 MHz sine, 5 MHz sine, 10 µs wide 1 pps pulse

Amplitude

0.5 Vrms, ±10 %

1 pps pulse amplitude

2.5 V into 50 Ω, 5 V into High-Z loads

Phase noise (SSB)

<−130 dBc/Hz (10 Hz), <−140 dBc/Hz (100 Hz), <−150 dBc/Hz (1 kHz), <−155 dBc/Hz (10 kHz)

Spurious

<−100 dBc (100 kHz BW)

Harmonics

<−60 dBc

Accuracy at shipment

±5 × 10⁻¹¹

Aging (after 30 days)

<5 × 10⁻¹¹ (monthly), <5 × 10⁻¹⁰ (yearly), 5 × 10⁻⁹ (20 years, typ.)

Short-term stability (Allan variance)

<2 × 10⁻¹¹ (1 s), <1 × 10⁻¹¹ (10 s), <2 × 10⁻¹² (100 s)

Holdover

72 hour Stratum 1 level (1 × 10⁻¹¹)

Frequency retrace

±5 × 10⁻¹¹ (72 hrs. off, then 72 hrs. on)

Settability

<5 × 10⁻¹²

Trim range

±2 × 10⁻⁹ (0 to 5 VDC), ±0.5 ppm (via RS-232)

Warm-up time

<6 minutes (time to lock), <7 minutes (time to 1 × 10⁻⁹)

Front-Panel Indicators (Green LEDs)

Power

“On” when AC power is applied

Locked

“On” when frequency is locked to Rb

1 pps input

Blinks with each 1 pps reference input applied to rear panel

1 pps sync

“On” when 1 pps output is synchronized within ±1 µs of 1 pps input

Receive

Blinks when RS-232 characters are received by FS725

Send

Blinks when RS-232 characters are sent by FS725

Rear-Panel Connections

Frequency adjust

0 to 5 VDC adjusts frequency by ±0.002 ppm (normally unconnected)

1 pps input

One 100 kΩ input. Requires CMOS level pulses (0 to 5 VDC). If an external 1 pps input is applied, lock is maintained between the 1 pps input and 1 pps output, with computer adjustable time constant from 8 minutes to 18 hours.

10 MHz outputs

Two 50 Ω isolated sine outputs

5 MHz output

One 50 Ω sine output

1 pps output

One 50 Ω pulse output

Optional outputs

Each option board provides four 10 MHz, one 5 MHz, and one 1 pps outputs. Up to 3 boards can be installed.

Alarm relays

Max. current, 3 A. SPDT, normally open or normally closed. May be wired in parallel with other relays to “wire-or” a single alarm.

Rb lock

Relay status matches the front-panel “Locked” LED.

1 pps

Relay status matches the front-panel “1 pps sync” LED.

RS-232

9-pin connector configured as DCE, 9600 baud. Windows RbMon software is provided.

Environmental

Operating temperature

+10 °C to +40 °C

Temperature stability

∆f/f < ±1 × 10⁻¹⁰ (+10 °C to +40 °C)

Storage temperature

–55 °C to +85 °C

Magnetic field

∆f/f < 2 × 10⁻¹⁰ (1 Gauss field reversal)

Relative humidity

95 % (non-condensing)

General

AC power

90 to 132 VAC or 175 to 264 VAC, 47 to 63 Hz, 50 W

Dimensions (W × H × L)

8.5" × 3.5" × 13"

Weight

9 lbs.

Warranty

One year parts and labor on defects in materials and workmanship

> **Important:** The rear-panel 1 pps input requires CMOS-level pulses (0 to 5 VDC) into its 100 kΩ input.

> Recommended pairing: add Option 01, 02, or 03 distribution amplifier boards (6, 12, or 18 outputs) to expand the FS725's available 10 MHz, 5 MHz, and 1 pps output count.

## About this Pre-Owned unit

This pre-owned unit is inspected and functionally verified in-house before it ships and is backed by our pre-owned warranty. To confirm its exact condition, firmware revision, installed options, or included accessories for your application before ordering, contact our Test Architects.

-   Backed by a ValueTronics warranty
-   Functional verification included
-   Standard Calibration Upgrade Options: No Calibration Required, NIST Traceable, Z540.1 or ISO 17025 with Data, Z540.3 Guardbanding with Data.
-   Note that unnecessary accessories may not be included (contact Test Architect to confirm).

ValueTronics supplies both new and used test and measurement equipment. New units ship factory-sealed, exactly as received from the manufacturer; every used unit is inspected and functionally verified in-house at our 20,000 sq ft secure facility in Elgin, Illinois before it ships. Our Test Architects can help you select the condition, calibration, and configuration that fit your application.

**Please review the Manufacturer's Data Sheet to verify published specifications. Feedback on this webpage is always welcome — please reach out to your Test Architect at any time for questions or concerns. Thank you, we truly appreciate you being our customer.**

## Details

- **Price:** 4795.0 USD - 5110.0 USD
- **Vendor:** Stanford Research Systems
- **Type:** Standard
- **Tags:** Frequency Standard, FS725, Inc., Manufacturers, Model 725, Other Equipment, Pre-Owned, product_manual_group_1, Product_used, SRS, Standard, Stanford Research, Stanford Research Systems

## Variants

| Variant | Price | Available |
|---------|-------|-----------|
| None | 4795.00 USD | In stock |
| NIST Traceable | 5040.00 USD | In stock |
| NIST Traceable with Full Data | 5110.00 USD | In stock |
| ISO IEC 17025 Accredited | 4795.00 USD | In stock |

## Images

- FS725 Stanford Research Standard

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> Source: [ValueTronics](https://valuetronics.com/products/fs725-stanford-research-standard-used)
> Updated: 2026-06-02