SR844锁相放大器用户指南：初学者入门教程

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本篇文档是斯坦福研究系统（SRS）公司出品的SR844型锁相放大器用户手册，适合初学者系统学习该设备的使用方法。锁相放大器是一种精密的电子仪器，用于测量微弱信号的频率成分，并将其与参考信号同步，从而提高信噪比，常用于信号检测、分析和测量等领域。首先，标题中的"锁相放大使用手册"表明了主要内容围绕SR844锁相放大器的操作原理、功能特性、设置步骤、以及可能遇到的问题解决方案展开。对于新手来说，这份文档详细介绍了如何正确安装和配置放大器，包括但不限于： 1. **基本介绍**：型号SR844的介绍，强调其作为一款高性能的RFLock-In放大器，专为科研和工程应用设计，具备高灵敏度和精确度。 2. **认证信息**：文档强调了产品经过严格的出厂测试，符合出版规格，并且校准数据可追溯至美国国家标准与技术研究院（NIST），确保了测量结果的准确性和可靠性。 3. **保修政策**：用户享有自发货日起一年的质保期，涵盖了材料和工艺缺陷。在维修或服务需求时，需通过SRS授权服务中心进行，确保专业支持。 4. **服务指导**：用户在遇到问题时，应先联系SRS或授权代表，了解正确的返修流程和信息更新，以确保问题能得到及时处理。 5. **版权和保留权利**：文档版权属于SRS，所有内容可能随时根据需要更新，提醒读者遵守版权法。 6. **技术细节**：手册深入讲解了锁相放大器的工作原理，如锁定机制、相位跟踪算法、信号输入输出接口的使用方法等，使读者能够理解和掌握基本操作。 7. **实例应用**：针对不同应用场景，手册可能提供实际案例和示例，帮助用户理解如何在实验室环境中有效地利用SR844锁相放大器。这份锁相放大使用手册为学习者提供了全面而深入的指导，无论是初次接触的用户还是有一定经验的工程师，都能从中受益匪浅。阅读并按照手册的步骤操作，可以有效提升在相关领域的工作效率和实验精度。

1-6 The Basic Lock-In

SR844 RF Lock-In Amplifier

Remember, the signal is a 0.5 Vpk (1.0 Vpp) square

wave. A square wave is composed of signals at all

odd harmonics. The SR844 is a square wave

detecting lock-in and detects all of the odd

harmonics of the fundamental. The amplitude of the

fundamental (at 1.00 MHz) is 4/

x Vpk. The

contribution from all odd harmonics is

1 + (1/3)

+ (1/5)

+ (1/7)

+ ...

≈

1.19

The detected amplitude is 4/

x 0.5 Vpk x 1.19 or

0.759 Vpk. The SR844 reads the signal in units of

Vrms (0.707 x Vpk) or 0.537 Vrms.

The CH1 display may not read exactly 0.54 V for a

number of reasons:

• The REF OUT amplitude is only a nominal

specification.

• The basic accuracy of the unit is

0.25 dB

(3%).

• Since the phase may not be exactly 0

, X=Rcos

is slightly less than R (amplitude).

4 Press Shift then Phase to perform

AutoPhase.

This adjusts the reference phase inside the

instrument. (The phase at which the signal is

detected changes, but Ref Out remains unchanged.)

This should set the value of Y (on the CH2 display)

to zero.

5 Press Phase. Display the reference phase. It should be close to

zero.

Press the +90° key.

This adds 90

to the reference phase. The value of

X should drop to near zero, while Y changes to

about –0.54 V (negative of the previous X reading).

7Use the knob to adjust the reference phase

until Y is zero and X is equal to the positive

amplitude.

While the reference phase is being displayed, the

knob can be used to change it. The adjustment

described should result in the phase returning to

nearly zero again.

In general, the knob is used to adjust the quantity

displayed in the Reference display (if it can be

changed). The keys below the display are used to

select the desired quantity.

The Basic Lock-In 1-7

SR844 RF Lock-In Amplifier

8 Press Freq. Now the display shows the reference frequency, still

1.00 MHz.

9 Rotate the knob left to get to 999 kHz and

998 kHz.

The internal frequency may be adjusted with 3-digit

resolution.

Rotate the knob right to get to 1.01 MHz and

1.02 MHz.

The actual frequency is within 1 count in the 4th

digit of the displayed frequency. For example, when

set to 4.56 MHz, the actual frequency is within

0.001 MHz of 4.56 MHz.

10 Use the knob to adjust the frequency to

96 kHz.

The X reading should vary less than 10%.

11 Press Sensitivity Down. The sensitivity changes to 300 mV (indicated below

the Sensitivity Down key). The OVLD indicators

in the CH1 and CH2 displays indicate that the

readings may be invalid due to an overload

condition. OVLD indicators in the Input, Time

Constant and Sensitivity areas are used to pinpoint

the source of the overload.

12 Press Shift then Sensitivity Up to perform

AutoSensitivity.

This adjusts the sensitivity so that the measured

magnitude, R, is a sizable percentage of full scale.

The instrument should end up on the 1 V scale and

the displays showing their previous values.

13 Disconnect the cable at the SIGNAL IN

connector.

Watch the CH1 display bargraph drop down to zero.

The time constant is 100 ms, the bargraph falls

quickly but not instantaneously.

14 Reconnect the cable to SIGNAL IN. Watch the CH1 bargraph come back up.

15 Press Time Constant Down six times until

the time constant is 100

The Time Constant is adjusted using the left hand

pair of keys in the Time Constant area. The

indicated time constant should be 1,

100,

µµ

s . The

CH1 and CH2 values remain nearly unchanged but

may be noisy in the last digit.

16 Disconnect the cable at the SIGNAL IN

connector, then reconnect it.

The bargraph falls and rises nearly instantaneously.

17 Press Time Constant Up until the time

constant is 3 s.

The indicated time constant should be 3, x1,s. The

CH1 and CH2 displays remain nearly unchanged.

18 Disconnect the cable at the SIGNAL IN

connector. Wait until the CH1 reading drops

to zero.

The bargraph falls slowly.

X, Y, R, θ

θθ

θ and dBm 1-9

SR844 RF Lock-In Amplifier

X, Y, R, θ

θθ

θ and dBm

This measurement is designed to use the internal oscillator and an external signal source to explore some

of the signal types. You will need a synthesized signal generator cable of providing 200 mVrms (0 dBm)

sine waves at 100 kHz into a 50

Ω

load (the DS335 from Stanford Research Systems will suffice), and

BNC cables.

Specifically you will display the lock-in outputs when measuring a signal that has a frequency close to,

but not equal to, the internal reference frequency.

Note: The last few items in this section require that the signal generator have a Sync output; if you are

using a signal generator that has a single output only, you can split the output using a BNC Tee (or a

power splitter or a directional coupler).

1 Disconnect all cables from the SR844.

If the power is off, turn it on. Wait for self-

tests to complete.

Turn on the unit.

2 Press Shift then Recall (PRESET) to restore

factory presets.

The factory preset configuration is:

1 Vrms sensitivity.

100 ms, 12 dB/oct time constant.

Internal Reference at 1.00 MHz.

Signal Input 50

Ω

3Use the knob to adjust the SR844 reference

frequency to 100 kHz.

We are using a low reference frequency so that the

intrinsic frequency difference between the SR844

and the signal generator has a smaller absolute

value.

4 Press Sensitivity Down.

Press Time Constant Down twice until the

time constant is 10 ms.

The SR844 sensitivity should now be 300 mVrms.

We need a shorter time constant to measure the

output signal.

5 Turn on the external signal generator and set

the frequency to 100 kHz exactly, and the

amplitude to 200 mVrms, 0 dBm, or

600 mVpp into 50

Ω

. The exact value doesn’t

really matter. Low-frequency signal generators

may have waveform selection (select sine

wave) and DC offset (set it to zero). If the

signal generator offers modulation, make sure

it’s off.

While not phase-locked, the signal generator and

SR844 should be at very nearly the same frequency;

the slight frequency difference will be manifested as

a changing relative phase.

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