Scitec 420 User manual

Boston Electronics Corporation, 91 Boylston Street, Brookline MA 02445
[phone] or [phone] * fax [phone] * [email protected] * www.boselec.com
Bartles Industrial Estate, North Street, Redruth, Cornwall TR15 1HR
Tel. [phone] E-mail: [e-mail]
Fax. [phone] Web: http://www.scitec.uk.com
MODEL 420 ANALOG DUAL PHASE
LOCK-IN AMPLIFIER
USER MANUAL

Boston Electronics Corporation, 91 Boylston Street, Brookline MA 02445
[phone] or [phone] * fax [phone] * [email protected] * www.boselec.com
Bartles Industrial Estate, North Street, Redruth, Cornwall TR15 1HR
Tel. [phone] E-mail: [e-mail]
Fax. [phone] Web: http://www.scitec.uk.com
CONTENTS
1 POWER SUPPLY
1.1 WARNING
1.2 Power Supply
1.3 To Change The Voltage Selection Switch
1.4 Earth
1.5 Fuse
2 OPERATION
3 INPUT STAGE
3.1 Accessing Input Stage Jumpers
3.2 Jumper Settings Overview
3.3 Single Ended DC Coupled Input (Default Setting)
3.4 Single Ended AC Coupled Input
3.5 Single Ended Current Input
3.6 Differential Input
3.7 Low Impedance Differential Input
4 SIGNAL GROUND, CASE GROUND AND EARTH
4.1 Accessing Input Stage Jumpers
4.2 Signal Ground Connected to Case and Electrical Earth (Default Setting)
4.3 Signal Ground Connected to Case and Electrical Earth Via 100ΩResistor
4.4 Signal Ground Isolated From Case and Electrical Earth
5 REFERENCE SECTION
5.1 1F/2F Reference Select Switch
5.2 Coarse Phase Control
5.3 Fine Phase Control
6 DEMODULATOR / OUTPUT SECTION
7 USING THE 420 AS A SINGLE PHASE INSTRUMENT TO TAKE
QUICK MEASUREMENTS
8 USING THE 420 AS A SINGLE PHASE INSTRUMENT TO TAKE
ACCURATE MEASUREMENTS
9 USING THE 420 AS A DUAL PHASE INSTRUMENT TO TAKE
ACCURATE MEASUREMENTS
10 HOW TO MAKE AMPLITUDE AND PHASE MEASUREMENTS
USING AN OSCILLOSCOPE

Boston Electronics Corporation, 91 Boylston Street, Brookline MA 02445
[phone] or [phone] * fax [phone] * [email protected] * www.boselec.com
Bartles Industrial Estate, North Street, Redruth, Cornwall TR15 1HR
Tel. [phone] E-mail: [e-mail]
Fax. [phone] Web: http://www.scitec.uk.com
1 POWER SUPPLY
1.1 WARNING
This instrument can be damaged if operated with the power
voltage selector incorrectly set.
Switch off and disconnect power before opening unit. Do not
operate the unit with the lid removed as there are power
voltages within the unit.
1.2 Power Supply
The unit will accept either: or:
115V AC ±5% at 50Hz or 60Hz 220V AC ±5% at 50Hz or 60Hz
The voltage selection switch is mounted inside the unit. The current voltage selection is marked
on the label on the back of the unit.
1.3 To Change The Voltage Selection Switch
(1) Switch off and disconnect power before opening unit.
(2) Remove the 10 screws holding the lid on then remove lid. Do not disconnect earth strap.
(3) Change the voltage selection switch mounted on the PCB to the rear of the unit next to the
transformer (large blue box). Available settings are 115V and 230V.
(4) Replace lid and the 10 screws.
(5) Update the label on the back of the unit to indicate the current voltage selection switch
position. It is very important that this is done to ensure there is no confusion in the future.
1.4 Earth
The instrument should be grounded via the power inlet cable.
The instrument is encased in a metal box, which is connected to
this earth. No modifications should be made to this earthing
mechanism. Without this earth connection it is possible for the
box to become live under fault conditions. The signal ground can
however be disconnected for details see section 4.
1.5 Fuse
The unit is fitted with a 1Amp anti-surge (T) 5x20mm fuse, incorporated into the power
connector on the rear of the unit.

Boston Electronics Corporation, 91 Boylston Street, Brookline MA 02445
[phone] or [phone] * fax [phone] * [email protected] * www.boselec.com
Bartles Industrial Estate, North Street, Redruth, Cornwall TR15 1HR
Tel. [phone] E-mail: [e-mail]
Fax. [phone] Web: http://www.scitec.uk.com
Only replace the fuse with another of the same type, size and value.
To access the fuse, first remove the power cable then open the fuse compartment with a finger
nail. When shipped from the factory the fuse compartment contains two fuses. The inside fuse
is the fuse in use, while the outside fuse is a spare and can be used to replace the inside fuse if
it is blown.

Boston Electronics Corporation, 91 Boylston Street, Brookline MA 02445
[phone] or [phone] * fax [phone] * [email protected] * www.boselec.com
Bartles Industrial Estate, North Street, Redruth, Cornwall TR15 1HR
Tel. [phone] E-mail: [e-mail]
Fax. [phone] Web: http://www.scitec.uk.com
2 OPERATION
After first ensuring that the current voltage selection is appropriate to your power supply, the
power can be connected to the rear of the unit. The power supply switch (16)1is mounted on
the front panel and causes the power on indicator to light (15).
Please note that the instrument has a number of circuits within it that require time to settle on
power up. This can cause the output meter to jump to full scale for a short time on power up.
This is most noticeable if the time constant control (3) is set to a high value (above 1S). To
speed up the settling process turn the time constant control (3) to 1S or below.
If there is no reference signal connected to the unit then the offset nulling circuitry within the
unit does not operate and hence an offset may be seen on the output. This is normal and will be
removed when a reference signal is applied.
It is safe to apply signals to the unit while the lock-in is switched off, though this is not
recommended for any length of time. However, the input impedance of the signal input and
reference input with unit switched off may drop producing an unacceptable load on the sources
of these signals. It is hence recommended that the unit is switched on whenever input signals
are applied to it.
Care should be made that the reference signal does not affect the input signal. The reference
signal is commonly many factors larger than the signal to be measured and it is therefore
relatively easy for the reference signal to be picked up on input signal cable. For this reason
keep the cabling for the input signal as far away as possible from the cabling for the reference
signal. Careful screening helps but it is still possible for the input cable to pick up the reference
signal if significant lengths are put into close contact.
1The numbers in brackets refer to the numbers used in Figure 18 at the back of this manual.

Boston Electronics Corporation, 91 Boylston Street, Brookline MA 02445
[phone] or [phone] * fax [phone] * [email protected] * www.boselec.com
Bartles Industrial Estate, North Street, Redruth, Cornwall TR15 1HR
Tel. [phone] E-mail: [e-mail]
Fax. [phone] Web: http://www.scitec.uk.com
3 INPUT STAGE
The input stage amplifies the input signal to a level suitable for demodulator section.
The input signal should be connected to the BNC signal input connector (2).
The input sensitivity is set by using the input sensitivity dial (1). Please note that it is not
possible to rotate the dial directly from the 1V setting to the 3µV setting.
The input stage of the unit can operate in a number of ways. By default, the unit is factory set
so the input stage acts as a single ended, DC coupled input as this is the lowest noise method of
operation. It is also possible to operate the input stage in AC coupled mode, differential mode
and current mode. To operate the lock-in in these different modes requires jumpers within the
unit to be modified. This can be achieved as follows:
3.1 Accessing Input Stage Jumpers
The input stage includes jumpers which enable the mode of operation of the input stage to be
modified. To access the jumpers:
(1) Switch off and disconnect power before opening unit.
(2) Remove the 10 screws holding the lid on then remove lid. Do not disconnect earth strap.
(3) Change the jumper settings as per the diagrams in the following sections. Do not operate
the unit with the lid removed due to the power voltages accessible within the unit.
(4) Replace lid and the 10 screws.

Boston Electronics Corporation, 91 Boylston Street, Brookline MA 02445
[phone] or [phone] * fax [phone] * [email protected] * www.boselec.com
Bartles Industrial Estate, North Street, Redruth, Cornwall TR15 1HR
Tel. [phone] E-mail: [e-mail]
Fax. [phone] Web: http://www.scitec.uk.com
3.2 Jumper Settings Overview
A simplified input stage circuit diagram is shown in Figure 1. This shows all of the jumpers in
the input stage.
Figure 1 Input Stage Jumper Options
A diagram of the position of the input stage jumpers is shown in Figure 2.
Figure 2 Input Stage Jumper Positions

Boston Electronics Corporation, 91 Boylston Street, Brookline MA 02445
[phone] or [phone] * fax [phone] * [email protected] * www.boselec.com
Bartles Industrial Estate, North Street, Redruth, Cornwall TR15 1HR
Tel. [phone] E-mail: [e-mail]
Fax. [phone] Web: http://www.scitec.uk.com
3.3 Single Ended DC Coupled Input (Default Setting)
The default setting is for a single ended DC coupled input. This mode gives the best noise
performance. Although the input is DC coupled, the input will not saturate with up to the
following DC offsets on the input:
Input Gain Setting Maximum DC Input Offset Before
Saturation Occurs
1V to 300µV ±10V
100µV to 10µV ±1V
3µV ±300mV
The input impedance of the lock-in in this mode is 1012Ω||1nF.
The jumper settings for this mode is given in Figure 3. The equivalent circuit is shown in
Figure 4.
Figure 3 Single Ended DC Coupled Input Settings
Figure 4 Single Ended DC Coupled Input Circuit
3.4 Single Ended AC Coupled Input
For applications where large amounts of DC offset are expected on the input, the input stage
can be AC coupled with an RC network. Please note that the high value resistor will add a
large amount of thermal noise to the input. (This mode of operation is common to a lot of other
manufacture’s lock-in amplifiers. When measuring the input noise of their instruments, it is
standard practice to short the input which has the nice result of removing the thermal noise of

Boston Electronics Corporation, 91 Boylston Street, Brookline MA 02445
[phone] or [phone] * fax [phone] * [email protected] * www.boselec.com
Bartles Industrial Estate, North Street, Redruth, Cornwall TR15 1HR
Tel. [phone] E-mail: [e-mail]
Fax. [phone] Web: http://www.scitec.uk.com
this resistor from their measurements. However, how often do you make measurements with
the input shorted in real life??)
In this mode of operation the input can have up to ±12V DC offset before saturation occurs on
all input gain settings.
The input impedance of the lock-in in this mode is 6.8x106Ω||100nF.
The jumper settings for this mode is given in Figure 5. The equivalent circuit is shown in
Figure 6.
Figure 5 Single Ended AC Coupled Settings
Figure 6 Single Ended AC Coupled Circuit
3.5 Single Ended Current Input
The input stage can also used as a current input due to the high impedance of the input stage
opamps. The current input is converted to a voltage by the 6.8MΩresistor which has a
tolerance of 1%. This mode of operation has not been full characterised by Scitec Instruments
and is not guaranteed.
The input impedance of the lock-in in this mode is 6.8x106Ω||1nF.
The jumper settings for this mode is given in Figure 7. The equivalent circuit is shown in
Figure 8.

Boston Electronics Corporation, 91 Boylston Street, Brookline MA 02445
[phone] or [phone] * fax [phone] * [email protected] * www.boselec.com
Bartles Industrial Estate, North Street, Redruth, Cornwall TR15 1HR
Tel. [phone] E-mail: [e-mail]
Fax. [phone] Web: http://www.scitec.uk.com
Figure 7 Single Ended Current Input Settings
Figure 8 Single Ended Current Input Circuit
3.6 Differential Input
The above 3 modes can all be converted to a differential input by removing the link between
pins 5 and 6.
It is useful to place the unit into differential mode if you do not wish the input signal ground to
be connected to the lock-in amplifier signal ground. This can be done to break a ground loop or
other such problem.
For example the jumper settings for the Differential DC coupled input are shown in Figure 9
and the equivalent circuit is shown in Figure 10. The other modes are similar.
Please note that it is important that the input signals are not left to float when in differential
mode but are externally coupled to ground by some method. If this is not done, the DC offset
on the input will rise (or fall) until the levels specified in section 3.3 are breached. The input
signal will then be distorted as it travels through the input stage and you will get an erroneous
result.