Krohn-Hite 3945 User manual

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MODEL 3945
170Hz to 25.6MHz
Dual Low-Pass
Programmable Butterworth Filter
Operating Manual
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Service and Warranty
Krohn-Hite Instruments are designed and manufactured in accordance with sound engineering practices
and should give long trouble-free service under normal operating conditions. If your instrument fails to
provide satisfactory service and you are unable to locate the source of trouble, contact our Service De-
partment at [phone], giving all the information available concerning the failure.
DO NOT return the instrument without our written or verbal authorization to do so. After contacting us,
we will issue a Return Authorization Number which should be referenced on the packing slip and purchase
order. In most cases, we will be able to supply you with the information necessary to repair the instrument,
avoiding any transportation problems and costs. When it becomes necessary to return the instrument to
the factory, kindly pack it carefully and ship it to us prepaid.
All Krohn-Hite products are warranted against defective materials and workmanship. This warranty ap-
plies for a period of one year from the date of delivery to the Original Purchaser. Any instrument that is
found within the one year warranty period not to meet these standards, will be repaired or replaced. This
warranty does not apply to electron tubes, fuses or batteries. No other warranty is expressed or implied.
Krohn-Hite Corporation reserves the right to make design changes at any time without incurring any obli-
gation to incorporate these changes in instruments previously purchased.
Modifications to this instrument must not be made without the written consent of an authorized employee
of Krohn-Hite Corporation.
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Model 3945
170Hz to 25.6MHz
Dual Low-Pass Butterworth
IEEE-488 Programmable Filter
Operating Manual
Serial No. ______________
Copyright 2007, All rights reserved. Contents of this publication may not be repro-
duced in any form without written permission of Krohn-Hite Corporation. Printed in
U.S.A. Ver. 021507.
Krohn-Hite Corporation
15 Jonathan Drive, Unit 4
Brockton, MA 02301
Tel: [phone]; Fax: [phone]
[email protected]; www.krohn-hite.com
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3945 Table of Contenets
Section 1 General Desciption ······················1-1
1.1 INTRODUCTION ·······················1-1
1.2 SPECIFICATIONS·······················1-1
1.2.1 Function........................1-1
1.2.2 Channels 1.1 and 1.2................1-1
1.2.2.1 Filter Characteristics ·············1-1
1.2.3 Channel 2.1......................1-2
1.2.3.1 Filter Characteristics ·············1-2
1.2.4 Amplifier Mode Characteristics (CH2.1 Only) . 1-4
1.2.5 High Frequency Band-Pass Operation .....1-4
1.2.6 General ........................1-4
1.2.7 Options ........................1-5
Section2 Operation····························2-1
2.1 INTRODUCTION ·······················2-1
2.2 TURN-ON PROCEDURE···················2-1
2.3 SELFTEST···························2-1
2.4 FRONTPANELCONTROLSANDDISPLAY·······2-1
2.4.1 DataKeysandDisplay...............2-1
2.4.2 ParameterandControlKeys...........2-2
2.4.3 Channel Selection..................2-3
2.4.4 Cutoff Frequency ..................2-3
2.4.5 Input Gain (Pre-Filter) ...............2-3
2.4.6 OutputGain(PostFilter)..............2-4
2.4.7 Digit Select/Increment and Decrement .....2-4
2.5 REARPANELCONTROLSANDCONNECTORS····2-4
2.5.1 Introduction......................2-4
2.5.2 Front Connector Selector Switch.........2-4
2.5.3 BNC Connectors...................2-4
2.5.4 DCLevelAdj.....................2-4
2.5.5 Power .........................2-5
2.5.6 GPIB Connector ...................2-5
i
Table of Contents
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2.6 FILTEROPERATION·····················2-5
2.6.1 Introduction......................2-5
2.6.2 Variable Band-Pass/Band-Reject Operation . . 2-5
2.6.2.1 Band-Pass ··················2-5
2.6.2.2 High Frequency Band-Pass ··········2-5
2.6.2.3 Band-Reject··················2-5
2.6.3 Amplitude Response ................2-6
2.6.4 Phase Response...................2-7
2.6.5 GroupDelay .....................2-7
2.6.6 Transient Response.................2-9
Section3 GPIBProgramming······················3-1
3.1 INTRODUCTION ·······················3-1
3.2 PRELIMINARYPROGRAMMINGINFORMATION····3-1
3.2.1 GPIBPrimaryBusAddress............3-1
3.2.2 IEEE-488 Programming Connector .......3-2
3.3 ASCIIDATACOMMANDS··················3-2
3.3.1 Format.........................3-2
3.3.2 Types of Data Commands .............3-2
3.3.3 Table of ASCII Commands.............3-3
3.3.4 Examples.......................3-4
3.3.4.1 Example 1···················3-4
3.3.4.2 Example 2···················3-4
3.3.4.3 Example 3···················3-5
3.4 IEEE-488 STANDARD COMMANDS············3-5
3.4.1 Multi-Line Messages ................3-5
3.4.1.1 Polling Commands ··············3-7
3.4.1.2 Parallel Polling ················3-7
3.4.1.3 Service Request and Serial Polling ······3-8
3.4.1.4 Serial Responses ···············3-8
3.4.2 Uniline Messages ..................3-9
3.5 TALKERFORMAT·······················3-9
3.5.1 ParameterInformationFormat..........3-10
3.5.2 Model Number and Software Version Format . 3-10
II
Table of Contents
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3.6 PROGRAMMINGEXAMPLES················3-11
3.6.1 Example1–MicrosoftQuickBasic.......3-11
3.6.2 Example2–BorlandTurboC...........3-12
3.6.3 Example3–National Instruments IBIC.....3-17
Section4 IncomingAcceptance ····················4-1
4.1 CHANNELS1.1AND1.2 ··················4-1
4.1.1 Introduction......................4-1
4.1.2 Required Test Equipment .............4-1
4.1.3 Procedure.......................4-1
4.1.3.1 Low-Pass/High-Pass Response ·······4-1
4.1.3.2 Cutoff Frequency Accuracy ··········4-2
4.1.3.3 Band-Pass/Band-Reject Response ······4-2
4.1.3.4 Stopband Attenuation ·············4-4
4.1.3.5 Pre-Filter and Post Filter Gain Accuracy ····4-4
4.1.3.6 Noise Check ·················4-5
4.2 CHANNEL2.1·························4-5
4.2.1 Introduction......................4-5
4.2.2 Required Test Equipment .............4-5
4.2.3 Initial Settings and Procedure ..........4-6
4.2.4 DCOutputLevel...................4-6
4.2.5 GainAccuracy....................4-6
4.2.6 Distortion.......................4-6
4.2.7 Squarewave Rise and Fall Times ........4-7
4.2.8 Frequency Calibration ...............4-7
4.2.9 Wideband Noise ...................4-7
4.2.10 Stopband Attenuation................4-8
III
Table of Contents
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Model 3945 Filter
IV
Table of Contents
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SECTION 1
GENERAL DESCRIPTION
1.1 INTRODUCTION
The Model 3945, Figure 1, programmable Butterworth/Bessel filter provides one Butterworth channel of
low-pass, tunable over the range from 170Hz to 25.6MHz; and two independent Butterworth or Bessel
channelsoflow-pass,high-pass,by-pasoronechannelofband-passorband-reject,tunableovertherange
form 3Hz to 2MHz.
The 3945 has been specifically designed for applications requiring high frequency band-pass filtering.
The high cutoffmay be set to any frequency between170Hz and 10MHz, and low cutoffto any frequency
between 3Hz and 2MHz.
Input gain is useful to users wanting to increase the amplitude of the signal and improve the sig-
nal-to-noise ratio before filtering.
Channels 1.1 and 1.2 furnish Bessel or Butterworth transfer functions can be programmed to operate as
twolow-pass,twohigh-pass,oneband-passorone-bandrejectfilterwithgainsupto40dBperchannel.
The filter is controlled by the front panel keyboard or over the IEEE-488 bus. Provided are LED displays
and indicators for input and output gain, cutoff frequency, channel selection, overload (channel 2.1 only)
and remote operation. Non-volatile, battery-backed, CMOS memory permits storing and recalling of 99
selectable groups of front panel settings. Storing and recalling group settings is accomplished with a sin-
gle command. Self-testing of the digital circuitry occurs upon power-up.
1.2 SPECIFICATIONS
1.2.1 Function
Channel 1.1 and 1.2: Two independent channels of low-pass, high-pass or bypass; one channel of
band-pass or band-reject.
Channel 2.1: One low-pass filter channel or amplified bypass.
1.2.2 Channels 1.1 AND 1.2
1.2.2.1 Filter Characteristics
Type: 4-pole Butterworth (maximally flat) or Bessel (linear phase).
Attenuation Slope: 24dB/octave per channel.
Tunable Frequency Range (fc): 3Hz to 2MHz.
Section1-GeneralDescription
1-1
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Frequency Resolution: 1Hz from 3Hz to 1kHz; 10Hz to 2kHz; 100Hz to 100kHz; 1kHz to 1MHz;
10kHz to 2MHz.
Cutoff Frequency Accuracy (fc): ±2% or least significant digit (which ever is grate) 20Hz to 500kHz;
±5% to 2MHz.
Relative Gain at fc: Butterworth, -3dB; Bessel, -7.58dB.
Bandwidth: dc to fc, dc coupled; 0.2Hz to fc, ac coupled (low-pass); fc to 10MHz (high-pass).
Passband Response (0dB, Input/Output gain): ±0.5dB to 2MHz.
Stopband Attentuation: -100dB to1MHz;-75dBat 10MHz;-55dBat30MHz; -50dB at50MHz;-40dB
to 100MHz.
Harmonic Distortion: -80dB at 1kHz at Vrms.
Noise (referred to input): <200mV with 2MHz bandwidth detector.
Input:
Pre-Filter Gain: 0dB or 20dB; ±0.2dB.
Coupling: ac or dc.
Impedance: 1 megohm in parallel with 100pf.
Maximum Signal (at 0dB gain): ±4.5V peak at fc <1MHz; ±4V peak at 2MHz.
Maximum DC Blocking Voltage: ±200V in ac coupled mode.
Output:
Post-Filter Gain: 0dB or 20dB; ±0.2dB.
Impedance: 50ohms
Maximum Voltage: ±6.5V peak into > 500 ohms; ±1.3V peak into > 50 ohms.
Maximum Current: ±25mA.
DC Level: Adjustable to Zero.
DC Stability: ±0.5mV/°C typical; ±1mV/°C max.
1.2.3 Channel 2.1
1.2.3.1 Filter Characteristics
Filter Type: 4-Pole, Butterworth, low-pass.
Attenuation: 24dB/Octave.
Section 1 - General Description
1-2
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Tunable Frequency Range: 170Hz to 25.6MHz.
Frequency Resolution: 10Hz from 170Hz to 2.56Hz; 100Hz from 2.6kHz to 25.6kHz; 1kHz from
26kHz to 256kHz; 10kHz from 260kHz to 2.56MHz; 100kHz from 2.6MHz to 25.6MHz.
Cutoff Frequency Accuracy: ±2% to 2.56MHz, ±5% to 25.6MHz.
Passband Response: ±0.2dB up to 256MHz, ±0.5dB to 25.6MHz.
Stopband Attenuation: 100dB to 1MHz; 80dB at 10MHz; 70dB at 30MHz; 30dB at 50MHz; 50dB to
100MHz.
Input/OutputCoupling: ACor DC. AC coupling cutoff is approximately16Hz at the input and10Hz at
theoutputwitha 50Wtermination. Notethat theinternal50ohms inputterminationisbefore theACcou-
pling.
NoiseSpectralDensity(10kHzto100MHzreferredtoinput): Below–128dBm/Hzinto50ohms. This
translates into a wideband noise power or voltage for a 30MHz BW of below –53dB or 500mVrms re-
ferred to input.
HarmonicDistortion(1Vrmssinewave): >-60dBbelowsignalupto100kHz(0.1%). Allharmonicsbe-
low 50dB to 1mHz; below 40dB above 1MHz.
SpuriousSignals: Below–80dBmto65MHz;below –75dBmto100MHz. Referredtoinputrepresented
in voltage form: 22mV and 40mV respectively.
DC Stability: ±0.5mV/°C referred to input.
Input:
Pre-Filter Gain: 0dB, +10dB, +20dB; ±0.1dB.
Impedance: Selectable 1M ohms or 50 ohms, ±2%, shunted by 65pF.
MaximumSignal: ±1.5Vpeakwith0dBinputgain,reducedinproportiontoinputgainselected.
Maximum Input Without Damage: 12Vrms with input terminator OFF, 7Vrms with input
terminator ON.
Maximum DC Blocking Voltage: 200V. Note that the internal input termination is before the
AC coupling and can only tolerate 7Vrms when ON.
Output:
Post-Filter Gain: 0dB, +6dB, +20dB, +26dB; ±0.1dB.
Impedance: 50 ohms, ±2%.
Maximum Signal: ±3V peak open circuit; ±1.5V peak into 50 ohms.
DC Level: Adjustable to Zero.
Section1-GeneralDescription
1-3
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1.2.4 Amplifier Mode Characteristics (Channel 2.1 only)
Bandwidth: >50MHz.
Input and Output: Same as Channel 2.1 Filter Characteristics.
Rise and Fall Time: <7ns with 0dB input gain 6dB output gain; <10ns with +20 input or output gain.
<5% ringing or overshoot.
1.2.5 High Frequency Band-Pass Operation
Connecting the output of channel 1.1 or 1.2 (set to high-pass) to the input of channel 2.1 (set to low-pass,
50 ohm input impedance, 6dB output gain), provides a band-pass filter with 0dB insertion loss and the
low-cutoff adjustable between 3Hz and2MHz, and the high-cutoff adjustable between 170Hz and 0mHz.
At high-cutoff settings above 10mHz, the passband response will deteriorate.
1.2.6 General
Frequency: Keypad entry or increment, decrement keys.
Memory: 99 selectable groups; memory is non-volatile battery-backed CMOS.
Overload Modes (Channel 2.1 only): Three selectable modes; non-latching, that monitors all channels
and displays the first channel to have an overload; latching, that maintains the overload display until it is
cleared; and no indications.
Overload Indicators (Channel 2.1 only): LEDS for input and output. Gaindisplay flashes when over-
load occurs on displayed channel.
Self-Test Diagnostics: MPU checks unit upon power-up. Display indicates failure mode.
Displays: 7 segment, green, LED; 0.3” high.
Remote Programming: IEEE-488/1978 GPIB interface. Subsets: SH1, AH1, T6, L4, SR1, RL1, PP1,
DC1, DT0, C0, E1.
Operating Temperature: 0°C to 50°C.
Isolation to Chassis: ±200Vdc.
Storage Temperature: -20°C to 70°C.
Input/Output Connectors: BNC, front and rear.
Power Requirements: 90-132/180-264 volts ac, 50Hz-400Hz, 35 watts.
Dimensions: 3 ½” (9cm) high, 8 ½” (21.8cm) wide, 18” (46.2cm) deep.
Weights: 12 lbs. (5.4kg) net; 14 lbs (6.3kg) shipping.
Accessories: 6 foot, 3 terminal line cord; operating and maintenance manual.
Section 1 - General Description
1-4
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1.2.7 Options
Rack Mount Kit: Part No. RK-37, permits installation of the Model 3945 into a standard 19” rack spac-
ing.
Specifications apply at 25°C ±10°C.
Specifications subject to change without notice.
Section1-GeneralDescription
1-5
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Section 1 - General Description
1-6
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Section 2
OPERATION
2.1 INTRODUCTION
The Model 3945 three channel filter covering the frequency range from 3Hz to 2MHz (channels 1.1 and
1.2)and170Hzto25.6MHz(channel2.1). Allfilterparametersareprogrammableviathefrontpanelkey-
board controls or remotely over the IEEE-488 (GPIB) bus.
The filter has five modes of operation: high-pass, low-pass, band-pass, band-reject, gain (channel 2.1
only) and bypass (channels 1.1 and 1.2 only).
2.2 TURN-ON PROCEDURE
The Model 3945’s line voltage range has been preset for either 115V or 230V operation. To change this
setting, remove the bottom cover to expose the line switch. Be sure to change the fuse to the proper rating
for the line switch setting selected.
Make certain the POWER switch on the front panel is off.
Plug the line cord into the unit, then the ac outlet.
If the Model 3945 is to be programmed remotely, connect the bus cable to the rear panel connector of the
unit.
After reading the Self-Test feature, described next, turn on the Model 3945.
2.3 SELF TEST
When the Model 3945 is turned on, the microprocessor performs a self-test routine whereby the entire
RAM and ROM operation is verified. During the test, the front panel LEDs and DISPLAYS will light se-
quentially. If there is a malfunction on the microprocessor board, such as a defective RAM or ROM, the
sequence will stop and the word “bad” will appear in the DISPLAYfollowed by a number 1, 2 or 3. Refer
to Section 6, Maintenance, to find which RAM or ROM is defective.
When the self-test program is complete, the Model 3945 will return to the last set-up prior to turning the
unit off. The Model 3945 is now ready to operate.
2.4 FRONT PANEL CONTROLS AND DISPLAY
2.4.1 Data Keys and Display
Data entry keyboard controls [0] to [9] and [.] set the numeric value of the parameter selected. To enter
1.5kHz, press the [1][.][5]keys and the parameter key [KILO] and [FREQ]. The cutoff frequency will be
indicated in the DISPLAY.
Section2-Operation
2-1
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2.4.2 Parameter and Control Keys
[KILO] When pressed, multiplies the numeric value of the key-
board entry by 103.
auto [MEGA] When pressed, multiplies the numeric value of the key-
board by 106.
[FREQ] When pressed, enters and/or displays frequency in Hertz.
[TYPE]
When pressed, DISPLAY will indicate the filter type in the
selected channel, “bu.” (Butterworth), “bES.” (Bessel) for
channels 1.1 and 1.2; and “bu.” only for channel 2.1.
Whenpressedagain,thetypewillchangetothenexttype
(i.e. “bu.” to “bES.”). There will be no change for channel
2.1.
When in band-pass or band-reject mode (channels 1.1
and 1.2 only), the type will be changed on each channel
simultaneously.
[MODE]
Whenpressed,DISPLAYindicatesthemodeofoperation
for the channel displayed. For channels 1.1 and 1.2, the
DISPLAY will indicate “bYP.” for bypass, “L.P.” for
low-pass, “h.P.” for high-pass, “b.P.” for band-pass and
“b.r.” for band-reject. Channel 2.1 will indicate “L.P.” or
gAin” (Amplifier mode).
Whenpressedagain,theMODEwillchangetothenextin
the order explained above.
[RECLL]
Whenprecededbyanumber,itwillrecalltheentireinstru-
ment set-up from the memory location selected.
Whenfirstpressed,theDISPLAYindicatesthenumberof
thenextmemorylocationtoberecalled. Forexample,the
DISPLAYwillindicatethefollowing: “n=09”. Pressingthe
[RECLL] key again will recall the entire instrument set-up
of from the next memory location.
When pressed to indicate the next memory location to be
recalled only, pressing the [CE] (clear entry key) will re-
store the DISPLAY to the cutoff frequency setting.
[ALL CH] When frequency, input/output gain, type, mode or cou-
plingareenteredorchanged,theLEDinthe[ALLCH]key
is lit, the new setting will be entered in all channels of the
filter (channels 1.1 and 1.2 only).
[SHIFT] The [SHIFT] key in conjunction with other keys (keys with
red lettering under them) provide additional filter charac-
teristics, and permits front panel entry of the type of GPIB
line termination and address.
Section 2 - Operation
2-2
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Store
When[SHIFT][RECLL]isfirstpressed,theDISPLAYindi-
cates that the number of the next memory location avail-
able. For example, the DISPLAY will indicate the follow-
ing: “n=09”. Pressing[RECLL]againwillstore the entire
instrument set-up into that memory location. If another
memorylocationisdesired,enterthatlocationonthekey-
board and then press [SHIFT][RECLL].
When [SHIFT][RECLL] is preceded by a number (0-98),
the filter will store the entire instrument set-up into the
memory location selected. The maximum number of
memory groups is 99.
When [SHIFT][RECLL] is pressed to indicate the next
memory location only, pressing the clear entry key [CE]
will restore the DISPLAY to the cutoff frequency setting.
AC/DC
Coupling
Pressing the [SHIFT] key followed by the [TYPE] key will
displaytheinputcoupling,indicating“AC”or“dC”,andwill
alternatebetweenthetwowheninthelow-pass, band-re-
ject, amplifier and bypass modes. High-pass and
band-pass modes (channels 1.1 and 1.2 only) will indi-
cate “AC” only.
GPIB
Address
When the [SHIFT] key followed by the [MEGA] key are
pressed, the DISPLAY will indicate the existing GPIB ad-
dress setting. To select a different address setting, enter
the address number in the data keys from [0] to [30] and
pressthe[SHIFT]and[MEGA]keys(seeSection3.2.1for
GPIB addressing information).
GPIB Line
Termination
When the [SHIFT] key followed by the [ALL CH] key are
pressed,theDISPLAYwillindicatetheexistingGPIBLine
Termination Code sequence. To select a different one,
enteranumberfrom[0]to [4]andpress[SHIFT][ALLCH]
keys(seeSection3.2.1forlineterminationinformation).
Software
Version
When the [SHIFT] key followed by the [KILO] key are
pressed, the DISPLAY will indicate the software version
installed (i.e. 3.5).
2.4.3 Channel Selection
The up [D] control key below the CHANNEL display cycles through the channel settings. When held,
channels will cycle through all channels continuously.
2.4.4 Cutoff Frequency
Dataentrykeyboard controls [0]to [9]and [.]setthenumericvalueofthecutofffrequency desired. To se-
lect 1.5kHz, press the [1][.][5]data keys and parameter keys [KILO] and [FREQ]. The cutoff frequency
for the channel selected will be indicated in Hertz on the four digit DISPLAY. The KILO and FREQ keys
will be lit. Also see 2.4.7.
2.4.5 Input Gain (Pre-Filter)
Up [D] and down [Ñ] INPUT GAIN SET controls increase or decrease the input amplifier by 20dB for
channels 1.1 and 1.2 and from 0dB to 20dB in 10dB steps for channel 2.1. The two digit DISPLAY
will indicate the gain selected.
Section2-Operation
2-3
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2.4.6 Output Gain (Post Filter)
Up [D] and down [Ñ] OUTPUT GAIN SET controls increase or decrease the output amplifier by
20dB for channels 1.1 and 1.2, and to either 0dB, 6dB, 20dB or 26dB on channel 2.1. The two digit
DISPLAY will indicate the gain selected.
2.4.7 Digit Select/Increment and Decrement
When the [SHIFT] key is pressed, followed by either the [D] or the [Ñ] keys under FREQ beside the IN-
PUT BNC connectors, the DISPLAYwill intensify either the least or the most significant digit. Pressing
the [D] or [Ñ] keys will then increment or decrement the intensified desired digit.
Pressing [SHIFT] and either [D] or the [Ñ] keys again will intensify the next digit in the DISPLAY. The
[D]key willmovethe intensifieddigittotheleftandthe[Ñ]willmovetheintensifieddigit totheright(di-
rection is labeled in red to the left of keys).
Input Ohms
When the [SHIFT]key is pressed, followed by either[D] or[Ñ] key under the InputGain display, the Cut-
offFrequency displaywillindicatethepresentInputOhmssetting,either “50”for50ohmsor“Hi”for1M
ohms. Pressing the [SHIFT] key and either [D] or [Ñ] key again, will toggle the setting.
Key Click Feature On/Off
When the [SHIFT] key is pressed, followed by the [D] key under the CHANNEL display, the key click
feature will either toggle on or off.
2.5 REAR PANEL CONTROLS AND CONNECTORS
2.5.1 Introduction
Model 3945 rear panel consists of the following: three input and output BNC connectors, dc level adjust-
ments,afuseholder,GPIBbusconnector,frontconnectorswitchesforChannel2.1andanacreceptacle.
2.5.2 Front Connector Selector Switch
For optimum performance when using the rear connectors, the switches on the rear near the connectors
should be in the OFF position. This will disconnect the cabling from the front panel connectors.
2.5.3 BNC Connectors
There are threeinput and output BNC connectors. Each connector (1.1, 1.2 and 2.1) isan independent fil-
ter channel. In the band-pass and band-reject modes, the input and output connectors are configured as in
paragraphs 2.6.2.1 through 2.6.2.3.
2.5.4 DC Level Adj
Proper procedure for adjusting input and output dc levels can be found in the Calibration section of this
manual.
Section 2 - Operation
2-4
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2.5.5 Power
Receptacle: Standard 3 pin.
Fuse: 3/8 amp slow-blow for 120V operation; 3/16 amp slow-blow for 230V operation. To change this
setting, refer to Section 2.2.
2.5.6 GPIB Connector
Standard IEEE-488 interface. Subsets are SH1, AH1, T6, L4, SR1, RL1, PP1, DC1, DT0, C0 and E1.
2.6 FILTER OPERATION
2.6.1 Introduction
The Model 3945 is a Multichannel filter with three channels that can function independently. Each chan-
nel can operate in either the low-pass mode or high-pass mode and provide 24dB/octave attenuation, or
channels 1.1 and 1.2 can be set to the same mode and connected in series, externally, to obtain 48dB/oc-
tave attenuation. The interconnection of two channels, by front panel data key entry, will provide
band-pass or band-reject (null) operation and no external connections are needed.
2.6.2 Variable Band-Pass and Band-Reject Operation
2.6.2.1 Band-Pass
Variable band-pass response is obtained by applying the input signal to channel 1.1. With the Channel
Displayon1.1,setthefiltertothe band-pass modebypressingthe [MODE]key until “b.P.” appearsinthe
DISPLAY. Enter the desired low-cutoff (high-pass) frequency, then set the filter to channel 1.2 and enter
the desired high-cutoff(low-pass) frequency. The band-pass response willappear at the output BNCcon-
nector of channel 1.1 simultaneously.
2.6.2.2 High Frequency Band-Pass
When a highfrequency band-passresponse above 2MHz is desired,the Model 3945 can be configured by
connecting, externally, the output of channel 1.1 or 1.2 (set to high-pass) to the input channel 2.1 (set to
low-pass, 50 ohm input impedance and 6dB output gain for 0dB insertion loss).
This provides a band-pass filter that has a low cutoff (high-pass) that is adjustable between 3Hz to 2MHz
and a high cutoff (low-pass) that is adjustable between 170Hz and 10MHz.
Note: At high cutoff settings above 10MHz, 3945 passband response will deteriorate.
2.6.2.3 Band-Reject
Variableband-reject responseisobtainedbyapplyingthe signaltochannel1.1. WiththeChannel Display
on1.1, setthefiltertotheband-rejectmodebypressingthe[MODE]keyuntil“b.r.”appearsinthedisplay.
Enter the desired low-cutoff (low-pass) frequency into channel 1.1, then set the filter to channel 1.2 and
enter the desired high-cutoff (high-pass) frequency. Anull can be obtained by setting the low cutoff fre-
quency to approximately 0.58 of the desired null frequency, the high cutoff frequency to approximately
1.7 of the null frequency. Fine tuning of both cutoff frequencies can be accomplished by varying the cut-
offfrequency usingtheincrement/decrementfeatureof theDIGITSELECTdescribedinparagraph 2.4.7.
The resolution of the Model 3945 will limit the extent of the null.
Section2-Operation
2-5
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