LabMaster Series

TECHNOLOGIES

Unistep LabMaster Series

DFM DIGITAL FREQUENCY METER USER MANUAL

Copyright © 2016 - Unistep Technologies

User Manual

DFM – Digital Frequency Meter

2

REVISION HISTORY REV

DATE

Description

BY

1.2 1.1 1.0

Nov 10, 2016 Nov 4, 2016 Oct 16, 2016

Typo corrections Modifications to synchronize with production changes Original draft

OE OE OE

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CONTENTS Section 1 – Introduction Features ............................................................................................................................... 4

Section 2 – Initial Setup 2.1 2.2 2.3 2.4

Power Supply ................................................................................................................ Jumper Configurations .................................................................................................. Test Setup .................................................................................................................... Input Amplifier Gain/Bias Adjustment ........................................................................

4 4 5 5

Section 3 – Range Selection 3.1 3.2 3.3 3.4

100 MHz Range ............................................................................................................ 10 MHz Range .............................................................................................................. 100 kHz Range .............................................................................................................. 10 kHz Range ................................................................................................................

6 6 6 6

Section 4 – Circuit Operation 4.1 4.2 4.3 4.4 4.5

Power Supply ............................................................................................................... Input Amplifier ............................................................................................................. Prescaler and Logic ....................................................................................................... Counter and Range Selection ....................................................................................... Display ..........................................................................................................................

6 7 7 7 8

Section 5 – Specifications 5.1 Specifications ............................................................................................................... 8

Section 6 – Appendices 6.1 Electrical Circuit Diagram ............................................................................................. 9 6.2 Assembly Diagram / Component Locator .................................................................... 10

Copyright © 2016 Unistep Technologies

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User Manual

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1. INTRODUCTION DFM module is a simple but very capable frequency counter that can measure digital input signal frequencies from 0.1 Hz to over 90 MHz, and analog signals from 1 Hz to over 20 MHz with signal amplitude as small as 500 mV p-p. With the recent changes introduced, version 2.0 now also includes features to enable electronic troubleshooting adventures in the lab. With the troubleshooting hints and circuit operation details included in this user manual, we hope that the students will be able to expand their understanding of electronic circuits by analyzing and investigating how the DFM circuits work.

2. INITIAL SETUP DFM modules are, naturally, fully tested after assembly and before shipment from our manufacturing facilities. In a typical school environment, however, these boards receive a lot of handling by different people. We, therefore, suggest that, if possible, the setup and testing explained in this section be carried out by each new user to make sure DFM is in good working condition. The schematic and the component locator diagrams are included at the end of this user manual and should be consulted while carrying out all tests and adjustments.

2.1 Power Supply On-board power supply is a linear regulator that regulates input power from 6-9 V DC down to 5 V DC. Since the difference between the input voltage and the 5 V internal power rail gets dissipated as heat by the regulator IC, it is best to keep the input power voltage between 7 V - 8 V for adequate voltage regulation without excessive heat dissipation. Please note that two lower input terminals of the CN2 terminal block are both tied to GND.

2.2 Jumper Configurations Make sure the jumpers in the circuit are set as follows for regular operation: J1 J2

No shorting plug; This provides a high input impedance for minimum signal loading. Shorting plug between pins 1-2; This routes the signal from the input amplifier to the prescaler circuit. 3 2 1

J2

J3

Shorting plug between pins 1-2; This configuration routes the prescaler output to the counter circuit. J3 1

J4

2 3

Shorting plug between pins 1-2, 3-4, 5-6, and 7-8; J4 is a programming connector for U7. 2

4

6

8

10

1

3

5

7

9

J4

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User Manual

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2.3 Test Setup The diagram below shows a common lab setup to experiment with the board and carry out calibration and functional testing.

2.4 Input Amplifier Gain/Bias Adjustment Input signal amplifier of the DFM circuit does not have any AGC (Automatic Gain Control) capabilities, so it is important to make sure the maximum gain of the amplifier stage is available with all input signal variations. As mentioned before, this gain/bias adjustment is already done at the factory and does not need to be repeated if the setting has not been changed and the frequency measurement is working as expected. If there’s reason to suspect that the R4 trimpot may have been shifted from its calibration setting, or if the unit has trouble measuring frequency, the bias/gain calibration may need to be carried out, as explained below: 1. 2. 3. 4. 5. 6. 7.

Apply a test signal to the input of the unit with frequency about 1.0 MHz and amplitude 0.8 V p-p. Connect Channel #1 of the scope to the input signal and obtain a clear triggered signal display with about 5 cycles visible on the screen. Make sure the probe attenuation is at 10X setting. Connect Channel #2 of the scope to the TP2 test point on the unit. Make sure that the probe attenuation setting is at 10X. Observe the signal on Channel #2 while slowly adjusting R4. At one extreme end of the adjustment range, you should see a waveform with a small average DC value and a very small AC signal on top of the DC. As you approach the other end of the R4 adjustment range you should see the waveform move closer to the positive supply voltage, with severe clipping of the positive half cycles. The optimum setting is when the signal amplitude swing (peak-to-peak amplitude) is at its maximum, just before the top clipping starts.

3. RANGE SELECTION DFM module has four display ranges, selected by the four switches just below the LED display. The label below the switch indicates the maximum signal frequency that can be displayed by that range selection. The two LEDs to the left of the display indicate whether the displayed value is in MHz or kHz, as follows: Amber LED (LD3) -- Displayed value is in MHz Green LED (LD2) -- Displayed value is in kHz.

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User Manual

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These LEDs are also used as gating indicators. Display will be updated with a new reading when the indicator blinks. Please note that the gating (and therefore, the counting) continues even when there’s no input signal. A frequency reading of zero indicates the absence of an input signal, or a signal with a frequency lower than 0.1 Hz. A string of dashes (‘-‘) indicate a measurement overflow which means that the signal frequency is greater than the maximum frequency for the range currently selected. 3.1 100 MHz Range (S1) - Maximum frequency that can be displayed is 99.999 MHz - Resolution is 1 kHz - Amber ‘MHz’ LED (LD3) is turned on and blinks at the gating interval of 1 second. 3.2 10 MHz Range (S2) - Maximum frequency that can be displayed is 9.9999 MHz - Resolution is 100 Hz - Amber ‘MHz’ LED (LD3) is turned on and blinks at the gating interval of 1 second. 3.3 100 kHz Range (S3) - Maximum frequency that can be displayed is 99.999 kHz - Resolution is 1 Hz - Green ‘kHz’ LED (LD2) is turned on and blinks at the gating interval of 1 second. 3.4 10 kHz Range (S3) - Maximum frequency that can be displayed is 9.9999 kHz - Resolution is 0.1 Hz - Green ‘kHz’ LED (LD2) is turned on and blinks at the gating interval of 10 seconds.

4. CIRCUIT OPERATION As can be seen on the electrical circuit diagram (‘schematic’), the DFM circuit is made up of the following functional circuit sections: • • • • •

Power Supply Input Amplifier Prescaler and Logic Counter and Range Selection Display

4.1 Power Supply Power supply section is a linear voltage regulator circuit built around the U4 IC. Power input within the range of 6 – 9 V DC from an external power source is normally connected to CN2-3 power input terminal while the negative output (Ground) is connected to CN2-1. P5 ground post provides a convenient location to attach the ground leads of measuring instruments. Internal power rail is a regulated 5.0 V DC line and can be measured at TP5. The red LD1 LED is a ‘power good’ indicator. U5 is an LDO (Low-Drop-Out) 5V linear regulator that is sometimes installed instead of U4 when the application calls for that. U5 is not installed in the educational version of the DFM.

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User Manual

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4.2 Input Amplifier Input amplifier provides voltage amplification to the input signal to make sure the digital logic circuits have a good signal to work with. The circuit is built around the Q1 and Q2 transistors and provides a fixed voltage gain determined by the ratio of R6 to R5. The R2 and R3 resistors provide an input impedance of around 1 M to the input signal and the back-to-back D1 and D2 switching diodes limit the signal applied to the base of Q1 to about 1.5 V p-p. If the input signal is supplied through a coax cable, the J1 jumper pins can be shorted to present an input impedance of 68 Ohms for better impedance matching. The C5 capacitor shifts the DC component of the amplified signal to about Vcc/2, provided by the voltage divider R7 and R8, and the D3 diode removes any negative voltage swings to protect the input of the U1d Schmitt Trigger gate. As mentioned in Section 2.4, R4 trimpot is used to adjust the bias setting of the Q1/Q2 amplifier configuration to obtain the optimum gain and operating point.

4.3 Prescaler and Logic Section Amplified signal coming from the input stage is converted to a digital signal by the U1d Schmitt-Trigger NAND gate and can be observed at TP3. The digitized signal has two paths to follow from this point to the input of the counter MCU, U7: The first path goes through R11, U2d and U2c, and is at the natural frequency of the signal. The second path is through the Divide-by-10 circuit U3, R14, U2b, and U2c. MCU selects the path to be taken by controlling the logic level at U1c-10 and U2b-5 (also observable at TP4). A high (or logic ‘1’) level on this signal selects 10:1 prescaling. For the 10 MHz, 100 kHz, and 10 kHz range selection, signal takes the first path at the natural frequency. For the 100 MHz range, signal is forced through the second path where it is divided by 10. The MCU multiplies the counted value by 10 to display the correct frequency. U2a gate forces the U3 4-bit counter to load a count of 6 when the counter reaches the final count of 15. This is how divide-by10 is obtained from the 4-bit counter. J2 jumper pins and the P3 test pin are provided as an alternate digital signal input path into the logic/prescaler block, bypassing the input stage. If the signal is applied through this test pin, the counter is capable of measuring signals as high as 90 MHz.

4.4 Counter and Range Selection Section The center of this circuit in this section is the U7 PIC MCU that does all the counting, scaling, and display management. The precise clock signal required for precise frequency measurement is provided by the U6 temperature-controlled crystal oscillator module. Circuitry to the right of the J4 jumper block, including the jumper block itself is the reset, power-on-reset, and the programming section. A short activation of the S5 resets the processor. It is important that the shorting plugs be present on the J4 pins to make sure the MPU receives power and the DP and G segments of the 7-segment displays are driven properly. MCU always wakes up in the default measurement range of 100 MHz, following a power-up or a reset. As mentioned before, S1-S4 switch bank does the range selection, and the LD2 and LD3 LED lamps are used as ‘kHz’ and ‘MHz’ indicators, respectively. Copyright © 2016 Unistep Technologies

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4.5 Display Section Display section is made up of the five multiplexed 7-segment LED displays (DS1-DS5), segment driver PNP transistors (Q3-Q10), and the digit driver MOSFET transistors (Q11-Q15). Segments, including the decimal points, are driven in parallel, all at the same time by the Q3-Q10 transistors. Since these driver transistors are of the PNP type, a logic ‘low’ (or logic ‘0’) signal at the MPU outputs turns the transistors on and applies power to the anodes of segment diodes. Current through the diodes is determined by the currentlimiting resistors R32-R39. R24-R31 resistors control the base currents of the driver transistors. Individual digits are multiplexed by turning the digit driver MOSFETS on and off in sequence. Since the LED displays are of the CC (Common Cathode) type, a digit is turned on and off by connecting and disconnecting the cathodes from the GND line by turning the Q11-Q15 MOSFET transistors on and off. These MOSFET transistors are of the NChannel variety, so they need positive voltages at their gates to turn on.

5. SPECIFICATIONS

6.

Power Supply:

6.0 - 9.0 VDC

Current Consumption:

Maximum approx. 260 mA with 7.5 V DC input and display all zeroes

Maximum Frequency:

20 MHz through the input amplifier 90 MHz through the digital input pin P3

Gating Window:

10 s for the 10 kHz range; 1.0 s for all other ranges

Frequency Resolution:

0.1 HZ in the 10 kHz range

Sensitivity:

0.5 V p-p at the input amplifier TTL level at P3

Accuracy:

Better than 0.005% on all ranges

Display Digit Multiplexing:

333 Hz

APPENDICES

6.1 Electrical Circuit Diagram 6.2 Component Locator

Copyright © 2016 Unistep Technologies

DFM-USM-1v2

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B

C

D

E

F

G

H

I

J

K

1

INPUT AMPLIFIER

PRESCALER Vcc

Divide-by-10 Counter

Vcc

Vcc

R9

SIGNAL INPUT

2

R4

R7 E

C2 D

C1

CN1 IN1 IN2 GND

R11

U2d

C5

R3

R5

+

C6

U2a

6.0 - 9 V DC GND GND

LD1 R15

C8

U1c

R8

C4

2

P5 Vcc

D3 R10 U2c

J1

TP6

C7

R13

D2

D1

P4

CN2

Vcc

P3

J2

R6

S

R2

D4 D5

TP5

TP2

G

R1

P2

C

Q1

U5

Vcc

Q2

B

TP1

P1

U4

Vcc

U3

TP3

U1d C3

1

POWER INPUT

U2b

3

C12

R14

R12

C13

C14

Decoupling Capacitors for U1, U2, U3, U6

C15

3

TP4

J3 P6 Prescale Control H : Prescale = 10:1 L : Prescale = 1:1

4

Vcc

Vcc J4

U6 R16

Vcc En

Out Gnd

40.000 MHz

1

2

3

4

5

6

7

8

9

10

4 C10

R21 S5

DS1 – DS5:

RST

Q3 – Q10

Vcc

Anode Drivers

C11

5

1 2

R17

R18 R19 R20

3 4 5 6 7

TP9

8 9

S1

S2

S3

S4

6

10

TP8

11 12 13 14

MCLR RA0 RA1 RA2 RA3 RA4 RA5 Vss OSC1 RA6 RC0 RC1 RC2 RC3

PIC18F25K22

U7 RB7 RB6 RB5 RB4 RB3 RB2 RB1 RB0 VDD Vss RC7 RC6 RC5 RC4

R24

28

18

15

E

C

D

B C

D

R34 R33

R22

C9

DS1

R32

R23

DS4 D

Q13 R42

6

D

Q15 G

G S

S

R41

DS5 D

Q14

G

G S

R40

DS3 D

Q12

G

10 kHz

100 kHz

10 MHz

100 MHz

LD2 (Grn) (kHz)

LD3 (Amb) (MHz)

DS2

D

Q11

17 16

E

C

D

A G

F

B

R35

Q3

20

E

C

D

A G

F

B

R36

Q4

R31

E

C

A G

F

B

R37

Q5

R30

22 21

D

A G

F

B

R38

Q6

R29

E

R39

Q7

R28

23

19

Q8

R27

25 24

A G

F

Q9

R26

26

5

Q10

R25

27

S

S

R44

R43

Range Selection

7

7

DISPLAY

COUNTER & RANGE SELECTION U1b

U1a

8

BF256 Top View

2N4403 Top View

G S D

E B C

2N3906 Top View

2N7000 Top View

E B C

S G D

Toronto, Canada Unistep Technologies DFM – DIGITAL FREQUENCY METER Circuit Diagram By: 1 of 2 8 OE Page: Sep 29, 2016 2.11A/C Release Date: Version: File: DFM-B20-ECD-2e11.vsd Print Date: Nov 10, 2016 Drawing:

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D

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