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Old 5th Aug 2009, 10:04 pm   #1
Runepune
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Default A prescaler for digital multimeters.

The 74HC390 is a versatile little counter/divider logic chip, and can be configured in many ways. That means we only need to use one chip for a 100 division, and two for a 1000 or 10000 division. No need to use three or four 16 pin DIL packages here.
http://home.online.no/~rumad/div/divideby100.jpg


My hand held DMM can reliably measure frequencies up to about 50kHz, With a 1000 division, I can then convert the displayed frequency(kHz), directly in MHz(ie. I just pretend it is in MHz). If you already have a DMM with frequency measurement capabilities and want to build this little prescaler, be sure to check how high the DMM can count. Then, use the appropriate output shown in the schematic. It will for sure make up a handy-dandy little counter that can be thrown in a bag and used for field service. With a current consumption of only 4mA at 10MHz, three AAA battery's(4.5Volts) will last for a long time.
The RF amplifier is probably a mess from an engineers point of view, so I'll guess there's a lot of possible improvements. This one works for me, but if you want to use american types of transistors, you may have to experiment a little with resistor values an such. I'm unsure of the transistor(BF199), but the BF245 JFET that I used, can probably be substituted with a 2N4416.
http://home.online.no/~rumad/div/prescalerschm.gif


Here is how the thing looks like, assembled on a board, under test and ready to be closed in with lids on top and bottom.
http://home.online.no/~rumad/div/prescalercounter.jpg
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Old 6th Aug 2009, 2:28 pm   #2
David G4EBT
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Default Re: A prescaler for digital multimeters.

Welcome to the forum - where you will find that you are amongst friends and kindred spirits!

Very neat and useful little project - thanks for the posting and clear diagrams and pics.

Best regards,

David,
G4EBT
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Old 7th Aug 2009, 3:03 pm   #3
Runepune
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Default Re: A prescaler for digital multimeters.

Thank you David. :wave
Well, not too many interested so far, I can see. Nah... That's OK
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Old 7th Aug 2009, 7:36 pm   #4
OErjan_S
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Default Re: A prescaler for digital multimeters.

I am interested, I have counters with much better precision but this is a nice tool in the field kit.
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Old 7th Aug 2009, 9:02 pm   #5
Runepune
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Default Re: A prescaler for digital multimeters.

Wow, is this Ørjan from The Radioboard? So you are here too?
Hi hi there, anyway
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Old 8th Aug 2009, 10:22 pm   #6
OErjan_S
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Default Re: A prescaler for digital multimeters.

yes it is, I am here and on a few similar forums
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