BNC input with 600 Ohm termination - How to drive?

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Chuck99
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Posted by Chuck99: Thu. Jun 23, 2016 - 02:05 AM
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I have a BK Precission 1856D Frequency Counter. 

It has a BNC input connector for a 10MHz external reference frequency input. 

The input is terminated with 600 Ohms.

 

     From the 1856D Manual

EXT. STD. IN: 10 MHz (External Standard Frequency Input)
LEVEL: 1.5Vrms to 5Vrms
IMPEDANCE: Approx. 600Ω

 

I want to connect an external reference oscillator using a coax cable that has a mating BNC connector. 

The coax cables I've found have either 50 Ohm or 75 Ohm characteristic impedance.

 

1) Why did they choose a 600 Ohm internal termination for the BNC?

 

2) How best to drive this input?

    I've considered using a BNC "T" with a 50 Ohm BNC Terminator at the input connector and using a standard 50 Ohm cable.

    Is there a better way?

 

 

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Learning and Information, General Electronics
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Bronze Markbobgardner
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Posted by bobgardner: Thu. Jun 23, 2016 - 02:19 AM
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If the generator has a 50ohm R, and its putting out a 5V square wave into unterminated air, and you connect it to a second 50ohm load at the counter, the volts drops to 2.5, but that might be ok. Into 600, it wont drop so much. I thought you didnt need to worry about transmission line effects until the line was > 1/4 wavelength? I bet Jim The Ham knows all the Rules Of Thumb. f*wavelength=c I tried to solve for wavelength using 3e8 as c and got .033m. That right?

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Chuck99
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Posted by Chuck99: Thu. Jun 23, 2016 - 02:27 AM
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At 10MHz, the free air wavelength is 300E6/10E6 = 30 meters.

 

In the coax the speed is somewhere around 200E6, so the wavelength is around 20 meters.

 

The cable I'll be using will at most be 3 feet.  So does that mean I can ignore the 50 Ohm impedance of the coax cable and just drive the input direct?

 

 

 

... and you connect it to a second 50ohm load

That is pretty much what I described as using a BNC "T" and 50 Ohm Terminator.

 

 

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davef
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Posted by davef: Thu. Jun 23, 2016 - 10:54 AM
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I would prefer to properly load the oscillator, ie your 2nd option ... as long as the signal provided to the counter is >1.5Vrms.  I assume the oscillator has been characterised with a 50 Ohm load.

Last Edited: Thu. Jun 23, 2016 - 10:55 AM
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Chuck99
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Posted by Chuck99: Thu. Jun 23, 2016 - 07:02 PM
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davef, thanks for your reply.

 

The oscillator that I'm using is the Abracon AOCJYR-10.000MHz-M5625LF which has LVCMOS output. 

I plan to use a 74ACT240 or similar to buffer the oscillator and 50 Ohm output impedance to match the coax.

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ka7ehk
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Posted by ka7ehk: Thu. Jun 23, 2016 - 07:13 PM
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If you are using a 50 ohm source and a 50 ohm coax, then add a BNC 50 ohm terminator. The extra 600 ohms will only reduce the load impedance to 46 ohms and that will be close enough. 

 

Actually, if you have a LVCMOS output, I would be happy just connecting it with a coax to the 600 ohm load. Adding the ACT buffer will speed up the rise/fall times and increase the presence of overshoot/ringing (because the rise/fall is now much shorter compared to the transmission line electrical length). 

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Thu. Jun 23, 2016 - 07:14 PM
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Chuck99
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Posted by Chuck99: Thu. Jun 23, 2016 - 07:27 PM
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The oscillator (Abracon AOCJYR-10.000MHz-M5625LF) specifies a 15pf load, so I need a buffer between the oscillator and the cable.

I chose the ACT buffer to handle the 50 Ohm cable (100 Ohm load on the buffer).

 

If I use an HC or HCT buffer, can I drive the cable direct to the 600 Ohm input - no 50 Ohm series resistor at the source or 50 Ohm terminator at the input?

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ka7ehk
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Posted by ka7ehk: Thu. Jun 23, 2016 - 08:45 PM
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I would still use a 50 ohm series resistor at the buffer output. And, in my mind, HC or HCT would be better than ACT due to the slower rise/fall times. 

 

To get within +/-10 ppb, it needs 15pf of load c (though there are other contributors). One HC buffer will have around 7pf of input C. Connect two in parallel. That will give you your desired C and give you half the internal load impedance and twice the drive current. 

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Chuck99
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Posted by Chuck99: Thu. Jun 23, 2016 - 09:01 PM
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Just so I have this clear, I should use a 50 Ohm series resistor at the output, but no 50 Ohm termination resistor at the input to the frequency counter.  Is that correct?

 

 

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ka7ehk
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Posted by ka7ehk: Thu. Jun 23, 2016 - 09:17 PM
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Hmmm. Think, oh my brain. Think!

 

Several years ago, I made a little test signal source out of an HC buffer chip. 3 for the RC oscillator and 3 for the output driver. Adjustable frequency. NO series OUTPUT R and it was just fine with at least a few feet of 50 ohm or 75 ohm coax. So, I would forget about the 50 ohm source resistor.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Chuck99
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Posted by Chuck99: Thu. Jun 23, 2016 - 09:34 PM
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OK.  Thanks Jim.

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Torby
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Posted by Torby: Sat. Jun 25, 2016 - 04:17 PM
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I bet it would work just fine if you plugged a coax between the two bnc connectors 

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