## BNC input with 600 Ohm termination - How to drive?

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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?

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?

Imagecraft compiler user

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.

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

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.

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

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?

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"!

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?

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"!