10 MHz reference clock

[r111]

I need a 10 MHz reference clock to feed into a 50 ohm back terminated SMA connection on an arbitrary waveform generator. The manufacturers specifications on the AWG specify a square wave clock signal with a power of 12 to 14 dBm. I have been looking at oscillators at some online shops but I guess I'm a little uncertain as to what exact specifications I need to get the specified power output.

What exactly is the load capacitance of an oscillator? Does that impedance need to be included when calculating the output power of the oscillator? Will varying values of the load capacitance affect the amount of reflections coming back from the waveform generator?

Would it be reasonable to buy an oscillator with a power output that is >14dBm and then use an RF attenuator to achieve the desired power output?

I guess I'm mainly just having trouble translating the various specifications for the various oscillator ICs available to the specifications that I need.

 

[D.A.(Tony)Stewart]

0 dBm = 1.000 [mW] {milliWatt} in 50Ω

50% d.f. Square wave = 2.0Vpp = 1Vrms ref = -2.22 dBV = 10.79 dBm @ 50Ω

so 13 dBm = ______ you do the math..

So al you need is a 5V Square wave with a 50Ω drive impedance or emitter follower with a series R or equiv. ..rough cut...

You can make a 1 PPM TCXO for about $1 in parts with CMOS inverter. 25 cent Xtal and 25 cent trimmer , fix ca and 1MΩ feedback resistor

Look up Pierce XO Crystal Oscillator and TCXO for AT Cut crystals < 30 ppm with temp,, 30~50 ppm offset to be nulled.
Oh I forgot you need a varicap calibrated for C(12v:1) ratio and look up table on PWM or DAC to tune varicap with thermistor....

Otherwise just get room temp version 15 ppm instead of wide temp...


Load capacitance is the equivalent circuit in parallel with the Crystal when in a Pi Filter Pierce Oscillator hig impedance resonant mode.. The inverter is eith unbuffered (x10) or buffered (x1000) gain .. both "should " give a clean square wave... some prefer unbuffered to prevent spurious harmonic osc mode wit some vendor IC's and some vendor Xtals... Most common is 50/50 ppm -40+70'C 20pF or so....@ 10MHz ..
If you want 0.1 PPM thats a longer story.... with a nano oven using SMT resistors and thermistor in copper and foam...and run above max ambient temp at null of AT Xtal curve F vs Temp.

 

[r111]

Sorry for the simple questions but I'm quite new to this. What is an effective way to create a 50 ohm driver impedance? Can I just do this by attaching some type of capacitive or resistive impedance to the clock output? Fast rise/fall times are mentioned as being important in the manual (which is about all that is mentioned unfortunately) so increasing R or C in series should slow down the rise/fall times. Are there other ways to accomplish 50 ohm driver impedance without adversely affecting the rise or fall times?

 

[D.A.(Tony)Stewart]

Easiest way is SN74128 is designed to drive 50Ω lines with logic levels.. Check with supplier if this is adequate. ~ $2 @1k
https://www.ti.com/product/sn74128



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Are you sure its a square wave> ? usually AWG's have a 0.1ppm TCXO 50Ω sine wave gen inside @10MHz, so just wondering...

 

[r111]

It definitely asks for a square wave. Apparently it uses the square wave in a Phase locked loop to generate its internal 4 GHz clock frequency.

 

[D.A.(Tony)Stewart]

ok pls confirm accuracy and lvl.