[Radio1950]

Hi Richard,

good to hear that you are progressing with that project.
I assure you that it will be a very important "standard" in your workshop that you can rely on.

As Wrangler says, the Zo of that circuit will be variable depending on a host of factors, and AmbientNoise has given a method of measurement.

The basic concept is that if you load up a Source with a test resistor of value equal to the source Zo, then the Source open circuit voltage falls to half value.
Best done at RF with a CRO and 10X probe, and with a load pot to start with, progressing to small resistors with low stray capacitance and inductance, once you have found a ball park figure.

The principle holds true for Nuclear Power Stations and "potato cells", although you would never carry out the actual test of either.

I would suggest you move away from the TTL driver and use a Video Driver chip or discrete amp.

Although it doesnt help you here with your circuit, here's what I did with my NEC PCB GPSDO which has a uBlox Lea 5-T and a 10 MHz output buffered by LVDS differential amps giving a 400mV PP square wave.
It may give you suggestions.

I broke quite a few design and construction rules, but the outcome is a working unit.

To take signals “off GPS PCB board” I just “isolated” them with 0.1 MF ceramic caps and sometimes with 100R resistors, or both C and R.

As suggested by others, I used a DIY 2 pole 10 MHz tuned LC filter to convert the square wave to sine.
I wouldn't bother with an RC filter.

The filter is two 9.5 uH coils on 1cm tubing, 50pF adj parallel tuning caps, top coupled with 22pF, and input and output taps at 10% up the windings for quasi Z match.
This works very well in practice, with a "sine looking" output.

SimSmith simulator gives approximate figures of -25dB and -35dB for second and third harmonics for the filter.
The loss is approx 5dB.

It is very worthwhile to have your GPSDO ref signal as sine, because some monitoring test equipment eg Freq Counters can be tricked by second harmonics etc of square waves.

I aimed for a final ref output of 10MHz and +5dBm50.
I used an amplifier after my 10 MHz filter.

The RF Amp is a cheap LNA PCB ex eBay with SPF51 MMIC type amp, with input and output coupling caps changed to 1000 pF and collector inductor changed to 1 mH approx.
This to modify the response “start” down from 50 MHz to below 1 MHz. The caps are just soldered in parallel with the existing SMDs.
Amp gain is approx 20 dB.
The output Z is a nominal 50R, but in practice, for this type of project, it can be 1-150R or so.
If you actually have a very low Zo, I would insert a 47R resistor in series with the output as protection against a host of unusual "load" scenarios.

My RF Amp has the output of the 10MHz filter signal as an input, and produces an unbalanced 50 ohm output at a nominal level.
The Amp output level is set by slightly mis-tuning the filter output tuned circuit, and watching the wave shape on a CRO for sine/level compromise.
I was lucky with my circuit levels.
In practice I get +7dBm.

An emitter follower is also very useful as a defined Zo output driver, after you set your level.
If you use an emitter follower and an output level of 0dBm, the signal input will have to be 0.25 V RMS and the stage has to handle a swing of approx 700 mV PP.

Is your uBlox ref Freq actually 1.0MHz or 10 MHz please??

Very good luck to you.
Hope some of this helps.