Zach, I got an answer to my own question, I aquired 74AC line drivers not 74ACT line drivers. The ACT devices are TTL compatible right there in the datasheet! The datasheet says 1.5V is enough to turn on the driver while the AC device at 5V needs at least 2.5V, which the Si5351a will never reach. Thanks for reading at least my ineptitude on this! 73

Thanks for that. Have you ever ran into the 74ac244 not 'turning on' or in other words, the si5351 output not meeting the 74's input threshold? The device is supposed to turn on with TTL voltages, but I cannot get my si5351 to output more than 2.8v peak to peak. Have you ever ran into that with your line drivers?

Got it. So I got a 74AC244PC and I measured the output Impedance to be around 14 Ohms. a 4:1 transformer now makes sense as it should be 55 Ohms after the transformer. Neat! I have a question though. In the schematic I see you outputted Pin 16 to TR1. My DIP version of the chip gives a nasty Mhz signal on pin 16 when fed 1 Khz. I probed around and I do see an amplified signal on Pin 17, not 16. Maybe its a slightly different pinout with the 74AC244PC? I will check again soon, but the signal is definitely on PIN 17 with the chip I have 73

Just wondering what the reason is for the Transformer after the line driver? If its 1:1, where does the impedance matching come in?

Amazing Well done!

Got thanks! One more question on the Quectel - When the thing is powered it always starts close to midnight 23:59:48 . I understand that this is because the GPS doesn't know what time it is because it hasn't received any satellite time stamp yet. I do notice that even though I might never get a fix, if at least one satellite is heard, this time will change in the $GPRMC sentence to the current time UTC. Is there any way to know this sudden change in time occured? I tried looking at the GPS Time in $GPRMC below, but without a good fix, the date will not change. Is there any NMEA sentence to say the time stamp was updated BUT there is not yet a fix? I'm only interested in the correct time. Jeff $GPRMC,220521,V,0.0,N,0.0,W,0.0,,060180,,W*70