cheap gps receiver for tx syncing applications

I'd like to know why you'd need a GPS to sync a transmitter in the first place.

"I'd like to know why you'd need a GPS to sync a transmitter in the first place."

You should know .... coverage. We can't use more power for coverage, so multiple transmitters is the only legal/viable solution. If you want the content to be on the same frequency in the same area, the content must be absolutely sync'd, else they interfere with each other. GPS is one way of sync'ing transmitters.

Where can the average Part 15 person find the instructions to do this?

Can A Part15'er really be average?

I thought we all knew and understood we are strange.
😉

The Crow, you are TOO TRUE. We are SO strange. But maybe strangeness has an average......

No, still don't know what I'm talking about.

Maybe, the original question really is:

What kind of electronic approach do you
use to achieve syncing of multiple transmitters?
How do you hook up the GPS and what info
from the GPS is used? What does the circuitry
look like? What is the theory behind it all?

Somewhere around Newport R.I. there were
a bunch of TIS transmitters that were synced
up, They were somewhere in the 1610 to
1700 range. Here's the funny part. The transmitters
appeared to be on the exact same frequency, but
the voice messages were not synced. So we drove
through an area in the car where two voices were
trying to talk on top of one another. There was no
hetrodyne at all, that I can remember.

Maybe they weren't synced but were very close together
in frequency just by luck.

Anyway, for whatever it's worth, I have no idea
how to use a GPS to sync a bunch of transmitters.
I guess the Hamilton Rangemaster has the circuitry
to allow for that.

I do remember using WWV to align the 100 kHz crystal calibrator
on my Heathkit HR-10B "communications" receiver.
I put the word communications in quotes because the
HR-10B was not a very good receiver. It had all kinds
of problems, as any ham who owned one would know.
Still, I loved the receiver anyway, and I do have one
in my current collection.

Best Wishes,
Bruce, Dog Radio Studio 2

Perhaps this will help:
http://www.lammertbies.nl/comm/info/GPS-time.html

It's not easy.

The GPS receiver module originally referenced, like many others that come and go on the market, has a "1 pulse per second" output pin that is typically "TTL" logic compatible.

The simplest way to sync multiple transmitters would be to use a PLL/counter circuit to multiply the 1 Hz reference frequency from the GPS up to the operating frequency. In reality this might involve one PLL/counter circuit to multiply 1 Hz to 10 kHz which would then serve as the reference for a second PLL/counter that multiplies the 10 kHz reference to the operating frequency (in increments of 10 kHz). Multiple transmitters will then be phase locked to the 1 Hz GPS reference, resulting in essentially identical frequency and phase at each transmitter.

This simple approach has a MAJOR drawback: loss of the integrity of the 1 Hz signal will cause the transmitter's frequency synthesizer to go into la la land. The transmitter's frequency will be unpredictable.

So, the safe, but more complicated, way to do it is to use the 1 Hz GPS signal through a PLL/counter frequency multiplier to produce the same frequency as the transmitter's crystal oscillator. The phase difference between the multiplied GPS reference and the crystal oscillator is then used to "pull" the oscillator frequency until the frequency and phase are identical. The oscillator needs to be a "VCXO" (Voltage Controlled Crystal Oscillator). A varactor diode is used to vary the capacitance across the crystal to adjust the frequency in a range of a few hundred Hz. A phase comparator circuit must be used to detect the phase difference between the GPS-based reference frequency and the crystal oscillator frequency. The phase-difference voltage is applied to the varactor diode resulting in locking the crystal oscillator to the GPS-based reference.

The crucial advantage here is that loss of the GPS-based reference will only result in the transmitter frequency being off by a few hundred kHz.

The crystal-controlled Rangemaster should work without the frequency circuit gymnastics ... it doesn't synthesize frequencies, so you need to be sure the one you order is the one you will use ... it's not frequency agile, you can't adjust the broadcast frequency other than replacing the crystal.