QUOTE from "Real Ham Radio"

The Z3801A has been more readily available from numerous sources, and appears to be more popular. The Z3816A is a newer version, and has some hardware improvements. It is very similar to the Z3801A, but with these major differences:

Receiver	Front Panel	Serial Data	Output Freq.	GPS RX Channels	Serial Port	Oven Oscillator	Power Required
Z3801A	Six LED	RS-422	10 MHZ	Six	Fixed	HP 10811	+27 or -54 VDC
Z3816A	Four LED On-Off Switch	RS-232	10 MHZ, 19.6608 MHZ	Eight	User Config.	MTI 260	27 VDC or 120 VAC

The Z3816A took advantage of a newer Motorola GPS receiver that had eight instead of the earlier six satellite channels. Also, the serial port in the Z3816A is already set up for RS232.

In the Z3816A, HP used an MTI Series 260 OCXO instead of their larger HP 10811 double oven oscillator that was in the Z3801A. The MTI oven oscillator is a high stability/high reliability oven controlled crystal oscillator (OCXO). MTI states that it offers thermal stabilities from 2.0E-010 to 4.0E-009 over a 100 degree C temperature range, rivaling Rubidium atomic clock performance. They also mention the 260 Series has been used in many applications worldwide as a direct replacement for atomic clocks.

So, which oscillator is better, the HP 10811 used in the Z3801A, or the MTI 260 used in the Z3816A? Since a GPS disciplined oscillator is compensated for long term crystal aging, all that remains is the temperature effect on frequency. The MTI oscillator design has better temperature stability, and is probably better on phase jumps, which is mainly what matters for GPS timing applications. The HP 10811 dual oven oscillator is a super low phase noise floor design, which is probably not important for most hams. This makes the MTI oscillator the preferred choice.

Since the Z3816A has these hardware advantages, many would choose it over the Z3801A. However, I doubt that many amateur users would be able to measure a difference in their respective performances. For most, the Z3801A performance is substantially better than what they used to have for a frequency or time standard.