GPSDO: a new 10 MHz distribution (and isolation!) amplifier

Many attempts have been made in the past to provide a low phase noise 10 MHz signal as a frequency reference, however recently I experienced some trouble because of ground loops. Normally no problem to decouple from DC voltages, but still the ground stays connected. The only way to avoid such ground loops is to use potential-free isolation, best using transformers. Capacitive coupling may be an option, but it is best avoided, at least it is though to get good isolation, say 2 kV or above, with capacitors that can transmit 10 MHz, at reasonable cost and size.

I am looking for about 1 V p-p, reasonably square shape output, into 50 Ohms, or TTL level (about 5 V) into high impedance. About 5-10 dBm at the 1st harmonic, 10 MHz. So we need to drive about 15 mA through a 50 Ohm load.

As amplifier elements, I am using 74HCU04 unbuffered inverters, these are balanced for propagation delay, and I have plenty of these in a box. The HCU04 is essentially a single stage inverter, a gate with a pretty good linear region – an amplifier. Propagation delay is about 5 ns at room temperature, so it is good solution to amplify clocks, and so on. We are using it to amplify a 10 MHz signal from an OCXO.

For isolation, looking for some small transformers (generally speaking ethernet transformers will work well), I found the PE-65612NL at low cost (list price is about 4 USD per piece, but some sellers have them at a small fraction of this cost, most likely, from surplus). These are 1:1, 2 kV min, signal transformers originally intended for digital audio signal separation. Good enough for our purposes.

A really affordable offer… sure you can substitute any other reasonable signal transformer that can cope with at least 20 mW, and is reasonably inexpensive.

The schematic – first, a single HCU04 is used to square up the OCXO output, and then distribute to 3 outputs, two are used to drive 2 isolated outputs each (4 outputs total), the other output is routed to a PLL circuit (because this isolation amp is part of a GPSDO). Any phase drift of the 1st stage HCU04 introduced by thermal and other slow effects will be canceled to some part by the GPS loop (because the sampling of the phase is very close to the isolated outputs, only followed by a set of paralleled-up gates) – although I don’t expect such drift.

The resistors were selected as 3×330 Ohm, giving about 100 Ohms source resistance and about 1.4 V pp when terminated in 50 Ohms.

Output power is fairly consistent, like, +-0.2 dBm when comparing 4 units. Fundamental output at 8 dBm is exactly the right range. Probably you can adjust it in the range of 5 to 10 nominal without changing much the other characteristics of the circuit, by changing the resistor values from the paralleled-up gates to the isolation transformer.

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