DCF77 vs. GPS time comparison: not a lot of uncertainty…

Some folks were asking about the accuracy of the DCF77 10 MHz standard described earlier, DCF77 10 MHz – which has an Piezo brand OCXO, steered by a long-time-constant PLL locked to the DCF77 77.5 kHz carrier.

But, how to assess the short and long term stability of such a ‘standard’ in practical terms? Well, short term accuracy – it will simply be that of the Piezo OCXO, and some noise injected by the power supply. Mid- and long term, the drift will be determined by the DCF77 master clock (which is dead accurate), and the propagation conditions of the long wave signal (which is by far worse).

With my location at Ludwigshafen, Germany, I’m reasonably close to the DCF77 transmitter – maybe 70 miles? So there is hope that the transmission induced effects are not all that bad.

To measure the mid and long term stability, see below two plots of the DCF77-locked phase of the Piezo OCXO, vs. the instantaneous phase of GPS, stable to 40 ns or better, and obtained from a Motorola M12+ timing receiver. Measurements were done by measuring the time interval from the GPS 1 PPS signal, to the rising edge of a 10 kHz signal – derived from the 10 MHz OCXO by a good divider (using a ADF41020 REF input – R divider routed to MUX output) by HP 5335A counter.

dcf dcf vs gps time day 57603

dcf dcf vs gps time day 57604

In short – DCF77 is tracking GPS extremely well, and the OCXO phase is stable to within a few 10 to 100 ns. In practical terms, 1 second of observation time would be well enough to calibrate any frequency standard to 1 ppm or better, by comparison with the DCF77 locked OCXO. In other words, the DCF77 locked OCXO instability appears to be dominated by the propagation of the DCF77 signal more then anything else.

ADF41020 18 GHz PLL: universal divider and PLL board

I cannot praise Analog Devices enough for the ingenious designs, and for providing parts like the ADF41020, a fully integrated 18 GHz PLL. This is actually part of a major design effort for a multi-channel frontend, here just a description of the small test board used to establish the general circuit layout and board design.

Probably interesting is also the hand-soldering of the LFCSP leadframe package, which is actually not as difficult as it seems. For soldering of the pad, there is a large via in the center, which does provide good heat-sinking and is easy to solder through the 1.2 mm board.

pll18d0 layout

Above, the layout, below, 10 boards – 14 dollars and a few weeks later.

pll18d0 pcbs

For soldering, best use 0.5 mm Ag-containing SMD solder, with Type 32 flux, which is halogen free resin flux.

pll18d0 solder

To mount the LFCSP, first apply some solder to the chip pads, but not to the center/heat sink pad. Apply some flux to the board (which is already pre-tinned; use any good SMD flux pen). Then align with a good magnifier, using some Kapton tape to hold the chip in place – leave one side exposed. Then solder, in one stroke, using a medium hot soldering tip. Reflow another time – one side done. Remove the Kapton tape, and solder the remaining 3 sides. Then stick down the chip with Kapton tape again (to avoid any remote chance of movement, in case all the solder melts during the next step). Turn around the boards, and solder through the via, with a fine solder tip.

pll18d0 via

pll18d0 adf41020 mounted

pll18d0 full board

For a test, just apply a test signal to the input, and use the “MUX” output to check for any pulses. There we go:

pll18d0 2215 pulses

These pulses aren’t quite long, so it is one of the few occasions where a scope more advanced than the 2215 Tektronix is really useful in the home shop… same pulses on a HP/Agilent/Keysight 54720A, 54713B plug-in, and 100 MHz 1:10 probe.

pll18d0 pulse out

These fast risetime pulses, and the various prescalers, dividers and good input sensitivity make the ADF41020 quite useful for any PLL and frequency counting applications.

pll18d0 2ghz in 25 khz out

2 GHz in, 25 kHz out — confirmed.

About the input sensitivity: the ADF41020 is specified over a 4 to 18 GHz range – how about lower frequencies? A quick look at the input circuit shows a 3 pF capacitor – which equals a reactance of about 53 Ohms, at 1 GHz (i.e., the capacitor and termination resistor will cut the input power available to the buffer approximately in half).

pll18d0 rf input

pll18d0 input sens

… quite useful down to 1 GHz, no problem or instability at all. Also checked the the reproducibility, for 3 devices – not a lot of scatter.

EIP 545A Microwave Counter: another power meter upgrade

This is just a brief update on an earlier post, related to the power meter option for EIP counters, EIP Power Meter Upgrade. The current 545A came without any options, even without GPIB, and I was not quite sure if the firmware power meter option firmware would work, even if no GPIB option is present. Well, easy enough, put it to test.

545a upgrade eproms

It didn’t take much, a few (rather old 2516, and 2532) EPROMs, a resistor, some spare ICs, and a few inches of solder…

545a board

Here, the result: all working just fine, counting away at several GHz, and power reading appears to be resonably accurate. I would not substitute it for a proper power meter, for calibration purposes, but the power meter option is a handy feature for any quick test and confirmation.

545a upgrade test

Major relocation, and numerous 230 V conversions…

Long time no post, not because there is nothing happening here, more to the opposite. Relocated from the US, East Coast, back to Germany, including the US section of my electronics shop, 40+ pieces of heavy test gear. All made it over the sea just fine, in a 20′ container. Now, changing all the fuses and converting everything to 230 VAC mains voltage. I will spare you the details, just a few impressions for some trusty HP power supplies. These actually require some re-wiring, you have to break to circuit traces, and install a wire bridge.

230v 6205c

230v 6209a

230v pcb traces

230v plugs cut off

Installing new plugs… wires properly protected.

230v plug

230v traces broken

Now just install a bridge between the middle solder points. Great that there are schematics and manuals, even for 50 year old devices!

230v schematic

230v fuse

…don’t forget to replace the fuse with one of the proper size for 230 VAC operation!