Pete iK's Blog: U3S Beacon Kit

I have spent a couple of days modifying my U3S, and tidying up the wiring to make it look presentable inside the case. I also used the spare panel switch to switch the power to the amplifier on and off.

Modifications:

As built from the Kit Instructions, and powered by a single 5V supply, the U3S will change frequency, when transmitting a Callsign or WSPR Frame. Or indeed any other mode of transmission. This can be quite problematic on WSPR as it can cause the receiver to fail to decode an otherwise good signal, due to excessive Drift. On QRSS it causes an ugly 'sag' in the frequency, which has been likened to Bow's or Hook's.

After hearing about WA5DJJ's problem of Bow’s at full power and when he decreased the power output, the Bow’s decreased. Bow's are frequency shifts caused when the U3S is transmitting, it occurs on WSPR, and QRSS. These shifts look like bows or hooks on a 4 hour QRSS Grabber image. I remembered my earlier tests when setting up the OCXO/Si5351, without any voltage on the amplifier, there were no Bow’s. On analysis, this led me to believe the problem lay with RF from the Amplifier being fed back into the OCXO and causing frequency shifts, via the common 5V supply rail. In fact that is how I frequency lock my IC-R75 receiver! Fortunately the 3 supply rails can be easily separated (it is a feature of G0UPL's design), by simply removing 2 links installed at assembly time.

This is how I did it:

1. I added three L7805CV, TO220 cased voltage regulators, inside the case to minimize voltage drops in the supply leads to the equipment itself. See photographs below.

2. The 7805’s are bolted to the inside of the lower outer-case half, and in a line just under the front edge of the installed GPS. My GPS board is mounted component side up. I use an external 3V3 amplified Antenna, not the supplied patch antenna. The supplied antenna wont work inside the aluminium case!

3. There is no need for an insulating washer under the mounting screw or a mica washer under the 7805 case; the cases are grounded, anyway.

4. Each 7805 has a 33µF 50V Tantalum capacitor directly across its input and ground, and a 0.1µF 100V Mylar capacitor directly across its output and ground. Each capacitor is soldered onto the leads of the 7805’s to minimize lead inductance.

5. The capacitors across the terminals of the Regulators are essential to stop the RF getting back into the other supply rails, and indeed into the Regulators themselves. No other bypass capacitors are required.

The three, 7805 Regulators feed:

1. The Amplifier (PA) – but could be exchanged for a 7809 or 7812 as the need arises.

2. The Logic Board and OCXO (+5 Volt rail).

3. The OCXO Oven Heater (Heat).

To feed these three sections separately, remove the 2 links adjacent to C1 on the main board. One connects the PA to the +5V rail and the other connects the Oven Heater to the +5V rail. I installed a 4 way plug on the outer row of holes in the main board marked +PA/+5V/Heat/Gnd. I added a 5th wire to the loom which is connected to the Filter/Relay board ground plane, and the 2 earth wires are connected to a common earth point at the 3 Regulators. The 3 regulators are bonded by their mounting screws to the case bottom half.

I installed a couple of Ferrite beads over the 5 wire supply cable (just in case) where it is routed over the top of the Display, the beads (and cable) push neatly into the gap between the Panel Facia and the Display board.

I’m currently using a mains power supply whose output is 12V; it is capable of 10 amps, but a supply that is capable of 2 amps would be entirely adequate. The output voltage is 12V, and it does not change when running 250 mW from the U3S.

Soak testing the unit today I have found there is minimal temperature rise from any 1 of the three 7805’s used, even when using the 12V supply. It will remain there.

I am very pleasantly surprised! The U3S no longer displays any tendency to cause Bow’s, Hooks, or what-ever you want to call them. The output frequency remains constant for the whole 7 minutes it takes to send my call, and restarts at the same frequency every time. Even when running full power. The PA can be switched on/off/on while it is transmitting and the unit will not reset, a problem I discovered when using a common supply rail!

Photos of the 'guts' below. I hope this answers a few problems and you find it useful.

Pete Mulhare

ZL2iK

Click to enlarge