Bringing up the NASA Interface Simulator test box (Patreon)

Quick update on the Apollo comms setup. We revived one more test box: the Interface Simulator. From what we can tell from the switches, it should generate the  subcarriers, both in the up and in the down direction. The up direction is under the STE MOD label (S-band Test Equipment Modulation). This section’s output connects to the ground test transmitter modulation input. The down direction is under the USBE (Unified S-Band Equipment) label, and connects to the input of the Apollo transponder.

After a day of tracing and reverse engineering, we thought we understood most of the beast.

Turn on was uneventful, with the usual few lights needing to be replaced and the power supplies testing good as always. We turned on the 30 kHz (voice) and 70 kHz (data) switches, and our two uplink subcarriers dutifully appeared. It workz!

We hooked up Ken's pseudo-random ranging generator. This was slightly more challenging, as the correct input was on the back panel, not in the front as you'd expect. But after a bit of monkeying around, we had a fully modulated uplink, including ranging. This original NASA setup will replace the pile of HP boxes that we used in episode 15 to reconstruct the uplink.

We then tried the downlink section, starting with the PCM data at 1.024 MHz. The bi-phase modulation did not work at first, so we opened up the module responsible for it. It had some early analog ICs in it!

We figured out that we must not have had enough signal amplitude at the input to trigger the comparator IC. So we progressively increased the digital input level, and all of a sudden we got the bi-phase modulation. You can see a phase shift in the middle of the scope trace. We have spacecraft data!

The downlink voice was slightly more challenging. There had been a modification. The subcarrier frequency for voice had been moved to 768 kHz from the original Apollo 1.25 MHz. Fortunately, the engineer, Mr. F. Landsburg, left us a terse note documenting what he did. We thank him very much, 50 years later.

There were enough clues in the note to help us guess how to undo the mod.

We guessed right the first time around. After a bit of re-tuning, it is back at the original 1.25 MHz - well, almost, 1.2499 MHz says my counter. We are ready for Apollo voice comms. We won't strictly need the box for the downlink once we have the original PMP working, but it's a "nice to have" for early tests.

I added my own notes, Landsburg's style.

Here you go, another NASA/Motorola box that's 100% good after 50 years! It's already been added to our setup.

Marc