Slight Course Change on X1M AGC

Been pre-occupied with other priorities lately, so haven’t spent much time on any of my ham projects. This past weekend was a rain washout here, so I got a few hours to get on the bench and play around with the X1M project.

As I expressed previously, it was my intention to embed an Arduino Nano inside the X1M and use it to (among other things) implement a firmware controlled AGC process. After actually embedding a Nano and seeing how cramped the installation is, reality set in and I started considering another approach. I’m now thinking more along the lines of a ‘back-pack’ unit containing the Arduno and other enhancement circuits, connected to the main radio via the existing DB9 connector. The DB9 presently has about 1/2 dozen of its pins connected to points inside the radio that I don’t see myself ever needing. My plan is to re-purpose these pins so that the functionality I want to implement can be accomplished in a less intrusive, more flexible way outside the box. Also, I’m scaling back my ambitions (for now) and am planning on putting a hardware AGC board inside the radio, but bringing out the AGC voltage for the Arduino to measure and turn into an external software-calibrated S-meter function.

In the process of looking at the best way to do all this, I think I discovered a design flaw in the X1M having to do with gain distribution. It turns out that the SA612 product detector overloads on strong signals about 15dB before any of the previous IF stages and the following AF stages, becoming a bottleneck for strong signal handling. If the radio had AGC, this might not be so important, but in a fixed-gain radio it’s very important to have as much strong signal capability as possible leading up to the first gain-control element. In this case that’s the volume control, at least when there’s no AGC add-on. I think this problem is due to the use of the MC1350 as an IF amp which, in my estimation, has far too much gain for most IF applications. I plan to look at this some more to determine if my reasoning is correct, but I might be suggesting an IF gain reduction mod, even if AGC isn’t added.

Anyway, look forward to a change in direction as I implement the AGC and add the external Arduino.

Joe
W3JDR

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Embedding the Arduino Nano Inside The X1M – Part 1

This weekend I got some time to try out my concept for embedding an Arduino Nano inside the X1M. You can find more details here.

X1M Transceiver in the Field

I just got back from a 5 day camping outing where I got a chance to try out my X1M field kit. You can read about it here.