SDR / Flex sequencer

This is a sequencer for use with VHF/UHF/microwave transverters controlled by an Arduino Mega and driven by a Flex-3000.

A mess -- but it's my mess

Mon Sep 7 21:53:22 CDT 2020

My goal is to cobble together enough hardware to run transverters on the following bands, 2M, 1.25M, 70cm, 33cm (902MHz), 23cm (1296MHz), 2304MHz (13cm) and 9cm (3456MHz). Some of these bands I have existing transverter hardware and others are a work in progress. These need some sort of an IF radio and I plan to use a Flex-3000. The Flex-3000 has I2C FlexWire output -- that can do the transverter bandswitching and logically key the proper transverter. An Arduino will handle all logic, timing and sequencing.

Huh. The 3000 doesn't do transverters. Well, not natively or as nicely as a 6000-series or 5000-series, but that's what I'm willing to take in the field.

What I have at the moment:

A PCA9555 that interfaces with the Flex-3000. The Flex-3000 speaks I2C to this chip and the chip has individual line outputs to indicate band information to the Arduino.

An Arduino Mega 2560 handles the logic. I'm using a 2560 beause I need a lot of pins to do relay switching, display and communicate with the PCA9555. Storage and speed are a bonus. Connected to the Arduino is a 2x16 LCD display shield that has some pushbuttons that will later be used to override the SDR -- and realistically will allow for use with any other radio and allow continued operation if the FlexWire connection gets goofy. I also have a breakout board in there to allow easier connection of wires.

Two 8-relay boards with isolated input. Eight will handle transverter interfacing (PTT?) and four will handle RF. RF? Yes, these are really crappy little SRD-12VDC-SL-C but they'll be handling 28MHz RF with signals that aren't particularly weak, considering the high-level transverter outputs. It'll be fine. If not, it'll be easy to switch to real RF relays. What it is -- is good enough for now!

Two of the cheap transverter boards you find on ebay -- 2M and 70cm. I need a 144MHz LO for 33cm, 23cm, 3456 and 9cm -- again -- cheap enough for a first pass. I have other options. 70cm transverter -- that's where my equipment falls short. I can do 70cm on my Yaesu FT-847, but unlike other bands I don't have an external 70cm transverter. This will do in the meantime.

Everything is wired up in a 2U rackmount enclosure. I had initially considered building the W1GHZ transverters in here but that's no longer part of the plan. Most of the assemblies are easily disconnected for modification or maintenance.

Code. See my github page for code. It used to interface with an SDR-1000 -- and still could -- but that code got pushed aside for the 3000.

Status is probably more accurately reflected in the github code, but hardware-wise I can decode bands properly and the inexpensive transverters appear to work fine for receive on 2M and 70cm -- so a promising start.

In parallel I'm working on the W1GHZ inexpensive transverters. The 902MHz one will be the first completed. I have a 1296 transverter, but the TR relay system needs to be worked out and same for an antenna.

Did I mention my plan is to do this all mobile?

Sat Nov 28 17:17:49 CST 2020

Much further along. I had to change a few things -- more relays for separate TX and RX ports on both my 222 and 144MHz DEMI transverters, power for the same, etc. etc.

But just how good are these junky SPDT relays when there's RF involved? I have five relays with the common all tied together with some 14ga solid copper, fed at the first relay with the 28MHz connection to the radio and various transverters on the NO port of the relays.

I did some testing and I think the answer is "good enough for me". I used my NanoVNA and NanoVNA saver to measure the S-parameters through the relays.

S11 Return Loss -- -11dB or so

S11 Smith Chart

The return loss is around -11dB around 28MHz where I'm interested. I think this results in a mismatch loss of 0.36dB or around 8% between the radio and transverter. If I can afford a loss anywhere in the system, it's here! Transmit power is already going through an attenuator and the transverter output should be plenty high. So I'll go with it. I tried adding some parallel capacitance -- the values I had were either sufficiently small that I couldn't measure an improvement or so large that the effect was detremental.

Next up, I think my 144MHz transverer isn't receiving. Then a TR output relay for the 1296 transverter and finish the 902MHz W1GHZ transverter. I'd like to add manual up/down in the software as well.

This page last modified Sat Nov 28 17:37:50 CST 2020 by timc!