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33 — Designing our next set of 80 m ARDF transmitters.

2015-08-16. By Patrick.

It's time to design our next set of more powerful 80 m transmitters. Several people have used similar designs. In particular, a design by Rik Strobbe appears to be the starting point. They all use crystals as the oscillator.

I would like to design a transmitter that is more agile. The Silicon Labs Si5351 is a frequency generator integrated circuit with which quite a few amateur radio designers are experimenting. It looks pretty complicated to control the frequency output. Silicon Labs provides software that runs on personal computers to figure out the optimal parameters for any frequency output. But being able to tune a transceiver means controlling the frequency with firmware running in the transceiver.

But do we really need such control for ARDF? For our purposes, we don't really need to tune across the bands. We just need a set of frequencies from which too choose with a selection switch. We want them spaced across the band so that we can choose any two without interference. We also want to include those frequencies that have traditionally been used in ARDF crystal-controlled transmitters. This is important in case we need to also use someone else's transmitters.

The next problem is much more interesting. It's a true electronics design problem that I haven't face yet. The circuits I have looked at are mostly pretty simple. That's what I need. They tend to follow a basic chain of information and energy flow. The chain starts with the crystal and goes directly to a transistor or goes to a driver followed by a transistor. Pretty simple. In order to get a bit more power than we had with our last set of transmitters, we're going with the crystal-driver-transistor chain. Rik Strobbe's worked as a basic template along with some other schematics. But his oscillator combines the crystal and part of a logic chip. And the logic chip is also part of the driver. I'm sure it's all pretty simple to him but it took a while for me to figure what was happening.

Some building and testing is going on. But basically, I think I have a handle on the major reasons for the driver and the obstacles I will face. The way I understand it, the driver is there for two reasons. First, the oscillator usually doesn't have enough voltage to turn on the power amplifier. The power amplifier will probably need 5 volts. Second, the oscillator won't be able to source enough current to turn on the power amplifier. This one stumped me for a bit. The transistor I'm using in the power amplifier is the IRF610, a MOSFET. The gate at the MOSFET has high input impedance. So why should current be an issue. Insert much studying of theory and head scratching here. Actually, much of the theory is irrelevant as it explains what happens under DC conditions. As best as I can figure it out, the MOSFET gate has capacitance. So, although the first thing you read about it everywhere is that it has a 'high impedance' input, that gate has to be charged and discharged with every cycle. The Si5351 may or may not be able to source enough current but it's a 3 V device and so I will still definitely need a driver.

2015-12-11 update: In blog entry 39 (Sep-27), I describe having just finished the ugly construction version of the transmitter.

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