Summer 2008 Issue

A USB Programmable, High-Stability Local Oscillator for Microwave Transverters

A problem facing microwave operators, especially rovers, is frequency drift. Here N5AC provides a solution that can be programmed from your computer.

By Steve Hicks, N5AC


Figure 4. Two high-stability 10-MHz reference oscillators.

The project described in this article received the 2008 Southeast VHF Society/Mini-CircuitsTM Annual Award for Design Achievement. A similar version was previously published in the Society’s 2008 conference Proceedings.

I picked up the mic and called again on 3456.1 MHz: “CQ CQ CQ Contest, N5AC, November Five Alpha Charlie, over.”
“There you are,” the voice came back, “but now you are about 3 kHz low.”
“Weren’t we about 3 kilohertz high this morning?” I asked.

The Problem

The above on-the-air scenario is familiar to many microwave operators, especially rovers. The amount of frequency drift on the microwave bands varies with band of operation, equipment, and temperature. The cause of the drift is almost always the local oscillator in the transverter, which is primarily responsible for setting the operating frequency of the transverter. A typical 2304-MHz transverter block diagram, shown in figure 1, will explain why.
With a local oscillator (LO) frequency of 2160 MHz, the incoming RF at 2304 MHz will be downconverted to 144 MHz, assuming low-side injection (2304 – 2160 = 144). 2304.1 MHz will be heard on 144.1 MHz, and 2304.2 MHz will be heard on 144.2 MHz. If the LO drifts to 2160.005 MHz, though, 2304 MHz would be heard on the IF radio at 144.095 MHz, 5 kHz low. The IF radio’s frequency stability is also important and can affect where signals appear, but with a lower frequency of operation (144 MHz for the IF radio in this case), the stability of the underlying oscillator has 15 times less impact than the microwave LO. Let’s try to understand why this is so.

Oscillator stability is often rated in parts-per-million (ppm) across a temperature range. This figure is derived from the design and underlying physics of how the crystal is cut and mounted. A typical rating might be 1 ppm (10–6) from 0–70° C. Parts-per-million indicates how many Hz a signal would move for every MHz of operating frequency. If this oscillator was the basis for a 144-MHz transceiver, the oscillator could move up to 144 Hz (ignoring for the moment mixing going on in the transceiver). That’s not far. This same oscillator used on 10 GHz could drift up to 10 kHz over the operating temperature range (10–6 ¥ 1010 = 104)! This is much more significant. Combine with this the fact that many home-brew amateur projects do not achieve 1 ppm and it’s evident where the drift comes from.

Therefore, by the time a 100-MHz crystal oscillator is multiplied up by a factor of close to a hundred, a small drift in the base oscillator becomes a massive movement in the microwave bands. Frequency stability on the microwave bands has been more of a luxury for amateurs. Few had it and the rest of us were always chasing everyone around the bands and trying to get on frequency. This problem is not unique to hams, though. Commercial enterprises, such as cellular phone carriers, have all of the same problems. They have to be able to hold a signal steady in the 1900-MHz band and demodulate a digital signal in a handheld transceiver, your cell phone. The solutions to these problems have opened up some new windows for hams to better control frequency stability.

Finding a Solution

By now you may have figured out that I am talking about synthesizers, VCO and PLL integrated circuits, and high-stability underlying oscillators. Many solutions in each of these product categories have emerged as the demand for quality cellular and other handheld communications has increased. More and more of this functionality is also being placed on a single chip to reduce size. It used to be that you would purchase a VCO and a PLL chip and be responsible for your own loop filter, but now many of the parts are incorporating all of the pieces inside them.

I recently stumbled upon the Si4133 family from Silicon Labs in Austin, Texas.1 This device has two on-board RF synthesizers that can operate in a selected range inside of the 750–1700 MHz range and an IF synthesizer that can go from 62.5–1000 MHz. The output from both of the RF synthesizers is not on at once. The part is designed to be able to jump from one to the other as you would in a multi-band handheld radio (phone) so they are multiplexed to a single output on the part. The last one programmed is sent out of the part through the multiplexer. The Si4133 family requires only an external inductor for each of the synthesizers for tuning and all the rest of the VCO, PLL, loop filter, etc., is all within the IC itself. The part also requires a reference frequency input that is used as the base oscillator for the PLL. A block diagram of the Si4133 is shown in figure 2.

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