Wednesday, July 19, 2006

Orion Keyclick Checks

Executive Summary: The Ten-Tec Orion (original model) CW characteristics are significantly different between versions 1.373b5 and 2.056 of the firmware. The older firmware shows better CW click suppression. The newer firmware has a wider range of rise and fall times available, however, it appears that the raised cosine waveform is less accurately generated.As a software-defined radio (SDR), the Ten-Tec Orion's personality has undergone significant changes since its initial release, particularly in the recent transition from version 1 to version 2 of the firmware. I was interested in the issue of key clicks for CW work. What is the role of the Orion "CW Rise/Fall" parameter, and what is the best setting?
It is hard to evaluate key clicks without high quality lab gear, but I resolved to see what I could do with a 20 MHz scope and an Icom R-8500 test receiver. The test setup was the Orion driving a dummy load at 18.090 Mhz. The R-8500 was set up attached to a few feet of coax. The Tektronix 2205 scope was attached to the dummy load through a small gimmick capacitor. A 10 Hz symmetrical keying waveform was provided externally by a Tektronics CFG250 function generator. This corresponds to 25 wpm, more or less. Measurements were performed using both the latest version 2.056 firmware and 1.373b5, the last version 1 firmware in circulation.

First, we need to characterize the test receiver. We are operating in CW mode. Using the internal 10 and 30 dB attenuators and tuning off the Orion's carrier frequency, it is possible to check the receiver bandpass. The measured results using a CW carrier were -5 dB at 1.35 kHz, -20 dB at 1.6 kHz, and -30 dB at 1.8 kHz. In CW mode, the bandpass is symmetric around 0 kHz, so we are in rough agreement with Icom's spec, which is IF bandpass of 2.2 kHz at -6 dB.

(Disclaimer: All level measurements in this article are quite rough, relying on the calibration of the '8500 S-meter and the built-in attenuators. We are looking for qualitative comparisons that will bear on the key click question.)

Results for v 2.056

The first table shows the measured CW pulse risetime in msec. from the Orion vs. the Orion's "CW Rise/Fall" menu setting. Without a storage scope, it is difficult to make accurate measurements, especially of the fall time, because of Orion's timing jitter. We can see that the actual risetime is somewhat shorter than the menu setting.

The second table shows S meter readings at various distances from the Orion Tx carrier frequency versus "CW Rise/Fall" setting. The receive audio is predominantly from "clicks", although there are some weak birdies and other noise. (The response to a steady CW note is very small, except as noted below.) It is not surprising that there are a lot of artifacts like that, because an S1 signal indication is about 100 dB below the Orion carrier.

The R8500 is much inferior to the Orion's receiver. According to ARRL, its IP3 is about -7 dB and third order IMD dynamic range is 86 dB. Even with these limitations, the R8500 is useful for making comparisons, and it's the only independent receiver available at my QTH.

We notice that the observed click power falls off significantly as frequency offset increases. The response at 10 kHz is about 6 S-units (nominally 36 dB) below that at 3 kHz. (Rise/Fall = 3.) Surprisingly, the click power does not clearly decrease with increasing Rise/Fall setting. What's more, at Rise/Fall = 9 or 10, the response is actually increases significantly. I believe this is related to visible "defects" in the keyed transmit waveform, which appears to have a discontinuous slope as it approaches full power. It departs visibly from a smooth raised cosine. (Other experiments show that this waveshape problem may depend on Orion power level. The waveform is better at higher power settings. This suggests a possible interaction between Orion's internal ALC processing and the keying modulation.)

At 15 kHz, no meaningful measurement could be made because the R8500 output was dominated by a strong white-noise-like signal that might indicate phase noise in the receiver.

Results for v 1.373b5

A similar bank of experiments using the older firmware produced the following results.
The first result is that the range of actual rise times is from 1.6 to 6 msec, somewhat shorter than in the first test.

The second table shows a major improvement in key clicks. Click power is almost undetectable beyond 5 kHz offset. With Rise/Fall set to 7 or higher, clicks are significantly reduced (compared to a setting of 3) at all frequency offsets, and are barely detectable even at 5 kHz offset. The anomaly seen in v 2.056 at settings or 9 or 10 is not present in v 1.373b5.

Conclusions

From a standpoint of CW signal transmission, version 1.373b5 firmware is significantly better than 2.056. Visually, this seems to correspond to a cleaner raised-cosine waveshape seen on the scope. A rise/fall setting of 7 will minimize key click interference. This should be adequate for my use at 20-25 wpm, at least.

There is a strong possibility, especially with the v 2.056 firmware, that signal quality depends on power output level. (This needs further study.) Using full 100 W power seems to give the cleanest results. However, users who need to run lower power to drive linear amplifiers should watch for excess key clicks.

More careful measurements with good lab equipment would be very helpful. Unfortunately, the results can depend strongly on the firmware version. Ideally, an SDR transceiver should be recharacterized after each software update.
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