CW Rise Time Settings and Key Click Effects with Icom IC-756ProII HF/6M Transceivers by John Seney, WD1V
Many Icom IC-756ProII radio users discover that there is a "CW Rise Time Setting" menu that enables them to adjust and set the rise time value for each keyed CW envelope. Too fast a rise (or fall) time can cause "key clicks". Key clicks are essentially analog distortions in a keyed CW waveform that contain out of band products that produce noise (clicks). They most often occur at the very beginning of the transmitter being keyed. If the keying circuit lets the transmitter go from key down zero power to full power too rapidly, the transmitter's output becomes distorted. Distortion is easier to understand and deal with when it can be seen vs. heard.
Actually Seeing Analog Phenomena with Digital RecordingSeeing and analyzing "clicks" with an analog scope is difficult. To capture single shot transient events requires a digital scope. But getting good envelope recordings with a digital storage scope can be a challenge due to typical memory depth limitations (many scopes have 50,000 points or less). Short memory reduces sample rates over long recording times. For instance, at a 2 ns per sample (500 MS/sec.) rate: 500 samples records for 1 microsecond 1000 samples records for 2 microseconds 50,000 samples records for 100 microseconds 500,000 samples records for 1 millisecond 2,500,000 samples records for 5 milliseconds If a long enough record is obtained (in this case at least 3 milliseconds or more is needed), another challenge is to display all of the data. Many digital scopes limit their displays to just 500 data points. So how do you observe 2.5 million or more of anything at once?
A Digital Viewing SolutionThe following screen shots and measurements give some indication of what effects various rise time settings actually create. The data was taken with a 1 GHz BW Teledyne LeCroy "WaveRunner" digital storage scope Model LT-584 set to .5 ms (time per division - 5 ms total recording time on each screen). The default Teledyne LeCroy display shows a compaction of the entire contents of memory using a patented max/min binning algorithm. Thus the envelope of 2.5 million samples can be seen as 1 continuous sweep. Each trace was recorded as a single shot capture using 2.5 million samples at 500 MS/sec. of a 3.558 MHz CW carrier. Several shots were taken to verify stable triggering and consistent results for each subsequent rise time setting change on the IC-756ProII. RF power was set to minimum (viva QRP!) output and fed directly to the scope's 50 ohm input. This first screen displays 4 traces annotated as follows: 1 @ 2 ms rise time setting B M2 @ 4 ms rise time setting C M3 @ 6 ms rise time setting D M4 @ 8 ms rise time setting Note the overshoot and undershoot effects in the first 2 traces. The "too fast" rise and overshoot create the unwanted "key clicks" effect. The next screen shot are the same 4 traces of data but now displayed "overlayed" - stacked on top of each other. Note that since the trigger point was in the middle of each 2.5 million point array, the shapes of the RF building up on the left hand side of the display makes comparative times from key down to full carrier output easy to see.
Digital Measurement ForensicsAn additional scope challenge is to process the data contained in long records and make it useable. A Teledyne LeCroy can extract the positive trend amplitude of each 2.5 million data point envelope, filter the trend, and then automatically measure its rise time in less than 1/10th of a second using built-in DSP math functions. An alternative is downloading the waveform data via an Ethernet connection and using MatLAB functions to extract desired measurement values. The next screen shot captures the envelope keyed at the 2 ms setting, filters the envelope, and measures the filtered response rise time of trace B. The next screen shot captures the envelope keyed at the 4 ms setting, filters the envelope, and measures the filtered response rise time of trace B. The next screen shot captures the envelope keyed at the 6 ms setting, filters the envelope, and measures the filtered response rise time of trace B. The final screen shot captures the envelope keyed at the 8 ms setting, filters the envelope, and measures the filtered response rise time of trace B.
Bi-Modal Effect Based on Keying SpeedNote these rise time response measurements are proportional to IC-756ProII settings but about 4 times faster than radio settings indicate. A difference was noted between single shot vs. multiple random CW characters. When any 2 characters were sent in rapid succession, it approximately doubled the rise time - reducing the effect. Thus shorter rise time settings may produce "speed of keying dependent" clicks.
Conclusions, Comments, & FeedbackWhile taking this data, I also listened to the zero beat on a separate receiver (Elecraft K2) and could easily detect clicks at the 2 and 4 ms. settings. but not at 6 or 8 ms. I've had my IC-756ProII since November, 2002 (sold in 2007) and find it to be an incredible radio. One of the fortunate aspects is the phenomenal range of its many settings - such as noise reduction. Listening to a IC-756ProII is an unusual "radio" experience. Tuning stations in clear spectrum is trivial. But it becomes possible to activate DSP (filters and noise reduction) that enables hearing specific QSOs in spectrum that is full of traffic, QRM, and or QRN. Radios without IF based DSP don't get you there. The sensation is like listening to something that you know is highly processed. Since you hear what it is like without the DSP, its very, very cool. It reminds me of the spy movies where "gadgetry" resolves a single conversation in a crowded restaurant. Bond. James Bond. I'm not certain what value the 2 and 4 ms. rise time settings afford but that one can legitimately tune out of a possible problem is the way it ought to be. Perhaps the default rise time setting should be 6 or 8 ms. I welcome your comments on this data at firstname.lastname@example.org 73 John Seney WD1V [FN42] FIST #5014 CC #1151 K3/P3 Flex 5000A http://www.wd1v.net
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