Tuesday, June 22, 2010

Can you help me better understand the nature of the audio/speech circuit in my Kenwood TS-140S transceiver? ...

Q Jeff Lawson, KD1WZ, asks, “Can you help me better understand the nature of the audio/speech circuit in my Kenwood TS-140S transceiver? My concern is that I am not getting full output when operating SSB. The ALC and power meters indicate that the rig is operating properly, but during tests with another station my signal is reported to be almost an S unit stronger when I whistle as opposed to when I am just speaking. Why would there be a difference?”

A The ALC (automatic level control) in your transceiver generates a control voltage to ensure that the peak RF output does not exceed its ratings. It does so by responding to the voltage peaks in the input signal (whether it is human voice or a whistle.) When you’re watching your rig’s ALC meter, you are observing the action of the gain limiting control. The meter is displaying the amount of feedback voltage that is reducing the gain of the transmitter. The ideal situation is to have as much gain as you can without limiting, or with only a tiny bit of limiting. So, the proper way to adjust your mike gain is to set it so that the ALC indicator shows just a little ALC action on your voice peaks.

The ratio between peak and average power in any signal is dependent on the nature of the signal. A whistle is almost a pure sine wave, and its peak and average power are nearly the same. A human voice, however, has a waveform that has a fairly high peak-to-average ratio. Since your radio’s wattmeter indicates full output power for both speech and whistles, it is a peak-reading wattmeter. Many wattmeters read average power, not peak power. Those wattmeters would indicate less power when you are speaking than when you are whistling.

On the receiving end, the S meter is actually measuring the receiver’s AGC (automatic gain control) voltage. A good receiver will generate an AGC voltage from the signal peaks, but this may not always show on the S meter, which may indicate average power, or somewhere in between. If your transmitter’s ALC settings are correct and your transmitter indicates full output power on voice signals, you should rely on that, not on the uncalibrated, unknown performance of a receiver S meter.

A good explanation about power is found in the article “Power—Watt’s It All About?” by Mike Gruber, WA1SVF (now W1MG), May 1995 QST.

From QST February 1999