Material on this web site copyright © 1995-2013, TelePost, Inc. All rights reserved.
Pricing and specifications subject to change without notice.

Back to Home

Issues related to using a digital wattmeter when tuning with a pulser

It has come to our attention from a few users that it is difficult to get a steady reading when using certain pulser devices, or "peckers", for tuning. There are several reasons for this.

Generically speaking, pulsers are free running oscillators which generate a square wave at about a 33% duty cycle to allow an amplifier to be tuned for peak power while mainatining a lower average power. This is a bit at odds with the way digital wattmeters work, since they take a momentary snap shot of the power (ie, sample) at regular intervals, which most likely are not in sync with the pulser intervals. This problem is minimized somewhat because the meter sampling rate is more than twice that of the pulses. However, the sampling window is still drifing relative to the pulses, which means that samples will vary in level unless the pulses are identical.

Unfortunately, in my testing I found that this is not the case. Two popular "peckers" that I have tested show very high spikes on the leading and trailing edges of the pulses. See scope pictures below.

The top picture is from brand "A", and the bottom two are from brand "B". The bottom right picture was taken at a faster sweep to show more detail. In both cases there is a large amount of overshoot, almost 100%. Double the voltage translates into four times the power, plus the levels between the spikes also vary. It's no wonder that each sample taken by a digital meter will display a different power level, and sometimes a VERY different power level.

What is the solution? Try sending dits from your keyer at high speed. Most transceivers send clean square waves with just a little shaping at the ends... but no overshoot. Still not perfectly square, but much closer. You can also just send a couple dits, make an adjustment, send another couple, adjust again, etc. The LP-100A will display full power from just a dit. Most people can fine tune an amp with a taps on the paddle, and it's easier on the amp than a pulser.

Another solution is to keep an old analog meter in line. The ballistics (inertia) of the analog meter smooths out the spikes of the pulser. This allows you to maintain the best features of the LP-100A (accurate peak power and SWR display at all times, fast SWR alarm protection for your amp, R+jX display, etc.)

The ultimate solution might be a new option for the LP-100A that's in the works. The current LP-100A has a pair of analog outputs on the PCB,
one for power and one for SWR, which are designed to feed an analog meter movement. These outputs are generated by two D/A converters and scaled for 0-5VDC. We showed a version of the LP-100A with a smallish built-in panel meter at Dayton a few years ago, but there was little interest. I suspect that the lack of interest was due in part to the small size of the movement. We are now looking at a larger external matching analog meter which would plug into the LP-100A to add an analog display. This would be usable with any LP-100A.