KK7UQ PSK IMD Meter
This article is for blind and vision-impaired amateur radio operators who want to monitor the intermodular distortion (IMD) of their PSK transmissions. IMD can cause a signal to splatter into adjacent frequencies and interfere with neighboring QSO's.
This review describes the accessibility of the KK7UQ IMD Meter, which has high visibility LED displays and audible alerts.
Modern transmitters produce very little distortion when the input audio level is within design limits. IMD measurements in that case are generally below -30 dB.
However, high input audio levels can cause significant distortion. The modulation envelope is flattened and the spectrum widens, or splatters, with spurious sidebands. Photos on KK7UQ's Web site show PSK31 spectra, waterfall displays, and waveforms with various amounts of distortion.
Indicators of IMD
Whether you feed the output of a sound card into the microphone jack or an accessory port, the audio level must be adjusted carefully to avoid over-driving the transmitter. Short of using an IMD meter or a second receiver and a spectrum analyzer, hams often depend on the ALC meter and signal reports when they set the audio level. Each method has limitations:
Ideally, a transmitter ALC reading of zero means that the audio input level is low enough to prevent splattering. Unfortunately, you can't always rely on an ALC meter. The ARRL HF Digital Handbook (Fourth ed. pp 4-8 ff) reports that "a number of rigs become decidedly nonlinear" [while the ALC meter shows no activity].
Operating with the ALC = 0 is good practice, but it does not guarantee a low IMD.
Many PSK programs report the IMD level of a received signal. In the IMD Meter Operating Manual, KK7UQ points out that these readings can be inaccurate for a number of reasons: low signal-to-noise conditions, radio receiver operating in a nonlinear range (where S-meter readings are too high), or receiver over-driving the sound card. Also, IMD measurements by PSK software are only accurate during Idle transmissions, when no characters are being sent.
The IMD Meter
The KK7UQ IMD Meter is a standalone, wide band receiver that automatically tunes to your PSK signal and measures the effective IMD. A whip antenna receives your transmitted PSK31, QPSK31, or BPSK63 signal. The receiver operates from 160 meters to 10 meters with no band selection required.
An LED digital display shows effective IMD readings from -10 dB to -34 dB. A reading of -24 dB or less (more negative) defines a good PSK signal—meaning that any distortion products are lower than the main signal by 24 dB or more.
The meter is designed to monitor signal quality accurately in the range where corrective action is indicated: Stated accuracy is
+/- 1 dB for readings of -23 dB and higher (less negative). In the Good range of -24 dB and lower, the accuracy is from 0 to 5 dB.
Visibility and Audible Alerts
The digits of the high-visibility, red LED readout are 0.5" high. Two 0.2" diameter LED's, one red and one green, indicate the range of the IMD: "Red only" means -19 dB or greater. "Green only" means -24 dB or less. At intermediate readings both LED's are illuminated.
An audible Morse "H" (four dots) is emitted when the IMD goes above -20 dB. A single Morse "I" (two dots) means the IMD has gone below -24 dB.
When the IMD increases to -23 dB or less and both LED's are on, the meter emits soft, audible clicks in sync with the digital LED readout as it refreshes (about once per second).
For More Information
www.navigator-interface.com has details about the meter, including the excellent operating manual.
kk7uq.com has slides from presentations, including practical information about operating with PSK modes.
The KK7UQ PSK IMD Meter gives blind and vision-impaired hams feedback for setting the optimum input audio level for PSK transmissions. It has a high-visibility digital readout, bright LED range-indicators, and audible events that signal some changes in the IMD.
Peter DeNeef, AE7PD, is an Extra Class amateur radio operator in the U.S. This Web site has no ads or conflicts of interest.
Email: HamRadioAndVision "at" gmail "dot" com.