WM2U's Digipan Workshop
IMD Meter
designer; Clint Hurd, KK7UQ
by Ernie Mills, WM2U.
Please print this article and use it as a hand out at Club talks and lectures

Finally, the last piece in the PSK puzzle. A meter that will read your own transmitted signal IMD. Perfect! Now you can operate without worry of splattering the bands and annoying other PSK users.
Many times we all see that wide signal. Indeed I have seen some showing side bars out to the 15th harmonic. I normally make a point of including an IMD measurement of the incoming signal during QSO, and in the case of a bad IMD, offer help by signal reporting while the operator adjusts his/her signal drive and parameters until an acceptable IMD is obtained. Some operators however take offence at being told their trace is too wide and this can lead to some uneasy situations.
Not any more. :)

The IMD Meter is a stand alone device, hence you need no connection to your computer at all. It measures the IMD of your signal while you are transmitting. It has a small whip antenna which picks up your transmitted signal, amplifies it with an AGC controlled RF amplifier (50 dB dynamic range capable of QRP to High Power operation), detects the wave form, feeds the low frequency continuous function from the detector to a micro controller A/D, performs analysis of the wave form and calculates the IMD of the transmitted signal. It displays the IMD on a 3 digit LED display. It works on BPSK31 QPSK31 BPSK63, and covers 160 to 10 meters.
It has many really nice features including;

  • Two LED's, Green and Red which provide a summary of Good or Bad signal quality.

  • An audio tone sends four dots (Morse H) when IMD goes above -20dB, and sends two dots (Morse I) when signal drops below -24dB. This is an excellent feature as it will warn you of the IMD status change without actually looking at the meter. Remember that as you change frequency within the waterfall indicator (pass band), the audio drive requirements change to maintain the same power output. It is very easy to go from a good operating IMD to a bad one without noticing.

  • Built in comprehensive self test routine which verifies operation of the control panel and mode functions.

  • A very comprehensive and complete Operating Manual.

So what more do ya! want?

Unfortunately this meter is no longer available in kit form. It is however available fully assembled and tested by Jim Nuytens, N3JJA at jim.kj3n@verizon.net
he product is sold through US Interface at http://www.navigator-interface.com/ or email: jim.kj3n@verizon.net

While you are there, check out the latest Digital Interface, The Navigator Interface. I've heard great reviews.

o what ya! waiting for?
Go and get the IMD Meter and stop relying on 'judgment' signal reports from QSO's. Think about it! your PSK station is not complete until YOU know what YOUR putting out.
Hope to see your clean trace in the waterfall soon eh! Enjoy, Ernie WM2U