Digital Voice Modem (audio filter and reference oscillator)
For Two-Way Ham Radio Applications

In stock, and accepting orders!


Click here for schematic, parts list, construction, installation and set-up instructions.

Ordering instructions on bottom of page.

The INADVM is an affordable, professional multi-mode digital
voice modem filter and reference board for use in amateur radio systems.

It's designed to be mounted onto a genuine Arduino DUE or clone.

The INADVM is intended to be an open-source Multi-Mode Digital Voice Modem,
which utilities the power of a ARM processor and a simple analog interface board.

 The INA Digital Voice Modem is combined hardware and software, developed to handle all amateur digital voice modes. 

This product is mostly thru-hole to allow assembly and repair by the common technician.

Designed by several hams.

Price $35.00 each - in kit form.
Also available assembled and tested - $55.00 each.
US shipping and handling $7.50 in small quantity.
PayPal accepted - order below.

Enlarged INADVM REV2 Top photo.
(Click for alternate view)

For several months now, a team of hams have been working on the development the INADVM. Mike N4IRR, Steve N4IRS and I have been working on a line of MMDVM (like) audio boards called INADVM. It is made and shipped from the USA. The first version of the board is for the Arduino Due. It uses multi-turn pots for receive and transmit audio. The radio connector is a DB9. A 12 MHz TCXO is included. On the board are LEDs for Power, PTT and Data detect.

Now for the fun part. The design is mostly through hole (not surface mount) except for the voltage regulator and TCXO. Those two parts are premounted, and the rest is a kit. You will receive the PC board with the regulator and TCXO mounted and all the rest of the parts needed to complete it. The schematic and board layout is available from the INADVM website. All ready to assemble. No surface mounted parts to solder. Quality machine sockets for the ICs. There will be an assembled and tested version available also.

We decided upon building a PC board that was mostly thru-hole instead of being like the other surface mount versions out there. This was done for several reasons, but mainly because the team saw other similar items be widely accepted because this form-factor is easier to build and modify by the average (ham) technician. There were about a dozen INADVM prototypes built and put through some rigorous beta testing. The result of this testing is what prompted changes and additions to make this REV 2 production model.

There are a few differences between the prototype and REV2. The major differences are the addition of RSSI support, and pads to make use of the unused pins (1, 7, and 9) of the DB9 radio connector. There are three spare hole traces that terminate to these pins of the DB9. These connections can be used for operations that are beyond the original scope of this board, to wire up things we may not have thought of. Also, the silkscreen was corrected, adjusted and moved to improve look and feel.

Difference between REV 2 and REV3:
There is very little functional difference between REV2 and REV3. The REV2 boards do not work correctly when JU1 is in the "B" or 5V position. While the REV2 boards work fine on receive with JU1 in the B position, the offset created in the TX path creates undesirable distortion. In REV3, the power jumper was reconfigured to allow the receiver audio path (only) to run at 5V instead of 3.3V. The REV3 TX path is unchanged with the setting of jumper JU1 and remains at 3.3V in either case. The added capacitor (C24) was necessary when the jumper JU1 is moved from the 3.3V position to the 5V position. Since there is probably no advantage of running the receiver path from 5V, we recommend using only the 3.3 volt setting of JU1 (JU1A) with either REV2 or REV3. While it was a good idea, it's not very practical, as there's no real increase in dynamic range - since the Due can't handle any more than 3.3V on its input anyway. REV3 boards started shipping by default since October 2017.

Click here for schematic, parts list, construction, installation and set-up instructions.

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Product of INAD Communications, all rights reserved.
The INADVM hardware is under KI6ZUM CC license BY/SA/NC
Specifications may change without notice.
Designed by several hams.
Images are property of INAD Communications
Board layout by Kevin Custer W3KKC
HTML as of date below - All Rights Reserved!