Thursday, August 23, 2018

Improving the TCXO for Duplex MMDVM Hotspots

Various MMDVM hotspot devices have been on the market for about 3 years now and only recently have there been duplex ones available, which create more capability but with more complexity.

Proper passive and/or active cooling is a must when it comes to proper
operation of the duplex hotspot devices currently on the market.
Pictured device was constructed by Steve K2GOG

This HVDN article will explore making some thermal improvements to the duplex MMDVM boards that have become widely available over the past few months and are replacing many of the older hotspot devices available over the past few years.

The TCXO is one of if not the most critical part on the duplex
 hotspot device that creates user satisfaction

Problems and Corrections For Duplex MMDVM Hotspot Operation

TDMA based modes such as DMR are very reliant upon very stable time references. In order to create the simultaneous TS1 and TS2 capability, the TCXO needs to be extremely stable and quality components are only part of the equation.

If a Raspberry Pi 3 or 3B+ is to be used with many of the duplex hotspots, its important to do the following:

More Power: Ensure you are using a 5V 2.5A continuous or higher power source. Your RPI3B+ draws enough current and when coupled with the duplex board creates a higher load.  You need ample enough current to ensure stable operation.
Remove Heat: Placement of the MCU on the Raspberry Pi can create enough heat that will rise to the bottom of the Duplex MMDVM board which can influence its stability. You must use a heat synch or heat spreader accordingly to keep temperatures stable and as low as possible. These techniques are often known as passive cooling.
Move Heat: If it is not possible to remove heat, you can move it by using active cooling through the use of a small fan.  If choosing this route for heat management, consider the current impact this may have on your power supply.

Passive cooling through the use of self adhesive heat
spreaders on the bottom of the MMDVM duplex board

Appropriate Enclosure:  Do not choose to mount your assembled RPI3B plus MMDVM Duplex hotspot in a plastic, Plexiglas or 3-D printed enclosure. This enclosed and insulated enclosure may limit the life of your hot spot as well as impact its overall performance. Consider a metal or open frame construction if there is no option for active cooling available.
Proper RF Grounding & Shielding: Proper selection of antennas and installation of the SMA connectors is important to top performance of your hotspot.  Selecting the correct frequency that will best perform with your antennas and characteristics of your MMDVM device are important. HVDN recommends using frequencies in the lowest part of the 70cm allocation in your country for transmit as well as receive with appropriate spacing and separation.

Inside of Steve K2GOG's passively cooled duplex hotspot. Changes include extended SMA antenna
connectors to pass through the top of metal vented enclosure, passive heat spreaders under the
TCXO and directly above where the MCU is directly underneath the MMDVM board.

Correct Firmware:
 The development teams that create the hardware and software constantly make revisions based on user feedback. As of the August 22nd 2018 revision, improvements for TCXO management have been included in the firmware and it is advised for users to upgrade to atleast FW. 1.4.7 to take advantage of this. Here is a good article on how to update the firmware by Joe N1JTA

If you choose to use the Raspberry Pi Zero series of single board computer, many of the heat related issues are not much of an issue.   There is only one duplex board available that has a similar footprint as the RPIZW and is made by N5BOC and is available for purchase on Tindie, but HVDN has not yet been lucky enough to get one just yet for testing.

The N5BOC  duplex board is the smallest and most stable hotspot
device on the market capable of dual time slot operation

To learn more about some of what has been discussed in this article, here are some additional links:


  1. Link to a Dec 21 2018 announcement made on Pi-Star Forum re upgrading the firmware of MMDVM_HS Nano Hat devices.

  2. somersetlevels@gmail.comJune 21, 2020 at 6:56 AM

    Interesting that all variants of Chinese MMDVM show heat related issues and drift affecting BER caused by heat.
    I wonder why white Zumspot boad users don't complain?


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