A few months ago I purchased a new (secondhand) car, a 2011 Toyota RAV4. Of course, my first priority was to kit it out with amateur radio gear! Fast-forward to the present, and I've now got an auxiliary battery system, a Kenwood TM-D710G, a Codan NGT SRx, and a Codan 9350 auto-tuner antenna installed.

Now, I've been following the development of Codec2 and FreeDV (Open-source digital voice for HF) for quite some time, and have been involved in testing of FreeDV over-the-air for a while. One of the early tests involved using FreeDV while mobile, on the Hume Highway near Yass, NSW. This required a fair bit of infrastructure to get going (laptop, rig interface, headset), and so the release of the "Smart-Mic" SM1000 promised to allow easy use of FreeDV in a mobile situation. So, at the last AREG meeting, I bought a SM1000 straight from the author of Codec2 and FreeDV himself - David Rowe.

Over the last few days I've been working on interfacing the SM1000 with the NGT SRx, and David suggested I document the process, so here we are!

One of my aims for using the SM1000 in the car was being able to really use it like a microphone. One cable, low clutter.

The SM1000 has an isolated rig interface via a RJ45 socket with a configurable jumper block, and has a DC supply on a separate connector. I didn't really want to use the DC supply connector - this meant soldering wires to the DC connector pads, connected to the patch panel. The final wiring diagram is below:

A few notes on the connections:

The audio input on the NGT is balanced. I experimented with grounding one side of the input and using it single-ended, but this resulted in feedback issues. With the MIC GND separated from the other grounds, it can be used in a balanced mode. I used one of the CAT5E pairs to carry the balanced signal.

All the grounds on the patch panel are separate from the main SM1000 DC ground, so to power the SM1000 and have the PTT and Speaker output work, they must be connected together. Again, more soldered wires.

The case of the SM1000 is nicely connected to the DC ground, which is now connected to the ground of the NGT. If the NGT isn't bonded to the car body well (i.e. braid to an lug or similar), you'll get an interesting tingle whenever the antenna is tuning!

The patch panel ended up looking like this:

We could definitely use a bigger patch panel in SM1000 revision F! Also breaking out the supply and ground rails to the header would definitely make what I was trying to do a lot easier!

Finally, the output level of the SM1000 is only 50mVp-p. This is fine if you are driving a microphone input, but the NGT requires an input a bit closer to line level (300mVp-p).

The output level is set by the trimpot R47, and fixed resistors R48 (10K) and R49 (4R7):

Switching out R48 with a 1K resistor results in a maximum output level of around 350mVp-p, which can easily be adjusted down with the trimpot. R48 is an 0402 SMT resistor, so a hot air rework station is recommended:

Unfortunately I only had 0603 resistors in stock, but I managed to shoe-horn one into position:

The day after modifying my SM1000, I headed over to Andy VK5AKH's QTH. Andy also owns a RAV4, and also has a NGT SRx, so we modified his SM1000 and made up a suitable interface cable.

To configure the SM1000's levels we set up our cars near each other, with the NGTs set to transmit on a clear frequency. We also took the whip antennas off the Codan 9350s to limit propagation (the 9350 will quite happily tune the antenna stub on 40m and above). First we tried a basic SM1000 to SM1000 contact, but it quickly became clear that setting levels would be easier with a laptop running the FreeDV software at one end. Luckily I had brought my Signalink USB. A quick modification to the Signalink's internal jumper block and it was made compatible with the NGT to SM1000 cable, and I was able to run FreeDV and look at SNRs.

A bit of tweaking of trimpots later, and we were able to get at least 15dB SNR between the cars. For some reason I was able to get better SNR from AKH->QI than QI->AKH. More investigation required!

After all this we still had pretty significant garbled voice issues, with conversations somewhat intelligible, but nowhere near as good quality as we had encountered with the FreeDV software. We decided to hit the road and headed over to David Rowe's house for a coffee and some more testing.

On the way we were able to maintain a conversation on 40m for a while via ground-wave, though still with the garbled voice issues. We arrived at David's place, where we started performing a few tests:

Carrier TX using SM1000 test mode: Worked fine. Clean RX tone, no distortion evident on either transceiver.

Worked fine. Clean RX tone, no distortion evident on either transceiver. Analog audio TX using SM1000: Pretty bad distortion on the audio, as received by the other car. Tweaking the mic gain didn't seem to help... (hmmmm)

Pretty bad distortion on the audio, as received by the other car. Tweaking the mic gain didn't seem to help... (hmmmm) Transmission of FreeDV test signal from David's home station: Decoded audio sounded great coming out of the SM1000. This means the receive chain on the transceiver + SM1000 checks out... must be something in the transmit chain.

Decoded audio sounded great coming out of the SM1000. This means the receive chain on the transceiver + SM1000 checks out... must be something in the transmit chain. FreeDV transmission using test audio into SM1000 External Mic socket: Sounded great! At this point we figured something must be wrong with the SM1000's internal microphone.

It was around this point when we started seriously looking at what was going on with the clip indicator on the SM1000. We had previously noticed light illuminating a lot, even with the mic-gain trimpot set on what we thought was the minimum. After a comment from David, we realised that the mic-gain trimpot actually operates in reverse, compared with the other trimpots on the SM1000. This mean that full anti-clockwise is maximum mic-gain - no wonder we were experiencing audio issues!

After some further adjustments, we had both SM1000's operating nicely with the Codan NGTs, giving good quality audio!

Over the next few weeks we'll be using the SM1000's during drive-time (probably on 80m or 40m), getting a feel for how usable they are in a noisy mobile situation. Stay tuned!

Update #1

HF conditions have been pretty poor, to the point that even SSB comms aren't working on either 80m or 40m in the morning or afternoon (bring on a 60m allocation!!). So, we haven't been able to really give the SM1000s a decent test (apart from some short ground-wave contacts).

I also had the need to interface my laptop with the NGT again, via my Signalink USB. This meant modifying the Signalink's internal jumper to be compatible with the NGT-SM1000 cable:

This jumper block should also be compatible with the 6-pin Mini-DIN cable which Tigertronics supply.

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