Minimalist VHF Software Defined Radio Part 2

Shortly after I published the first post on a simple VHF SDR, Brady KC9TPA started making suggestions about optimising the code. So I encouraged him to have a look into the transmit side. How can we take a baseband modem signal (like GMSK) and convert it up to a HF IF frequency like 10.7 MHz using the STM32F4 DAC?

After a busy month (and not much sleep) Brady has done it! The following figures explain how it works:

Normally we would use a baseband DAC, mixer, LO, and crystal filter to generate a signal at HF (top). However Brady has shown it is possible to use a much simpler architecture (bottom).

So with the STM32F4, some clever software, and a buffer amplifier, he has generated a 10.7MHz HF signal. The DAC runs at 2MHz, which creates images (aliases) spaced every 1 MHz. The Band Pass Filter (BPF) selects just the image you want, e.g. 10.7 MHz in our case. The BPF doesn’t have to be very demanding like an Xtal filter, as the other images are 1MHz away. It is possible to tune the exact frequency a few hundred kHz in software.

Compared to a baseband IQ design this architecture doesn’t need two DACs, and doesn’t have have any IQ balance issues.

He used a GMSK modem signal as the baseband signal, however it could have easily been SSB, analog FM, or FreeDV. This is basically a baseband to HF SSB exciter. With a suitable BPF it could easily be tuned to anywhere on the lower HF bands. Software could then be used to tune the tx frequency within that band.

Brady received and sampled the 10.7MHz signal using an off the shelf SDR and it demodulated perfectly. Here are few photos showing his experimental set up, just a STM32F4 Discovery board and a buffer amplifier connected to the DAC. Note the sharp edges on the scope plot – this indicates lots of juicy HF content that we can tune to. He hasn’t added a BPF yet. The last plot is the GMSK signal as received by our demodulator running in Octave.

Our next step will be to mix this signal to VHF and add a PA to produce a 1 Watt 2M signal, to support our VHF FreeDV work. Please contact us if you can help us with a VHF PA design!

This design and the previous post that demonstrated the HF rx side suggests that the SM1000 could be modified to be a HF SDR transceiver. It already has a microphone and speaker amplifier, and even runs FreeDV out of the box! We would need to add a BPF, PA, and some gain on the rx side.

There is still a question over the STM32F4 internal ADC, e.g. it’s inter-modulation performance when used in over-sampled mode (thanks Glen English for pointing this out). Some more work required there. However this architecture is not limited to the STM32F4 – any uC connected to a few M-sample/s DAC and ADC (internal or external) will do. That’s the great thing about radios based on gcc C code and nearly no hardware!

SM1000 Part 12 – Testing in the US

Walter, K5WH has one of the 3 pre-beta SM1000 units. He writes:

Here’s a pic of the operations setup of the SM1000 on the air today from Houston Texas, into my HPSDR radio. With average Power down to 3 watts even. Made successful contacts to Mel-K0PFX and Gerry-N4DV. After working the audio levels a . . . → Read More: SM1000 Part 12 – Testing in the US

FreeDV and Codec 2 2015 Road Map

Last week I had a great chat with Gary Pearce KN4AG from Ham Radio Now:

Which brings me to my plans for 2015……..

2015 Open Digital Voice Road Map

I’m pretty excited about where Open Source Digital Radio is going in 2015. My goals for this year are:

A “sub zero” negative SNR FreeDV HF mode.
VHF FreeDV mode(s) that . . . → Read More: FreeDV and Codec 2 2015 Road Map

SM1000 Part 11 – Accepting Pre-orders!

The first batch of 100 SM1000s are being built in China right now and we estimate shipping will start in late March. Due to popular demand I am accepting pre-orders right now!

Australian customers can buy directly from my Store, rest of the world please use the Aliexpress Store for direct shipping from Shenzhen, . . . → Read More: SM1000 Part 11 – Accepting Pre-orders!

Minimalist VHF Software Defined Radio Part 1

I think the future of radio hardware is a piece of wire connected to a GPIO pin.

The rest of the radio will be “gcc compilable” free software running on commodity CPU horsepower. I spoke about this at length in my recent linux.conf.au 2015 talk.

For the last two weeks I’ve been developing a simple radio architecture . . . → Read More: Minimalist VHF Software Defined Radio Part 1

Codec 2 and GMSK over VHF Radio Part 1

In the previous post comparing GMSK modem algorithms, I had some results suggesting we can build a Codec 2 VHF “mode” that outperforms legacy analog FM by 10dB (that’s a factor of 10 in power). It seemed to good too be true. So for the past few weeks I’ve been working with Daniel, VA7DRM, to . . . → Read More: Codec 2 and GMSK over VHF Radio Part 1

SM1000 Part 10 – First Over the Air Tests

Last night I visited Matt, VK5ZM, with a SM1000 Beta. He configured the SM1000 to interface to his IC706 using the CN12 connector patch socket. This allowed an RJ45 cable from the SM1000 to connect to the RJ45 mic/audio connector on the IC706. A few level adjustments and he was using FreeDV to . . . → Read More: SM1000 Part 10 – First Over the Air Tests

GMSK Modem Simulation

Modems are an interface between theoretical physics and what can actually be built. The laws of physics set the limits of modem performance, and ultimately the amount of power you need for a certain bit error rate at a receiver. With the right algorithm, we can reach the limits of modem performance.

I think that’s kind . . . → Read More: GMSK Modem Simulation

FSK over FM

I’m interested in developing a VHF mode for FreeDV. One intriguing possibility is to connect a modem to legacy analog FM radios, which would allow them to be re-purposed for digital voice. One candidate is FSK at 1200 bit/s, which is often used over FM for APRS. This operates through FM radios using . . . → Read More: FSK over FM

SM1000 Part 9 – First Betas

Edwin and his team at Dragino have hand assembled the first two SM1000 Beta units in Shenzhen. I’m working with him to perform some initial tests while we wait a few days for the prototype enclosures to be made. Then Rick and I will both get a SM1000 shipped to us for testing. . . . → Read More: SM1000 Part 9 – First Betas