The Acorn II is a nice entry level board into software defined radio (SDR). Although this comes as a kit, mine was already assembled. Based on the looks of the board it is well laid out, and with a few exceptions, parts are well spaced for easy assembly. The board comes ready to run on 40M with the supplied crystal. Part of the board layout allows easy modifications to an external oscillator circuit , alternate band filtering along with headers for alternate connections to antenna, IF output, antenna, and power.
To begin using it computer software is needed but is free and readily available via the internet. We will only discuss those with Windows operating systems and have not gone into details on setup.
- First up, as mentioned in the manual, is the SDRadio program from I2PHD Software.
- The software is rather easy to have up and running. The biggest tip is to click on the displayed frequency in the center of the screen above the spectrum window. You can then enter your center frequency of your LO (or LO / 4) in which the dial at the bottom will now display the correct frequencies. Also not shown but discovered by accident is by clicking on a finer resolution (IE 6KHz) then draging the sides of the highlighted selected frequency will allow changing the signals bandwidth (to reduce for CW). If your are having problems then click “Options” in the upper left above the spectrum to select your sound card, resolution, I<>Q swapping, etc.
- The downfall is only up to ~96KHz bandwidth, or ~48KHz on each side of the center (crystal) frequency can be accessed (or about 6960-7060KHz).
|40M showing 6KHz Bandwidth Setting
|40M showing 6KHz Bandwidth Setting
- SDR# is one of my favorites and never seems to disappoint. A little more difficult to set up but has many good features. The best part is this, and others, can handle higher sound card sample rates. For my laptop I can set the rate up to 192000 and get almost double the frequency range of SDRadio.
- +/-~85KHz from center (at 192000 sample rate) or from about 6938-7092KHz.
- A little more difficult to set up but also more available options.
- Another nice SDR very similar to SDR#.
- Never could make it work right, and not work good on my SDR dongles either. I can see the spectrum but can’t tune in a single frequency to listen to. This is worth a mention because some people have had good success with it.
A good question that comes up is “Do I need software to use this receiver?”. The answer is “No, but…..”. In effect this is a DC receiver as the I and Q channel outputs are in the audio range. So yes on an active band you should be able to use a little speaker amplifier and hear signals. But……..
- You can hear a bit with earbuds but how far you hear is only as good as your hearing (a few KHz), it will act as a DC receiver hearing on both sides of the center frequency.
- Amplification is needed (for speakers) and additionally filtering if the band is busy.
- Like a crystal controlled DC Receiver (AKA Pixie) you are limited to the one frequency and what your ears can hear. You CAN SVXO the crystal, and say you can bend an additional 4KHz. But since the oscillator operates at 4X the frequency and is divided down, the resulting VXO is only 1/4 of the 4KHz or 1KHz, enough for RIT.
- You can tune more with an external oscillator, such as an AD9850 or si5351.
The kit comes with a 28.060 crystal. Divides by 4 gives us a 7.015MHz center frequency. Next up we bypassed the 40M on board filter and tried various crystals just to see what it would do. It is recommended to use a band filter as images from other frequencies will be present. The below table is common QRP crystals, along with some potential crystals from various distributors. Frequency coverage may vary depending on the quality of your sound card and software being used. In some cases you can pick out stations without a filter but using an appropriate filter for the band is highly recommended. For example interference from the AM BCB will prevent you from hearing anything in the 472-479KHz area.
|DIP style Oscillators
(requires 5V supply)
After having success with crystals we decided to try a couple of external signal generators. First up was the Owon AF202F function generator which is my benchtop generator. This has a limit of 50MHz (or 50 / 4 = 12.5MHz as the highest receiving frequency). The generator worked fine and was able to tune in a couple of SW broadcast stations in the process. One thing noticed was a rise in the overall noise floor (more below). We also tried the si5351 board from Etherkit with good success. One we have not tried (yet) was the M0XPD IOT DDS board as we have yet to build one up.
One noticeable difference between internal and external generators was the overall noise floor levels. I was suprised to see the Owon had a great deal of noise when used compared with the si5351 clock generator board. Below are a few screen shots of noise floors. Note for external oscillators I also disabled the internal clock by jumpering one pin of the crystal area to ground. The oscillator circuit with NO crystal installed will free run self-oscillate around 59-60MHz drifting at several KHz per second and slightly increase the noise floor in the process. On the subject of measuring noise floors, with an SDR receiver you can use this is a mini test tool to check your spectrum bandwidth or other spurpious signals within your tuning range. I would not recommend to use one as an accurate gospel but usable for a quick check.
|Sound Card Only
(not connected to receiver)
|No Clock Applied
(connected but disabled oscillator)
|Internal Crystal 28.060MHz
|si5351 at 28.060MHz
|Owon AG2053F at 28.060MHz