OpenRadio Part 3 – Filters

Over the past week I’ve built my own OpenRadio prototype, using the construction notes Mark has put together as a guide.

To help others I measured a few DC voltages and recorded them. I found one small bug in my assembly: one of the flip-flop pins was not soldered correctly, leading to erratic signals. After that I set the LO to receive a 14 MHz signal and managed to receive a carrier from my FT-817, via about 60dB of in-line attenuation. At the moment I am using SpectraView running under Wine as the SDR software, however we really should get a Linux/Open Source SDR program running for the mini-conf. I only have a mono input sound card in my laptop so I’m getting a mirror image of the received spectrum. Still, good enough to get started.

I connected the radio to an external antenna and tuned to a local AM station on 1310 kHz. This sounded very strong but distorted. When I tuned to 7.150 MHz I could still hear AM radio signals, which suggests very strong local signals overloading the mixer. I tested this idea by inserting a 20dB attenuator in line with the antenna and sure enough the AM signal on 1310 kHz became clear and I could no longer hear AM stations on 7.150 MHz.

I could even see the AM signal on my oscilloscope – it measures 1Vrms (20mW) on the antenna terminals! That’s enough to light a LED (10mA at 2V).

However an attenuator is not ideal, so using the tables from my trusty copy of RF Circuit Design I built a simple High Pass Filter to attenuate broadcast signals by about 20dB, but pass other HF signals above 3 MHz. This consists of a 1nF capacitor and two 4.5uH inductors (21 turns on a 7mm diameter pencil) in a “Pi” arrangement. This worked well, the AM signals sound fine and no break through on other HF frequencies.

I also tested Mark’s 27 MHz Low Pass transmit filter, this cleaned up the PSK31 tx signal nicely, 2nd harmonic at least 30dB down with about 1Vrms into 50 ohms (20mW) transmit power. Here is a photo of my OpenRadio with both filters on the right. The larger coils at the top are part of the 3 element 3MHz high pass filter, which then connects to the 7 element 27MHz low pass filter.

Mark and I even had a OpenRadio to OpenRadio PSK31 QSO on the 40m Ham Band! I had about 40mW transmit power on 40m. This was actually NVIS propagation so 100km up and down to the ionosphere and 10km across Adelaide!

So our #2 prototype helps us confirm that the design is working. I’ve followed Mark’s construction notes and made some of my own, and obtained experience in setting up the Arduino and Spectraview software. The broadcast HPF design may be useful for others who experinece strong local interference.

Well done Mark on a fine job designing OpenRadio and writing the support Arduino software. He has put in a tremendous amount of work to develop and test the hardware, written a lot of software, and carefully documented everything on the OpenRadio Wiki. This is a great resource that will be useful to many others. Next step is the kit production. Right on schedule for in January.

One thought on “OpenRadio Part 3 – Filters”

  1. Hi David,

    Most cool!! You might consider getting samples into linrad?

    I have 14,106.5mhz I&Q i-> audio card -> linrad#1 -> ethernet -> linrad #2 -> audio -> fldigi -> ssh text window on my desktop pc running right now. ei5fs talking to hb9bdm as seen from Concord, NH USA !! Still don’t have xmtr hooked up :(.

    4 computers !! Crazy. But you can REALLY see what is going on with linrad !!

    Might be worth the effort. You can most probably convince Leaf (sm5bsz) to help with Linrad interface?

    Nice work :).


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