Let’s check out SDR medium wave performance with a variety of front end designs which all work differently and well, most of the time.
You may recall the time when proficient DX’ers recommended tube rather than solid state receivers for best medium wave listening. Classics like the old Hammarlunds and Collins R-390A received rave reviews. More recently, solid state rigs like the AOR AR-7030 hit the spot. Today, it’s all about SDR receivers, especially since the Perseus arrived with its full band recording capabilities.
As you know, key factors for medium wave reception are good selectivity and high dynamic range. Generally, this means at least 12 bits ADC and a well designed signal chain at the front end. Once you have converted signals to spurious-free digital I/Q, almost any software will do. The trick is keeping the front end linear.
So, let’s take a look at some SDR medium wave performance. I ran these tests on daytime listening to CBK Regina on 540 kHz. This 50 kW transmitter is 600 km. east of my home and puts in a decent daytime signal of -60 dBm or around S9+10dB. In particular, I was examining how good SNR I could achieve while avoiding any overload from strong local broadcasters, using a 1 meter active loop antenna.
My Flex 6300 (green above) performed best, running wideband open without any attenuation. Adding external attenuation reduced signal strength and SNR as expected, but did not change the noise floor. While many ham rigs cripple medium wave performance to avoid IMD, the Flex does not. It’s also a good performer down in LW.
Next up, my Perseus (orange) worked well but required -10 dB attenuation to prevent in-band overload on medium wave. Without some small attenuation, everything crashed. Those extra 2 bits in the Flex ADC sure make a difference. But, as a system with its step attenuation, you will find Perseus great for AM BCB DX.
SDR Medium Wave Performance – Dual Channel
My Afedri is a good performer but trickier to use. As shown above in blue, you use a digitally controlled RF amplifier to control MW overload. If you don’t use an external attenuator, you need to manually reduce front end gain. Alternatively, you can leave the AGC loop inserted and that finds the sweet spot automatically. Your SNR is about the same, either way. The Afedri CPU does a good job of measuring ADC clipping and adjusting RF gain appropriately.
Finally, the RSPduo (above red) provides good SDR medium wave performance, but you must adjust Gain Reduction manually to avoid overload, especially with an amplified loop antenna. Without external attenuation, I needed Gain Reduction of -42 dB to turn the overload indicator off. My best signal to noise ratio came from inserting -10 to -20 dB attenuation externally with Gain Reduction of -18 or -12 dB. And, of course, make sure you turn the MW bandstop filter off!
Two final notes. First, all these receivers are quite accurate, with the same signal strength reading within ±2 dB. Second, the dual channel radios (RSPduo and Afedri) let you control the gain distribution of each channel independently, which is necessary if each tuner uses a different type of antenna.