Congratulations to Joe N9OK on his homebrew build of the SolderSmoke direct conversion receiver. Welcome to the Hall of Fame Joe.
Joe makes some very nice comments on the benefits of doing this in a homebrew way:
I've build many dozens kits over the years, esp Knight, Heath, K2, K3, and others. This was my first receiver built from scratch. It works surprisingly well and has decent sensitivity. Thanks @Bill N2CQR and Dean @Soldersmoke for creating this challenge. It was fun and non-trivial. The only stage I had issues with was the amplifier, which was too compact on v1. v2 works well.
I think Joe should hold onto that smaller version of the AF amp. I think he could get that stablized and it could become the heart of yet another FB DC RX.
Joe went the extra mile and measured the minimum discenable signal of the receiver:
I see no problem with Joe's measurment technique. I think the key word here is "discernible." Often people will focus on the RF signal that will produce a 3 db or 10 db increase in audio signal. But Joe has just looked at the RF signal level that will create a discernible audio signal. That is fine. As Joe demonstrates, this simple DC receiver is remarkably sensitive.
Joe N9OK's measurement is what psychoacoustics calls the absolute threshold of hearing (ATH). Alan W2AEW, did the measurement in line with the more objective MDS. More or less, usually in the same range, but the former can be lower depending on the person.
ReplyDeleteIf we put an audio filter at the output we can squeeze a few more dB for CW if desired. Probably get below -130 dBm or less. Weak levels like this makes the measurement isolation critical.
good chatty chat stuffnFluff gentlemen, but the antenna is what matters
ReplyDeleteAgreed. Splitting hairs on simple equipment is a good passtime? A poorly sensitive receiver that offers good dynamic range may handle low-level signals without degrading them from intermodulation distortion better than a receiver with so called 'high sensitivity' but lower dynamic range performance.
DeleteAgreed, IMD, selectivity and other parameters are important. The above comments are confined to the bench measurements. At 7 MHz, a dipole with (per ITU paper) Fa~ 40-45 dB or more will quickly change how much less important the MDS is, at least for those HF bands.
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