Chinese "Pixie" kit minimum detectable signal - Page 1
Forum section: electronics / rf-microwave

author: rhb
author_rank: Super Contributor
timestamp: August 14, 2020, 02:16:53 am
content: I made a JFET switch using an MPF-102 and used it to pulse modulate a 7.110 MHz signal fed to the Pixie antenna connection through a 60 dB by 1dB JFW rotary attenuator. Minimum detectable signal was -59 dBm. Minimum comfortable signal was -50 dBm. This is without any QRM or QRN so I'd expect in practice the signal would need to be stronger for a QSO. Probably by quite a lot given the lack of any real AF filtering in the Chinese Pixie.My 33622A would only go to -5 V, so I'll need to build a -8 V pulse source to fully turn off the JFET for testing more sensitive receivers. Admittedly not much, but I'm starting from nothing, so it's progress even if very limited.Have Fun!Reg

author: fourfathom
author_rank: Super Contributor
timestamp: August 14, 2020, 06:33:27 am
content: -59 dBm = 252 uV. This is spectacularly poor sensitivity. With a halfwave dipole antenna you are probably going to have an atmospheric background noise floor somewhere around -110 to -130 dBm (assuming a 10 MHz frequency), so your receiver should be at least that sensitive.

author: rhb
author_rank: Super Contributor
timestamp: August 14, 2020, 01:08:31 pm
content: It's so poor as to be pretty much unusable in my view. Why it ever got popular is beyond me. You can do far better with very little effort and a similar BoM cost. To go with the poor sensitivity, it radiates 10 mW into the antenna when receiving and violates FCC spurious emissions rules when transmitting.The sensitivity problem is because the Pixie 2 uses an LM386 to provide *all* the gain which is limited to 46 dB. I've designed a quad op amp active filter with adjustable gain up to 100 dB, adjustable frequency and Q.For my first scratch design effort I picked a very low bar. Not hard to beat a Chinese Pixie at a similar price point. Having pots to adjust VXO, RIT, AF gain, AF frequency and Q will probably double the price, but the increase in function justifies it and one can always use trimmers or fixed resistors for the AF and RIT frequency and Q to cut the cost at the loss of some functionality.Have Fun!Reg

author: rhb
author_rank: Super Contributor
timestamp: August 16, 2020, 05:06:44 pm
content: A couple of additional notes:I neglected to account for the loss of the JFET switch I used. That appears to be ~ 7 dB. So adjusting for that puts the audio MDS at ~ -66 dBm. Better, but not good.I measured the 10 dB (S+N)/N point at -55 dBm using a Keysight 33622A and 34401A with coaxial cables, no switch. I measured N, then measured S+N.Have Fun!Reg

author: fourfathom
author_rank: Super Contributor
timestamp: August 16, 2020, 06:03:59 pm
content: Quote from: rhb on August 16, 2020, 05:06:44 pmIf the Pixie had AGC then I think your measurement would be off. In that case I think the correct S+N/N method would require notching out the signal freq for the noise measurement. Since the Pixie has no AGC then your method ought to be accurate. For my own casual tests I just eyeball the spectrum analysis display of the audio output.

author: rhb
author_rank: Super Contributor
timestamp: August 16, 2020, 07:21:40 pm
content: AGC would not affect the measurement in this case as the signal is not changing. I thought about using an SA, but in the end, what you can hear is the ultimate arbitrar for old school CW. I can hear the signal well below the noise level.Have Fun!Reg

author: fourfathom
author_rank: Super Contributor
timestamp: August 16, 2020, 09:00:54 pm
content: Quote from: rhb on August 16, 2020, 07:21:40 pmI must have misunderstood, I thought you were measuring noise with no signal input. Yes, 10dB is pretty arbitrary, the MDS (Minimum Discernible Signal) is usually well below that (but not as well defined).

author: rhb
author_rank: Super Contributor
timestamp: August 16, 2020, 11:26:19 pm
content: Rather depends on how acute someone's hearing is :-(I'm beginning to think that MDS is almost in the same class as audiophool specifications.I'm still collecting parts, but getting close to building a clone of the Chinese Pixie with the LO, PA/mixer and AF sections all separate so I can substitute each individually as I redesign it. I feel pretty confident I can do a lot better within the same basic cost constraint.The follow on design will probably use 4-6 transistors and a quad op amp. Possibly a bit more. My goal is portability and low power drain without seriously compromising performance or dependency on parts likely to become unavailable.Have Fun!Reg

author: richard.cs
author_rank: Super Contributor
timestamp: August 17, 2020, 10:16:15 am
content: Well, the human ear has a spectral resolution around 30 Hz, and the brain can also be trained to do some interesting time domain correlation - e.g. for CW it knows where the edges should be quantised to one of a small number of steps. I think that's mostly where an apparent 10 dB or better MDS is coming from - it's not nearly such an impressive SNR if you're looking at the noise in the 30 Hz bandwidth your brain is using rather than in a 100-200 Hz receiver bandwidth.