Any advice for a Tuned RF receiver ? - Page 1
Forum section: electronics / projects

author: MathWizard
author_rank: Super Contributor
timestamp: December 15, 2023, 09:00:31 pm
content: For a 5.226MHz BJT TX/RX I'm making "from scratch", I'm having a hard time getting the TRF RX to work the way I want. Some of the effect's I'm seeing are fairly new to me like all the frequency distortion and mixing that happens, especially when inductors are added, and the RX is just on breadboard. Before when I think of frequency distortion, I'm thinking of a sine wave turning into a square wave from saturation. But some of these frequency effects happen well before that, and IDK if it's an oscillation or highly amplified noise. The TX looks good enough for now, it's the RX.And I'm just getting into noise sources in circuits. And after 4-5 stages of CE amps, well IDK what's to be expected, or how good/bad my BJT's and other parts really are, or my circuits.Today with some ~4ft piece of wire as an antenna, hooked to my 10x probe, local AM stations are mostly from -25 to -52dBV, and my TX is at -52dBV, and FM shoots up to that level once and a while.With the various input tanks and filters I've been trying, I've had the AM stations down under -60dBV and my signal up to into the -40's before amplification. For now the audio is from my AWG.I've been trying BP, LP, HP filters between stages, and a few times I thought I made the perfect RX, with -1dBV at 5.24MHz, and all the next strongest signals were -50dBV and less right around that freq. But today my real TX signal is at 5.226MHz, and it was just an oscillation, that was at +3dBV when the TX was off. When I sent 1Khz audio from the TX, instead of nice modulation, I was getting round balls with other effects.One of the pictures is at 5.232MHz, with just the input tank, and 2 amps with an LC tank on each collector, and hardly any the 1kHz modulation shows up in the time domain, since this cant really be my signal TX I guess. But it's all around it on the FFT plots.Then there's the ones with a broader BW, and I do have the real TX signal there and modulation, but it's just very noisy, and lot of extra signals get modulated in and sounds bad.I worked out the transfer functions w/ ro, Cpi Cu and Ce all included for Av, Zin, Zout, and I get the same answers as LTSpice, but my parts are not perfect parts, and neither is my measuring equipment like LCR meter that only goes to 100kHz, and 10x probes with enough capacitance to mess up even this 5MHz project. So I don't know what to trust besides if it will sound ok.I've built better sounding AM receivers by accident,then the best I've had this doing so far, and also 2 and 3 BJT receivers that sound better too.

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 15, 2023, 09:27:18 pm
content: The breadboard probably isn't helping.

author: Benta
author_rank: Super Contributor
timestamp: December 15, 2023, 09:47:10 pm
content: Quote from: BrokenYugo on December 15, 2023, 09:27:18 pmUnderstatement of the year.

author: MathWizard
author_rank: Super Contributor
timestamp: December 15, 2023, 09:57:02 pm
content: But it's such a black box, it's so hard and time consuming to be soldering and unsoldering it all. What works fairly good on paper and the sim, doesn't really match my parts yet.Yeah and I have 100k and 27k for the 1st base divider, not those /10.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 17, 2023, 01:56:41 pm
content: It sounds like the amp is overloading creating a string of harmonics and that there is possibly poor isolation between input and output.Excessive peaking?BTW Murata make a supercheap filter for 5.5MHz, SFT5.5MA.

author: MathWizard
author_rank: Super Contributor
timestamp: December 17, 2023, 04:33:46 pm
content: No no no, I want to do it the old fashioned way, and with my old parts.I'm just getting used to how bad 20pF of the scope probe can be. Now I don't go probing the input or base of circuits and expect good results, but yeah it will screw up outputs too.So I made a copy of the Bob Pease HiZ, low capacitance buffer probe.In LTSpice it works beautifully as a 1:1 buffer. I made it on a copper clad board, with good quality BC550/560's, but my 2N5485 JFET's are from ebay.But it needs some improvement, and better matching/balancing of the parts, but it's not bad for an overnight attempt.Below is all with 100mVrms in, = -20dBV . The input goes up and down a bit though, and plus I'm just using cheap alligator lead, coax cable. I have my scope probe in little copper coils on the board, so no GND clip. And for the buffer probe tip, I soldered on a safety needle, it's springy too so that's great.

author: MathWizard
author_rank: Super Contributor
timestamp: December 17, 2023, 04:35:00 pm
content: And here's some plots of the numbers I recorded.Ok so I'm really limiting the bandwidth of my scope on 10bit, when looking at this 120MHz stuff.Actually I wired it up wrong, I had the lower 3.3k going to GND, and at DC my output could not buffer negative inputs. I was wondering why my low frequency response was so different to the sim.But after correcting the circuit, the mistake circuit gives a much much flatter response, than the correct version, so it could be useful for something. I can see adding a cap here or there to the correct circuit, can alter the HF crest, but will usually lower the BW.With some tweaks it will be fine enough for me for now., but all these pic's are from the mistaken circuit.