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Causes of IF feedthru
In article , Joel Koltner wrote: Hi Dave, Thanks for the information; it's quite helpful! I was (am) committing the faux pas of feeding the mixer output directly into a narrow bandpass filter... For example, the popular Mini-Circuits SBL-1 mixer seems to have LO-to-IF isolations of anywhere from better than 65 dB (at HF) down to around 30 dB (at UHF). If you're trying to tune a weak signal (say, 80 or 90 dB weaker than the LO signal) then the residual LO feedthrough can cause the sort of swamping you're seeing. So... say I'm using a 45MHz IF, trying to tune 414MHz using the SBL-1 and an LO of 414-45=369MHz (low-side injection) at the SBL-1's recommended +7dBm. Presumably I'll see a 7dBm - 30dB (LO-IF isolation) = -23dBm signal at the IF, but it should still be at 369MHz, right? How does the LO "bleed through" to the 45MHz IF... and what power level should I expect to see there? I'd say that you could be seeing the 369 MHz LO signal in your IF pathway through a number of mechanisms. First: some of it may be getting past your bandpass filter. If you're using a crystal filter, you may find (if you look) that the filter's stopband attenuation is neither wonderful nor flat at some frequencies. Crystals often have secondary resonances at (or near) the harmonics of their fundamental frequency - that's how overtone filters work - and your LO of 369 MHz isn't all that far from the 8th harmonic of your IF frequency. You'd probably need to sweep your filter's response (wideband oscillator plus an RF detector or a spectrum analyzer) to determine if this is occurring. If this is the problem, you might remedy it by adding a one- or two-stage passive lowpass filter (maybe LC, maybe just RC) somewhere after the mixer and before the final detector. This would attenuate out the residual LO signal that the bandpass filter doesn't. Maybe run the mixer output into that grounded-base/source 50-ohm buffer amp I suggested, gain a few dB, and then run the signal into a slightly lossy low-pass filter whose output presents your bandpass filter with the termination impedance it expects? Second: you may be getting parasitic transmission of the LO signal *around* the bandpass filter - either capacitive or inductive. If your bandpass filter is a tuned LC type rather than a crystal filter, there might be coupling between the inductors. Third: you might be getting some LO pickup by the gain stages in the IF path, direct from the local oscillator - wire-to-wire coupling or something like that. Fourth: you might be getting LO wandering around on the power supply lines or in the ground paths/planes - insufficient power-supply decoupling, shared power-supply wires, shared ground paths, and so forth. I get the impression that careful attention to layout, shielding, and decoupling are very important for higher-performance receivers. Some builders seem to prefer to place the LO, mixer, and filter in separate shielded sub-compartments (soldered together from double-sided PC board material if nothing else), to carry the signals between these compartments on shielded coax, and to use feedthrough caps and ferrite beads on the power wiring. This helps minimize the various forms of parasitic coupling between stages. -- Dave Platt AE6EO Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
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