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Causes of IF feedthru
In article ,
Joel Koltner wrote: that a significant amount of energy ends up at my 1st IF frequency that seems to be coming from the LO. While I expect to see energy from LO+/-IF end up at IF, of course, I've checked the LO+/-IF spurs (the LO is coming from a PLL-based synthesizer), and in general it seems that a lot more energy ends up at the IF than what the spurs alone would suggest. I remember a talk I attended where the presenter mention that one of the biggest problems with building receivers was "the LO getting into the IF," so I'm thinking this is what he meant? Are there other less obvious paths for the LO getting into the IF than just the LO+/-IF spurs? The signal right at the IF eventually gets turned into DC and hence filtered out, so in theory it doesn't really matter that much, but in practice with very weak signals eventually the IF feedthru is stronger than the weak signals, so it limits how much amplification I can provide and hence limits the ultimate sensitivity of the receiver. Local-oscillator "blowthrough" is almost inevitable in non-balanced mixers. Even in single- and double-balanced mixers it's fairly common - it's an indication of less-than-perfect balance. Commercial DBMs usually quote an IF-isolation spec. 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. Homebrew DBMs with hand-wound toroids, and less-than-well-matched diode sets may have significantly worse LO-to-IF isolation. With diode-and-transformer DBMs, performance can degrade significantly if the mixer ports don't "look into" a nice broadband 50-ohm impedance. If, for example, you connect the IF port directly to a bandpass filter, the filter's impedance at the LO frequency is likely to be very much different thatn 50 ohms, and this may affect LO feedthrough. The higher-performance superhet designs I've seen, generally take care to ensure that each mixer port sees the proper termination impedance. In the case of the LO port, the easiest way is to generate a stronger-than-necessary LO signal (maybe 13 dBm) and then feed it to the mixer through a 50-ohm pad (6 dB in this case). In the case of the RF and IF ports, you can either use a resistive pad with modest attenuation (perhaps 3 dB), or use a diplexer. For the IF port you might want to feed the mixer into a common-gate or common-base broadband amplifier stage, run at a current level which gives it a 50-ohm input impedance... and then feed the output of this amp to your IF bandpass filter. I've seen one fairly expensive device (a Tek spectrum analyzer) which had horrendous RF-spur problems. Turns out that its front end uses a simple non-balanced diode mixer with essentially no RF-to-IF or LO-to-IF isolation. The LO-to-IF isolation isn't a problem due to the nature of the analyzer design... but the analyzer uses a relatively low swept-IF frequency range, and the incoming RF signals can blow right through and be detected as if they were IF mixing products. It's an OK analyzer for bench testing of many radios, but is pretty much useless for trying to look at VHF over-the-air signals. -- 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! |
#2
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Causes of IF feedthru
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? ---Joel |
#3
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Causes of IF feedthru
On 11 Jun, 22:52, "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? ---Joel DBMs should be terminated in 50 ohms over a wide bandwidth. MCL has recommendations concerning this. Leon |
#4
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Causes of IF feedthru
On Jun 11, 5:52*pm, "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? Ah - here you hit something on the head. I didn't know you were asking about UHF, but now I can contribute! SBL-1 balance isolation gets substantially worse as you move from HF to VHF to UHF. Looking at the spec sheet, as you move from 3MHz (where I use the SBL-1) to 369 MHz (where you use the SBL-1) the isolation gets 34 dB worse. *AND* the typical and guaranteed numbers are only if all the ports are terminated nicely. So while my gut feeling was originally "LO-IF isolation, that's never been a problem for me at 3.5 or 7 MHz with the SBL-1" now I look at the spec sheet and think "wow, isolation is a much bigger problem at UHF than anything I've ever dealt with in the SBL-1 at HF"! If you can arrange the IF stuff at the output of the mixer such that you have a good flat termination match and such that you hit an IF filter that will get rid of the LO before any substantial gain, I think you'll come out way ahead of just switching to a different variety of mixer. Tim N3QE |
#5
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Causes of IF feedthru
Thanks for the advice Tim; much appreciated!
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#6
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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! |
#7
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Causes of IF feedthru
Thanks Dave -- great post. I think I need to take the tiny little ball of
mixer/filter I have, spread it out a bit, terminate all the ports properly, and start making some measurements before connecting the IF filter, etc. back up a piece at a time! |
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