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AGC signal/noise question...
On Fri, 18 Aug 2006 19:54:00 +0200, "i3hev, mario held"
wrote: Michael Black wrote: In that case though, you should be worried about that nearby strong signal dropping the gain of the receiver so you can't hear the weak signal... may be you are not thinking of cw... Provided the SNR is good enough, you can filter and post-amplify your weak signal, e.g. with a good AF filter, or a dsp. Of course, if the IF stages are (reasonably) linear in response, you can disable the AGC, but this would be no answer to the original question Since the SNR is established by the frontend the IF system can have a more relaxed SNR. However be wary of ICs like the MC1350 as the gain reduction occurs the internal noise is bad. I've built several recievers using this part and at ~10db gain reduction the noise jumps way up. I've gone to cascode JFETs as the noise is more predictable and generally lower. The device used does make a difference. Allison |
#2
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AGC signal/noise question...
wrote: On Fri, 18 Aug 2006 19:54:00 +0200, "i3hev, mario held" wrote: Michael Black wrote: However be wary of ICs like the MC1350 as the gain reduction occurs the internal noise is bad. I've built several recievers using this part and at ~10db gain reduction the noise jumps way up. I've gone to cascode JFETs as the noise is more predictable and generally lower. The device used does make a difference. Allison I have to disagree on the MC1350 and way back 30 years to its predecessor, MC1590. The prototype HF receiver presently on my workbench has a NF of 5.5 and that hardly rises more than that with AGC current applied to the AGC pin. BTW, that receiver, single-conversion with one IF at 21.4 MHz, uses only MC1350s up to the detector, including the one mixer stage. [ LO is a separate PLL board ] |
#3
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AGC signal/noise question...
On 19 Aug 2006 20:19:19 -0700, "
wrote: wrote: On Fri, 18 Aug 2006 19:54:00 +0200, "i3hev, mario held" wrote: Michael Black wrote: However be wary of ICs like the MC1350 as the gain reduction occurs the internal noise is bad. I've built several recievers using this part and at ~10db gain reduction the noise jumps way up. I've gone to cascode JFETs as the noise is more predictable and generally lower. The device used does make a difference. Allison I have to disagree on the MC1350 and way back 30 years to its predecessor, MC1590. The prototype HF receiver presently on my workbench has a NF of 5.5 and that hardly rises more than that with AGC current applied to the AGC pin. Read EMRFD page 6.16 (ARRL press) they tested the 1350 and at the point where the gain cell has equal conduction on both legs the noise rises significantly. I duplicated the test fixture and yes, it's noisy, from around 6db to around 11db in my fixture when gain is reduced by 10db and that was at 16mhz. In a reciever that used it I went to two cascode stages using JFETs and the difference noise was notable for weak signals just into the agc range. I restrict the 1590/1350/ca3028 for lower perfomance recievers now. I also verified that the 1590 does same and also the CA3028 wired as differential AGC. Even tried three 2n3904s and same result. The agc range was good and at full gain the noise was ok but the noise increase at partial agc was surprizing. BTW, that receiver, single-conversion with one IF at 21.4 MHz, uses only MC1350s up to the detector, including the one mixer stage. [ LO is a separate PLL board ] I do most of my RX experimentation at 6/ 2M and 70cm SSB so noise and overload perfomance are important to me. Images are also a big problem as I'm near a lot of VHF/hf broadcast. Allison |
#4
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AGC signal/noise question...
From: on Sun, Aug 20 2006 9:06 am
On 19 Aug 2006 20:19:19 -0700, " wrote: wrote: On Fri, 18 Aug 2006 19:54:00 +0200, "i3hev, mario held" wrote: Michael Black wrote: However be wary of ICs like the MC1350 as the gain reduction occurs the internal noise is bad. I've built several recievers using this part and at ~10db gain reduction the noise jumps way up. I've gone to cascode JFETs as the noise is more predictable and generally lower. The device used does make a difference. Allison I have to disagree on the MC1350 and way back 30 years to its predecessor, MC1590. The prototype HF receiver presently on my workbench has a NF of 5.5 and that hardly rises more than that with AGC current applied to the AGC pin. Read EMRFD page 6.16 (ARRL press) they tested the 1350 and at the point where the gain cell has equal conduction on both legs the noise rises significantly. I duplicated the test fixture and yes, it's noisy, from around 6db to around 11db in my fixture when gain is reduced by 10db and that was at 16mhz. In a reciever that used it I went to two cascode stages using JFETs and the difference noise was notable for weak signals just into the agc range. I restrict the 1590/1350/ca3028 for lower perfomance recievers now. Apparently I hit some nerve on my disagreement. My first experience with the MC1590 was in 1973 and a need to operate over 55-64 MHz. Electronic gain control was essential and it had to be fast. Motorola supplied some additional information which was later incorporated into appnotes. The MC1350 was marketed around '73 along with the MC1330 video detector as a TV IF system. It didn't sell that well in quantities (presumably) and both were dropped from active production (Lansdale acquired masks and now makes the MC1350). The 1350 (8-pin DIP) should use the same die in the metal can MC1590. While neither one was ever touted as a super-champ low-noise device, it is what I consider respectable as to NF. The fact that it has differential input and differential output is convenient from the standpoint of circuit design. Especially so when input impedances (each side) has a dependable 5K R in parallel with about 5 pF total capacitance. Gain of both begins to fall above 75 MHz with output loads of 100 Ohms resistive. I've found no noticeable difference between differential input v. single-ended. That IC is what I term a "double Gilbert cell" in that AGC control current affects both differential inputs equally (or very nearly so). Whether one connects to both inputs or just one shouldn't make any difference other than output gain. I also verified that the 1590 does same and also the CA3028 wired as differential AGC. Even tried three 2n3904s and same result. The agc range was good and at full gain the noise was ok but the noise increase at partial agc was surprizing. I've never encountered any "surprising" increase in noise at any AGC input to a 1590 or 1350 causing partial gain reduction. That is as true in 2005 as it was in 1973. If there is a SNR of 10 db at an RF carrier input of 3 uV and a gain reduction of 10 db for a 10 uV RF input results in 3 db more noise in the front end, the SNR with a 10 uV input is still higher than the one at 3 uV. What has been "lost" there? Let's look at the original problem starting this thread: There was a claim of "increased noise" with AGC on, but no quantifiable data. The sudden segue to stating that a certain IC is "bad" is a leap that defies good design practices to me. I'm not impressed that the ARRL had some test data in a publication; having been hands-on with this Motorola design for a number of years, I have a number of RCA lab notebook pages filled with my testing of it along with a patent involving it granted 1974...besides my own hobby notebooks. Low-noise input amplifier design is an entirely separate subject and there are a number of other active devices which can do lower NFs than 5. What was orignally needed was some way of getting some numbers and test configuration of Andrea's problem...to pin down a possible reason for alleged increased noise with AGC applied, presumably a "partial AGC" application. [I can't quantify "partial" as a numeric value...maybe others can?] I do most of my RX experimentation at 6/ 2M and 70cm SSB so noise and overload perfomance are important to me. Images are also a big problem as I'm near a lot of VHF/hf broadcast. [shrug I live about 6 miles from 50 KW KMPC on AM...] If we can get back to the original claim of "increased noise with AGC applied" we might be able to help Andrea some. We don't know what Andrea has for a main receiver and interjecting some "badness" remarks by the ARRL about a certain IC isn't going to help clarify Andrea's problem. |
#5
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AGC signal/noise question...
On 20 Aug 2006 12:46:25 -0700, "
wrote: shortend Read EMRFD page 6.16 (ARRL press) they tested the 1350 and at the point where the gain cell has equal conduction on both legs the noise rises significantly. I duplicated the test fixture and yes, it's noisy, from around 6db to around 11db in my fixture when gain is reduced by 10db and that was at 16mhz. In a reciever that used it I went to two cascode stages using JFETs and the difference noise was notable for weak signals just into the agc range. I restrict the 1590/1350/ca3028 for lower perfomance recievers now. Apparently I hit some nerve on my disagreement. My first experience with the MC1590 was in 1973 and a need to operate over 55-64 MHz. Electronic gain control was essential and it had to be fast. Motorola supplied some additional information which was later incorporated into appnotes. The MC1350 was marketed around '73 along with the MC1330 video detector as a TV IF system. It didn't sell that well in quantities (presumably) and both were dropped from active production (Lansdale acquired masks and now makes the MC1350). The 1350 (8-pin DIP) should use the same die in the metal can MC1590. While neither one was ever touted as a super-champ low-noise device, it is what I consider respectable as to NF. Wide open and at 20db reduction the noise figure is not bad at all. Only that the apparent increase is noteable. I've seen it occur with other topologies including simple bipolar or FET stages (even tubes). The fact that it has differential input and differential output is convenient from the standpoint of circuit design. Especially so when input impedances (each side) has a dependable 5K R in parallel with about 5 pF total capacitance. Gain of both begins to fall above 75 MHz with output loads of 100 Ohms resistive. I've found no noticeable difference between differential input v. single-ended. Those are the feature of the part that makes them desireable. That IC is what I term a "double Gilbert cell" in that AGC control current affects both differential inputs equally (or very nearly so). Whether one connects to both inputs or just one shouldn't make any difference other than output gain. I call that circuit an analog four quadrant multiplier for what it does not how it's made. The feature of the Gilbert cell that applies for the AGC use is the lack of DC shift at the output points keeping downstream DC coupled stages at their undistrubed bias points. However the active device that is in the agc control positions is still a noise generator (as are all active devices) and as agc increases it's contribution is additive to the RF path devices. Makes little difference if the node where outputs are combined see no DC shift the various diff amp transistor as individual pairs do see a significant shift (100% collector current to near 0). It's easier to see using the older MC1550 or CA3028 diffamps or even discretes in a diffamp with current source. I also verified that the 1590 does same and also the CA3028 wired as differential AGC. Even tried three 2n3904s and same result. The agc range was good and at full gain the noise was ok but the noise increase at partial agc was surprizing. I've never encountered any "surprising" increase in noise at any AGC input to a 1590 or 1350 causing partial gain reduction. That is as true in 2005 as it was in 1973. If there is a SNR of 10 db at an RF carrier input of 3 uV and a gain reduction of 10 db for a 10 uV RF input results in 3 db more noise in the front end, the SNR with a 10 uV input is still higher than the one at 3 uV. What has been "lost" there? The surprize is that I'd not considered the possibility that the SN+N/N could degrade unevenly due to applied agc. So I'd never paid attention until I was trying to improve an earlier reciever design (ca1978) of my own and at the same time aquired a copy of EMRFD and did some testing to verify their resuts. Since the design was optimized for low RF gain and high overload thresholds I was revisiting anything that could better the design without loosing those features. Note it's a single conversion system with high IF. The problem was a MDS of -136dbm but the 10db Signal+N/N point was around -110dbm and at ~121dbm it was worse than at -130! The front end was common gate RF amp (2n4416s) driving a pair of 4416s in a single balanced mixer. Low noise but limited gain for better overload performance. No agc before the IF. If needed there are switchable resistive attenuators (3, 6,12db or 19db total). Measured gain from antenna to IF is only 16db (after all losses). Disable the agc or increase the threshold and it wasn't as measurable or appeared to disappear.. The hunt was on. The results were a surprize as there is no data for noise output with no input or signal to noise with gain reduction. My Moto databooks go way back, as do my National, RCA and Signetics library. More current datasheets do not reflect any improved information. The revised RX used two stages of mpf102 Jfet in cascode plus a diferential pair of 2n3904s to resolve the 5-10V agc to be compatable with the new fet amp to replace the two MC1350s and the problem of decreasing signal to noise as signal increased with agc active disappeared. Not to say the fet amps did not do the same thing only that the rate of noise increase was a smoother curve from max gain to min gain. In retrospect a delaying AGC to the first of the two 1350s could potentially have the same effect but was not investigated. I may revisit it at some time as I still have the original if module in the junkbox. Let's look at the original problem starting this thread: There was a claim of "increased noise" with AGC on, but no quantifiable data. Thats a problem, the lack of data or information on the circuit. Also I've repaired a few commercial radios that due to component failure or "golden screwdriver" had the various operating conditions sufficiently altered as to cause a similar problem. The sudden segue to stating that a certain IC is "bad" is a leap that defies good design practices to me. I'm not impressed that the ARRL had some test data in a publication; having been hands-on with this Motorola design for a number of years, I have a number of RCA lab notebook pages filled with my testing of it along with a patent involving it granted 1974...besides my own hobby notebooks. That's nice but are the test results in error from two different sources? No. However, it's was a noteable weak point. But calling it bad is your words. It's a point that needs to be understood and allowed for. In a design with more RF gain and/or less mixer noise it many not have been a factor or less of one. Also in the case that brought it to a point for me even altering how agc is applied might have achieved a better result. Since the 1350 is at IF for most designs the noise is likely from front end causes should be investigated first. One would hope the design had secured the system noise performance before the IF. However in low gain systems or system with no gain before the mixer and first filters this may be problematic. Low-noise input amplifier design is an entirely separate subject and there are a number of other active devices which can do lower NFs than 5. What was orignally needed was some way of getting some numbers and test configuration of Andrea's problem...to pin down a possible reason for alleged increased noise with AGC applied, presumably a "partial AGC" application. [I can't quantify "partial" as a numeric value...maybe others can?] I do most of my RX experimentation at 6/ 2M and 70cm SSB so noise and overload perfomance are important to me. Images are also a big problem as I'm near a lot of VHF/hf broadcast. [shrug I live about 6 miles from 50 KW KMPC on AM...] 10 miles from the Needham towers in MA. Not less than 8 VHF broadcasters, then the usual crowd of UHF and now the HDTV-UHF broadcasters and no small party of FM broaccasters. Oh and WKOX 1200 AM three miles away. Then I have 9 hams within a 1 mile circle and two within 1500ft running KW level at VHF. RFI are us. I understand overload as +15dbm on coax is common here. It's an interesting design challenge to do low noise figure RX and at the same time be overload resistant in a harsh environment. If we can get back to the original claim of "increased noise with AGC applied" we might be able to help Andrea some. We don't know what Andrea has for a main receiver and interjecting some "badness" remarks by the ARRL about a certain IC isn't going to help clarify Andrea's problem. Having tested and understood the problem I would say the authors of EMRFD did a fair job of pointing out the points where a device needs better understanding. A blanket "it's great" is lore, testing it and understanding it is engineering. Having done the work to understand it better I can appreciate the perfomance of the part and it's limitations. I still use it and have a tube of them because it's a useful part. Just like the often reviled SA602 mixer. It's relevence is I've seen this before and understood it's origin and also elsehere. The other aspect is that if a commonly accepted part is not fully understood and can lead to undesired effects then, why not others. AGC is not a trivial thing to be tacked on and considered a problem solver. Protects the ears but it's place in the reciever is not always understood. It does not always solve things like gross overload at the front end or possibly further down or outside the agc detectors bandwidth. But a lack of information about his radio doesn't help us either. We do not know for instance what topology is used for RF and if any agc is even applied to it. Allison |
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AGC signal/noise question...
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#7
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AGC signal/noise question...
On Sun, 27 Aug 2006 13:56:50 -0700, Roy Lewallen
wrote: wrote: . . . It's relevence is I've seen this before and understood it's origin and also elsehere. The other aspect is that if a commonly accepted part is not fully understood and can lead to undesired effects then, why not others. . . Very nearly 30 years ago, I was looking into "feed forward" circuits, a technique developed by someone at Tektronix for ultra-low distortion amplification. It turns out that the topology of the MC1350 is similar to what's needed, and a feed forward amplifier can be made from one plus just a few external components. But even by then, I'd learned that it's risky to use components for other than their intended purpose. So I collected 8 or 10 samples from various vendors (the part was widely sold then), and opened them up. Those in cans were easy, using a little can opener that worked like a tubing cutter. Some of the plastic DIP ones were more difficult, but one of the labs at Tek was able to dissolve the plastic while leaving the chip intact. Then I examined them carefully with an inspection microscope. Here's what I found: 1. There were at least three very different designs. The chip size of the largest was several times that of the smallest. 2. Some designs were inherently better balanced than others. Some had resistive "cross unders" where traces cross, which weren't the same on both sides of the circuit. Based on this, I decided it was too risky to make a design based on that part number, since a vendor could change chip suppliers or designs without notice. Interestingly, about six months later, I got a call from the component engineering group asking if I still had the chips. It seems that one or more of the vendors supplying that part (which was used for other applications at Tek) had changed their design, causing failure of some products and the shutting down of their production lines. Tek was big enough that vendors were often required to give advance notice before such changes, but they hadn't given any notice in this case. I'm bringing this up because I'm hearing the MC1350 being spoken of as though all are the same. It wouldn't surprise me if, after all these years, they're now all being made with one design from one foundry. But those ones in your junk box might be way more different than you think. This is almost certainly true of just about any IC. Roy, That is my engineering experience as well. At the time I did my testing I had Motorola, National and Hitachi parts Some fairly current date codes and a few from early 80s and and while the general behavour was similar I noted differences in gain, overall noise and DC balance as well. The noise increase was enough to be noteable in a particular case but on analysis understandable and to be expected. Then again I date back to when the Fairchild UA703 was a breakthrough gain block for RF. Allison KB!GMX |
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AGC signal/noise question...
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#9
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AGC signal/noise question...
On Mon, 21 Aug 2006 16:15:01 -0500, Andrea Baldoni
wrote: wrote: : If we can get back to the original claim of "increased : noise with AGC applied" we might be able to help Andrea : some. We don't know what Andrea has for a main receiver : and interjecting some "badness" remarks by the ARRL about : a certain IC isn't going to help clarify Andrea's problem. Sorry for the short answer. The receiver is a Yaesu FR-101 with his internal 2m converter. Actually, I'm doing some recalibration but I didn't found at now nothing really out. I just noted that, mostly noticeable in FM mode (by noise increasing) but present in all modes looking s-meter, the response of the IF was not a gaussian with a flat top. In the center there was a little decreasing area. If you tune it perfectly, you had a sligtly less signal than little up or down in freq. Dual conversion for HF and triple conversion of 2m. There would not be RF agc on the 2m converter. In FM modes you should see limiting so agc is behavour is different. So I started recalibrating. I had stopped because I had trouble with the noise blanker. In the manual, there is a indication of a test point where hook a voltmeter, but there is not any test point nor any indication on the schematic where the exact point is. The transformer is T116, maybe someone could help. Tomorrow I'll figure out what to do, I think I'll hook the voltmeter as to measure the voltage rectified, at the gate of the FET. If you have the schematic (it's freely downloadable in many places) maybe you could tell if I'm right. Likely a test point that is not marked on the board. I don't have a print handy. Anyway, thankyou very much you want to help troubleshooting my receiver, but I was using what I noted (increase in noise with AGC on) mainly to start a general discussion about AGC implementation and effects, and his future in digital receivers as well as in homebrew very high-end ones. It has started, and it's very interesting. Having experimented with recievers since before EE school and still many decades later I find it challenging. Always looking for and at new ideas. Allison |
#10
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AGC signal/noise question...
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