Re: [Repeater-Builder] Re: The GLB Preselector- Preamplifier
I lived up in Rome NY for a couple of years when I was stationed at Griffiss Air Force Base. Six inches of snow only got a brief comment during the local weather cast. They considered it a dusting. 73, Joe, K1ike Chuck Kelsey wrote: Tonawanda doesn't get near as much snow as a bit further south - Chautauqua County. We had 295 here in Mayville last season. Chuck WB2EDV
Re: [Repeater-Builder] Re: The GLB Preselector- Preamplifier - SNOW
Yep, pretty much a non-event. Chuck - Original Message - From: Joe k1ike_m...@snet.net To: Repeater-Builder@yahoogroups.com Sent: Saturday, November 28, 2009 9:42 AM Subject: Re: [Repeater-Builder] Re: The GLB Preselector- Preamplifier I lived up in Rome NY for a couple of years when I was stationed at Griffiss Air Force Base. Six inches of snow only got a brief comment during the local weather cast. They considered it a dusting. 73, Joe, K1ike Chuck Kelsey wrote: Tonawanda doesn't get near as much snow as a bit further south - Chautauqua County. We had 295 here in Mayville last season. Chuck WB2EDV
Re: [Repeater-Builder] Re: The GLB Preselector- Preamplifier
At 11/27/2009 08:35, you wrote: Re: The GLB Preselector- Preamplifier I think we've covered this before, but I'll say it again: the noise figure spec, the most important spec on any preamp, is missing! Would you buy a power amplifier that only spec'd 10 dB gain? Yes, if it's composite performance includes very good/great 3r Order Intercept Performance. ...but the P1dB for the GLB isn't given, so we don't have that either. How do we know (other than anecdotally) what the 3rd order intercept performance of the GLB within its passband is? Yes it has narrow passband characteristics, but my point is that that characteristic can be replicated with superior noise performance using a GaAsFET (or PHEMT) preamp coaxial cavity filters. Perhaps these are good preamps nonetheless, but be advised: you'll never get as good of a noise figure as using an ordinary GaAsFET preamp with a 0.25 or even 0.5 dB loss pass cavity in front of it. Not true... the Phempt Device is considered higher performance over a conventional GaAs Fet. Perhaps marginally, but insignificant for the purposes of this discussion. I sometimes use the terms interchangeably, since most PHEMTs are types of GaAsFETs. Although the low noise figure ranks near number 1 as a desired spec, the application is always a trade-off and a low noise device with poor high level performance is not a good thing. Agreed. But generally the GaAsFETs/PHEMTS have better P1dBs than bipolars MOSFETs. http://en.wikipedia.org/wiki/HEMT http://groups.yahoo.com/group/Repeater-Builder/post?act=replymessageNum=96145 Perhaps this isn't so important on antenna noise-limited VHF bands, but in SoCal on 220 440 it is. The only place I'd use a GLB preselector would be a space-constrained application where a 1/4 wave resonator simply couldn't fit. A cavity would normally always be considered helpful... but we are/were talking just about the basic composite preamplifier packages. IE no external components... If your discussion excludes use of separate filter assemblies ahead of the preamp, then I think we're talking apples oranges. Again, my point was that the combination of a pass cavity followed by a low-noise preamp will outperform the GLB units. The GLB Pre-selector Preamp also has trailing tuned circuits, and they greatly improve its 3rd order performance. How does a filter placed AFTER the active device help improve it's P1dB? It may protect the receiver(s) downstream, but they do nothing to improve the power-handling performance of the preamp/preselector. Bob NO6B
Re: [Repeater-Builder] Re: The GLB Preselector- Preamplifier
At 11/27/2009 13:08, you wrote: Yes, if it's composite performance includes very good/great 3r Order Intercept Performance. ...but the P1dB for the GLB isn't given, so we don't have that either. How do we know (other than anecdotally) what the 3rd order intercept performance of the GLB within its passband is? Two ways that I know about... the first is to measure it or locate someone else's data as I have also done. About a year or so back when I solicited here on the group for anyone wishing to part with spare GLB units... I found a web page with very well done 3rd order data and his minor circuit (bias) improvements. I then performed and closely verified his data with my own testing. Actually getting into the details of the GLB Pre-selector Preamplifier Operation in high signal levels was quite an eye opener (learning experience). Great! Too bad the manufacturer didn't do this; would've saved you others the trouble of having to characterize a brand new component. Yes it has narrow passband characteristics, but my point is that that characteristic can be replicated with superior noise performance using a GaAsFET (or PHEMT) preamp coaxial cavity filters. Maybe your missing two small items... one is the GLB units are offered with GaAs devices and two... I have yet to see other brand amplifiers with the integrated same number and type leading and trailing tuned circuits (inside the preamp box) I didn't miss that at all. You seem to be missing my point that the leading, small resonators in front of the GLB will either degrade the NF far more than a 1/4 wave coaxial resonator, or offer far less out-of-band rejection. Although the low noise figure ranks near number 1 as a desired spec, the application is always a trade-off and a low noise device with poor high level performance is not a good thing. Agreed. But generally the GaAsFETs/PHEMTS have better P1dBs than bipolars MOSFETs. What the signal coming out of the pipe (the amplifiers amplifier) actually is at and above P1dB (The 1 dB compression point) is what I'm more concerned with. An unhappy (or circuit designed to be) GaAs and Phempt device also make a killer mixer (grunge generator). The gain data on paper can be excellent but the buck-shot out the amplifiers tail pipe is what I'm paying more attention toward... There are cases where some bipolar device out perform GaAs Fet in rather harsh 3rd order (overloaded) applications. Only if the GaAsFET preamp is maldesigned. Some will break into oscillation at different source/load complex impedances. I've also seen some bipolar designs (the Kendecom RX front ends come to mind) where the 1st RF amp is biased at a rather high DC current value. Apparently the goal was to trade off NF for more dynamic range. So yes if you look far wide enough you'll find exceptions to just about everything. A cavity would normally always be considered helpful... but we are/were talking just about the basic composite preamplifier packages. IE no external components... If your discussion excludes use of separate filter assemblies ahead of the preamp, then I think we're talking apples oranges. Again, my point was that the combination of a pass cavity followed by a low-noise preamp will outperform the GLB units. Not in every case... what is behind the Amplifier matters. Behind? I assume you mean the RX (after). I don't worry about my RX's dynamic range - I use GEs :) But if you're not, more pass cavities, or even your GLB preselector, after the preamp are an easy fix, since loss is less of an issue there. The GLB Pre-selector Preamp also has trailing tuned circuits, and they greatly improve its 3rd order performance. How does a filter placed AFTER the active device help improve it's P1dB? It doesn't... but it can improve it's 3rd order operation. Well, again we're talking (OK, typing) but simply not communicating. P1dB 3rd order intercept are closely related, as they are both measures of a widget's dynamic range/linearity, I use the terms somewhat interchangeably for the purposes of this discussion. It may protect the receiver(s) downstream, but they do nothing to improve the power-handling performance of the preamp/preselector. If you consider the entire GLB unit as one composite amplifier... yes it can and most often does. Again, the ONLY filtering that will improve an amplifier's resistance to IMD is filtering on its input, not its output. The tuned stages that are after the GaAsFET in the GLB serve only to protect the following device (RX or another preamp) from overload by out-of-band signals. In the case of a good commercial RX like our GE Mastr IIs Motorola Micors, this is almost always unnecessary, since they already have integral high Q ( lossy!) helical resonators. If I can easily locate the web page I found the demonstration circuit on... I'll post it here. It is/was quite an
Re: [Repeater-Builder] Re: The GLB Preselector- Preamplifier
FWIW the BF981 is a dual-gate MOSFET, not a GaAsFET. I assume Aria switched to a GaAsFET for the current-production unit? Bob NO6B I bought several of these when they were first on the market. When this unit was first in production from Gil, he made it available like this: Under 200 MHz was Bi Polar, over was GaAs FET - HOWEVER, you could custom order under 200 MHz the GaAs option. The Bi Polar device was MRF-901 Motorola. I don't know what the FET was... I only ever owned one GLB Preselector/Preamp above 200 MHz (it was custom built for 222 MHz) and it had a MRF-901 in it -- go figure. GLB stand for Gilbert L. Boelke W2EUP inventor of the Hybrid Ring Duplexer - a design sold to Sinclair Radio Labs in Tonawonda NY Kevin Custer
Re: [Repeater-Builder] Re: The GLB Preselector- Preamplifier
skipp025 wrote: Smart guy... cumbersome type of duplexer that worked fairly well. I survived living near Tonawanda NY in Cheektowaga where snow is no fun. Yes, Gil was a very smart guy - somewhat ahead of his time. Here is the only information I can find (that is still available) about his untimely passing: http://barra.hamgate.net/links/jun2001.pdf Snow? Oh yes, about 6 inches here is Friedens PA today - it was 50 degrees yesterday. Kevin
Re: [Repeater-Builder] Re: The GLB Preselector- Preamplifier
At 11/27/2009 15:27, you wrote: Great! Too bad the manufacturer didn't do this; would've saved you others the trouble of having to characterize a brand new component. I actually ended up verifying what another person on a similar path did before me. I didn't miss that at all. You seem to be missing my point that the leading, small resonators in front of the GLB will either degrade the NF far more than a 1/4 wave coaxial resonator, or offer far less out-of-band rejection. We were talking about two different directions. Yes to all the above if the topic is pre-amplifiers and external cavities. A GLB Pre-Selector Pre-Amplifier at a high RF Site parked bare naked between a duplexer and a receiver can out perform some bare preamplifiers. Of course - no argument there. But as you mention above, I'm not making that comparison. If you have the luxury of the extra typical High Q band-pass cavities then the NF will be better... but again the 3rd order performance might not be and what happens after the active device plays a much more important part of the realized high signal level performance. ...if you're considering overall system performance, which is largely a function of the RX you're using. If you're using one of the bulletproof older commercial RXs, the preamp's dynamic range becomes much more important that the RX. Only if the GaAsFET preamp is maldesigned. Some will break into oscillation at different source/load complex impedances. The key is the manufactures data sheet for the device and the completed pre-amplifiers real world measured 3rd order performance. This reply appears to be in the context of one designing their own preamp. I was thinking in terms of commercially available preamps. I don't worry about my RX's dynamic range - I use GEs :) But if you're not, more pass cavities, or even your GLB preselector, after the preamp are an easy fix, since loss is less of an issue there. Receiver/antenna system design and construction can be just this side of voodoo magic. I think this is where/why we seem to be missing each other's points. To me it's not magic at all: it's simple, straightforward engineering. If you have a ballpark antenna noise temperature, RX noise temperature (deduced from 12 dB SINAD), knowledge of nearby (in freq. location) potential problem radiators, you can calculate the necessary hardware (gain, required noise figure filtering) to get the most noise-free signal detected. I suppose the more unknowns you have in the above equation, the more it becomes voodoo magic. Well, again we're talking (OK, typing) but simply not communicating. P1dB 3rd order intercept are closely related, as they are both measures of a widget's dynamic range/linearity, I use the terms somewhat interchangeably for the purposes of this discussion. Closely related but not necessarily the same. Where they are different in high level operation can be and sometimes is a big factor in the system performance. Irrelevant; we're really getting off track on this one. Neither P1dB nor 3rd order intercept of an amplifier is affected by filtering placed after it. Again, the ONLY filtering that will improve an amplifier's resistance to IMD is filtering on its input, not its output. The tuned stages that are after the GaAsFET in the GLB serve only to protect the following device (RX or another preamp) from overload by out-of-band signals. Nope, the trailing stages do contribute to the GLB IMD Performance. The out of band issue is an additional side benefit. Incorrect. If a large signal is presented to the active device, how do tuned stages on the OUTPUT of the active device protect it from overload? They can't. In the case of a good commercial RX like our GE Mastr IIs Motorola Micors, this is almost always unnecessary, since they already have integral high Q ( lossy!) helical resonators. The key issue is the band-width of the above radio front end circuits, which are fairly wide in the real world. The measured 3 dB bandwidth of a G.E. MVP front-end helical resonator assembly is 1.8 MHz. I don't consider that fairly wide. FWIW the BF981 is a dual-gate MOSFET, not a GaAsFET. Yep, but the GLB trailing tuned circuits would also improve the realized GaAs Fet device equipped 3rd order performance... as would properly set cavities between a pre-amplifiers output and the receivers input. No, they don't. They only improve the 3rd order performance of whatever is downstream. I suspect the point being missed here... is when a preamplifier is generating spectral buckshot in a very toxic condition... the filtering after the active device improves/reduces what the receiver sees/has to deal with. If the preamp is generating spectral buckshot, you're already screwed. The typical symptom here is on-channel crap being generated by the preamp, which of course cannot be filtered out. Mucho better to have
Re: [Repeater-Builder] Re: The GLB Preselector- Preamplifier
Tonawanda doesn't get near as much snow as a bit further south - Chautauqua County. We had 295 here in Mayville last season. Chuck WB2EDV - Original Message - From: Kevin Custer To: Repeater-Builder@yahoogroups.com Sent: Friday, November 27, 2009 7:27 PM Subject: Re: [Repeater-Builder] Re: The GLB Preselector- Preamplifier skipp025 wrote: Smart guy... cumbersome type of duplexer that worked fairly well. I survived living near Tonawanda NY in Cheektowaga where snow is no fun. Yes, Gil was a very smart guy - somewhat ahead of his time. Here is the only information I can find (that is still available) about his untimely passing: http://barra.hamgate.net/links/jun2001.pdf Snow? Oh yes, about 6 inches here is Friedens PA today - it was 50 degrees yesterday. Kevin -- No virus found in this incoming message. Checked by AVG - www.avg.com Version: 9.0.709 / Virus Database: 270.14.85/2531 - Release Date: 11/27/09 14:39:00
RE: [Repeater-Builder] Re: The GLB Preselector- Preamplifier
It comes down to where are the IM products really being generated. I think what Skipp is trying to say is that if the preamp generates spurious products from overload that fall outside of the center frequency, that filtering behind the preamp will help keep those products out of the receiver. While this may be true, keep in mind that any IM products that are generated by the preamp are going to be at fairly low levels because of the inefficiency of the mixing action in the preamp. A mixer is what it becomes when you get into the non linear range of the preamp. But as with any mixer its product amplitude is going to be way down from the signals that cause the mixing. The off frequency products generated by a non linear preamp are not the real problem. It is the on frequency products that get thru. They are going to be relatively weak also but because they are on frequency everything in the chain is going to amplify them. Filtering at the output of the preamp will do nothing to reduce any mixing action in the preamp as that is dependent on input level. Measuring 3rd order products is done at the output of the device so with filters at the output it is going to look like the filters are helping reduce these off frequency products but that is not how you measure IM performance of a device. They are usually referenced to on frequency levels. Also keep in mind when reading IM specs for an amplifier that some manufacturers reference to the input and some reference to the output of the amplifier. Referencing to the output makes the spec look better by the amount of gain that the amp has. Adding filters to the output of the device can help reduce the IM tendencies in the following receiver however by keeping off frequency signals out of the receiver. It is the total amount of power that reaches a device that causes overload. But any off frequency IM products that may be generated in the preamp will be much weaker than direct off frequency signals So the addition of filters after the preamp may seem like they help the preamp but they are really helping the receiver from generating IM in its first active stage. Remember that when you add a preamp you destroy the IM performance of the receiver by the amount of gain in the preamp. 73 Gary K4FMX Letting the preamp generate poop then filtering the off-channel garbage you've already generated in that preamp is a poor solution; Ah, now were getting close... now assume the preamplifier generates really bad unwanted products in extremely overloaded conditions that don't occur most of the time. When the buckshot flies for relatively modest times... the trailing internal/external filters (regardless of location) would help a good receiver better deal with the event. the in-band garbage generated in the preamp goes right on through. And now we ask how much F-center and close adjacent in band garbage actually gets to the receiver front-end and how well do the receiver(s) handle this event? With the right hardware layout a lot better than you might assume at first glance.
Re: [Repeater-Builder] Re: The GLB Preselector- Preamplifier
At 11/27/2009 18:19, you wrote: Yep, but the GLB trailing tuned circuits would also improve the realized GaAs Fet device equipped 3rd order performance... as would properly set cavities between a pre-amplifiers output and the receivers input. No, they don't. They only improve the 3rd order performance of whatever is downstream. Bingo... but also in reality the trailing filters would also contribute toward the preamplifiers 3rd order performance. There is a way to prove the above: measure the P1dB (or the 20 kHz 2-tone IMD response) of the GLB as a function of frequency. If it tracks the overall gain curve of the unit, then you're right. However, I'd expect the above response to be considerably wider, tracking the response of the input filtering only. Bob NO6B
