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Cobra 2010 loses Tx audio
"This method has been used in the real world for many years, and it is still being used. Better ways? Several. Long story short, the power-to-voltage ratio of a signal is always higher than the power-to-voltage ratio of noise. Most RF front ends are voltage amps. But a -power- amp on the left can dig the signal out of the noise on the order of 2-4dB, sometimes more. I like using a common-base for the 1st RF, but you can re-bias a common emitter and make pretty good improvements. And, as I stated before, a low input impedance will reduce or eliminate the impedance transformation prior to amplification. The objective is not low gain but low input impedance. Closer to the impedance of the feed, to keep the first impedance transformation as small as possible. With a common emitter, the only way to do that is by reducing the gain. And just at the first RF stage, not necessarily everything else in front of the first mixer. As long as we are on that subject, an RF stage isn't even needed at frequencies below 30MHz. As an example, you can use a Mini-Circuits SRA-3 doubly balanced diode ring mixer, that has only 4.77dB conversion loss at 11M. You also have approximately 35dB of port to port isolation. You can do better with discretes from Radio Shaft, which is really sad when you realize that those are their lab numbers. The only advantage I've seen to Mini-Circuits is the size. For performance, their stuff sucks. From the above statement, I can tell that you have very little experience with doubly balanced mixers, especially the ones from Mini-Circuits. The LAVI-XXX series of mixers have IP3s in the +33 to +40dBm range. The only type of discrete mixer that can even come near this type of performance is something that uses either a quad JFET ring, a quad CATV bipolar ring, or a dual power FET type that uses something like the Siliconix VN66. Your typical balanced dual JFET mixer, as used in some of the Yaesu and Icom transceivers will achieve IP3s in the +10 to +15dBm range, which isn't bad. This is without having the preamp switched in. Now, to even be able to measure that type of performance, you need to have all of your RF sources very clean. This means at least -65dBc for all RF signals. Special attention must be paid to the 6th and 9th harmonics of the LO, as these artifacts can cause poor return loss of the I.F. port and also, 2nd order IMD measurements can be degraded. The test setup must have an intermodulation free dynamic range of at least 10dB better than the device you will be testing. This includes connectors, attenuators used for isolation, etc. Attenuators with transverse heat sink fins have the best IMD characteristics. The only advantage that an RF amplifier would provide in this situation is minimizing 1st LO radiation through the antenna port of the radio. It also serves as a buffer to the mixer, which is essential for reducing mixer IMD. The RF amp is generally a good idea. The RF amp will not reduce IMD..........it will actually degrade the IMD performance of the mixer by the amount of gain that the RF amp provides. It is very easy to see this if you are making IP3 measurements on a mixer. Add 10dB of gain ahead of that mixer, and IP3 degrades by 10dB. |
Cobra 2010 loses Tx audio
Well, I have replaced a few of the electrolytic caps in the modulator
section, and I have SSB audio. Now, on to the AM section............. "Pete KE9OA" wrote in message . .. Thanks...........that sounds super. I have the radio apart on my bench downstairs. Gosh........what a brick! Isn't is great, when you have to undo everything that the "technician" did to upgrade the radio? There were several other problems that I have found. Why the manufacturer chose to use hot glue on all of the solder lugs of the switches escapes me. This caused several solder connections to fatigue and break off. It looks like this repair will be an hour here and there. I might get it fixed in a few months. The one redeeming quality is the great Rx section. Pete "Telstar Electronics" wrote in message oups.com... On Feb 17, 1:59 am, "Pete KE9OA" wrote: Another eBay special that "worked perfectly". It doesn't matter if I put in an external audio source from an audio oscillator or a microphone. I've tried three good microphones, and have the same problem. When I first key the mic, Tx audio is fine, but it quickly, almost abruptly fades after about 2 seconds. Has anybody seen this problem? Pete It certainly could be many things... but what you describe would lead me to replace the high power audio IC. I'm assuming that it has one... and uses a high-level modulation scheme. I have seen these fail in the way you have described. I believe what happens there is that the internal die has somehow lost contact with the heat sink... and therefor heats rapidly... sending semiconductors within the IC all over the bias map. This IC is normally failrly cheap, available, and is usually easy to replce. Hope that helps... www.telstar-electronics.com |
Cobra 2010 loses Tx audio
On Sat, 24 Feb 2007 22:24:56 -0800, Frank Gilliland
wrote: +++On Sat, 24 Feb 2007 18:03:18 -0600, "Pete KE9OA" wrote in : +++ +++How about the real world above 1GHz? It is very easy to model these +++"insignificant" reactances in a program such as ADS and see the effects on a +++real world circuit design. +++ +++ +++Did you miss this part? +++ +++ +++ When a resistor is used at its intended frequency..... +++ +++ ********** How about used below its self resonant frequency? james |
Cobra 2010 loses Tx audio
On Sat, 24 Feb 2007 18:01:09 -0600, "Pete KE9OA"
wrote: +++Resistors can have complex impedances, especially film resistors. Carbon +++film resistors can get by up to 30MHz or so, and metal film resistors +++shouln't be used above 10MHz. The problem with these devices is that they +++consist of a sprial etched resistance material that has a fair amount of +++reactance as you go up in frequency. +++Carbon composition resistors are preferable in RF applications, but even +++their lead length becomes too reactive at higher frequencies. +++Nowadays, we use 0603 or smaller size components at higher frequencies. 0402 +++geometry is presently being used at higher frequencies, with 0201 size soon +++to become the norm. This is what I have been working with for the last +++couple of years. +++ +++Pete +++ ************* And carbon composition should be avoided. They absorb moisture and change resistance with time. I have seen to many 100K carbon comps measue around 60K with time. I would aviod them like the plague. I have worked with chip components for over 20yrs. I stay away from the samllest one unless the board density constraints or the design dictates it. james james +++"Frank Gilliland" wrote in message m... +++ On Mon, 19 Feb 2007 19:12:33 GMT, james wrote +++ in : +++ +++On Sun, 18 Feb 2007 18:24:33 -0800, Frank Gilliland wrote: +++ ++++++Conjugate match is needed for maximum power transfer. ++++++ ++++++ ++++++IMPEDANCE match... for maximum power transfer. A 'conjugate' match is ++++++when the impedances are complex, which isn't always the case. +++*********** +++ +++I have found that it is rare in the real world that impeadances are +++not complex. Outside transimission lines, there is little that is not +++complex. +++ +++ +++ You just said that resistors have complex impedance and transmission +++ lines are flat. +++ +++ +++ Then again when you conjugate match, the imaginary part of +++the complex impedances is nulified and you are then left with the real +++part. +++ +++ +++ Reactances don't just disappear. They create a current between the +++ source and load that must be assessed to see if it is going to cause +++ any problems. Sometimes it doesn't and sometimes it does. +++ +++ +++ +++ |
Cobra 2010 loses Tx audio
On Sat, 24 Feb 2007 22:22:56 -0800, Frank Gilliland
wrote: +++This method has been used in the real world for many years, and it is still +++being used. Better ways? +++ +++ +++Several. +++ +++Long story short, the power-to-voltage ratio of a signal is always +++higher than the power-to-voltage ratio of noise. Most RF front ends +++are voltage amps. But a -power- amp on the left can dig the signal out +++of the noise on the order of 2-4dB, sometimes more. I like using a +++common-base for the 1st RF, but you can re-bias a common emitter and +++make pretty good improvements. And, as I stated before, a low input +++impedance will reduce or eliminate the impedance transformation prior +++to amplification. ************ That is true in most cases. Most of my RF work in the front end dealt around using small loop antenna( less than 1/8 wave) for paging recievers and those puppies have very low radiation resistance. You need some impedance transformation even if you do use common base. james |
Cobra 2010 loses Tx audio
On Sun, 25 Feb 2007 16:40:37 -0600, "Pete KE9OA"
wrote in : "This method has been used in the real world for many years, and it is still being used. Better ways? Several. Long story short, the power-to-voltage ratio of a signal is always higher than the power-to-voltage ratio of noise. Most RF front ends are voltage amps. But a -power- amp on the left can dig the signal out of the noise on the order of 2-4dB, sometimes more. I like using a common-base for the 1st RF, but you can re-bias a common emitter and make pretty good improvements. And, as I stated before, a low input impedance will reduce or eliminate the impedance transformation prior to amplification. The objective is not low gain but low input impedance. Closer to the impedance of the feed, to keep the first impedance transformation as small as possible. With a common emitter, the only way to do that is by reducing the gain. And just at the first RF stage, not necessarily everything else in front of the first mixer. As long as we are on that subject, an RF stage isn't even needed at frequencies below 30MHz. As an example, you can use a Mini-Circuits SRA-3 doubly balanced diode ring mixer, that has only 4.77dB conversion loss at 11M. You also have approximately 35dB of port to port isolation. You can do better with discretes from Radio Shaft, which is really sad when you realize that those are their lab numbers. The only advantage I've seen to Mini-Circuits is the size. For performance, their stuff sucks. From the above statement, I can tell that you have very little experience with doubly balanced mixers, especially the ones from Mini-Circuits. You're right. I ran some of their stuff through the bench many years ago and was disappointed, so I never used it. As for size, Analog Devices has been making some remarkable stuff in the last few years. The LAVI-XXX series of mixers have IP3s in the +33 to +40dBm range. You used dB before, which I assumed was carrier attenuation. Still, I'm not impressed. The only type of discrete mixer that can even come near this type of performance is something that uses either a quad JFET ring, a quad CATV bipolar ring, or a dual power FET type that uses something like the Siliconix VN66. Your typical balanced dual JFET mixer, as used in some of the Yaesu and Icom transceivers will achieve IP3s in the +10 to +15dBm range, which isn't bad. This is without having the preamp switched in. Now, to even be able to measure that type of performance, you need to have all of your RF sources very clean. Exactly! That's why I pointed out those numbers are "lab numbers". If you want to get some realistic numbers you have to test it under realistic conditions, which isn't that hard. The only drawback is that the numbers will be relative; i.e, it's a comparison test against other circuits. But if you do you will find that what I'm saying is true -- discretes perform much better. And yes, you have to carefully match the curves. This added labor, along with higher assembly costs and parts counts, are the primary reasons why discretes are rejected over mini-bricks; it rarely has anything to do with performance. This means at least -65dBc for all RF signals. Special attention must be paid to the 6th and 9th harmonics of the LO, as these artifacts can cause poor return loss of the I.F. port and also, 2nd order IMD measurements can be degraded. The test setup must have an intermodulation free dynamic range of at least 10dB better than the device you will be testing. This includes connectors, attenuators used for isolation, etc. Attenuators with transverse heat sink fins have the best IMD characteristics. The only advantage that an RF amplifier would provide in this situation is minimizing 1st LO radiation through the antenna port of the radio. It also serves as a buffer to the mixer, which is essential for reducing mixer IMD. The RF amp is generally a good idea. The RF amp will not reduce IMD..........it will actually degrade the IMD performance of the mixer by the amount of gain that the RF amp provides. It is very easy to see this if you are making IP3 measurements on a mixer. Add 10dB of gain ahead of that mixer, and IP3 degrades by 10dB. I can see that you are locked into a voltage-only mode. Feed your mixer under test with signals of varying impedance. I think you will be suprised, if not shocked. |
Cobra 2010 loses Tx audio
On Mon, 26 Feb 2007 02:03:09 GMT, james wrote
in : On Sat, 24 Feb 2007 22:22:56 -0800, Frank Gilliland wrote: +++This method has been used in the real world for many years, and it is still +++being used. Better ways? +++ +++ +++Several. +++ +++Long story short, the power-to-voltage ratio of a signal is always +++higher than the power-to-voltage ratio of noise. Most RF front ends +++are voltage amps. But a -power- amp on the left can dig the signal out +++of the noise on the order of 2-4dB, sometimes more. I like using a +++common-base for the 1st RF, but you can re-bias a common emitter and +++make pretty good improvements. And, as I stated before, a low input +++impedance will reduce or eliminate the impedance transformation prior +++to amplification. ************ That is true in most cases. Most of my RF work in the front end dealt around using small loop antenna( less than 1/8 wave) for paging recievers and those puppies have very low radiation resistance. You need some impedance transformation even if you do use common base. Well, yeah, with a 1/8 wave loop? LOL! Anyway, a common base with a single transistor can get you in the neighborhood of 100 to 500 ohms, depending on the transistor. With a 50 ohm input that leaves you with a transformation ratio from 2:1 to 10:1, which is -way- better than the typical 1000:1 to 10000:1 range needed for a bipolar voltage amp (I won't even mention FET's). The lower the ratio the better. Put two or three transistors in parallel and you can divide that ratio down even further. Take a half-hour or so and sift through your pile of schematics. I'm sure you'll find a few radios that do this. Even some HF tube radios used a grounded-grid triode on the front end -- not for stability as might be assumed, but for performance. |
Cobra 2010 loses Tx audio
On Sun, 25 Feb 2007 16:23:27 -0600, "Pete KE9OA"
wrote in : There is no such thing as an intended frequency. Now, if you said that if a resistor is used, taking into account its limitations. Must we continue this silly bantering? My apologies. I'll rephrase: When a resistor is used within it's intended operating frequency range..... When a resistor is used below the frequency where it no longer behaves like a resistor..... When a resistor is used in the frequency range for which it was designed..... Better? And if you don't like "silly bantering", why are you in this group? |
Cobra 2010 loses Tx audio
On Sun, 25 Feb 2007 18:55:29 -0800, Frank Gilliland
wrote: +++On Mon, 26 Feb 2007 02:03:09 GMT, james wrote +++in : +++ +++On Sat, 24 Feb 2007 22:22:56 -0800, Frank Gilliland wrote: +++ ++++++This method has been used in the real world for many years, and it is still ++++++being used. Better ways? ++++++ ++++++ ++++++Several. ++++++ ++++++Long story short, the power-to-voltage ratio of a signal is always ++++++higher than the power-to-voltage ratio of noise. Most RF front ends ++++++are voltage amps. But a -power- amp on the left can dig the signal out ++++++of the noise on the order of 2-4dB, sometimes more. I like using a ++++++common-base for the 1st RF, but you can re-bias a common emitter and ++++++make pretty good improvements. And, as I stated before, a low input ++++++impedance will reduce or eliminate the impedance transformation prior ++++++to amplification. +++************ +++ +++That is true in most cases. Most of my RF work in the front end dealt +++around using small loop antenna( less than 1/8 wave) for paging +++recievers and those puppies have very low radiation resistance. You +++need some impedance transformation even if you do use common base. +++ +++ +++Well, yeah, with a 1/8 wave loop? LOL! +++ ********* Actually not that difficult. Definitely the frontend transistor were bipolar. Often configured in cascode and operating at 0.95VDC and narrow band operation (5 MHz wide) anywhere between 30 and 1000 MHz. +++Anyway, a common base with a single transistor can get you in the +++neighborhood of 100 to 500 ohms, depending on the transistor. With a +++50 ohm input that leaves you with a transformation ratio from 2:1 to +++10:1, which is -way- better than the typical 1000:1 to 10000:1 range +++needed for a bipolar voltage amp (I won't even mention FET's). The +++lower the ratio the better. Put two or three transistors in parallel +++and you can divide that ratio down even further. +++ +++Take a half-hour or so and sift through your pile of schematics. I'm +++sure you'll find a few radios that do this. Even some HF tube radios +++used a grounded-grid triode on the front end -- not for stability as +++might be assumed, but for performance. +++ *********** true. I still like depletion mode MOSFETs as they operate more like vaccum tubes than bipolar transistor do. Ever tried a common gate depletion mode MOSFET amp in any RF AMP? james |
Cobra 2010 loses Tx audio
On Mon, 26 Feb 2007 21:38:24 GMT, james wrote
in : snip I still like depletion mode MOSFETs as they operate more like vaccum tubes than bipolar transistor do. Ever tried a common gate depletion mode MOSFET amp in any RF AMP? With a JFET, not a MOSFET. I'll bet it's as sensitive as a gay attorney watching a Snicker's commercial. It sounds a bit scary though..... maybe if it's DC coupled to a bipolar 2nd stage? However it's used it would definately need some sort of protection that doesn't contribute to noise. And imagine the possibilities using a dual-gate MOSFET (too bad they don't make them anymore). |
Cobra 2010 loses Tx audio
My original question was to see if anybody had any experience with the "no
Tx audio" problem. As it turned out, there was a bad 330uF 10V electrolytic cap that was causing the problem. As it turned out, this unit turned out to be another unit that was "peaked and clipped" by somebody who didn't know what they were doing. In other words, another piece of eBay junk. Pete "Frank Gilliland" wrote in message ... On Sun, 25 Feb 2007 16:23:27 -0600, "Pete KE9OA" wrote in : There is no such thing as an intended frequency. Now, if you said that if a resistor is used, taking into account its limitations. Must we continue this silly bantering? My apologies. I'll rephrase: When a resistor is used within it's intended operating frequency range..... When a resistor is used below the frequency where it no longer behaves like a resistor..... When a resistor is used in the frequency range for which it was designed..... Better? And if you don't like "silly bantering", why are you in this group? |
Cobra 2010 loses Tx audio
I like to use 0805 series, at higher frequencies, I see 0603 and smaller
sizes being used. As far as carbon comp resistors, they are the best through-hole components for RF, at least up to a certain point. True, they can and do change value over the years, and they can absorb moisture. Pete "james" wrote in message ... On Sat, 24 Feb 2007 18:01:09 -0600, "Pete KE9OA" wrote: +++Resistors can have complex impedances, especially film resistors. Carbon +++film resistors can get by up to 30MHz or so, and metal film resistors +++shouln't be used above 10MHz. The problem with these devices is that they +++consist of a sprial etched resistance material that has a fair amount of +++reactance as you go up in frequency. +++Carbon composition resistors are preferable in RF applications, but even +++their lead length becomes too reactive at higher frequencies. +++Nowadays, we use 0603 or smaller size components at higher frequencies. 0402 +++geometry is presently being used at higher frequencies, with 0201 size soon +++to become the norm. This is what I have been working with for the last +++couple of years. +++ +++Pete +++ ************* And carbon composition should be avoided. They absorb moisture and change resistance with time. I have seen to many 100K carbon comps measue around 60K with time. I would aviod them like the plague. I have worked with chip components for over 20yrs. I stay away from the samllest one unless the board density constraints or the design dictates it. james james +++"Frank Gilliland" wrote in message om... +++ On Mon, 19 Feb 2007 19:12:33 GMT, james wrote +++ in : +++ +++On Sun, 18 Feb 2007 18:24:33 -0800, Frank Gilliland wrote: +++ ++++++Conjugate match is needed for maximum power transfer. ++++++ ++++++ ++++++IMPEDANCE match... for maximum power transfer. A 'conjugate' match is ++++++when the impedances are complex, which isn't always the case. +++*********** +++ +++I have found that it is rare in the real world that impeadances are +++not complex. Outside transimission lines, there is little that is not +++complex. +++ +++ +++ You just said that resistors have complex impedance and transmission +++ lines are flat. +++ +++ +++ Then again when you conjugate match, the imaginary part of +++the complex impedances is nulified and you are then left with the real +++part. +++ +++ +++ Reactances don't just disappear. They create a current between the +++ source and load that must be assessed to see if it is going to cause +++ any problems. Sometimes it doesn't and sometimes it does. +++ +++ +++ +++ |
Cobra 2010 loses Tx audio
From the above statement, I can tell that you have very little experience with doubly balanced mixers, especially the ones from Mini-Circuits. You're right. I ran some of their stuff through the bench many years ago and was disappointed, so I never used it. As for size, Analog Devices has been making some remarkable stuff in the last few years. I have worked with some of their newer stuff, and it has been very good. AD got their act together pretty well, in the RF arena. The Analog Devices AD831 isn't a bad design; it does have a good IP3, but in order to reach the NF of a Mini-Circuits SRA-3 however, you need to have a preamplifier ahead of it. With its 12dB NF, it isn't a bad mixer for HF use up to 30MHz. I had started a receiver design using the 831, but things got so busy at work that I shelved that project for awhile. The LAVI-XXX series of mixers have IP3s in the +33 to +40dBm range. You used dB before, which I assumed was carrier attenuation. Still, I'm not impressed. I thought the only reference to dB was port to port isolation and SSB conversion loss. The only type of discrete mixer that can even come near this type of performance is something that uses either a quad JFET ring, a quad CATV bipolar ring, or a dual power FET type that uses something like the Siliconix VN66. Your typical balanced dual JFET mixer, as used in some of the Yaesu and Icom transceivers will achieve IP3s in the +10 to +15dBm range, which isn't bad. This is without having the preamp switched in. Now, to even be able to measure that type of performance, you need to have all of your RF sources very clean. Exactly! That's why I pointed out those numbers are "lab numbers". If you want to get some realistic numbers you have to test it under realistic conditions, which isn't that hard. The only drawback is that the numbers will be relative; i.e, it's a comparison test against other circuits. But if you do you will find that what I'm saying is true -- discretes perform much better. And yes, you have to carefully match the curves. This added labor, along with higher assembly costs and parts counts, are the primary reasons why discretes are rejected over mini-bricks; it rarely has anything to do with performance. I agree on those points. Unless the LO in the actually is actually filtered to the point where all higher terms are at least -65dBc, that performance won't be realized. The manufacturers I worked for over the years were quite happy with -25dBc for the 2nd harmonic of the LO. It also serves as a buffer to the mixer, which is essential for reducing mixer IMD. The RF amp is generally a good idea. The RF amp will not reduce IMD..........it will actually degrade the IMD performance of the mixer by the amount of gain that the RF amp provides. It is very easy to see this if you are making IP3 measurements on a mixer. Add 10dB of gain ahead of that mixer, and IP3 degrades by 10dB. I can see that you are locked into a voltage-only mode. Feed your mixer under test with signals of varying impedance. I think you will be suprised, if not shocked. You do make a good point; an unconditionally stable low gain RF amplifier will satisfy this requirement. I have done the measurements that you mention, and I have noted some level of disparity between real world conditions and manufacturers' specs. I know................too many manufacturers play the "numbers game". As long as they stick to the same standards, one can use these numbers to initially select a product but the devices still need to be characterized before those numbers are actually believed. Pete |
Cobra 2010 loses Tx audio
On Sun, 4 Mar 2007 11:09:00 -0600, "Pete KE9OA"
wrote in : My original question was to see if anybody had any experience with the "no Tx audio" problem. As it turned out, there was a bad 330uF 10V electrolytic cap that was causing the problem. 15 volt? As it turned out, this unit turned out to be another unit that was "peaked and clipped" by somebody who didn't know what they were doing. In other words, another piece of eBay junk. Too many of those out there. |
Cobra 2010 loses Tx audio
On Sun, 4 Mar 2007 11:09:00 -0600, "Pete KE9OA"
wrote: +++My original question was to see if anybody had any experience with the "no +++Tx audio" problem. As it turned out, there was a bad 330uF 10V electrolytic +++cap that was causing the problem. As it turned out, this unit turned out to +++be another unit that was +++"peaked and clipped" by somebody who didn't know what they were doing. In +++other words, another piece of eBay junk. +++ +++Pete ******** Yep, Seen many an electrolytic go bad. Especially in power supplies. Heat is the enemy of electrolytics. As for EBAY CB radios, mostly useless other than parts. james |
Cobra 2010 loses Tx audio
On Sun, 4 Mar 2007 11:23:03 -0600, "Pete KE9OA"
wrote in : From the above statement, I can tell that you have very little experience with doubly balanced mixers, especially the ones from Mini-Circuits. You're right. I ran some of their stuff through the bench many years ago and was disappointed, so I never used it. As for size, Analog Devices has been making some remarkable stuff in the last few years. I have worked with some of their newer stuff, and it has been very good. I'll have to run some of the new stuff across the bench. AD got their act together pretty well, in the RF arena. The Analog Devices AD831 isn't a bad design; it does have a good IP3, but in order to reach the NF of a Mini-Circuits SRA-3 however, you need to have a preamplifier ahead of it. With its 12dB NF, it isn't a bad mixer for HF use up to 30MHz. I had started a receiver design using the 831, but things got so busy at work that I shelved that project for awhile. Call me old-fashioned but I still prefer discretes. The LAVI-XXX series of mixers have IP3s in the +33 to +40dBm range. You used dB before, which I assumed was carrier attenuation. Still, I'm not impressed. I thought the only reference to dB was port to port isolation and SSB conversion loss. Port to port isolation or carrier rejection, whatever you want to call it..... you can easily get 60 dB or better using discretes. Heck, some of the old DSB-SC tube rigs were even designed to mix in a -power- stage! Anyway, you used dBm in one post and dB in another; not the same thing. The only type of discrete mixer that can even come near this type of performance is something that uses either a quad JFET ring, a quad CATV bipolar ring, or a dual power FET type that uses something like the Siliconix VN66. Your typical balanced dual JFET mixer, as used in some of the Yaesu and Icom transceivers will achieve IP3s in the +10 to +15dBm range, which isn't bad. This is without having the preamp switched in. Now, to even be able to measure that type of performance, you need to have all of your RF sources very clean. Exactly! That's why I pointed out those numbers are "lab numbers". If you want to get some realistic numbers you have to test it under realistic conditions, which isn't that hard. The only drawback is that the numbers will be relative; i.e, it's a comparison test against other circuits. But if you do you will find that what I'm saying is true -- discretes perform much better. And yes, you have to carefully match the curves. This added labor, along with higher assembly costs and parts counts, are the primary reasons why discretes are rejected over mini-bricks; it rarely has anything to do with performance. I agree on those points. Unless the LO in the actually is actually filtered to the point where all higher terms are at least -65dBc, that performance won't be realized. The manufacturers I worked for over the years were quite happy with -25dBc for the 2nd harmonic of the LO. And then they moved on to designing CB amps? It also serves as a buffer to the mixer, which is essential for reducing mixer IMD. The RF amp is generally a good idea. The RF amp will not reduce IMD..........it will actually degrade the IMD performance of the mixer by the amount of gain that the RF amp provides. It is very easy to see this if you are making IP3 measurements on a mixer. Add 10dB of gain ahead of that mixer, and IP3 degrades by 10dB. I can see that you are locked into a voltage-only mode. Feed your mixer under test with signals of varying impedance. I think you will be suprised, if not shocked. You do make a good point; an unconditionally stable low gain RF amplifier will satisfy this requirement. Hence my recommendation to use a low-impedance front end. I have done the measurements that you mention, and I have noted some level of disparity between real world conditions and manufacturers' specs. I know................too many manufacturers play the "numbers game". As long as they stick to the same standards, one can use these numbers to initially select a product but the devices still need to be characterized before those numbers are actually believed. I won't even use a 2-cent resistor until I destroy it on the bench first. A lot of the manufacturer specs look really good on paper but don't mean squat beyond the ideal conditions of a lab test. Even a supposedly identical component made by different manufacturers will behave differently in the actual circuit, especially under failure analysis (which can be a very expensive lesson if not learned before designing or repairing power equipment..... don't ask!). |
Cobra 2010 loses Tx audio
It was actually a 10V unit. I don't know why they would use something like
that either. All I've got to do is isolate the problem with whatever sets the parameters for AM mode. It receives fine in AM mode, transceives fine in SSB mode. Looking at the schematic, I saw a 220 Ohm resistor that is switched in for biasing the output stage for SSB. In the AM mode, the mode switches bypasses this resistor. Could be a bad mode switch. I have been working 60+ hours every week, so I get about an hour each weekend to work on this thing. Unless they send me out to Singapore for awhile.................... Pete "Frank Gilliland" wrote in message ... On Sun, 4 Mar 2007 11:09:00 -0600, "Pete KE9OA" wrote in : My original question was to see if anybody had any experience with the "no Tx audio" problem. As it turned out, there was a bad 330uF 10V electrolytic cap that was causing the problem. 15 volt? As it turned out, this unit turned out to be another unit that was "peaked and clipped" by somebody who didn't know what they were doing. In other words, another piece of eBay junk. Too many of those out there. |
Cobra 2010 loses Tx audio
"Frank Gilliland" wrote in message ... On Sun, 4 Mar 2007 11:23:03 -0600, "Pete KE9OA" wrote in : From the above statement, I can tell that you have very little experience with doubly balanced mixers, especially the ones from Mini-Circuits. You're right. I ran some of their stuff through the bench many years ago and was disappointed, so I never used it. As for size, Analog Devices has been making some remarkable stuff in the last few years. I have worked with some of their newer stuff, and it has been very good. I'll have to run some of the new stuff across the bench. AD got their act together pretty well, in the RF arena. The Analog Devices AD831 isn't a bad design; it does have a good IP3, but in order to reach the NF of a Mini-Circuits SRA-3 however, you need to have a preamplifier ahead of it. With its 12dB NF, it isn't a bad mixer for HF use up to 30MHz. I had started a receiver design using the 831, but things got so busy at work that I shelved that project for awhile. Call me old-fashioned but I still prefer discretes. The LAVI-XXX series of mixers have IP3s in the +33 to +40dBm range. You used dB before, which I assumed was carrier attenuation. Still, I'm not impressed. I thought the only reference to dB was port to port isolation and SSB conversion loss. Port to port isolation or carrier rejection, whatever you want to call it..... you can easily get 60 dB or better using discretes. Heck, some of the old DSB-SC tube rigs were even designed to mix in a -power- stage! Anyway, you used dBm in one post and dB in another; not the same thing. The only type of discrete mixer that can even come near this type of performance is something that uses either a quad JFET ring, a quad CATV bipolar ring, or a dual power FET type that uses something like the Siliconix VN66. Your typical balanced dual JFET mixer, as used in some of the Yaesu and Icom transceivers will achieve IP3s in the +10 to +15dBm range, which isn't bad. This is without having the preamp switched in. Now, to even be able to measure that type of performance, you need to have all of your RF sources very clean. Exactly! That's why I pointed out those numbers are "lab numbers". If you want to get some realistic numbers you have to test it under realistic conditions, which isn't that hard. The only drawback is that the numbers will be relative; i.e, it's a comparison test against other circuits. But if you do you will find that what I'm saying is true -- discretes perform much better. And yes, you have to carefully match the curves. This added labor, along with higher assembly costs and parts counts, are the primary reasons why discretes are rejected over mini-bricks; it rarely has anything to do with performance. I agree on those points. Unless the LO in the actually is actually filtered to the point where all higher terms are at least -65dBc, that performance won't be realized. The manufacturers I worked for over the years were quite happy with -25dBc for the 2nd harmonic of the LO. And then they moved on to designing CB amps? It also serves as a buffer to the mixer, which is essential for reducing mixer IMD. The RF amp is generally a good idea. The RF amp will not reduce IMD..........it will actually degrade the IMD performance of the mixer by the amount of gain that the RF amp provides. It is very easy to see this if you are making IP3 measurements on a mixer. Add 10dB of gain ahead of that mixer, and IP3 degrades by 10dB. I can see that you are locked into a voltage-only mode. Feed your mixer under test with signals of varying impedance. I think you will be suprised, if not shocked. You do make a good point; an unconditionally stable low gain RF amplifier will satisfy this requirement. Hence my recommendation to use a low-impedance front end. I have done the measurements that you mention, and I have noted some level of disparity between real world conditions and manufacturers' specs. I know................too many manufacturers play the "numbers game". As long as they stick to the same standards, one can use these numbers to initially select a product but the devices still need to be characterized before those numbers are actually believed. I won't even use a 2-cent resistor until I destroy it on the bench first. A lot of the manufacturer specs look really good on paper but don't mean squat beyond the ideal conditions of a lab test. Even a supposedly identical component made by different manufacturers will behave differently in the actual circuit, especially under failure analysis (which can be a very expensive lesson if not learned before designing or repairing power equipment..... don't ask!). I do understand your thinking.........Maxim used to love us to do their "beta testing" on their new devices. I don't remember using dB and dBm in the same context. It must have been late.......probably a typo on my part. Just 100 more hours, and I can undo everthing that the other guy did to my fine radio!!!!!!!!!!!! Pete |
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