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#1
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Impedence mismatch into FET preamp
Hi all.
I have a FET preamp, tuned by means of two fixed capacitors and one variable inductor that serves as an impedance transformer. Am I correct in thinking that if there is an impedance mismatch (say the input circuit was tuned into a 50 ohm load, then was finally presented with say a 105 ohm load), then the preamp's noise figure would increase and the FET would start to become artificially noisy? Then if the FET was pushed into overload by an adjacent strong signal, the noise would then start to reduce. I have this odd situation on a repeater, and I can only put it down to mis-tuned cavities. I've never come across it before though, where the repeater actually IMPROVES in sensitivity when the transmitter keys up (?!?!), so I would like to know what's going on before I fix it. Thanks all, Sam M1FJB |
#2
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Impedence mismatch into FET preamp
In article , "Samuel
Hunt" wrote: Am I correct in thinking that if there is an impedance mismatch (say the input circuit was tuned into a 50 ohm load, then was finally presented with say a 105 ohm load), then the preamp's noise figure would increase and the FET would start to become artificially noisy? Then if the FET was pushed into overload by an adjacent strong signal, the noise would then start to reduce. I have this odd situation on a repeater, and I can only put it down to mis-tuned cavities. I've never come across it before though, where the repeater actually IMPROVES in sensitivity when the transmitter keys up (?!?!), so I would like to know what's going on before I fix it. Sam- There may be some measurable increase in a preamp's noise output, but a 2 to 1 mismatch will probably not cause such a serious degradation in its noise figure. Perhaps I misunderstand you. When the noise reduces due to overload, that is NOT the same as an improvement in sensitivity. That is commonly refered to as desensitization (desense). An amplifier's gain is reduced to zero when it is in saturation (or oscillating). Inadequate or mistuned cavities can cause desense. It can also be caused by poor shielding, often from poor co-axial cable. You might benefit if you switched to double-shielded cable. Also look for poorly attached connectors. 73, Fred, K4DII |
#3
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Impedence mismatch into FET preamp
Hi Fred and all....
It's not due to desense. When I tune the cavities up properly, there's about 10dB noise figure on the preamp (reciever is 0.8uV sensitive). Then when it transmits, the noise figure doesn't change. When I tune the notches off slightly so the preamp starts to desense, the noise figure goes down as it starts to desense, then the noise figure goes up high as it pushes the preamp well into desense. I'm going to retune the cavities for minimum SWR at some point anyway (we've got problems with the frequency and a deaf repeater is actually quite a good idea at the moment, so it's not an issue), and see what that does. I just wanted to try to understand why it's doing this, does the FET noise figure increase with an impedance mismatch on the input, and so pushing it into desense reduces the amount of noise on the preamp? It's just an interesting thing I've never come across before and would be interested to find out a technical explanation to it all. Thanks all, Sam "Fred McKenzie" wrote in message ... In article , "Samuel Hunt" wrote: Am I correct in thinking that if there is an impedance mismatch (say the input circuit was tuned into a 50 ohm load, then was finally presented with say a 105 ohm load), then the preamp's noise figure would increase and the FET would start to become artificially noisy? Then if the FET was pushed into overload by an adjacent strong signal, the noise would then start to reduce. I have this odd situation on a repeater, and I can only put it down to mis-tuned cavities. I've never come across it before though, where the repeater actually IMPROVES in sensitivity when the transmitter keys up (?!?!), so I would like to know what's going on before I fix it. Sam- There may be some measurable increase in a preamp's noise output, but a 2 to 1 mismatch will probably not cause such a serious degradation in its noise figure. Perhaps I misunderstand you. When the noise reduces due to overload, that is NOT the same as an improvement in sensitivity. That is commonly refered to as desensitization (desense). An amplifier's gain is reduced to zero when it is in saturation (or oscillating). Inadequate or mistuned cavities can cause desense. It can also be caused by poor shielding, often from poor co-axial cable. You might benefit if you switched to double-shielded cable. Also look for poorly attached connectors. 73, Fred, K4DII |
#4
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Impedence mismatch into FET preamp
In article , "Samuel
Hunt" wrote: It's not due to desense. When I tune the cavities up properly, there's about 10dB noise figure on the preamp (reciever is 0.8uV sensitive). Then when it transmits, the noise figure doesn't change. Sam- What frequency is this repeater on? A sensitivity of 0.8 uV doesn't sound very good to me. On 146 MHz I'd expect that without any external preamp, although it might be good at 1200 MHz??? 73, Fred, K4DII |
#5
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Impedence mismatch into FET preamp
On Sun, 26 Feb 2006 09:54:49 +0000 (UTC), "Samuel Hunt"
wrote: Hi all. I have a FET preamp, tuned by means of two fixed capacitors and one variable inductor that serves as an impedance transformer. Am I correct in thinking that if there is an impedance mismatch (say the input circuit was tuned into a 50 ohm load, then was finally presented with say a 105 ohm load), then the preamp's noise figure would increase and the FET would start to become artificially noisy? Then if the FET was pushed into overload by an adjacent strong signal, the noise would then start to reduce. I have this odd situation on a repeater, and I can only put it down to mis-tuned cavities. I've never come across it before though, where the repeater actually IMPROVES in sensitivity when the transmitter keys up (?!?!), so I would like to know what's going on before I fix it. Thanks all, Sam M1FJB How are you measuring this? How do you hook the signal generator to the system when you check the sensitivity with and with out the transmitter on? It sounds like a mis tuned duplexer but by what you are saying it is hard to tell what you are seeing. 73 Gary K4FMX |
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