A preamp that is tuned with a noise figure meter is in most cases not going to be the same when installed in the system as the input load is going to be different. How much the input load changes is going to depend on what it is hooked to.
Best noise figure performance usually doesn't yield a 50 ohm input impedance. Noise figure is best adjusted with the preamp connected in the circuit in which it is going to operate. 73 Gary K4FMX _____ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Jeff DePolo Sent: Friday, February 09, 2007 7:26 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Re: Advanced Receiver Research Preamp 144-148 > The better question is... have you looked at the input section of > the circuit diagram? Looks like a high Z input doesn't it... :-) Don't think I've seen an ARR schematic, but most GaAsFET preamp designs I've evaluated will tend to produce least noise figure with an input Z quite a ways away from 50+j0, and with gain a few dB below the maxima. > It will for a number of reasons at part of it being the preamp > input impedance and part of it related to the cavity "loading"... > plus the number of series cavities. Well, yeah, of course I know WHY. I was just stating that the detuning effect is what prompted me to evaluate the preamp further. Without having seen it on the VNA I wouldn't have had any reason to bother testing the preamp. > They are adjusted for low noise below the defined F-center as > probably are Chip's preamps (per my last conversation with him). The > resultant gain is at least the rated/spec value. Gain is the least of my concerns, and in a repeater installation, giving up a fraction of a dB of NF or a couple of dB of gain is no big deal if it means getting a better match. I would think it would behoove ARR to ask the customer what the intended use was, and align the preamp accordingly, if their design can't provide a decent match when the preamp is tuned for low NF. For weak-signal work, where you typically have nothing in front of the preamp to avoid any increase in NF caused by losses ahead of the gain stage, this might not be that big of a deal, but you still have to consider the degraded power transfer and VSWR on the feedline (yes, VSWR in a receive situation) when the input match gets to be that bad. Again, I don't know if *ALL* ARR preamps have this problem, or if I just happened to stumble across two bad ones by accident. That's why I originally posted the question - has anyone else swept an ARR, particularly P432VDG models? > Also depends on the cavity filter... ...and you can show me a coaxial cavity filter that won't be out-of-tune when connected to a load Z different than what it was properly tuned for? And, no, futzing with cavity tuning as a makeshift means of conjugate matching doesn't count.
