cyamamot@nunki.usc.edu (Cliff Yamamoto) (11/30/88)
Greetings
I have an arbitrary question regarding RF preamps, splitters, and
combiners. Does the following look feasible?
______________
_____ | GaAs FET amp |
\ | / __________ | 400-450 Mhz | __________
V | 3 dB |------|______________|------| 3 dB |
|____| splitter | _______________ | combiner |----> to receiver
|__________|------| GaAs FET amp |------|__________|
| 450-500 Mhz |
|______________|
Would there be too much noise introduced by the splitter and combiner
to be of any use? Or is it better to select one amp and operate it at 450Mhz
without the splitter/combiner? If I use ONE amp, is it possible for an amp to
have a negative dB effect (i.e. attenuation) at the extremes?
Example : if I wished to monitor two signals at 425Mhz and 475Mhz would:
1) the above diagram using two amps having a noise figure of 0.5dB
and some gain X dB be better
OR is
2) one amp with a 400-500Mhz range, but higher NF and lower gain
better. (tuned to center of 450Mhz)
In short my question is : What do you do if you want to pre-amplify a wide
frequency range with high gain and low noise, but DO NOT have an amp with such
a wide BW available.
Cliff Yamamoto
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cyamamot@nunki.usc.edu cyamamot%nunki.usc.edu@oberon.usc.edudya@unccvax.UUCP (York David Anthony @ WKTD, Wilmington, NC) (12/01/88)
In article <2038@nunki.usc.edu>, cyamamot@nunki.usc.edu (Cliff Yamamoto) writes: > Greetings > I have an arbitrary question regarding RF preamps, splitters, and > combiners. Does the following look feasible? > ______________ > _____ | GaAs FET amp | > \ | / __________ | 400-450 Mhz | __________ > V | 3 dB |------|______________|------| 3 dB | > |____| splitter | _______________ | combiner |----> to receiver > |__________|------| GaAs FET amp |------|__________| > | 450-500 Mhz | > |______________| Feasable and doable, but probably not as good as a well designed 400-500 mHz amplifier. You should be able to build a reasonable GaAs preamp with an 0.5 - 0.7 dB noise figure with conventional microwave transistors. The first splitter will have an insertion loss associated with it (-3 dB); add it to your noise figure cal- culation. That signal is gone **forever**. As long as your amp is quieter than 3.5 dB, the single amp is better. Also, without good design, there will be differing propagation delays through the two amplifiers. When the two signals finally do sum in the other combiner, you may find marked gain reduction for some frequencies and excessive gain for others. If you **must** listen to 425 and 475 mHz, you could do separate amplifiers for each frequency, and switch between them. This will give some improvement, due to lower noise bandwidth and the ability to obtain a conjugate impedance match for the antenna system at the amplifier input. The broadband amp stinks in one respect, though; if you have a strong RF source nearby (say, a TV station on channel 18) it will be overloaded. Conventional slotted line tecniques can null out that junk, though. There are numerous other considerations, but the fact that there is attenuation in the input of the first amplifier simply adds that attenuation to the first stage noise figure. York David Anthony WKTD Wilmington, NC
dvh@cci632.UUCP (David Hallidy) (12/01/88)
I wouldn't recommend a 3dB splitter before either preamp. If you do this, you are automatically degrading the system noise figure by the 3dB loss of the splitter- same as if you had 3dB of cable loss between the antenna and the preamp. I believe the answer is to build (or purchase) a wider bandwidth preamp. Avantek has an application note describing a GaAsFET preamp with about a 0.4dB NF and about 20dB gain which has an untuned input (matched to 50 ohms) using the ATF10135 device and which will have excellent performance over AT LEAST 100 MHz. Check it out. Put the preamp as close as possible to the antenna and then run about as much coax down to the receiver as you want- you'll have plenty of excess gain. This eliminates all the splitters and combiners. In fact you could put a splitter after the preamp and use it to drive a couple of receivers and still have enough gain for both. Hope this helps. 73 Dave KD5RO
dvh@cci632.UUCP (David Hallidy) (12/01/88)
I forgot to add that the Avantek preamp circuit is specified from 400-500 MHz. They also, in the same app. note, talk about designs for the 900 MHz and 1300 MHz bands. Also, the reason you don't want the preamp after the lossy splitter (or cable) is because the system noise figure is primarily determined by the gain and noise figure of the first stage. In this case, the first stage has a 3dB NF, and -3dB gain. Running the noise figure cascade on this system, with a 0.4dB preamp after the splitter, will yield a noise figure cascade somewhere in the 2dB range. After the preamp, you can play a lot of games,for the same reason- the first stage determines the overall noise figure- assuming the preamp has enough stage gain to overcome the loss and noise of the following stage(s). Therefore, if you have a preamp with a 0.4dB NF and 20dB gain at the antenna, you can overcome a lot of loss after the preamp and still retain most of that excellent performance. I think I got it all out this time. 73 Dave KD5RO
dvh@cci632.UUCP (David Hallidy) (12/07/88)
In article <23212@cci632.UUCP>, dvh@cci632.UUCP (David Hallidy) writes: > > > I forgot to add that the Avantek preamp circuit is specified > from 400-500 MHz. They also, in the same app. note, talk > about designs for the 900 MHz and 1300 MHz bands. > > Also, the reason you don't want the preamp after the lossy > splitter (or cable) is because the system noise figure is > primarily determined by the gain and noise figure of the > first stage. In this case, the first stage has a 3dB NF, and > -3dB gain. Running the noise figure cascade on this system, > with a 0.4dB preamp after the splitter, will yield a noise > figure cascade somewhere in the 2dB range. > > After the preamp, you can play a lot of games,for the same > reason- the first stage determines the overall noise figure- > assuming the preamp has enough stage gain to overcome the > loss and noise of the following stage(s). Therefore, if you > have a preamp with a 0.4dB NF and 20dB gain at the antenna, > you can overcome a lot of loss after the preamp and still > retain most of that excellent performance. > > I think I got it all out this time. > > 73 Dave KD5RO > Boy, sometimes I REALLY do it!!! It was pointed out to me that the noise figure cascade as described in the second paragraph of this follow-up is incorrect. IT SURE IS!!! The correct system noise figure for the scenario described above is actually (by my now questionable calculations) 3.3dB. It isn't possible for the system, as described, to have a noise figure lower than the noise contribution of the first stage, and since the second stage isn't totally noiseless, it also has to be factored in (which I did do, just wrong). In the cascade formula, the gain of the first stage is required in the denominator of the part of the equation relating to the second stage contribution. I did include it, but as a decibel expression, not as a RATIO- since the equation works in raios, this really changes the result!!! As I said, sometimes I really do it. Bottom line is, put the preamp at the antenna and you won't have any problems. 20% bandwidth is no problem to achieve and you should keep an excellent system noise figure across the 400-500 MHz range. I hope this is finally right. (THANX GLEN FOR THE EMAIL POINTING THIS OUT) 73 Dave KD5RO