Greetings,
I believe David is correct here, the effect of the mismatch between
antenna and preamp is still there and so all the available energy from
the antenna will not be transferred to the preamp. However, the effects
of the length of cable will be gone as with zero length there can be no
standing wave.
So if the preamp has a pretty good 50 input impedance you will gain
nothing by adding a 3dB pad. This could be a trade off if the input
impedance is terrible - which has less loss (= lower noise) the 3dB pad
and its mismatch losses to both antenna and preamp, or the single
antenna to preamp mismatch loss? At least with zero cable it would be a
stable value. If different cable lengths are used, the transferred
impedance will be also be different meaning that the mismatch losses
could well change too.
Getting the preamp as close as possible to the antenna is always best I
think. With a zero, or very short fixed length cable the measurement
set up will be as repeatable as possible while reducing the noise floor.
Does this help or confuse?
Colin..
-----Original Message-----
From: Pommerenke, David [mailto:davi...@ece.umr.edu]
Sent: Friday, November 16, 2001 9:10 AM
To: 'Ken Javor'; 'Ravinder Ajmani'; marti...@appliedbiosystems.com
Cc: emc-p...@majordomo.ieee.org
Subject: RE: Pre-amps
Dear Ken,
I am thinking you are missing something (hope that I am correct).
If you simply shorten the cable to zero, the problem of the missmatch,
and
the effect of an error that is not corrected for by the antenna factor
still
remains.
David Pommerenke
-----Original Message-----
From: Ken Javor [mailto:ken.ja...@emccompliance.com]
Sent: Thursday, November 15, 2001 5:51 PM
To: Pommerenke, David; 'Ravinder Ajmani'; marti...@appliedbiosystems.com
Cc: emc-p...@majordomo.ieee.org
Subject: Re: Pre-amps
If input VSWR for preamp and output VSWR for antenna are both bad, then
it
seems a simple solution is to connect preamp input to antenna output and
let
50 Ohm output of preamp drive cable, solving two problems at once. Am I
missing something?
----------
>From: "Pommerenke, David" <davi...@ece.umr.edu>
>To: "'Ravinder Ajmani'" <ajm...@us.ibm.com>,
marti...@appliedbiosystems.com
>Cc: emc-p...@majordomo.ieee.org
>Subject: RE: Pre-amps
>Date: Thu, Nov 15, 2001, 7:49 AM
>
>
> On the amps:
>
> For emissions there are the following electrical criteria:
>
> - Noise figure expect about 4 dB for a broadband amp 30 MHz - 2 GHz.
> The noise figure is often larger at the lower frequencies if the
> amplifier goes up to many GHz.
>
> - Input SWR. This is important. Most broadband amps (especially if
they
go
> up to many GHz)
> have a bad input match at low frequencies. As the log-per antennas
have
a
> bad mismatch too,
> you will have multiple reflections on the cable between the antenna
and
> the pre-amp.
> This reflections will influence your measurement and cannot be
corrected
> for by the antenna
> factor. They may be as large as a few dB below 100 MHz. For that
reason,
> you may be forced
> to add a 3 dB attenuatore at the antenna. This increases your noise
> figure by 3 dB.
>
> - Gain. Of course, you need only as much gain as is needed to
overcome
the
> cable loss (cable to
> the spectrum analyzer) and the noise figure of the spectrum
analyzer.
> More gain will not help
> you.
>
> David Pommerenke
>
> -----Original Message-----
> From: Ravinder Ajmani [mailto:ajm...@us.ibm.com]
> Sent: Wednesday, November 14, 2001 1:27 PM
> To: marti...@appliedbiosystems.com
> Cc: emc-p...@majordomo.ieee.org
> Subject: Re: Pre-amps
>
>
>
>
> Joe,
> HP (Agilent) make good Pre-amps for different frequency ranges. If
you
are
> looking for an economical solution then you may try Com Power Corp. at
> (949) 587-9800.
>
> Regards, Ravinder
> PCB Development and Design Department
> IBM Corporation
> Email: ajm...@us.ibm.com
>
************************************************************************
***
> Always do right. This will gratify some people and astonish the rest.
> .... Mark Twain
>
>
>
>
>
> MartinJP@appliedbiosyst
>
> ems.com To:
> emc-p...@majordomo.ieee.org
> Sent by: cc:
>
> owner-emc-pstc@majordom Subject:
Pre-amps
>
> o.ieee.org
>
>
>
>
>
> 11/14/2001 10:14 AM
>
> Please respond to
>
> MartinJP
>
>
>
>
>
>
>
>
>
> I am having some difficulties locating manufacturers that provide
preamps
> with a 20-22dB gain.
>
> What manufacturer/model do you recommend? Why?
>
> Your assistance is appreciated.
>
> Regards
>
> Joe Martin
> Applied Biosystems
>
>
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