[vna] FW: 3 small questions and 1 BIG question

[tom.mcdermott4 at sbcglobal.net]

Hi,
 
Make sure you have the latest software version 2.0 installed, and pull a
copy of the latest Instruction Manual from the website.
In the VNA application  Help->About will check for the latest software
automitically.
 
http://www.tapr.org/software_library.php?dir=/pub/n5eg
 
 
 
1. R41 is listed in the bill-of-material (Excel spreadsheet), as 10 ohms.
It is on the tapr web site in the buildDocuments folder:
 
ftp://ftp.tapr.org/pub/n5eg/buildDocuments/VNA_R3_BOM_Jan-01-2006.xls
 
The schematic is a little out-of-date on just a couple values, the BOM is
accurate.
 
2. I don't know which resistor you are looking at. Proabably best to look at
the BOM excel spreadsheet.
 
3. There are some resistors that need to be 1%. Some manufacturers
standardize on 1% to increase the volume, that makes them effectively the
same cost as 5%.
 
4. It is hard to tell what the sweep speed value is from the JPEG image. For
narrow filters, you would need to set the sweep speed correspondingly
slower. Your filter appears to be about 6 KHz wide, so it will have a time
delay on the order of a millisecond or so. Sharp edges can require even
slower sweep speeds. You should also set the Tx Level to 0 dB, and attenuate
if your filter has gain.  Can't tell from the image, but if the word
'Uncalibrated' appears on-screen - that's an indication that the controls
need to be reset. These cautions are described in the instruction manual.
You can get the latest one at the TAPR web site (URL above).
 
    -- Tom, N5EG
 
 
 

-----Original Message-----
From: vna-bounces at lists.tapr.org [mailto:vna-bounces at lists.tapr.org] On
Behalf Of Jack Klinker
Sent: Wednesday, September 05, 2007 7:01 PM
To: vna at lists.tapr.org
Subject: [vna] FW: 3 small questions and 1 BIG question


1. R41 10 ohms (BOM) vs. 27 ohms (schematic)?  Which is correct?
 
2. I figured the SGA-5386 Rbias as 24 ohms.  5v - 3.6v @ 60 mA = 23.3 ohms.
Did I do this right?
 
3. All resistors are specified a s  1%.  It looks like that 5% would be fine
in most(all) places.  Am I missing something?
 
The BIG question:
4. The VNA seemed to be working fine, but then things changed.  It seems to
be a strange gain effect: about a  20 dB jump when there is a higher level
input.  See attached  .jpeg print screen image.  The filter under test (18
MHz 4 element XTAL filter) was showing a normal transfer function, then this
started happening.  I tried a different filter.  I mostly re-built the
filter.  I replaced the front-end SGA-5386 in the VNA, but nothing changed.
Did something die in the 8302 log detector?

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