On Fri, 13 Dec 2019 at 22:58, <roncraig1@...> wrote:
You are not going to be able to improved reading of high impedance very
much as the primary limitation is the 50 ohm impedance bridge and signal
level differences get very small at the higher load impedance.(refer to
right hand, high Z side point of Smith chart. The difference in reflection
coeff. is very small at higher Z).
The best solution is to make a transmission measurement. That allows high
impedances to be measured better than a reflection measurement
Probably the best thing to improve unit's overall accuracy is the
replacement of the cheap thick film 49.9 ohm chip resistors (EIA96 marking
'68X') in the input bridge with 50.0 ohm 0.1% tolerance high frequency thin
film resistors. They are about $1.50 each at Mouser Elect. for Vishay
FC0603E50R0BTBST1 part number.
I don¡¯t know the tolerance of the 49.9 ohm resistors, but 49.90000000 ohms
is plenty good enough
Here are a couple of equations for VSWR that I personally find convenient
VSWR=R/50 for R>=50
VSWR=50/R for R <= 50.
So for 49.9 ohms
VSWR=50/49.9=1.002
You will find it is a 60 dB return loss if you use an online calculator
such as;
The guaranteed return loss of the loads in my $13,000 Agilent 85052B 3.5 mm
calibration kit is 48 dB at low frequencies.
IMHO, unless the 49.9 ohm resistors have a poor tolerance, *the difference
between 49.9 and 50 ohms is not worth worrying about. *
I measured the source match at port 1 using an HP 8720D VNA with 85052B
calibration kit. The match is *excellent* - it far exceeds the uncorrected
performance of the 8720D. On port 2, the match is poorer, but still quite
good.
Dave
--
Dr. David Kirkby,
Kirkby Microwave Ltd,
drkirkby@...
Telephone 01621-680100./ +44 1621 680100
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Dr. David Kirkby, Kirkby Microwave Ltd