?
You could certainly do this. The professionl way is
to use a ganged source-detector. You might achieve a sufficient sensitivity by
using a logaritmic detector rather than a diode rf voltmeter. The problem using
this set up for antennas as I mentioned earlier is that the null is compromised
by any strong signals you may receive and also by the source harmonics which
will not be nulled of course. You will really need, from my experience, around 3
to 5 volts source drive for the bridge. I have wonder about using the same
source as an LO for a direct converstion receiver. Here screening would be of
paramount importance. Maybe optical fibre coupling the source to the
receiver??
?
Alan
G3NYK
----- Original Message -----
Sent: Thursday, October 08, 2015 8:06
PM
Subject: Re: [hp_agilent_equipment] RF
Impedance Measurement
I have another idea.? The purpose
of using noise as a signal source is so that the frequency of interest can be
selected by the detector/receiver.? The same thing can be accomplished
with a signal generator and voltmeter, with the signal generator providing the
frequency of interest, the only issue being that of getting a sufficiently
sensitive detector (the same receiver would work here).
So now the problem is reduced to
fabricating the bridge itself.? The excitation and detection has become
simple and thus the balance of the drive signal has to be considered.?
Further, I wonder how to calibrate the capacitor; it will not read the unknown
reactance directly but a calculation must be made.? If the basic impeda
nce of the bridge (standard value) is 50 Ohms, then the ratio of the unknown
(measured) resistive component to this needs to be the scaling factor for the
capacitance (or negative capacitance).
Has anyone done the arithmetic for
this?? I am agonizing over the what might be the best way to mark the
bridge dials.? I presume that marking them with the value of the
component and calculating later might work best.
If I make several
transformers, I won't have to make a very wide band transformer.? If I
use several values of standard resistance I can cover a wide impedance
range.? And with a couple of variable capacitors the frequency and
impedance range can be extended.? Sensitivity at balance is also an
issue.
This has moved from a difficult project to a simple one and now
is back to being difficult.
Bob
On Thursday, October 8, 2015 11:55 AM,
Bob Albert wrote:
Robert,
Thanks for telling me,
but I suspect the shipping cost (based on your callsign) would make the cost
prohibitive.? I am in Los Angeles.
So my intent is to use
the main dial on an old cheapie signal generator and custom calibrate
it.
Bob K6DDX
On Thursday, October 8, 2015 11:17 AM,
"robert8rpi@... [hp_agilent_equipment]"
wrote:
Hi Bob,
I have some
Hammarlund 25pF to 250pF precision variables. They are straight law with
built-in anti-backlash reduction gears. They came from aircraft fuel quantity
gauge test sets and look like new. A lot of variable capacitors have non
linear laws to give a linear frequency scale on receivers. let me know off
list if you are interested in one of the Hammarlunds.
Robert G8RPI.
