Don:
My thought was a single conversion receiver with a high IF (45 MHz) with
a monolythic crystal filter, and an AD8307 as the IF amp and detector.
Use an NE602 or 612 oscillator/mixer. Throw in a 30 MHz LP filter to
knock down images. Local oscillator runs 45-75 MHz.
These filters are about 30 KHz wide, and not very expensive. Their
impedance is a good match for an AD8307 and the NE602.Use a PIC as a
frequency meter, maybe a huff-n-puff stabilization arrangement with the
PIC as well, or partial stabilization as used in the HP8640B signal
generator's PLL circuit.
If the objective is to come in at the $100 level or less, this approach
might make it. There's not a lot of budget for parts with that target price.
Jack
Don Wilhelm wrote:
Jack,
How about putting a sharp tuned circuit between the detector output from the
bridge and the AD8307 - a single parallel tuned circuit with a variable cap
and a winding on a toroid can serve with high Q over a wide frequency range
(depending on the variable capacitor size) - not much different than tuning
the receiver. An input link on the toroid can match the 50 output of the
bridge and a third link can match the input impedance of the AD8307.
Of course the detector need not be so exotic if a narrow band signal source
is used (like a QRPp transmitter), and that can simplify not only the
bridge, but the detector too since more power is available.
Ideas and alternatives abound - all with tradeoffs for accuracy, ease of
construction, ease of use.
Yes, the number of knobs grows, and likely limits use at the top of a tower,
but half wavelengths (or multiples) of coax are handy in that situtation and
allow the measurements to be made on the ground - ideally by a ground crew
member and the antenna tuning done by the person at the tower top.
73,
Don W3FPR
-----Original Message-----
Don:
I've been giving the subject a bit of thought as well and a noise
bridge came to mind rather quickly. I have an old MFJ 202 noise bridge I
bought 25 years ago.
However, I believe a broad band detector such as the AD8307 will fail.
The null condition is rather narrow frequency wise, and the noise
generator detector output is broad, so the different in total power at n
AD8307 detector between no null and perfect null will be tiny,
particularly if the bridge is constructed to have good balance, which
is, of course, the objective.
This is a case where one needs a narrow band tuned detector, I believe.
Or, a narrow band source.
Jack K8ZOA
www.cliftonlaboratories.com
Don Wilhelm wrote:
Those who have both an Elecraft N-Gen and a KX1 have everything needed
except the bridge elements to construct and use a relatively
compact version
of that Improved Noise Bridge described by John Grebenkemper in
that August
1989 QST article.
The N-gen is the noise source and the KX1 can serve as the
detector - listen
for the null. Of course, it is easy to construct the noise
generator, so
those with only a portable receiver like the KX1 can have an accurate
impedance meter once it is calibrated.
Detectors other than a receiver can also be used - an AD8307 based power
meter should work just fine and fit into a small case.
The 'disadvantage' of the noise bridge is there are two knobs
to deal with,
but when a deep null is achieved one can read both the resistive and the
reactive component of the unknown impedance.
73,
Don W3FPR
-----Original Message-----
Dave G3YMC wrote:
I would also make a plug for a noise bridge. I built the one by KI6WX
which appeared in QST in August 1989 (available to download from the
ARRL site). If you have a suitable portable rig to couple to it does
most of what the fancy analysers can do at a fraction of the cost.
It should not be too difficult to design a fairly simple receiver
drawing a
small amount of current to be enclosed with a noise bridge to
make an 'easy
to carry' instrument. Top of head thinking.
73,
Geoff
GM4ESD
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