MD design, GEB, OP amp.

[BJ]

Iiich,,,new to this splendid forum and pretty
green to detector design, but fairly grey into
electronics in general,i have designed a MD on
request for a friend (for 2 mont's now) and
i wish i had found out about this forum sooner
rather then later, inventing the wheel several
times over is rather waste of brain cells! Ugh,

Now to the question, the design is/was "scratch"
basic IB, funkgen tri TX VCO with sine shaping
and tuned RX coil at 5khz. Now i have designed
a GEB where one can turn the phase angle from
0 to 360deg, is this range neccecary?
(im aware of metal-phase relationship)

Is there any usage of this 360deg, one pot turns
0-360 divided into a 0-180 deg and 180-360 by
a switch.

I use a single sync demod but switching between
a buffered and one inverted and buffered RX
signal, i concidder PW'ing the sync signal less
of usage since it affects demodulated signal
(DC goes up or down) What do you have to say
about PW modulated sync signal?
(PW affects sampling time of RX signal,
(calibrating usage could offcourse be an asset)

What RX pre amps do you prefear?
I find most LM158, LF353 etc having bad
offset temp drift, any one fancy in Cmos?
Still LM158 LF353 are hard to bet in price
performance.

A cheap LMC662 performs pretty well, far more
neat than most 70'ies OP designs, particularly
in the input offset drift department.

Reg
BJ

[Dave J.]

Re: MD design, GEB, OP amp.

Sorry, don't have time right now for a detailed reply, but hope this little bit helps.

In general, the best preamps are low noise bipolar units. OP-27 is a good choice if you have at least 6 volts. The ST LS404 types are popular. MAX410, MC33077, and MC33178, will run on 5 volts split down the middle.

Assuming that your design is intended to find all metals (no discrimination), buried in the ground, then the phase range needs to cover the range of ground quadrature. Ground can run all the way from 0 to 90 degrees loss angle, but at 5 kHz, a range of 10-15 degrees will balance just about everywhere but on an ocean beach. Ferrite has a loss angle of 0 degrees, and most ground is close enough to 0 degrees that machines which use fixed-phase ground balance are adjusted using a piece of ferrite.

For single-frequency machines, the industry nowadays uses square wave phase reference signals, and CMOS analog transmission gates as demodulators, with very few exceptions.

--Dave J.