Hi All,
will post some pics and diagrams of the transformer itself, as per usual I have to
make plans after I've made it!
The two circuit diagrams above are first the circuit I'm using currently and the second
is the next evolution, as yet unbuilt.
Refering to the first circuit the amp IC3A, biasing, gain setting and voltage limiting
components feeback into the LC tank C1,C2,L1 and R3.
R3 is a current sense and is amplified by IC3B by a gain of 50 to drive the lvdt primary
and the all-pass phaseshift amp IC3D.
The double balanced modulator is driven by the phaseshift amp via R16 and keeps the
ring diode currents to a few mA.
R15 and C13 lowpass the DC while damping out the LO+RF signal and IC3C is DC buffer.
The osc amp, gain amp and phaseshift amp all require a fast opamps, I chose OPA1654AIPW
with gainbandwidth of 20MHz. I ordered these only to realise that they are TSSOP with .6mm
pitch. Real sods to hand solder!
The net evolution will use similar amps but probably duals rather than a quad and SOIC being a lot
easier to handle. The only other difference is using two amps each with a gain of 7 rather than one with
a gain of 50. If I'm using two amps why not introduce some filtering.... hence the extra resistors/caps
as 4 pole Butterworth. Most if not all components on the 'evo' board will be SMD.
I'll draw some diagrams and be back when done.
Craig