ther resistor in the collector, of equal value as the emitter resistor.
.
d NO
Low.
Good thinking. At your frequency you'd want to make the transformer as a si ngle layer winding where the parallel capacitance can get down to 1pF.
The optimum winding for the best - lowest - capacitance for the best - high est - inductance put a one wire-diameter gap between every turn which is tr icky to manage.
The single layer winding prevents you from using bifilar winding, so the 1:
1 ratio depends on getting your winding geometry exactly right.
Bifilar wound transformers offer matching to one part in a billion, but tha t costs of the order of 100pF of interwinding capacitance (treat the bifila r winding as a twisted pair of the necessary length - not a perfect proxy, but good enough for many purposes).
The frequency minimum is set by the coupling capacitors-- it's a Gilbert cell multiplier. I've used them at a few hundred kHz.
Best regards, Spehro Pefhany
--
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single layer winding where the parallel capacitance can get down to 1pF.
ighest - inductance put a one wire-diameter gap between every turn which is tricky to manage.
To wind such a coil bifilar - in the sense that I used later the post to ge t a one part per billion matching between the two halves of the coil - you have to twist the two wires enough that they cross one another several time s per turn. Cutting out the reversal then becomes tricky.
I agree that winding on a flat pair of wires is a thoroughly practical way of getting to an optimally spaced single layer single wire coil, and I'm ki cking myself for not thinking of it. I was thinking in terms of nylon fishi ng line, which would probably work, and offer a cheaper solution, but one h arder to realise in practice.
However, it isn't a bifilar winding as is usually understood in this contex t.
The center-tapped transformer idea is good, nice and symmetric. Mini-Circuits makes super-wideband transformers, cheap.
Again, I'd vote for phemts as the best fast switches to ground. Their gate-drain capacitance is absurdly small. NE3508, enhanced a tiny bit, will get down to about 5 ohms. If you want to work at 10 MHz, it would be good to keep capacitances down and have everything switch in a couple of nanoseconds.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Switching to ground has a major problem. It's ok if you are in phase. The signal on the other end of the secondary is more positive than the centre tap.
As soon as you start to go out of phase, for example, in quadrature or full out-of-phase, the signal on the other end goes negative with respect to ground. If the device is bidirectional, the negative end should appear as the source. Then the gate would be positive and turn the device on. The transformer would have both ends of the secondary tied to ground.
Perhaps one way to solve it is to drive the gate more negative than the open end of the secondary. This runs into problems with breakdown in fast devices, which limits the maximum signal swing and thus the available dc output and dynamic range.
Here is what it looks like using LTspice switches:
Version 4 SHEET 1 1196 1240 WIRE 416 688 400 688 WIRE 448 688 416 688 WIRE 832 688 448 688 WIRE 832 704 832 688 WIRE 400 720 400 688 WIRE 448 720 448 688 WIRE 752 720 736 720 WIRE 784 720 752 720 WIRE 176 784 144 784 WIRE 272 784 256 784 WIRE 304 784 272 784 WIRE 784 784 784 768 WIRE 144 800 144 784 WIRE 832 800 832 784 WIRE 400 848 400 800 WIRE 448 848 448 800 WIRE 448 848 400 848 WIRE 528 848 448 848 WIRE 560 848 528 848 WIRE 672 848 640 848 WIRE 688 848 672 848 WIRE 304 864 288 864 WIRE 528 864 528 848 WIRE 688 864 688 848 WIRE 288 880 288 864 WIRE 400 880 400 848 WIRE 448 880 448 848 WIRE 144 896 144 880 WIRE 528 944 528 928 WIRE 688 944 688 928 WIRE 400 1008 400 960 WIRE 416 1008 400 1008 WIRE 448 1008 448 960 WIRE 448 1008 416 1008 WIRE 832 1008 448 1008 WIRE 832 1024 832 1008 WIRE 736 1040 736 720 WIRE 784 1040 736 1040 WIRE 736 1056 736 1040 WIRE 784 1104 784 1088 WIRE 832 1120 832 1104 WIRE 736 1152 736 1136 FLAG 752 720 IN FLAG 416 688 UL2 FLAG 416 1008 UL3 FLAG 144 896 0 FLAG 288 880 0 FLAG 672 848 R1C2 FLAG 688 944 0 FLAG 528 848 R1C1 FLAG 832 800 0 FLAG 784 784 0 FLAG 736 1152 0 FLAG 784 1104 0 FLAG 832 1120 0 FLAG 272 784 U11 FLAG 528 944 0 SYMBOL voltage 736 1040 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 WINDOW 0 18 18 Left 2 WINDOW 3 -92 135 Left 2 SYMATTR InstName V1 SYMATTR Value SINE(2.5 2.5 9980 0 0) SYMBOL sw 832 800 M180 WINDOW 0 11 106 Left 2 WINDOW 3 17 9 Left 2 SYMATTR InstName S1 SYMBOL voltage 144 784 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 WINDOW 0 -8 -9 Left 2 WINDOW 3 -98 190 Left 2 SYMATTR InstName V2 SYMATTR Value SINE(0 2 1e4 0 0 -90) SYMBOL sw 832 1120 M180 WINDOW 0 11 106 Left 2 WINDOW 3 17 9 Left 2 SYMATTR InstName S2 SYMATTR Value SW2 SYMBOL res 656 832 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 1k SYMBOL cap 672 864 R0 SYMATTR InstName C2
SYMBOL res 272 768 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R2 SYMATTR Value 50 SYMBOL cap 512 864 R0 SYMATTR InstName C1 SYMATTR Value 200p SYMBOL ind2 288 768 R0 SYMATTR InstName L1 SYMATTR Value 10mh SYMATTR Type ind SYMBOL ind2 416 704 M0 SYMATTR InstName L2 SYMATTR Value 10mh SYMATTR Type ind SYMBOL ind2 416 864 M0 SYMATTR InstName L3 SYMATTR Value 10mh SYMATTR Type ind SYMBOL res 432 704 R0 SYMATTR InstName R3 SYMATTR Value 1k SYMBOL res 432 864 R0 SYMATTR InstName R4 SYMATTR Value 1k TEXT 488 632 Left 2 !.tran 0 75m 25m TEXT 640 1224 Left 2 !.MODEL SW SW(RON=1 ROFF=1E6 VT=2.5) TEXT 464 600 Left 2 ;'Synchronous Detector Sine Wave Input Gnd Ref TEXT 640 1200 Left 2 !.MODEL SW2 SW(RON=1E6 ROFF=1 VT=2.5) TEXT 200 680 Left 2 !K1 L1 L2 L3 0.99
But as I tell my customers when they get ambitious like that, everybody's second digital lock-in design is much better than their first. ;)
I've seen a few people's first tries.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
This reminds me of what the Ham radio community calls the H-mode mixer. Series opposed nmos bidirectional switches where the source-source is conveniently arranged to be at system ground. Later variants used devices like the FST3125 fast bus switch.
The phemts don't have equivalent substrate diodes, so allow negative drain swing, as long as you pull the gates negative. They are depletion devices.
That would work with peak signal swings in roughly the 2 volt range, which is reasonable.
Post-detector gain is cheap, so the thing to optimize is peak swing over offset crud. At 10 MHz, the transformer+phemt thing looks pretty good.
Another config could be a transformer with CT grounded, driving a fast SPDT cmos mux. Or output from the CT and use the mux to ground the ends. That's not going to be as clean as the phemt thing.
There are ICs designed for zero-IF (direct conversion) I/Q receivers that might make really good PSDs.
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
Hey, my first try at digital lock-in was an LVDT signal conditioner. 16 bit ADC and a 68332 uP, all the work done in software. It worked first time. Using an ancient Gertsch decade transformer box to simulate the LVDT, it's accurate to 1 LSB at 16 bits.
formatting link
I drove a stepper, belt reducer, and ball screw into an LVDT and clearly resolved 90 nm steps. The temperature had to be very stable.
Those old AC ratio boxes are cheap and accurate to PPMs. All tapped transformers and switches inside.
formatting link
--
John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
A current conveyor is pretty much just four current mirrors in a box. An OTA is the same, except with a Gilbert cell on the front.
The only true OTA that's widely available is the LM13700, which is too slow (1 MHz, even at high current). (TI makes something they call an OTA, but it doesn't have differential inputs and its linearity is crap.)
The SA602 is a Gilbert-cell mixer that's much faster, though it's a bit of a pain to bias for DC applications, I expect.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
IME most folks don't pay enough attention to linearity and decoupling when they do their first digital lock-in. You need _good_ ADCs, strong ADC drivers, a good dither mechanism, and really simultaneous sampling.
Hanging some random ADC on some random MCU won't do it, and that's what I see all the time.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
The post detector gain is DC. With low DC signal output, you are stuck between the offset voltage and a small DC voltage. This limits the dynamic range.
I wonder if a simple balanced modulator might give less offset and higher output voltage. Miniscribe has really cheap ones in the low RF range.
They would easily operate at higher frequencies and avoid the difficulty of having to swing the gate negative. Drive them hard and don't bother terminating in 50 ohms.
The mux would have to accept negative voltage.
Any favorite model numbers? The LTC5584 only goes to 30 MHz:
formatting link
The TI TRF3711 goes to 300 MHZ
formatting link
10259062.pdf
There doesn't seem to be much call for a zero IF at 1 MHz, let alone 100 KHz.
Good to know, thanks. When I need something like that I usually build it, but your way would save a lot of parts.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics
160 North State Road #203
Briarcliff Manor NY 10510
hobbs at electrooptical dot net
http://electrooptical.net
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