GHz oscillator

fredag den 30. april 2021 kl. 21.31.52 UTC+2 skrev John Larkin:

or PM

but then your channel will have varying DC

How about a 4 - 20 ma current loop? The transmission line is shielded if you are going to carry 2 GHz signals. You can terminate in 50 ohms to match the cable impedance. Then all you need is a wide bandwidth current source which should be easy. The output voltage is simply E = I * R.

You can beat the bandwidth of PWM due to the filtering required at the receiver. Even extensive filtering leaves some ripple, which reduces the accuracy of data transmission.

With controlled current, you don't have to worry about voltage drops along the transmission line. You can set a current value that lasts for years, even with multiple powerups. Unlike PWM, the overall accuracy is determined by relatively few components.

The signal on an AC-coupled channel always averages zero. If you poke a PWM square wave through it, you get an asymmetric amplitude square wave with zero DC average.

Telecom-type optical signals are usually transmitted by switching a laser on/off, following the digital input. The transmitters are usually AC coupled, so need continuous, roughly 50% digital activity to work right.

That is typically received by a photodiode, an AC-coupled AGC amplifier, and a zero crossing detector, to typically make an AC-coupled differential CML output.

Ethernet type signals are transformer coupled on both ends.

That gets interesting to push a DC coupled signal through, theoretically and experimentally.

I'm interested in inherently AC-coupled channels, namely telecom type fiber links or ethernet-style transformer-couples twisted pairs.

We already sell fast DC coupled fiber links, laser on/off things. But the receive threshold has to be manually set and the effective threshold varies with path attenuation. Somebody tie-wraps a fiber down, or adds a bulkhead feedthrough, and the threshold is trashed.

lørdag den 1. maj 2021 kl. 01.19.47 UTC+2 skrev John Larkin:

unless that channel have limited bandwidth, no?

Of course. A typical SFP module might be rated for 10 GBPS (passes a 5 GHz square wave) and only work down to a couple of MHz on the low end.

On Tue, 27 Apr 2021 12:35:17 -0700, John Larkin <jlarkin@highland_atwork_technology.com> wrote:

Here's a 2 GHz oscillator with PWM modulation. First cut, needs work.

Costs about $2.50.

Version 4 SHEET 1 880 680 WIRE 0 -176 -640 -176 WIRE 144 -176 0 -176 WIRE -640 -128 -640 -176 WIRE 0 -128 0 -176 WIRE 144 -112 144 -176 WIRE -640 32 -640 -48 WIRE -592 32 -640 32 WIRE -464 32 -512 32 WIRE -336 32 -464 32 WIRE -224 32 -272 32 WIRE -144 32 -224 32 WIRE 0 32 0 -48 WIRE 0 32 -144 32 WIRE 144 32 144 -32 WIRE -144 80 -144 32 WIRE -640 96 -640 32 WIRE -416 96 -640 96 WIRE -384 96 -416 96 WIRE -640 112 -640 96 WIRE 0 112 0 32 WIRE -464 192 -464 32 WIRE -464 192 -592 192 WIRE -384 192 -384 96 WIRE -336 192 -384 192 WIRE -144 192 -144 160 WIRE -144 192 -272 192 WIRE -48 192 -144 192 WIRE -464 240 -464 192 WIRE -144 240 -144 192 WIRE -640 272 -640 208 WIRE 0 272 0 208 WIRE -464 352 -464 320 WIRE -384 352 -464 352 WIRE -224 352 -384 352 WIRE -464 384 -464 352 WIRE -144 384 -144 320 WIRE -224 400 -224 352 WIRE -192 400 -224 400 WIRE -192 448 -224 448 WIRE -464 512 -464 464 WIRE -224 512 -224 448 WIRE -144 512 -144 464 FLAG 144 32 0 FLAG -640 272 0 FLAG 0 272 0 FLAG -416 96 A FLAG -224 32 B FLAG -464 512 0 FLAG -224 512 0 FLAG -144 512 0 FLAG -384 352 PWM SYMBOL mesfet -48 112 R0 WINDOW 0 81 19 Left 2 WINDOW 3 59 55 Left 2 SYMATTR InstName Z1 SYMATTR Value SAV551 SYMBOL voltage 144 -128 R0 WINDOW 0 59 34 Left 2 WINDOW 3 56 69 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value 2.5 SYMBOL mesfet -592 112 M0 WINDOW 0 -56 24 Left 2 WINDOW 3 -94 54 Left 2 SYMATTR InstName Z2 SYMATTR Value SAV551 SYMBOL cap -272 16 R90 WINDOW 0 -41 28 VBottom 2 WINDOW 3 -37 27 VTop 2 SYMATTR InstName C1 SYMATTR Value 2p SYMBOL cap -272 176 R90 WINDOW 0 -44 26 VBottom 2 WINDOW 3 -39 26 VTop 2 SYMATTR InstName C2 SYMATTR Value 2p SYMBOL res -496 16 R90 WINDOW 0 -38 55 VBottom 2 WINDOW 3 -36 53 VTop 2 SYMATTR InstName R1 SYMATTR Value 200 SYMBOL res -128 176 R180 WINDOW 0 55 72 Left 2 WINDOW 3 50 41 Left 2 SYMATTR InstName R3 SYMATTR Value 200 SYMBOL res -16 -144 R0 WINDOW 0 -60 43 Left 2 WINDOW 3 -58 72 Left 2 SYMATTR InstName R4 SYMATTR Value 40 SYMBOL res -656 -144 R0 WINDOW 0 52 40 Left 2 WINDOW 3 53 70 Left 2 SYMATTR InstName R5 SYMATTR Value 40 SYMBOL voltage -464 368 R0 WINDOW 0 -86 66 Left 2 WINDOW 3 -288 112 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V2 SYMATTR Value PULSE(0.2 5 1n 25n 0 1) SYMBOL res -480 224 R0 WINDOW 0 -56 40 Left 2 WINDOW 3 -55 75 Left 2 SYMATTR InstName R2 SYMATTR Value 2k SYMBOL e -144 368 R0 WINDOW 0 68 35 Left 2 WINDOW 3 70 71 Left 2 SYMATTR InstName E1 SYMATTR Value -1 SYMBOL res -160 224 R0 WINDOW 0 -56 40 Left 2 WINDOW 3 -55 75 Left 2 SYMATTR InstName R6 SYMATTR Value 2k TEXT 16 384 Left 2 !.MODEL SAV551 NMF( vto=0.08, Beta=0.3,\n+ Lambda=0.07, Alpha=4 B=0.8, Pb=0.7,\n+ Cgs=0.997E-12, Cgd=0.176E-12, Rd=0.084,\n+ Rs=0.054, Kf=5e-11, Af=2) TEXT 208 288 Left 2 !.tran 0 25n 0 100f TEXT 192 168 Left 2 ;2 GHz PWM Oscillator TEXT 216 216 Left 2 ;JL Apr 30 2021

If you want a continuous signal but only have AC channels, consider a sine/cosine oscillator, or multiphase oscillator, where the sum of the squares of the signals is constant (good for gain control) but the frequency can mismatch against a local oscillator (giving you an FM decoded value, or phase-shift value if that's preferable).

Thermistor sensing of a sine and cosine pair of heaters is a fairly easy way to do gain control.

Filtering PWM can be made less onerous using Woodward's cancellation scheme.

SED last discussed this on 19 Feb 2021 under title "PWM Output With RC Coupled Inverted Signal"

I wonder if at 2GHz the inverter could be replaced with a 180deg length delay line? Could make very simple demodulator.

piglet

I can speak to how optical Ethernet ensures that their signals can be transformer-coupled, regardless of what data is sent.

The basic trick is that the code space of the line symbols is large enough that there are at minimum two line symbols that can be used to represent each possible data symbol (or control symbol). The line symbols are sent by OOK of the laser, it taking multiple on-off flashes to encode each line symbol. The transmitter keeps a running sum of the ones and zeros sent, and chooses which of the pair of line symbols to use depending on the running sum - if the sum is growing in one direction, the chosen symbols will drive the sum back towards zero average.

There is a founding article that I read twenty years ago on the design of such codes. If I recall, this was invented at IBM. While I may have a copy of that article, I'll probably never find it now.

Joe Gwinn

Thanks for the link. ISTR another analysis with actual waveforms.

However, doing this at 3 GHz might be a problem.

Optical ethernet is almost always SFP modules. Their i/o is capacitive coupled differential CML, no transformers.

That's 8b10b or longer versions. The idea is to keep the 1/0 balance exact long-term.

I want to send PWM, which doesn't, which is why it's interesting.

A typical FPGA serdes engine natively does 8b10b and longer balanced codes. We have used the serdes engines to make PWM and programmable pulses too. You have to get inside the serdes blocks and work near the end.

Exactly. Just LC lowpass filtering was what I had in mind.

Hand-wavy, seems totally reasonable.

It is one way of measuring the phase noise of amps.