In the Hilbert transformer-based phase shifting network implementation a la:
They measure the shift of the final y output as a relative phase with respect to the I output of the digital Hilbert transformer.
It looks like it's possible to also make an absolute phase shift with respect to the input signal to the transformer by adjusting the coefficients for the I/Q multiplier stage and reducing the calculated shift in proportion to the number of delay taps in the transformer, yes?
Sorry, I meant FIR in the title, not "IIR."
Yes, but the sampling theorem applies; it's only an absolute phase shift when the Hilbert coefficients are adequate for resolving the signal. With 2 coefficients, the filter could do an absolute TIME shift, which is different phase for all frequencies; going to 16 coefficients can get absolute phase shft over more range, with spurious outputs only outside the carrier-plus-modulation bandwidth that one ends up using.
If I'm reading the math correctly, you want to have an eight-cycle delay (latency) in the I path to match the FIR because H-transformer output has eight samples of nominally 'future' data on which it depends.
In fig 2, given an I and Q output from the phase-shift network, one can rotate the output any desired angle.
That's high-school trig. The problem is the "Hilbert", whose relative phase output is 90 degrees but the absolute phases squirm as a function of frequency.
An actual Hilbert transform box would output true 0 and 90 relative to the input at all frequencies. Unfortunately, a true Hilbert transform is non-causal hence impossible to make. An FIR approximation to the Hilbert transform adds time delay, which wrecks the phase shifts. It's like trying to simulate an ideal lowpass filter: the better the filter response, the longer the time delay.
There are lots of ways to make a network that shifts phase a programmable amount, but the programming has to change as a function of frequency. A variable delay line will do that too.
We recently finished up an all-analog dual IQ modulator box that our user programs by putting in I and Q as DC levels (actually waveforms) from one of our 4-channel ARBs. It only works at one frequency, so the "Hilbert" is just two RCs, one a 45 degree lead and one a 45 lag.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Ok, I think I have it. So I can have a relative phase shift between the I output and the Y output of e.g. 90 degrees over some bandwidth, but the y output will shift in absolute phase as a function of frequency wrt the input signal.
If I wanted otherwise I'd have to dynamically adjust the HT delay line coefficients.
As I understand it how good an approximation the constant relative phase shift is between the I and y outputs, and what bandwidth, depends on how many taps (and hence delay) you're willing to put into the transformer. Since the HT kernel is infinite you have to window it somehow which leads to ripple in the constant-phase pass band and the Gibbs phenomena at the edges, etc.
Fortunately Matlab/Octave provides design tools for that
The client would like a constant 90 degree phase shift over about an octave of the telephone voice band, 300-3kHz, and would prefer digital implementation. If they're OK with a delayed relative shift and not absolute sounds like it should be feasible, the DSP API they're using supports an enormous number of taps in its FIR building-block.
If they must have an absolute phase shift then it sounds like a tough row to hoe.
Right. The analog all-pass works the same way. One network has a sloping, slightly wiggly phase-frequency response. A second one is the same but offset a bit. The difference is close to 90 degrees over some frequency range. It's impressive: you can get about 1 degree max error over an 80:1 frequency with just 6 opamps. See the Williams book 3e, sec 7.5.
I think the two legs of the phase shifter can be designed to wiggle a bit, like designing a Chebychev filter.
11:1 frequency and 1.3 degree error takes four opamps! No ADCs, no DACs.
Yeah, causality sucks.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Hilbert transform filters (constant 90 degree phase shift) only work well on fairly narrow-band signals. The trouble is that there's an infinite singularity at DC, and the long high-frequency tail also has infinite energy.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Sometimes punting on the fancy stuff is a big win.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Yeah I tried to sell 'em on that route, not interested. it's an mod to some already extant ADC + DSP solution that also does EQ and dynamic range compression talking about analog daughterboards doesn't seem to win many hearts and minds, however straightforward they may be.
If they must have closer to being able to read the future than digital can provide in this case they'll go for the analog solution I expect they won't have a choice
So I'm still a little unclear on how the number of taps/coefficients in the FIR Hilbert transformer affects the performance vis a vis relative phase error in-band between the I and Y outputs, and absolute phase error between the Y output and the input signal.
Probably time to just fire up Matlab and experiment and look at the plots.
Tell me about it. Rev A had fancy allpass phase shifters made with screaming opamps, and everything oscillated. Rev B has half the parts and just works. Rs and Cs don't oscillate.
We shipped the first one, but we need a good sinewave source for production testing, discussed elsewhere.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
The analog solutions do not peek into the future, either.
-- -TV
I recall the thread we had about that.
I'm a fan of the Mini Circuits PAs for lab purposes. You're only talking about a couple of watts. Alternatively I've had good luck with RFbayinc.com, and they're a fair amount cheaper. Specifically this one:
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
That claims a low end of 20 MHz, but the graphs suggest it will work lower. We need about 14.5 MHz.
I may have justified building my own sine source, if I can have a version that's a high voltage pulse booster too.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
On a sunny day (Sun, 21 Jul 2019 11:33:29 -0700) it happened John Larkin wrote in :
If you had a RF ham license, and not much electronics design experience.. I have not used these guys but it is YAO bunch of QRP (low power RF) stuff
5W RF amp kit 20$and the signal generator 30$:
And it is in USD but I have no idea where they are located... And I have not tested these, but should be more than enough for 14 MHz 2W.
5W in 50 Ohm makes 15 Veff or 44.7 Vpp ?? Those signal generators I have seen on ebay too, and cheaper.ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.