Can passive phase shifters be implemented without a variable delay element?

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It?s my impression that wideband phase shifters in radar systems are most  
often implemented as some kind of variable delay line. In one common case,  
the variable delay is implemented as a binary-length set of switchable delay  
lines. In another traditional case, the delay element is magnetized ferrite  
in a waveguide - the speed of propagation in the ferrite varying with  
magnetic field intensity. In yet another case, the delay is implemented as a  
lumped-element low pass filter with voltage-variable diode capacitors.

The question is if it is possible to implement a phase shifter that is not  
ultimately based on some kind of variable delay. Upon what physics principle  
would this be based?

Joe Gwinn


Re: Can passive phase shifters be implemented without a variable delay element?
On Sunday, January 14, 2018 at 2:26:41 PM UTC+11, Joseph Gwinn wrote:
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If you need a variable delay, you need a variable element. If you only want
 a fixed phase shift you don't.

Radar systems use the variable delay to scan the beam, so the need for vari
ability comes from the requirement that you want them to scan.

I can conceive a of "radar" system with a  fixed wide-angle illumination an
d a lot of fixed detectors, each of which was most sensitive to reflections
 coming in an a different lines of sight, and that wouldn't need variable d
elays, but you'd need a lot of fixed different delays.

--  
Bill Sloman, Sydney


Re: Can passive phase shifters be implemented without a variable delay element?

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All you say above is correct, but I?m asking a physics question, not a  
radar design question:

Is there any way to build a passive variable phase shifter (a physical  
device) that does not ultimately rest on some kind of variable delay device?

Joe Gwinn


Re: Can passive phase shifters be implemented without a variable delay element?

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Any finite pole-zero system cannot be truly said to have delay, but perhaps  
fortunate for your question, there are no truly real pole-zero systems; all  
real systems have delay sooner or later.

The phase shift of a real system may be exaggerated due to pole-zero-like  
behavior, indeed to the extent that we prefer to analyze them exclusively  
that way (RLC circuits), but it's important to keep this in mind at  
frequencies and scales where it matters.

So even though you might analyze a physical passive variable phase shifter  
as a pole-zero system, it will contain true delay (which will probably be  
variable).

Is that a satisfactory answer?

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: Can passive phase shifters be implemented without a variable delay element?
On 01/15/2018 01:08 AM, Tim Williams wrote:

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Not true of systems with zeros, because a zero can go anywhere, but  
poles can only go in one half plane for a stable system.  So if you have  
a function with a zero in the unstable half plane, when you invert it to  
get the inverse operator, you wind up with a pole in the unstable half  
plane, i.e. an attempt at a time machine.

Cheers

Phil Hobbs
--  
Dr Philip C D Hobbs
Principal Consultant
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Re: Can passive phase shifters be implemented without a variable delay element?
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Ah, true -- well, to be fair I didn't stipulate that the system had to be  
stable as well, but that's obviously relevant to the "real physical" (and  
add "practial" as well) case. :)

More reading:
https://en.wikipedia.org/wiki/All-pass_filter

Tim

--  
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Contract Design
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Re: Can passive phase shifters be implemented without a variable delay element?
On Saturday, January 13, 2018 at 7:26:41 PM UTC-8, Joseph Gwinn wrote:
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UM... why is that 'wideband'?   A fixed delay is a different phase shift
at each frequency.

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one can phase-shift based on the identity

sin(w*t + phase) = sin(w*t) cos (phase) + cos(w*t) sin(phase)

using mixer techniques (mixer :== analog multiplier) with a suitable
way to generate (like, a lookup table) the phase-dependent parts.

Re: Can passive phase shifters be implemented without a variable delay element?

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Because many radar waveforms are wideband, one gigahertz being common in  
X-band radars. Typically, linear-phase response is required to avoid undue  
waveform distortion, which causes range blurring.

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This approach is widely used, but it is active, not a passive physical  
device.

Joe Gwinn


Re: Can passive phase shifters be implemented without a variable delay element?
On Sunday, January 14, 2018 at 10:51:37 AM UTC-8, Joseph Gwinn wrote:
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If  you start with both sin(w*t) and cos(w*t), it's just two potentiometers
and a sum junction (nothing active there).   The adjustable components are two
coefficients, not 'delay elements'.

Re: Can passive phase shifters be implemented without a variable delay element?
On Monday, 15 January 2018 07:26:55 UTC, whit3rd  wrote:
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That's true as far as it goes. But I have a question. How do you get both sin(wt) and cos(wt) without one being a time delayed version of the other?


NT

Re: Can passive phase shifters be implemented without a variable delay element?
On Monday, January 15, 2018 at 5:26:36 AM UTC-8, snipped-for-privacy@gmail.com wrote:
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Are you asking about oscillators with more than one output?   You build 'em with
that in mind.   It doesn't use a time-delay element in general, an LC tank can be outfitted
with a voltage follower for sine, and a current transformer pickoff on the inductor
current gives cosine.

Whether sine and cosine ARE time-delayed equivalents, is a philosophical question.   Whether
they're generated by using a time-delay, is an engineering question.   Which do you intend?

Re: Can passive phase shifters be implemented without a variable delay element?
wrote:

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Or a "quadrature oscillator", which uses two integrators to generate
the sin and cos outputs as part of the oscillator feedback.
<https://www.sciencedirect.com/science/article/pii/S2090123212000124
If you're careful, you can get some pretty low distortion with this
sort of oscillator, too.

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Re: Can passive phase shifters be implemented without a variable delay element?

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One can certainly use the above methods, and they are widely used in radar,  
at least for sine waves. However, typical pulsed radar waveforms are far  
wider band than a sine wave. The various filter methods are wideband enough  
for many applications, but not high (range) precision radars.

I?ve also used a 90-degree hybrid, two attenuators, and a summing junction  
to implement a variable phase. The hybrid is based on filters. I?ll have to  
model this and see how the group delay varies with phase.

Joe Gwinn


Re: Can passive phase shifters be implemented without a variable delay element?
On Monday, 15 January 2018 19:30:40 UTC, whit3rd  wrote:
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meters
s are two
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th sin(wt) and cos(wt) without one being a time delayed version of the othe
r?
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em with

that certainly answers nothing.

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ted
e inductor

interesting, hadn't thought of that approach

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question.   Whether
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ich do you intend?

the latter.
AFAIK all oscillators are based on time delay one way or another, but your  
v versus i outputs are interesting. I was thinking more of a classic ring o
scillator with however many outputs at different phases - there each output
 is a time delayed version of the previous one. I suppose you could say tha
t output A is time delayed B, B is time delayed C and C is time delayed A!


NT

Re: Can passive phase shifters be implemented without a variable delay element?
On Tue, 16 Jan 2018 07:40:55 -0800 (PST), snipped-for-privacy@gmail.com wrote:

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Or, instead of a time delay, you add a phase delay.  ;-)

Re: Can passive phase shifters be implemented without a variable delay element?
On Tuesday, January 16, 2018 at 10:41:03 AM UTC-5, snipped-for-privacy@gmail.com wrote:
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e:
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iometers
nts are two
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both sin(wt) and cos(wt) without one being a time delayed version of the ot
her?
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 'em with
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itted
the inductor
l question.   Whether
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Which do you intend?
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r v versus i outputs are interesting. I was thinking more of a classic ring
 oscillator with however many outputs at different phases - there each outp
ut is a time delayed version of the previous one. I suppose you could say t
hat output A is time delayed B, B is time delayed C and C is time delayed A
!

Hmm OK I'm seeing things different.  If there's no time delay in an RC
then there's no delay in an L/R. and there are lots of LC oscillators.

And if there's no delay in an RC there should be no delay in a collection
of them... something like a phase sequence filter... an RC network.

However I think all these things have a transient response.  
So steady state you get nice stable oscillators or phase shift.

If you want a phase shift with no transient response, then do a time delay.
  
At least that my view,  
as always I welcome corrections.  

The first time I really bumped into this was trying to make acoustic
impedance matching waveplates... I'd have to look up the details,
but Brass to plastic with aluminum in between.  (or something like that.)
With a single wavelength of the 'right' frequency.. nothing.
(no improvement)  
But as I added more wavelengths, (I love digital sig. gens.)  
I could see more transmitted power and less reflected.  
(Except at the beginning and end.)    

Waveplates, work in the steady state.  
  
George H.  
I've enjoyed this thread.  
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Re: Can passive phase shifters be implemented without a variable delay element?

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Transient response. There is a key question, a question equivalent to the  
issue of distortion of wideband signals. Pulse radar waveforms are  
transients. This too should be modeled.

Joe Gwinn



Re: Can passive phase shifters be implemented without a variable delay element?
On Sunday, January 14, 2018 at 1:51:37 PM UTC-5, Joseph Gwinn wrote:
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ems are most
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e  
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X-Band is 10GHz territory, 1GHZ is L-Band. Bands do not refer to *bandwidth
*, those radars may be quite narrow band operating at a high frequency that
 is tightly controlled. You're not likely to find a "wideband" phase shifte
r at mm-wavelength frequencies.


Re: Can passive phase shifters be implemented without a variable delay element?
On Mon, 15 Jan 2018 09:28:09 -0800 (PST),
snipped-for-privacy@gmail.com wrote:

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The radar receiver characteristics must mach transmitter signal
characteristics for best range. In the simplest case that would be the
transmitter at a fixed frequency and a receiver at the same frequency
with a few NHz bandwidth to get somewhat accurate distance
measurements.

At least for military radars, you do not want to use a fixed
frequency, but move around both the transmitter as well as the
receiver to avoid jamming. If you can move the frequency across the
whole X-band helps a lot. For this reason the front end should be
wideband, while a single frequency is used at a specific time. If some
form of spread spectrum is used, the total spectrum can be quite wide,
while the despread receiver signal can be quite narrow.


Re: Can passive phase shifters be implemented without a variable delay element?

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It?s the waveforms that are wideband, to achieve very high slant range  
resolution. As a practical matter, it?s difficult to implement the radar if  
the bandwidth exceeds 1/10 to 1/8 of the carrier frequency (loosely defined  
by radar band).

Joe Gwinn


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