IIR constant phase shift network, absolute vs. relative phas

[bitrex]

In the Hilbert transformer-based phase shifting network implementation
a la:

<https://www.dsprelated.com/showarticle/1147.php>

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?

 

[bitrex]

On 7/19/19 8:08 PM, bitrex wrote:

In the Hilbert transformer-based phase shifting network implementation
a la:

Sorry, I meant FIR in the title, not "IIR."

 

[John Larkin]

On Fri, 19 Jul 2019 20:08:16 -0400, bitrex <user@example.net> wrote:

In the Hilbert transformer-based phase shifting network implementation
a la:

https://www.dsprelated.com/showarticle/1147.php

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?

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

 

[bitrex]

On 7/20/19 11:56 AM, John Larkin wrote:

On Fri, 19 Jul 2019 20:08:16 -0400, bitrex <user@example.net> wrote:

In the Hilbert transformer-based phase shifting network implementation
a la:

https://www.dsprelated.com/showarticle/1147.php

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?

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.

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.

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.

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

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.

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.

 

[John Larkin]

On Sat, 20 Jul 2019 12:46:41 -0400, bitrex <user@example.net> wrote:

On 7/20/19 11:56 AM, John Larkin wrote:
On Fri, 19 Jul 2019 20:08:16 -0400, bitrex <user@example.net> wrote:

In the Hilbert transformer-based phase shifting network implementation
a la:

https://www.dsprelated.com/showarticle/1147.php

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?

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.

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.

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.

If I wanted otherwise I'd have to dynamically adjust the HT delay line
coefficients.

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.
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.

I think the two legs of the phase shifter can be designed to wiggle a
bit, like designing a Chebychev filter.

Fortunately Matlab/Octave provides design tools for that

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.

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.

11:1 frequency and 1.3 degree error takes four opamps! No ADCs, no
DACs.

If they must have an absolute phase shift then it sounds like a tough
row to hoe.

Yeah, causality sucks.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Phil Hobbs]

On 7/20/19 11:56 AM, John Larkin wrote:

On Fri, 19 Jul 2019 20:08:16 -0400, bitrex <user@example.net> wrote:

In the Hilbert transformer-based phase shifting network implementation
a la:

https://www.dsprelated.com/showarticle/1147.php

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?

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.

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

 

[Phil Hobbs]

On 7/20/19 1:53 AM, whit3rd wrote:

On Friday, July 19, 2019 at 5:08:21 PM UTC-7, bitrex wrote:
In the Hilbert transformer-based phase shifting network implementation
a la:

https://www.dsprelated.com/showarticle/1147.php

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...

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.

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

 

[bitrex]

On 7/20/19 5:34 PM, John Larkin wrote:

On Sat, 20 Jul 2019 12:46:41 -0400, bitrex <user@example.net> wrote:

On 7/20/19 11:56 AM, John Larkin wrote:
On Fri, 19 Jul 2019 20:08:16 -0400, bitrex <user@example.net> wrote:

In the Hilbert transformer-based phase shifting network implementation
a la:

https://www.dsprelated.com/showarticle/1147.php

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?

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.

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.

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.










If I wanted otherwise I'd have to dynamically adjust the HT delay line
coefficients.

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.
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.

I think the two legs of the phase shifter can be designed to wiggle a
bit, like designing a Chebychev filter.





Fortunately Matlab/Octave provides design tools for that

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.

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.


11:1 frequency and 1.3 degree error takes four opamps! No ADCs, no
DACs.

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 an absolute phase shift then it sounds like a tough
row to hoe.

Yeah, causality sucks.

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

 

[bitrex]

On 7/20/19 6:45 PM, Phil Hobbs wrote:

On 7/20/19 1:53 AM, whit3rd wrote:
On Friday, July 19, 2019 at 5:08:21 PM UTC-7, bitrex wrote:
In the Hilbert transformer-based phase shifting network implementation
a la:

https://www.dsprelated.com/showarticle/1147.php

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...

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.


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

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.

 

[John Larkin]

On Sat, 20 Jul 2019 18:46:43 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 7/20/19 11:56 AM, John Larkin wrote:
On Fri, 19 Jul 2019 20:08:16 -0400, bitrex <user@example.net> wrote:

In the Hilbert transformer-based phase shifting network implementation
a la:

https://www.dsprelated.com/showarticle/1147.php

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?

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.



Sometimes punting on the fancy stuff is a big win.

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

 

[Tauno Voipio]

On 21.7.19 03:02, bitrex wrote:

On 7/20/19 5:34 PM, John Larkin wrote:
On Sat, 20 Jul 2019 12:46:41 -0400, bitrex <user@example.net> wrote:

On 7/20/19 11:56 AM, John Larkin wrote:
On Fri, 19 Jul 2019 20:08:16 -0400, bitrex <user@example.net> wrote:

In the Hilbert transformer-based phase shifting network implementation
a la:

https://www.dsprelated.com/showarticle/1147.php

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?

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.

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.

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.










If I wanted otherwise I'd have to dynamically adjust the HT delay line
coefficients.

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.
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.

I think the two legs of the phase shifter can be designed to wiggle a
bit, like designing a Chebychev filter.





Fortunately Matlab/Octave provides design tools for that

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.

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.


11:1 frequency and 1.3 degree error takes four opamps! No ADCs, no
DACs.

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 an absolute phase shift then it sounds like a tough
row to hoe.

Yeah, causality sucks.



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

The analog solutions do not peek into the future, either.

--

-TV

 

[Phil Hobbs]

On 7/21/19 12:07 AM, John Larkin wrote:

On Sat, 20 Jul 2019 18:46:43 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 7/20/19 11:56 AM, John Larkin wrote:
On Fri, 19 Jul 2019 20:08:16 -0400, bitrex <user@example.net> wrote:

In the Hilbert transformer-based phase shifting network implementation
a la:

https://www.dsprelated.com/showarticle/1147.php

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?

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.



Sometimes punting on the fancy stuff is a big win.

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.

I recall the thread we had about that.

We shipped the first one, but we need a good sinewave source for
production testing, discussed elsewhere.

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:
<http://rfbayinc.com/products_pdf/product_1_186.pdf>

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

 

[John Larkin]

On Sun, 21 Jul 2019 12:52:27 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 7/21/19 12:07 AM, John Larkin wrote:
On Sat, 20 Jul 2019 18:46:43 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 7/20/19 11:56 AM, John Larkin wrote:
On Fri, 19 Jul 2019 20:08:16 -0400, bitrex <user@example.net> wrote:

In the Hilbert transformer-based phase shifting network implementation
a la:

https://www.dsprelated.com/showarticle/1147.php

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?

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.



Sometimes punting on the fancy stuff is a big win.

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.

I recall the thread we had about that.


We shipped the first one, but we need a good sinewave source for
production testing, discussed elsewhere.

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:
http://rfbayinc.com/products_pdf/product_1_186.pdf

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

 

[Jan Panteltje]

On a sunny day (Sun, 21 Jul 2019 11:33:29 -0700) it happened John Larkin
<jjlarkin@highlandtechnology.com> wrote in
<3sb9jel0a92esas8rva2vohh0g6ohnifkd@4ax.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.

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$
http://shop.qrp-labs.com/pa

and the signal generator 30$:
http://shop.qrp-labs.com/vfo
and here:
http://qrp-labs.com/vfo

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.