Reply by Jan Panteltje ●July 21, 20192019-07-21
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.
Reply by John Larkin ●July 21, 20192019-07-21
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
Reply by Phil Hobbs ●July 21, 20192019-07-21
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
Reply by Tauno Voipio ●July 21, 20192019-07-21
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
Reply by John Larkin ●July 21, 20192019-07-21
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
Reply by bitrex ●July 20, 20192019-07-20
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&nbsp; 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.&nbsp; 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.
Reply by bitrex ●July 20, 20192019-07-20
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
Reply by Phil Hobbs ●July 20, 20192019-07-20
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
Reply by Phil Hobbs ●July 20, 20192019-07-20
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
Reply by John Larkin ●July 20, 20192019-07-20
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