Phase shifting a low noise sinewave

[Greysky]

Hi, any ideas you may have on the following appreciated.

I have a 5 MHz sine wave oscillator which was specially built to have a low
close-in phase noise (-115 dB/c @ 1 Hz offset). What I would like to do is
phase shift the output +- 90 degrees, without totally destroying the high
quality of the output waveform. Most of the circuits I've used introduce too
much noise. I am beginning to think I need to steer clear of any phase
shifters that have op amps in the design. Any ideas out there?

TIA

 

Have you looked at passive all-pass filters?

Single-side band transmitters use exactly this sort of filter to
suppress one or other of the modulated side-bands, and at 5MHz you can
make quite nice inductors as single-layer windings on nickel-zince
ferrites.

Horowitz and (Win) Hill's "The Art of Electronics" (ISBN 0-521-37095-7)
shows a wide-band phase-shifting RC network in section 5.16 on
Quadrature Oscillators.

-----------
Bill Sloman, Nijmegen

 

[Ken Smith]

In article <EllJd.5682$8Z1.469@newssvr14.news.prodigy.com>,
Greysky <greyskynospam@sbcglobal.net> wrote:

Hi, any ideas you may have on the following appreciated.

I have a 5 MHz sine wave oscillator which was specially built to have a low
close-in phase noise (-115 dB/c @ 1 Hz offset). What I would like to do is
phase shift the output +- 90 degrees, without totally destroying the high
quality of the output waveform. Most of the circuits I've used introduce too
much noise. I am beginning to think I need to steer clear of any phase
shifters that have op amps in the design. Any ideas out there?

A thought is to make a 20MHz squarewave oscillator instead and use
flip-flops to make the 5MHz frequencies as squarewaves. This requires
figuring out how to use squarewaves in place of your sinewaves.

Any form of two pole low pass filter will make a 90 degree phase lag.
Unfortunately it also magnifies the resistor noise. You may be better off
if you go for 2 shifted versions of the 5MHz. Shift one early by 45
degrees and the other late by 45 degrees. These would still be 2 pole
circuits but their noise peaks would be away from the carrier, so you may
be able to get the needed low noise performance.


You could put two copies of the oscillator in the circuit. One of them
will have to be modified to added a varactor diode. You can then make a
PLL that brings them into the 90 degree relationship. This assumes that
the two oscillators naturally run at nearly exactly the same frequency.
It may mean using a common even. The varactor is a way for noise to get
into the second oscillator. If the oscillators are near exact, the
varactor can be deleveraged enough.

Also, it may be posible to copy all but the feedback of the existing
oscillator and figure out how to sneek a signal from it to drive the
second one to run at 90 degrees.

--
--
kensmith@rahul.net forging knowledge

 

[Jim Thompson]

<Ian_Buckner_not_@agilent.com> wrote:

"Greysky" <greyskynospam@sbcglobal.net> wrote in message
news:EllJd.5682$8Z1.469@newssvr14.news.prodigy.com...
Hi, any ideas you may have on the following appreciated.

I have a 5 MHz sine wave oscillator which was specially built to have a
low
close-in phase noise (-115 dB/c @ 1 Hz offset). What I would like to do is
phase shift the output +- 90 degrees, without totally destroying the high
quality of the output waveform. Most of the circuits I've used introduce
too
much noise. I am beginning to think I need to steer clear of any phase
shifters that have op amps in the design. Any ideas out there?

TIA

Do a google search on "phase shifter" and "voltage control".
There are commercial ones out there that cover your frequency,
or you could roll your own with quadrature hybrids and varactors.

A lot of the descriptions/theory are for very high frequencies, but
this still applies down at 5MHz, you just need to use an appropriate
hybrid and varicaps.

Regards
Ian

Do a google on "all pass" and "phase shift". I have even built them
on-chip for, 1.6GHz and up, for making image-reject mixers.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
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