Using a nanoVNA as DMTD, simulation only
After reading about DMTD and how the VNWA is doing frequency measurements I
was curious if it would be possible to use a nanoVNA to create a DMTD by
only changing the SW.
The nanoVNA has two input channels (S11 and S21) and a reference channel.
By disabling the output of the reference LO in SW the S11 and S21 channels
become two independent inputs. One via the reflection bridge (S11) into a
mixer and one directly into another mixer. Both mixers also have the
offset_LO as input which should be tuned so both mixers output close to the
IF frequency.
The output of the mixers is converted using a 16bit stereo ADC running up
to 96kHz. The 16 bit samples streams are converted to phase and amplitude
by doing a SW I/Q downmix to DC.
The number of samples to combine into one phase/amplitude measurement is
defined in the SW.
As I did not want to put a lot of effort into creating embedded SW I
created a one input channel simulation in Octave of the processing after AD
conversion.
The simulation uses a 1kHz input signal with added noise and a 48kHz sample
rate and combines 1k samples into one angle measurement. All sample data,
I/Q data, cosine and sine tables are rounded to 16bits as used in the
nanoVNA. The 48 angle measurements per second limit the frequency
difference between the input signals and the tuned frequency because if the
frequency difference is too high the unwrapping of the angle will fail.
After unwrapping the 48 angle measurements per second a linear regression
uses the angle measurements to calculate the angular speed per second,
dividing this speed by 2*pi gives the frequency deviation of the input
signal from the reference signal.
It would also be possible to output the 48 angle measurements per second
(or any subsampled number) as raw phase difference measurements and do the
rest of the processing in something like Timelab
Using 48kHz sample rate and 16 bit accuracy of the data and an added noise
level of 1e-5 (is this -100dBc/Hz (?)) the minimum observable delta
frequency in the simulation is about 1e-6Hz. Any lower delta frequency
falls below the 16 bit numerical resolution. A higher noise level, such as
1e-4, hides the 1e-6Hz difference.
To make a complete DMTD one would have to do this angular measurement for
both channels and subtract the measured angle.
It is assumed the internal reference cancels out in a dual channel setup
comparing the two inputs so the simulation assumes a perfect internal
reference.
Some questions.
1: The measurement of the angle (phase) is actually a combination of 1k
samples over a 1/48 second period. Is this a valid way to measure the phase
of an input signal? A frequency offset will cause phase rotation over the
measurement period. Is this causing systematic errors?
2: With a 10MHz input signal and a minimum observable frequency difference
of 1e-6Hz over a one second period the frequency resolution with a "gate
time" of one second seems to be in the order 1e-13. Could this be correct?
Is the noise level realistic? Would this translate into a phase resolution
of below 1 ps or am I making a big mistake?
Erik.
Hi
The “typical” gotcha doing this is channel to channel isolation.
Folks have tried it with various devices and that seems to be
the first barrier they run into. There may be others further down
the road …..
Often tossed up isolation numbers from various sources get into
the > 120 db range for signals that are very close to the same
frequency. If they are not close, then you start talking about how
close this or that harmonic is.
Simple test is the same one you now are very familiar with. Step
one input across the other and see what happens ….
Bob
On Sep 19, 2022, at 10:47 AM, Erik Kaashoek via time-nuts time-nuts@lists.febo.com wrote:
After reading about DMTD and how the VNWA is doing frequency measurements I
was curious if it would be possible to use a nanoVNA to create a DMTD by
only changing the SW.
The nanoVNA has two input channels (S11 and S21) and a reference channel.
By disabling the output of the reference LO in SW the S11 and S21 channels
become two independent inputs. One via the reflection bridge (S11) into a
mixer and one directly into another mixer. Both mixers also have the
offset_LO as input which should be tuned so both mixers output close to the
IF frequency.
The output of the mixers is converted using a 16bit stereo ADC running up
to 96kHz. The 16 bit samples streams are converted to phase and amplitude
by doing a SW I/Q downmix to DC.
The number of samples to combine into one phase/amplitude measurement is
defined in the SW.
As I did not want to put a lot of effort into creating embedded SW I
created a one input channel simulation in Octave of the processing after AD
conversion.
The simulation uses a 1kHz input signal with added noise and a 48kHz sample
rate and combines 1k samples into one angle measurement. All sample data,
I/Q data, cosine and sine tables are rounded to 16bits as used in the
nanoVNA. The 48 angle measurements per second limit the frequency
difference between the input signals and the tuned frequency because if the
frequency difference is too high the unwrapping of the angle will fail.
After unwrapping the 48 angle measurements per second a linear regression
uses the angle measurements to calculate the angular speed per second,
dividing this speed by 2*pi gives the frequency deviation of the input
signal from the reference signal.
It would also be possible to output the 48 angle measurements per second
(or any subsampled number) as raw phase difference measurements and do the
rest of the processing in something like Timelab
Using 48kHz sample rate and 16 bit accuracy of the data and an added noise
level of 1e-5 (is this -100dBc/Hz (?)) the minimum observable delta
frequency in the simulation is about 1e-6Hz. Any lower delta frequency
falls below the 16 bit numerical resolution. A higher noise level, such as
1e-4, hides the 1e-6Hz difference.
To make a complete DMTD one would have to do this angular measurement for
both channels and subtract the measured angle.
It is assumed the internal reference cancels out in a dual channel setup
comparing the two inputs so the simulation assumes a perfect internal
reference.
Some questions.
1: The measurement of the angle (phase) is actually a combination of 1k
samples over a 1/48 second period. Is this a valid way to measure the phase
of an input signal? A frequency offset will cause phase rotation over the
measurement period. Is this causing systematic errors?
2: With a 10MHz input signal and a minimum observable frequency difference
of 1e-6Hz over a one second period the frequency resolution with a "gate
time" of one second seems to be in the order 1e-13. Could this be correct?
Is the noise level realistic? Would this translate into a phase resolution
of below 1 ps or am I making a big mistake?
Erik.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Bob,
Thanks for the hint.
After adding overlapping ADEV calculation and extending the simulation to a
100 seconds measurement period I did some simulations using -80 dB to -120
dB leakage of another signal at 10, 1, 0.1, and 0.01Hz difference and
varying the noise level up to -80 dBc/Hz.
Worst case is a delta frequency of the two inputs of 1 Hz and noise and
leakage at -80 dB but even under these conditions the ADEV at tau of 1
second stays below 1e-12.
Given the above leakage and noise conditions the minimum reliable
observable frequency difference is 1e-5 Hz which is very promising.
The nanoVNA (or its better cousin the LiteVNA) do have at least 80dB
isolation between the inputs so I'm tempted to implement this on the actual
HW for validation.
Given the HW, without modifications, it can only work for (almost) equal
frequencies but this should be sufficient for many relevan use cases.
One area of concern are the close-in spurs of the SI5351 when used at small
offsets from 10MHz. Too difficult to simulate.
Erik.
Op ma 19 sep. 2022 om 22:02 schreef Bob kb8tq kb8tq@n1k.org:
Hi
The “typical” gotcha doing this is channel to channel isolation.
Folks have tried it with various devices and that seems to be
the first barrier they run into. There may be others further down
the road …..
Often tossed up isolation numbers from various sources get into
the > 120 db range for signals that are very close to the same
frequency. If they are not close, then you start talking about how
close this or that harmonic is.
Simple test is the same one you now are very familiar with. Step
one input across the other and see what happens ….
Bob
On Sep 19, 2022, at 10:47 AM, Erik Kaashoek via time-nuts <
time-nuts@lists.febo.com> wrote:
After reading about DMTD and how the VNWA is doing frequency
measurements I
was curious if it would be possible to use a nanoVNA to create a DMTD by
only changing the SW.
The nanoVNA has two input channels (S11 and S21) and a reference channel.
By disabling the output of the reference LO in SW the S11 and S21
channels
become two independent inputs. One via the reflection bridge (S11) into a
mixer and one directly into another mixer. Both mixers also have the
offset_LO as input which should be tuned so both mixers output close to
the
IF frequency.
The output of the mixers is converted using a 16bit stereo ADC running up
to 96kHz. The 16 bit samples streams are converted to phase and amplitude
by doing a SW I/Q downmix to DC.
The number of samples to combine into one phase/amplitude measurement is
defined in the SW.
As I did not want to put a lot of effort into creating embedded SW I
created a one input channel simulation in Octave of the processing after
AD
conversion.
The simulation uses a 1kHz input signal with added noise and a 48kHz
sample
rate and combines 1k samples into one angle measurement. All sample data,
I/Q data, cosine and sine tables are rounded to 16bits as used in the
nanoVNA. The 48 angle measurements per second limit the frequency
difference between the input signals and the tuned frequency because if
the
frequency difference is too high the unwrapping of the angle will fail.
After unwrapping the 48 angle measurements per second a linear regression
uses the angle measurements to calculate the angular speed per second,
dividing this speed by 2*pi gives the frequency deviation of the input
signal from the reference signal.
It would also be possible to output the 48 angle measurements per second
(or any subsampled number) as raw phase difference measurements and do
the
rest of the processing in something like Timelab
Using 48kHz sample rate and 16 bit accuracy of the data and an added
noise
level of 1e-5 (is this -100dBc/Hz (?)) the minimum observable delta
frequency in the simulation is about 1e-6Hz. Any lower delta frequency
falls below the 16 bit numerical resolution. A higher noise level, such
as
1e-4, hides the 1e-6Hz difference.
To make a complete DMTD one would have to do this angular measurement for
both channels and subtract the measured angle.
It is assumed the internal reference cancels out in a dual channel setup
comparing the two inputs so the simulation assumes a perfect internal
reference.
Some questions.
1: The measurement of the angle (phase) is actually a combination of 1k
samples over a 1/48 second period. Is this a valid way to measure the
phase
of an input signal? A frequency offset will cause phase rotation over the
measurement period. Is this causing systematic errors?
2: With a 10MHz input signal and a minimum observable frequency
difference
of 1e-6Hz over a one second period the frequency resolution with a "gate
time" of one second seems to be in the order 1e-13. Could this be
correct?
Is the noise level realistic? Would this translate into a phase
resolution
of below 1 ps or am I making a big mistake?
Erik.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Hi
With signals in the < 0.1 ppb offset range, you should see effects at the
-80 db isolation level. They should show up as ripples in what otherwise
should be a straight line ( ADEV drops vs tau in a straight line ….. ).
Bob
On Sep 20, 2022, at 11:12 AM, Erik Kaashoek erik@kaashoek.com wrote:
Bob,
Thanks for the hint.
After adding overlapping ADEV calculation and extending the simulation to a 100 seconds measurement period I did some simulations using -80 dB to -120 dB leakage of another signal at 10, 1, 0.1, and 0.01Hz difference and varying the noise level up to -80 dBc/Hz.
Worst case is a delta frequency of the two inputs of 1 Hz and noise and leakage at -80 dB but even under these conditions the ADEV at tau of 1 second stays below 1e-12.
Given the above leakage and noise conditions the minimum reliable observable frequency difference is 1e-5 Hz which is very promising.
The nanoVNA (or its better cousin the LiteVNA) do have at least 80dB isolation between the inputs so I'm tempted to implement this on the actual HW for validation.
Given the HW, without modifications, it can only work for (almost) equal frequencies but this should be sufficient for many relevan use cases.
One area of concern are the close-in spurs of the SI5351 when used at small offsets from 10MHz. Too difficult to simulate.
Erik.
Op ma 19 sep. 2022 om 22:02 schreef Bob kb8tq <kb8tq@n1k.org mailto:kb8tq@n1k.org>:
Hi
The “typical” gotcha doing this is channel to channel isolation.
Folks have tried it with various devices and that seems to be
the first barrier they run into. There may be others further down
the road …..
Often tossed up isolation numbers from various sources get into
the > 120 db range for signals that are very close to the same
frequency. If they are not close, then you start talking about how
close this or that harmonic is.
Simple test is the same one you now are very familiar with. Step
one input across the other and see what happens ….
Bob
On Sep 19, 2022, at 10:47 AM, Erik Kaashoek via time-nuts <time-nuts@lists.febo.com mailto:time-nuts@lists.febo.com> wrote:
After reading about DMTD and how the VNWA is doing frequency measurements I
was curious if it would be possible to use a nanoVNA to create a DMTD by
only changing the SW.
The nanoVNA has two input channels (S11 and S21) and a reference channel.
By disabling the output of the reference LO in SW the S11 and S21 channels
become two independent inputs. One via the reflection bridge (S11) into a
mixer and one directly into another mixer. Both mixers also have the
offset_LO as input which should be tuned so both mixers output close to the
IF frequency.
The output of the mixers is converted using a 16bit stereo ADC running up
to 96kHz. The 16 bit samples streams are converted to phase and amplitude
by doing a SW I/Q downmix to DC.
The number of samples to combine into one phase/amplitude measurement is
defined in the SW.
As I did not want to put a lot of effort into creating embedded SW I
created a one input channel simulation in Octave of the processing after AD
conversion.
The simulation uses a 1kHz input signal with added noise and a 48kHz sample
rate and combines 1k samples into one angle measurement. All sample data,
I/Q data, cosine and sine tables are rounded to 16bits as used in the
nanoVNA. The 48 angle measurements per second limit the frequency
difference between the input signals and the tuned frequency because if the
frequency difference is too high the unwrapping of the angle will fail.
After unwrapping the 48 angle measurements per second a linear regression
uses the angle measurements to calculate the angular speed per second,
dividing this speed by 2*pi gives the frequency deviation of the input
signal from the reference signal.
It would also be possible to output the 48 angle measurements per second
(or any subsampled number) as raw phase difference measurements and do the
rest of the processing in something like Timelab
Using 48kHz sample rate and 16 bit accuracy of the data and an added noise
level of 1e-5 (is this -100dBc/Hz (?)) the minimum observable delta
frequency in the simulation is about 1e-6Hz. Any lower delta frequency
falls below the 16 bit numerical resolution. A higher noise level, such as
1e-4, hides the 1e-6Hz difference.
To make a complete DMTD one would have to do this angular measurement for
both channels and subtract the measured angle.
It is assumed the internal reference cancels out in a dual channel setup
comparing the two inputs so the simulation assumes a perfect internal
reference.
Some questions.
1: The measurement of the angle (phase) is actually a combination of 1k
samples over a 1/48 second period. Is this a valid way to measure the phase
of an input signal? A frequency offset will cause phase rotation over the
measurement period. Is this causing systematic errors?
2: With a 10MHz input signal and a minimum observable frequency difference
of 1e-6Hz over a one second period the frequency resolution with a "gate
time" of one second seems to be in the order 1e-13. Could this be correct?
Is the noise level realistic? Would this translate into a phase resolution
of below 1 ps or am I making a big mistake?
Erik.
time-nuts mailing list -- time-nuts@lists.febo.com mailto:time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com mailto:time-nuts-leave@lists.febo.com
Hi Bob,
Indeed, see attached image with -80dB leakage of second signal at 0.1 Hz
offset.
The weird spikes are caused by 16 bit resolution of most of the calculations
[image: image.png]
Erik
Op wo 21 sep. 2022 om 00:20 schreef Bob kb8tq kb8tq@n1k.org:
Hi
With signals in the < 0.1 ppb offset range, you should see effects at the
-80 db isolation level. They should show up as ripples in what otherwise
should be a straight line ( ADEV drops vs tau in a straight line ….. ).
Bob
On Sep 20, 2022, at 11:12 AM, Erik Kaashoek erik@kaashoek.com wrote:
Bob,
Thanks for the hint.
After adding overlapping ADEV calculation and extending the simulation to
a 100 seconds measurement period I did some simulations using -80 dB to
-120 dB leakage of another signal at 10, 1, 0.1, and 0.01Hz difference and
varying the noise level up to -80 dBc/Hz.
Worst case is a delta frequency of the two inputs of 1 Hz and noise and
leakage at -80 dB but even under these conditions the ADEV at tau of 1
second stays below 1e-12.
Given the above leakage and noise conditions the minimum reliable
observable frequency difference is 1e-5 Hz which is very promising.
The nanoVNA (or its better cousin the LiteVNA) do have at least 80dB
isolation between the inputs so I'm tempted to implement this on the actual
HW for validation.
Given the HW, without modifications, it can only work for (almost) equal
frequencies but this should be sufficient for many relevan use cases.
One area of concern are the close-in spurs of the SI5351 when used at
small offsets from 10MHz. Too difficult to simulate.
Erik.
Op ma 19 sep. 2022 om 22:02 schreef Bob kb8tq kb8tq@n1k.org:
Hi
The “typical” gotcha doing this is channel to channel isolation.
Folks have tried it with various devices and that seems to be
the first barrier they run into. There may be others further down
the road …..
Often tossed up isolation numbers from various sources get into
the > 120 db range for signals that are very close to the same
frequency. If they are not close, then you start talking about how
close this or that harmonic is.
Simple test is the same one you now are very familiar with. Step
one input across the other and see what happens ….
Bob
On Sep 19, 2022, at 10:47 AM, Erik Kaashoek via time-nuts <
time-nuts@lists.febo.com> wrote:
After reading about DMTD and how the VNWA is doing frequency
measurements I
was curious if it would be possible to use a nanoVNA to create a DMTD by
only changing the SW.
The nanoVNA has two input channels (S11 and S21) and a reference
channel.
By disabling the output of the reference LO in SW the S11 and S21
channels
become two independent inputs. One via the reflection bridge (S11) into
a
mixer and one directly into another mixer. Both mixers also have the
offset_LO as input which should be tuned so both mixers output close to
the
IF frequency.
The output of the mixers is converted using a 16bit stereo ADC running
up
to 96kHz. The 16 bit samples streams are converted to phase and
amplitude
by doing a SW I/Q downmix to DC.
The number of samples to combine into one phase/amplitude measurement is
defined in the SW.
As I did not want to put a lot of effort into creating embedded SW I
created a one input channel simulation in Octave of the processing
after AD
conversion.
The simulation uses a 1kHz input signal with added noise and a 48kHz
sample
rate and combines 1k samples into one angle measurement. All sample
data,
I/Q data, cosine and sine tables are rounded to 16bits as used in the
nanoVNA. The 48 angle measurements per second limit the frequency
difference between the input signals and the tuned frequency because if
the
frequency difference is too high the unwrapping of the angle will fail.
After unwrapping the 48 angle measurements per second a linear
regression
uses the angle measurements to calculate the angular speed per second,
dividing this speed by 2*pi gives the frequency deviation of the input
signal from the reference signal.
It would also be possible to output the 48 angle measurements per second
(or any subsampled number) as raw phase difference measurements and do
the
rest of the processing in something like Timelab
Using 48kHz sample rate and 16 bit accuracy of the data and an added
noise
level of 1e-5 (is this -100dBc/Hz (?)) the minimum observable delta
frequency in the simulation is about 1e-6Hz. Any lower delta frequency
falls below the 16 bit numerical resolution. A higher noise level, such
as
1e-4, hides the 1e-6Hz difference.
To make a complete DMTD one would have to do this angular measurement
for
both channels and subtract the measured angle.
It is assumed the internal reference cancels out in a dual channel setup
comparing the two inputs so the simulation assumes a perfect internal
reference.
Some questions.
1: The measurement of the angle (phase) is actually a combination of 1k
samples over a 1/48 second period. Is this a valid way to measure the
phase
of an input signal? A frequency offset will cause phase rotation over
the
measurement period. Is this causing systematic errors?
2: With a 10MHz input signal and a minimum observable frequency
difference
of 1e-6Hz over a one second period the frequency resolution with a "gate
time" of one second seems to be in the order 1e-13. Could this be
correct?
Is the noise level realistic? Would this translate into a phase
resolution
of below 1 ps or am I making a big mistake?
Erik.
time-nuts mailing list -- time-nuts@lists.febo.com
To unsubscribe send an email to time-nuts-leave@lists.febo.com
Bob,
I extended the simulation to fully include both channels.
Using some careful math it was possible to use two different IF frequencies
for the downconversion to DC. This implies that any leakage after the two
mixers can be reduced by using different IF frequencies because he phase
calculations for each measurement sample acts as a one bin FFT effectively
filtering out the leakage from the other mixer output
The errors due to the 16 bit quantization and mismatch between IF frequency
and downconverted input signals can mostly be removed by choosing IF
frequencies that are very close to the frequencies of the downconverted
input signals, which, due to the new calculation method, is now possible.
It's like creating a PLL where you tune the IF for best accuracy, e.g.
identical to the input frequency after downmixing
The only leakage that is still relevant is the leakage from one channel
into the other channel before the mixers and, probably, leakage between the
two LO frequencies (to be investigated)
I've also tested the impact of having harmonics in the mixer output and as
these are all blocked by the one bucket FFT there is no visible impact.
It's weird that a frequency measuring device becomes more like a dual
heterodyne receiver with the final IF at zero Hz acting like an
analog/digital PLL where you can measure the phase errors very accurately
ADEV with 0.1 Hz input frequency difference after the mizers, -80dB leakage
and with equal IF frequencies
[image: image.png]
Same with different IF frequencies
[image: image.png]
Whether this method will enable to measure ADEV in any useful way (e.g. are
the ADEV pictures above a good representation of the actual performance)
is still to be investigated.
Erik.
Op wo 21 sep. 2022 om 00:20 schreef Bob kb8tq kb8tq@n1k.org:
Hi
With signals in the < 0.1 ppb offset range, you should see effects at the
-80 db isolation level. They should show up as ripples in what otherwise
should be a straight line ( ADEV drops vs tau in a straight line ….. ).
Bob
Hi
With different IF frequencies ( at least if I understand things right ….), I
think you risk the frequency difference between the two IF’s getting into
the result. Depending on how high the IF’s are that may be a big deal or
maybe not so much of a deal …..
Bob
On Sep 23, 2022, at 12:11 PM, Erik Kaashoek erik@kaashoek.com wrote:
Bob,
I extended the simulation to fully include both channels.
Using some careful math it was possible to use two different IF frequencies for the downconversion to DC. This implies that any leakage after the two mixers can be reduced by using different IF frequencies because he phase calculations for each measurement sample acts as a one bin FFT effectively filtering out the leakage from the other mixer output
The errors due to the 16 bit quantization and mismatch between IF frequency and downconverted input signals can mostly be removed by choosing IF frequencies that are very close to the frequencies of the downconverted input signals, which, due to the new calculation method, is now possible. It's like creating a PLL where you tune the IF for best accuracy, e.g. identical to the input frequency after downmixing
The only leakage that is still relevant is the leakage from one channel into the other channel before the mixers and, probably, leakage between the two LO frequencies (to be investigated)
I've also tested the impact of having harmonics in the mixer output and as these are all blocked by the one bucket FFT there is no visible impact.
It's weird that a frequency measuring device becomes more like a dual heterodyne receiver with the final IF at zero Hz acting like an analog/digital PLL where you can measure the phase errors very accurately
ADEV with 0.1 Hz input frequency difference after the mizers, -80dB leakage and with equal IF frequencies
<image.png>
Same with different IF frequencies
<image.png>
Whether this method will enable to measure ADEV in any useful way (e.g. are the ADEV pictures above a good representation of the actual performance) is still to be investigated.
Erik.
Op wo 21 sep. 2022 om 00:20 schreef Bob kb8tq <kb8tq@n1k.org mailto:kb8tq@n1k.org>:
Hi
With signals in the < 0.1 ppb offset range, you should see effects at the
-80 db isolation level. They should show up as ripples in what otherwise
should be a straight line ( ADEV drops vs tau in a straight line ….. ).
Bob
Bob,
The IF frequencies for downmixing to DC are inside the dsp part so they are
guaranteed to be correct as demonstrated in the simulation.
The risk is in generating the two LO' s for the two mixers as these need to
be coherent otherwise the noise from the LO's won't cancel out.
Thanks for pointing this out
At least I have the math now correct to have some freedom in choosing an IF
frequency.
Erik
On Fri, Sep 23, 2022, 20:34 Bob kb8tq kb8tq@n1k.org wrote:
Hi
With different IF frequencies ( at least if I understand things right ….),
I
think you risk the frequency difference between the two IF’s getting into
the result. Depending on how high the IF’s are that may be a big deal or
maybe not so much of a deal …..
Bob
On Sep 23, 2022, at 12:11 PM, Erik Kaashoek erik@kaashoek.com wrote:
Bob,
I extended the simulation to fully include both channels.
Using some careful math it was possible to use two different IF
frequencies for the downconversion to DC. This implies that any leakage
after the two mixers can be reduced by using different IF frequencies
because he phase calculations for each measurement sample acts as a one bin
FFT effectively filtering out the leakage from the other mixer output
The errors due to the 16 bit quantization and mismatch between IF
frequency and downconverted input signals can mostly be removed by
choosing IF frequencies that are very close to the frequencies of the
downconverted input signals, which, due to the new calculation method, is
now possible. It's like creating a PLL where you tune the IF for best
accuracy, e.g. identical to the input frequency after downmixing
The only leakage that is still relevant is the leakage from one channel
into the other channel before the mixers and, probably, leakage between the
two LO frequencies (to be investigated)
I've also tested the impact of having harmonics in the mixer output and as
these are all blocked by the one bucket FFT there is no visible impact.
It's weird that a frequency measuring device becomes more like a dual
heterodyne receiver with the final IF at zero Hz acting like an
analog/digital PLL where you can measure the phase errors very accurately
ADEV with 0.1 Hz input frequency difference after the mizers, -80dB
leakage and with equal IF frequencies
<image.png>
Same with different IF frequencies
<image.png>
Whether this method will enable to measure ADEV in any useful way (e.g.
are the ADEV pictures above a good representation of the actual
performance) is still to be investigated.
Erik.
Op wo 21 sep. 2022 om 00:20 schreef Bob kb8tq kb8tq@n1k.org:
Hi
With signals in the < 0.1 ppb offset range, you should see effects at the
-80 db isolation level. They should show up as ripples in what otherwise
should be a straight line ( ADEV drops vs tau in a straight line ….. ).
Bob