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Old 24th December 2013, 10:28 PM   #3451
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
Since it can lock and hold to 5+ decimal points it must servo. Open loop would not get there. It has an optical feedback to the microprocessor for the frequency trim and a 12 bit DAC to tune with. I could look at the tuning voltage the next time I have one open to see what it does.

Does a DAC into an analog multiplier count as an "MDAC"?
Probably not by definition. All the Mdacs I've seen are R2R dac that allow an analog signal to there reference pin. The DAC is analogous to the Vc of an analog multiplier. If used this way.

A 12 bit dac is only 4096. This must just be used for fine frequency tuning in the Boonton.
Is the dac the tuning element?
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Old 25th December 2013, 04:56 PM   #3452
1audio is offline 1audio  United States
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Default Boonton 1120 oscillator

Here is the complete circuit. There are 8 fets for resistor selection and 4 caps that are switched with FETs for the main tuning. I think the frequency selection is binary, not decimal but the micro hides that. The DAC is used for fine tuning. Its on a different board and comes in through the fine tune connection.

The track and hold/sample and hold agc is on the second sheet. Its pretty simple really and works from 10 Hz to 140 KHz.

The Boonton is a mid-1980's design and I'm sure there are better parts for the special functions today.
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Old 25th December 2013, 05:34 PM   #3453
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
Here is the complete circuit. There are 8 fets for resistor selection and 4 caps that are switched with FETs for the main tuning. I think the frequency selection is binary, not decimal but the micro hides that. The DAC is used for fine tuning. Its on a different board and comes in through the fine tune connection.

The track and hold/sample and hold agc is on the second sheet. Its pretty simple really and works from 10 Hz to 140 KHz.

The Boonton is a mid-1980's design and I'm sure there are better parts for the special functions today.
It's a typical track and hold and proportional controller used. What's interesting is the very very long integrator time constant. Shunting the integrator input to ground with those resistor values will produce a very low glitch. This might be a better way to do a pulsed integrator. I find leaving an integrator input open for most of the cycle time allows pick up of noise because integrator gain increases proportionally as the frequency approaches DC. I've noticed a high sensitivity to 60Hz and 120Hz and other low frequency noise. This noise is communicated to the multiplier and in turn to the oscillator. The sensitivity lessens considerably if the integrator is in a state of change. Inducing a small controlled amount of ripple,(sawtooth waveform), on the integrator achieves this even if it's just 10uV or so.
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Last edited by davada; 25th December 2013 at 05:39 PM.
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Old 25th December 2013, 05:59 PM   #3454
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
Here is the complete circuit. There are 8 fets for resistor selection and 4 caps that are switched with FETs for the main tuning. I think the frequency selection is binary, not decimal but the micro hides that. The DAC is used for fine tuning. Its on a different board and comes in through the fine tune connection.

The track and hold/sample and hold agc is on the second sheet. Its pretty simple really and works from 10 Hz to 140 KHz.

The Boonton is a mid-1980's design and I'm sure there are better parts for the special functions today.
I found the manual in my archives.

Both the ALC multiplier and FineTune multiplier are in the leveling loop. This loop does not effect frequency. Are you sure this isn't a fine tune of the level?
Both the 'source' and 'fine tune' connect to the output board.
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Old 25th December 2013, 06:34 PM   #3455
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Sect. 4.79 explains the fine tune function. It ties to the second integrator, not the first so it affects the phase of the second vs. the first. It can tune in 25 milliHertz steps at 1 KHz.
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Old 25th December 2013, 07:24 PM   #3456
davada is offline davada  Canada
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Originally Posted by 1audio View Post
Sect. 4.79 explains the fine tune function. It ties to the second integrator, not the first so it affects the phase of the second vs. the first. It can tune in 25 milliHertz steps at 1 KHz.
Okay I see what they did now.

So you want me to add servo tuning?
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Last edited by davada; 25th December 2013 at 07:33 PM.
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Old 25th December 2013, 07:28 PM   #3457
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Old 26th December 2013, 06:14 AM   #3458
1audio is offline 1audio  United States
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Originally Posted by davada View Post
Okay I see what they did now.

So you want me to add servo tuning?
I think the software needed plus a frequency counter and reference oscillator makes that more of a project than the oscillator. Its a nice but not necessary feature. However I really like the synthesizer accuracy and flexibility it provides.
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Old 26th December 2013, 07:53 AM   #3459
davada is offline davada  Canada
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I think the software needed plus a frequency counter and reference oscillator makes that more of a project than the oscillator. Its a nice but not necessary feature. However I really like the synthesizer accuracy and flexibility it provides.
The Booton is nice piece of engineering.

A counter is no problem and tuning can be driven to a set point but it would have to be shut down for measurement. This would be for convenience only.
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Old 26th December 2013, 11:10 AM   #3460
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Originally Posted by davada View Post
I wounder if a gyrator type filter would perform better than a Sallen key type.

This is what Filter Solutions came up with.
Passive, GIC, Akerberg Mossberg.
I'm not sure about gyrator/GIC--both use noninverting opamp configurations and thus are unlikely to be suitable for low distortion. Akerberg Mossberg is probably an improvement, but at this complexity level one could use a state-variable filter as well.

Quote:
Originally Posted by Mark Johnson View Post
Looks to me like the LT1468 is just an upside-down AD797. They didn't even bother to change the transistor numbering scheme.
The numbering scheme seems pretty natural. As others have noted, the schematic of at least the AD797 is substantially simplified--you can get a much more detailed view (just omitting bias details IIRC) in Scott Wurcer's AES preprint (no. 3231). A more detailed schematic (which I believe is pretty accurate for the main signal path) for the LT1468 is found in http://cds.linear.com/docs/en/lt-jou...8_1198_Mag.pdf.

Besides the same basic topology, there are substantial practical differences between the two parts:

* The AD797 gives the choice of adding a 50 pF distortion cancellation capacitor, which is not used in the LT1468 (The 10 pF cap C2 is part of the compensation and not used for distortion cancellation). Fortunately the latter uses quite high class B quiescent current in its output stage and thus offers reasonably good load driving capability.
* The AD797 uses much higher bias for the input stage, which results in low voltage noise. Thus it is more suitable for low impedance (< 1 kOhm) sources WRT noise, while the LT1468 offers low current noise and thus excels at higher source impedances.
* The AD797 can be difficult to get stable, particularly with low Z/capacitive feedback networks, low noise gains and with the distortion cancellation capacitor added. The LT part in turn is very well behaved WRT stability, as long as basic good design practice is followed.
* The AD797 offers lower common-mode distortion effects, thus is the part of choice for noninverting configurations.

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