Selction issues - looking to build a sweep marker generator

Hello,

Can you clarify a bit more on what you need?  I myself am lost on what you are talking about.  The most I can venture to say is it sounds like you might need two DDS outputs?  One for the 'marker' and the other for the sweep?  You could synchronize two single DDS chips but it might just be better to go with a dual.  We currently only make one dual part, the AD9958, which has a max SYSCLK of 500MHz.

- Kevin

What do you want the output to look like during the 1ms 'marked' period?  When you mention a variable marker, what do you want to vary about it?  I am not certain that what you are looking for can be done simply with just one of our DDS products, but the question doesn't give me (us) enough detail to determine that with finality.

OK, take or example IF frequencies are typically 262 kHz, 455 Khz and 10.7

Mhz. So for one it would be nice to be able to generate those frequencies

exactly.

Now suppose I have some way to set the center frequency and the bandwidth

(minus and plus limits the same.

That then becomes the injected signal for the sweep with variable amplitude

not exceeding 4V RMS.  Likely much less.

Now,let's say the system sweeps at a rate of about 8 steps/sec.  Some sort

of slow sweep, so the AGC control of the tuner doesn;t take hold.

Now, display the 10.7 MHz marker on something.  Now it could be from a D/A

converter driving the X-axis and blanking being a 1mS period, possibly

larger for this frequency, on the scope.

Another way would be to graph it on an LCD.

Initially, the task would be to vary the center frequency such that the -3db

points are identically placed.

Downloading to a computer might also be possible based on implementation.

Basically, you want to be able to determine the bandwidth and center

frequency of say a filter,  If the filter variable, you would like to be

able to move the peak of the filter.

So, I was thinking about a small display where the marker (center frequency)

can be moved and the -3db points found manually.  You move the center freq

and the -3db point is found with the center to edge steps shown.  You just

move the marker, such that they are the same.

Obviously, I've had soem to time to think about the design.  Initially, all

i wanted is a scope output and a marker, hence the idea of having the DDS

sweep and create a marker at mid span.  This does not appear to be possible.

So, the next thought is to do it on a small digital LCD display with some

sort of user interface that would  use encoders.  The computations could be

done on the display.  The data could be downloaded.

I've also read that at least one of the DDS chips if you use a 135 MHz

crystal, you cab get non fractional cycle values.  Not sure how easy it is

to obtain a 135 MHz crystal.

So, now I was sort of thinking of twp probes.  One containing the DDS with a

control for amplitude and one containing the DAC to do the measurement with

the AD8810 Log amp. This amp seems difficult to use when measuring voltages.

So, there are my thoughts:

Two possible designs:

One is effectively old technology and one is much newer technology.  Drive

the X position of a scope with a D/A and drive the blanking signal of an

analog scope.

The newer technology would use a small LCD display and actually try to

perform some calculations on the date.

I've done a fair amout of reseach since posting.  This

http://www.njqrp.org/dds/waystouse.html daughtercard using the AD9850 or the

AD9851 seems suitable.

The DDS daughtercard schematic is here:  http://www.njqrp.org/dds/

Computer controlled variable gain for the AD9850, AD9851 seems difficult to

do too.  The range would typically be 262 kHz to 10.7 Mhz with some edges.

If the design is done right, it might be able to be turned into an audio

spectrum analyzer with a different AD9850/9851 front end.

On Wed, Sep 14, 2011 at 10:08 AM, JLKeip <

Hmmm, this sounds like an amateur radio project -- I recognize some of the IFs as being used in my Boatanchor collection.

(My HQ-170 used a third IF of 60 kHz. )

There was a scalar network analyzer design in QEX (www.arrl.org) using the AD8302 and one of our DDS ICs a few years ago.

This article probably contains all the information you need. I wouldn't worry about exact frequency values for your readout; the DDS ICs

have more than enough resolution to get with a few (milli)Hz of the exact frequency you want. You can round off the value for the display

to the nearest kHz, which should still give you 455.000 kHz.

I have not done this myself but if you digitize the output of the detector (AD8302 or other ADI log amp) and keep the sample and associated frequency value in a buffer than after each sweep you'd find the maximum value (and thus center frequency) and the two values that 0.707 of the maximum value and you'd have the center frequency and -3 B points to add to the display via software before you update the display. In other words, crunch all the number during the blanking interval and you'll have them available for the new display update.

When we first started doing log amps, it turned out that filter delay through 455 kHz filters wasn't insignificant, so you're on the

right track with a slow sweep signal. (A couple of us built an FM IF strip at 455 kHz using the AD608 but unfortunately Murata

discontinued the discriminator we used so we had to drop the application from the data sheet.)

Although the AD9850 is used in a lot of amateur radio projects, you might want to look at the AD9913, which is much newer.

Regards,

Bob