DDS documentation
Contents
Links to general information
- Digikey product training module about the DAC of the DDS (or in general)
- AD9951 product information on digikey contains the DAC - and other - training modules
- AN-912: Driving a Center-Tapped Transformer with a Balanced Current-Output DAC (refered to in the DAC training module).
- AN557: Using the AD9850 as a LO of a transceiver.
- AD9951 VFO design
- 150MHz VNA
- Antenna analyzer DDS + AD8302
- Digital representation of signals
AD9850 DDS Board
PHSNA
PHSNA is a SNA design closely related to the design I have in mind, as pointed out to me by Nick Kennedy.
Selected excerpts from the mailing list
2) The RF amplifier that you added has limited harmonic distortion that will cause erroneous readings when the main signal is attenuated by the tested circuit but the harmonics will pass unattenuated. For example measure a narrowband Band Pass Filter frequency response and tune the SNA to a frequency band that includes the filter central frequency divided by 3. You will see there a spurious band pass curve (maybe 30 - 40 dB down) at third the filter frequency. I solved that by discarding the RF amplifier and using directly the DDS output, which has an excellent rejection of harmonics, at a cost of using lower power level.
3) The DDS module output power gets lower at higher frequencies. I have added a diode detector at the RF output and used the detected signal to stabilize the output power by comparing the detected signal to a reference voltage and feeding back the error signal through an integrator to the level pin in the AD9851. Now the accuracy is around +/1 dB across all the frequency range. Victor - 4Z4ME
I SPICE modeled the filter on the DDS module and its (ideal) response rolls off at around 60 MHz. I'll post that result to the Files in a little while. We opted to add a filter that rolls off just above 30 MHz. One other possibility is to pick off the complementary output available on the DDS module (no LPF in line there) and go straight the filter on the shield board.
Actually I'm still puzzled a bit about the decrease in level. The 9850 data sheet (and an Analog Devices guru) cite the sinc function response. But calculating the sinc function as per the data sheet only yields about a one dB drop from 1 to 30 MHz. And the LPF on the DDS "should" be pretty flat well beyond 30 MHz. But Jerry saw about 10 db roll off over 1 - 30 MHz. Our curve fit compensation took care of that, but it's like treating the symptom instead of the cause. Inquiring minds....
I'll find the time to take pictures of the SNA and post them here. The level control of the DDS chip is by controlling the Rset resistor connected to pin 12 of the AD985x. A circuit similar to the one that I use is presented here: http://www.vk5tm.com/homebrew/dds/dds.php However such a circuit will not be good for controlling a 10 dB RF level variation.
"building something without experimenting is just solder practice"
- I also have observed that the decrease in level with frequency is greater than expected. I think that it can be caused by the low Q of the surface mount inductors in the original filter.
- I also plan to put a better filter, and maybe use both outputs of the DDS with a balanced to unbalanced transformer.
- The AD9851 is almost identical to the AD9850, and doesn't have a level control feature. The amplitude is controlled with the current injected into pin 12 (Rset). That is what apparently Victor has done, but I would like to see his circuit.
A couple of days ago I made a revision on the PCB to add a couple of pads that could be used if you wanted to pick off the DDS signal at J4 on the module (complementary output - without LPF) Even considered hacking the module and removing the filter components. But I really like the idea of using both outputs (after ditching the LPF) and a transformer.
The amplifier characteristics that will influence the harmonics are the 2nd and 3rd intermodulation intercept point and the mmics will not differ by much from the amplifier that you use for a given similar power consumption. Why are you so worried for the isolation of the output? The DDS chip output practically is a current source and is not sensitive to its load if its output amplitude does not reach a high voltage. The low pass filter is your only concern for load termination. If you would use only the DDS 7th order filter, its cutoff frequency should be around 50 MHz so even if it is affected by load mismatch at its output, it would not affect its frequency response at 30 MHz and lower.
I think that I used only a 6dB attenuator at the RF output. As mentioned earlier, the level drop at high frequency might be caused by the low Q of the SMT inductors in the DDS board. You might try and replace them with small toroid inductors with the same inductance value.
We briefly thought about a MMIC but didd't go any further. In fact another 9850 DDS generator I have (KG6CYN design) uses a ERA1-SM1 MMIC.
An update version of the schematic is shown on this webpage (very similar to the QST article):
It is posted with Wes' W7ZOI permission. There is a small change in the battery supply but the power measuring circuit is essentially the same.
Actually, now that I've pulled up the original article, there are several small changes from the original schematic (that'll teach me to rely on my memory at 5 AM). However, they do originate from Wes, so I'm sure these changes are worth considering by anyone that needs to build one now.
Since I do have a parallel universe version of the PHSNA working, I've been measuring a few things. Most of my filters I'd already characterized point by point before developing the ability to auto scan. But I have several commercial filters in my junkbox from hamfests: Heath, Kenwood, Yaesu, ICM etc, which I've not checked. Some don't specify their design terminating resistance requirements and some are pretty high, like one from Heath that's 2000 ohms. To match it to my 50 ohm system, I could use some sort of resonated transformer or maybe an L-match but that sounded complicated and might affect the filter's response. Another way is to use minimum loss resistance pads. Minimum loss is pretty high when you're going all the way from 50 to 2000 though. It looks like 22 dB on the input and 22 on the output , so -44 dB total. That could put the signal down in the dirt, so I need some amplification. A while back I built a little broadband amplifier with 38 dB gain, using three MMICs in a line.
With this much difference in resistances, you just about don't have to calculate the matching pad. Just put 50 ohms shunt to ground at the input and output connections, and 2000 ohms series from there to the filter In/Out terminals. I had a fair amount of trouble with my noise floor while using the amplifier. Initially it was only -25 dBm but just by moving components and cables around physically I got to -40 dBm. Still way too high to show ultimate attenuation, but the shape and flattness and 3 dB BW ought to be OK. I don't know though, this filter is marked 2.1 kHz but I showed about 1.2 kHz. Is it my measurement method, or is it the filter? Guess I should try some others. What about filters that don't reveal their termination resistance requirement? From what I understand, you make measurements with various resistance values until the shape looks "right" and say, "that's it". Sounds like a lot of work to me. I wonder if we'll be able to do 455 kHz filters with this gizmo? Hope so.
I'd think you could use a broadband transformer. It ought to do OK over the fairly limited bandwidth of the filters. For 50:2000 you'd need about a 1:6 turns ratio. And the reactance of the high-turns winding ought to be around 2000 ohms or better. That's around 70 uH at 5 MHz. Ferrite cores should serve.
Or maybe a combination of techniques... 4:1 quadrafilar transformer to give 50:800 then attenuator pad to handle the remaining transformation. Wouldn't have to introduce so much loss, so maybe the level would be high enough for decent dynamic reange. link
This is a easy way to design matching circuits for your filters. But you will need a VNA not a SNA to produce s-parameter Touchstone file to load into RFsim99 software.
It has been my experience that an L network works a lot better than a transformer. However, I have never tried to build one for 50:2000 ohms. The transformers introduce more loss than an L network. I typically use ZMAT a software program which comes with the book, “Experimental Methods in RF Design” to design the network. I then wind the toroid and use a combination of a fixed and a variable cap. I temporarily mount the L network on a piece of copper clad and use a resistor as a temporary load. I then connect my RF generator with my Return Loss Bridge to the 50 ohm side. I set the frequency on the RF Generator correctly and adjust the variable capacitor for the best return loss (typically -25-30 db). Then, without changing the variable cap, I mount the L network in place. I learned this method from my good Aussie friend Kerry.
I am not sure if a 50:2000 ohm “L” network would work well. The Q might be too high. You might try a pair of L networks in series. Maybe a 50: 350 ohm and then a 350:2000 ohm.
Nick, as far as going down to 455 KHz, the SSNA has not been tested below 1 MHz. You may have to modify the amp or substitute a MMIC for the amp.
Well, I usually think a transformer should have 5 to 10 times the load resistance in the reactance of its magnetizing inductance, which is just to say the inductance of the winding connected to the load. That's hard to get if you need 10,000 to 20,000 ohms at 3.395 MHz, even with high permeability ferrites. You could use something with a lot less inductance, maybe iron powder, and then parallel resonant the winding, but now you're fusing with tuning.
But scratching around a bit I found that I have a few ferrite toroids with an AL value of 1100 to 1200, so I'd only need 20 turns to get the 10,000 ohms minimum I think I need. I wound a couple with 3 turns on the 50 ohms side and tried again, without the amplifier. Now my floor is at about -50 dBm - not great but a big improvement for sure. The filter shape looked about the same. Still 1200 Hz at -3dB points for a filter specified as 2100 Hz. Maybe it's just not a very good filter.
I have this other filter of unknown termination requirements that came with a nice plot. It's 2.1 kHz at 10.7 MHz and the plot shows it nice and flat on top with steep sides. I thought I'd try it in a 50 ohm environment and see how it looks. The main thing was a large amount of ripple in the passband - at least 3 dB. It had a little shelf or shoulder as it sloped off on the low side. And the loss inside the passband was about 19 dB. I wonder if there's some way from these observations to deduce the required terminating R, or if it's all trial and error.
Regarding using a L-match for 2000 to 50 ohms -- I've done it for an 80 meter EFHW. Don't know how efficient it is, but it seems to do OK.
I also thought about some kind of active matching for the 2000 ohm filter. Maybe simple JFET buffer stages with 50 in / 2000 out and 2000 in and 50 out impedances. But that was going to be more effort than I wanted to exert just for playing around with junkbox filters.
Jerry and I had been discussing a version of the SSNA that substituted a MMIC buffer amplifier instead of the discrete 2N5109 design. I've done the layout for such a version using a MiniCircuits ERA-3+ MMIC and can get boards made of either type. They will still be combined with a power meter board. I just have to get multiples of ten of each kind. No difference in cost. The MMIC offers wider bandwidth and needs a couple fewer parts.It could be useful if you wanted to use an AD-9851 DDS module.
The ERA-3+ costs less than $2, but MiniCircuits has a pretty steep minimum order of 20 pieces. But a couple of DX ops said it was easier to get them than to hunt up a 2N5109 or 2N3866.
Look in the files section for "COMBO ERA3" and "COMBO 2N5109" to see the schematics and layouts of each version to see what you'd be getting.
Yes, was thinking about it and then saw how much vendors were asking for the AD822. The AD820 is pricey enough.
But finally I decided the amp wasn't being loaded enough to worry about needing independent outputs, and two trimpots would serve to make the analog meter and the A/D output independently adjustable anyway. As long as you first set the gain to define the A/D port span, and then trim out the series resistance to the meter.
I was really considering the dual op amp in order to subtract an offset so that the bottom end of the dynamic range would map to zero volts, but decided that was needlessly complex just to recover about 10% of the A/D range.
If you go to Mini-Circuits website, you can register and received free samples of the ERA-3 chip (they automatically give you 4). Here is their address:
http://www.minicircuits.com/homepage/homepage.html
Just follow the registration instructions. They will ask for your application when you ask for the "EZ Samples." Just tell them the truth, don't try to make them think you're a defense contractor. :-)
I chose -10 dbm as the output to prevent damaging crystals. I think -10 dbm is good maximum based on some emails I had with Kerry Powers (can’t remember his call). However, the extra attenuation was put in so that people could decide how much power they wanted. You can get up to +20 db more out of the SSNA. Of course if you decide to increase the power out of the SSNA, you will have to change the amount of voltage you get out of the power meter. A value of +5 VDC is the absolute maximum you can feed back into the SSNA.
Thin brass 'shim stock' is available at most hobby and craft stores. It can be cut with scissors and bent to shape. The holes all along those strips are 'plated through' and tie the top and bottom strip together. Small pieces of wire may be soldered through several to solder to the 'shield' material.
Yes, That was the intent. To allow you do do at least some shielding around the front end components. I have one in the works at the home QTH and will post photos when I'm done, but that will be at least a week.
I had built a prototype (homemade PCB) without shielding and it workes fine, but I thought... maybe more is better :^))
Now that a lot of you have your boards, you will probably want to get building. There is a preferred building sequence that will help you test your system as you build. First, build the power meter. Calibrate it and set the voltage output to produce +5.0 VDC MAX at -10dbm. On my power meter, the output is set a little lower. The maximum input to the UNO R3 is +5.0 VDC. Anymore and you run the risk of damaging the UNO. So, my meter is set at 4.55 VDC for -10 dbm. When you guys get to the calibration point, there are several ways to do it based upon the equipment you have. If you have access to a HP8640B generator or other high quality generator, you can use it to do the calibration. Or if you have a good 5 watt power meter and a QRP transmitter, you can use them. There are a lot of different ways to do the calibration. We will be discussing them with you.
Once the power meter is built and operational, you can then build the SSNA. Before you build the SSNA, try downloading the software, compiling it and installing it on your UNO R3. That will give your little computer a quick test. Actually, this test can be done anytime. After building the “shield” board (the board that plugs into the computer), decide where you want to take the output from the DDS board. Jim, has written up some options for you. Depending on where you extract the output, you may have to modify T1 on the shield board. More on that later. You will be able to test the output level of your SSNA using your power meter. You will be able to set the calibration of the DDS by using a well calibrated Frequency meter or by listen for zero beat against WWV on a receiver.
One important thing to remember about the AD8307 rf detector is that it is VERY sensitive. It will measure down around -70 dBm, which is one ten-billionth of a watt! Or 71 millionths of a volt.
So you have to take extra special care to keep stray rf away from it. First and foremost is a VERY well shielded enclosure. One of those cast aluminum boxes is probably best, but the "minibox" style will be OK if the halves fit tightly together. I used an "Altoids tin on steroids" with a tight fitting slip-on lid, from PaperMart.com.
Anything coming into the box (other than the signal, of course) needs to be well bypassed for rf, hence the posts you've read recently about feedthru capacitors. Best bet is to keep the power supply (battery) inside the box. Then the only other penetrations are for input signal and DC out to the Arduino. You can probably get away without a feedthru cap for the dc output if you use an rf type connector and bypass it very close to the box wall with a couple of caps, maybe a 0.001 uF in parallel with a 0.1 uF There is another bypass cap on the PC board, but don't rely on that one.
If you use an analog meter with yours, the meter represents a big hole in the box (unless its one of those great mil surplus full-metal-jacket types). For a plastic meter, maybe some copper foil tape or real aluminum foil duct tape can be applied to the back to cover most of the gaps.
The filter components on the DDS module are "correct" if what you want is a LPF with a 70 MHz cutoff.
In the Files section I earlier had posted a SPICE model of that filter.
The problem is that the DDS ought not be used higher than 1/3 its clock frequency because of the spurs that are generated. So for the 125 MHz clock, about 40 MHz ought to be the upper limit. A 70 MHz cutoff will let some spurs through when you are operating at the upper end of the range. Jerry's filter is much better matched for the useful range of this DDS.
There has been speculation that these modules are goofs. Somebody put a 9850 on a board meant for a 9851. Maybe that's we can get 'em so cheap.
Look in the "Links" section for a link to an article from Analog Devices.
Key point is that if the AD8307 is driven with a DC source, it will read 3.01 dBm too high, compared to a sine wave. This gives you a way to calibrate your power meter using a battery, trimpot, and a DC voltmeter.
Of course the regular input to the 8307 is capacitively coupled, so to do this you have to connect the DC source directly to pins 1 and 8, and the source has to float.
I have done this with a previous build of a power meter and it appears to work, but I'd like to hear from others who try it, especially if you have a good independent means to verify the result.
