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Old 30th Sep 2017, 2:39 pm   #1
Wendymott
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Red face SSB Signal Generator

Hi peeps...... While messing about building My 40M SSB transceiver and subsequent Multiband SSB transceiver.... I found that I needed a definitive SSB signal, both of a known frequency and output level. As I could see no commercial equipment ..... " I am sure there must be some"..I decided that I would build one.
It is based on my Multiband transceiver TX exciter, but is dedicated to Low output levels suitable for receiver inputs. I had tried using my FT817 with lots of attenuation... but somehow it seemed like using a "steam hammer" to open a walnut.
I will post the other modules as they become, proven, but for now the Bandpass filter block is up and running.
I decided upon 80 m - 10m.. UK bands only... leaving out "Top band" as I have no use for it.
Thanks go to Jeremy G0HZU for the 40 M filter design, and me for the others. The Multiband RX/TX had Micro relays to switch the bands, but this version uses BA282 diodes, much cheaper, but power limited. I used BC547's to buffer select each band as the micro controller grounds the selected band pin.
Hopefully the schematic will be readable, but if not, i can PM anyone interested. All filters were aligned using my ROGOL spectrum analyzer with tracking generator. If anyone is interested I can post the display waveforms.
As it is a long term project, it may be a while before "part two".
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Old 30th Sep 2017, 4:21 pm   #2
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Default Re: SSB Signal Generator

I've not come across a sig gen with true SSB modulation, though some of the new vector modulation ones can have I/Q data files downloaded to them which will create SSB.

The nearest to a real time SSB sig gen would be a Redifon GK203 or SD1 exciter unit. Just add an output attenuator.

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Old 30th Sep 2017, 4:22 pm   #3
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Default Re: SSB Signal Generator

A very worthwhile project, and nicely produced! But no Topband? Wash your mouth out!
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Old 1st Oct 2017, 11:40 am   #4
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Default Re: SSB Signal Generator

Haa haa Andy..... Sorry.. No Top band.. David... I am not trying to make a professional piece of kit... just something I can use to get me "near enough" I know that is an athema to you.. the reasoning is this.... when I made the 40M unit, I was and still am concerned that my digital readout was not the same as the readout of the station I was communicating with.. it could be all of 2 Khz adrift..so if I make a generator that I know correlates to the FT817 or similar, then I can adjust the offset in the software to show the same.
It may be a sledge hammer etc... but it passes the time, keeps my supplier happy, me buying PCB material... and is a breathing space for my multiband transceiver on the stocks.
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Old 2nd Oct 2017, 2:03 am   #5
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Default Re: SSB Signal Generator

That looks really great, well done. I also admire your attitude to the problem and the way you decided to get on and design & make what you needed, also love the remark about the steam hammer & walnut.
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Old 2nd Oct 2017, 7:23 am   #6
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Default Re: SSB Signal Generator

Quote:
Originally Posted by Wendymott View Post
David... I am not trying to make a professional piece of kit.
My post was just a response to 'I'm sure there must be some'

For just creating an audio tone from a receiver, a simple unmodulated carrier tuned to the appropriate offset will do, of course. Many signal generators have external modulation inputs and will apply anything you want to their output, so you can use them as a pantry transmitter, but I've never seen any sig gen with a proper SSB modulator.

The latest all singing all dancing vector sig gens can do it if fed a pair of processed signals to I and Q inputs, forming a phasing type modulator, but then you need to add a lot of complex circuitry (ooh the pun!)

The nearest to a sig gen which can do SSB are probably the low power drive units like the Redifon GK203 (200mW 1-30MHz in 100Hz steps) which do CW SSB and all sorts of AM.

I wouldn't suggest you build one. The old drive units turn up as affordable surplus now and then, or just use a spare amateur transciever into a big attenuator.

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Old 2nd Oct 2017, 9:02 am   #7
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Default Re: SSB Signal Generator

Some radio "service monitors" have proper SSB mod, as they have to test SSB transceivers and other aeronautical systems. Like this Rohde and Schwarz unit.
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Old 2nd Oct 2017, 12:23 pm   #8
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Default Re: SSB Signal Generator

The trouble with R/S and other high tech companies is the number of zero's behind the sign. For amateurs like me,it is wayyyyyyyyyyyyy out of my remit.... No.... home construction is the way.
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Old 8th Oct 2017, 12:25 pm   #9
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Default Re: SSB Signal Generator

Another module completed...The "Up Converter" module requires 60m/V rms input to pin 8 to limit carrier "leak through"........ The SSD oscillator module AD9850 outputs a rather large 500 m/V after filtering and a Mar 4 amp.......... and this is not constant.... as the carrier reaches 30 Mhz it starts to drop off due to the low pass filter characteristic. The actual upper frequency required is 39 Mhz..and by then the output has dropped to about 50m/V rms.
As I want a relatively constant level I made a switched PIN attenuator, which is "channelized". The problem is that the impedance changes made it impossible to add the attenuator without an impedance match at the output, thus I used a MAR 6 on the output.
Maybe I am making a rod for my own back using the MC1496... but they are in stock.
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Old 9th Oct 2017, 9:29 pm   #10
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Default Re: SSB Signal Generator

Interesting solution but I'm not sure you need to go to these lengths to get a flat output from the 9850 DDS chip.

It's about 20 years since I last did anything at work with the AD9850 but there are a few design rules that can indicate the flatness at the raw DDS output pin of the chip. If you adopt the default 10mA Iout and feed this into the default shunt 50R resistor (and then connect your load circuit across this) then the level should be about 88mV rms at low frequencies.

This is because 10mA pkpk is 5mA pk and this is 3.54mA rms. With half of this going into your 50R load and half into the onboard 50R shunt resistance the power into the load will be 0.00177*0.00177*50 = 0.157mW or -8.0dBm.

This should be the level at a low frequency, eg 1MHz. If we assume the DDS is clocked at 125MHz then the amount of droop/rolloff at 39MHz can be predicted in a simple excel spreadsheet because the output should follow a sinx/x response. I've attached a quick and dirty excel sheet below that shows this in a graph of frequency response vs level for a given clock frequency of 125MHz.

In reality, a reconstruction (lowpass) filter will need to be added and this will introduce even more droop at 39MHz but probably only enough to give about 2.2dB total droop at 39MHz. i.e. 1.5dB due to sinx/x and 0.7dB droop due to the LPF. This 2.2dB total droop at 39MHz can be compensated for with a very simple equalisation circuit.

So I'd be tempted to look backwards into your (ebay?) DDS module to see how the lowpass filter has been implemented. From what I can see the majority of these cheapo ebay PCBs adopt the 'clockgen' circuit rather than the 'synth' circuit for the 9850. This looks to be a copy of the 200R elliptic filter used on the clockgen eval board and this is meant to be fed to the onboard comparator to generate a squarewave. But most hams would surely want to use the 'synth' circuit version with a 50R output?

So I'd be tempted to go back to basics and fit the parts that give a raw 50R output and then design a simple 50R LPF and equalisation circuit. Typically this would give out about -11dBm across LF through 40MHz and this would be flat to maybe +/-0.5dB. Then just add a buffer amp and pads as required?

Note that it's 20 years since I last messed with this old DDS chip so the above may contain nuts but I think it is about right
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Old 10th Oct 2017, 6:12 am   #11
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Default Re: SSB Signal Generator

Jeremy has it right.

If the output of a DDS's DAC was a series of theoretically perfect impulses, it would give a flat output across the frequency range (filters allowing!) but that would be very difficult to arrange.

Instead, the output is a series of steps and the voltage is held for the time between steps. this is called a 'Zero-order hold' and is the cause of the (sine(x))/x roll-off which also includes a series of periodic notches. The same function also crops up in the frequency response of tape heads due to the the thickness of their gap.

Outside the theoretical world, practical people ran into trouble with DAC glitches on every clock edge because multiple bits changed at once and they did not all have equal settling times, so the steps occurred with disproportionate twangs which made a right muck of the plans for controlling the output spectrum. In the 1980s, Sony DACs were the ones to have for low glitch energy. DDS chips were coming on the market with digital outputs only.. add you own DAC.

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Old 10th Oct 2017, 9:41 am   #12
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Default Re: SSB Signal Generator

Hi Jeremy and David. Wow Jeremy... you have certainly put a lot of work into that spreadsheet of which I am very grateful.... It is not an Ebay bought module, I buy the devices from Farnell or similar and make my own boards with an onboard filter and buffer. Which I add below....
The Filter layout and components were "borrowed" from a well known kit supplier.........
I am quite happy with the Atten module, as it does work, but if I can improve, I will.
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Old 10th Oct 2017, 11:29 pm   #13
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Default Re: SSB Signal Generator

The circuit values in your lowpass filter do look a bit odd and I'd expect a poor response from that filter, especially at 39MHz.

Maybe there were typo errors in the original circuit? To get a fairly flat response up to 40MHz from an AD9850 clocked at 125MHz should be quite easy if a simple LPF is followed by a lossy equaliser circuit. The output signal level should be quite predictable and it should be fairly easy to get within +/- 0.5dB flatness across 0-40MHz. The equaliser circuit would cost maybe 3dB in output level so the DDS would typically output -11dBm when flattened by the LPF and equaliser. This could then be followed by something like your MAR amplifier to boost the level as required.

I can show you how to design all this if it helps? The sinx/x plot in the excel sheet can also help predict the level of the unwanted alias term when the output is set to 39MHz. This is because all the major terms from the DDS DAC will follow this sinx/x envelope shape.

For example, at 39MHz Fout the main (unwanted) image will be at 125-39 = 86MHz. The excel spreadsheet shows that 86MHz will have about 8.3dB droop wrt 1MHz.

But there is already 1.5dB droop at 39MHz so the relative level of the 86MHz term to the 39MHz term will be just 8.3-1.5 = 6.8dB.

So to get 60dB rejection of the 86MHz term the LPF would need to provide 60-6.8 = 53.2dB rejection when designed. Note that there will be other spurious terms from the DDS such as 125MHz - (2*RF). Therefore, at 39MHz there will be a spurious term that will be at 125-39-39 = 47MHz. This will be harder to filter out and this term could easily be in the order of -50dBc. The AD9850 isn't a very clean DDS so there will also be a carpet of spurious terms down at maybe -70dBc and these can be seen in the datasheet plots.


I think a suitable LPF + equaliser design could be fitted to your existing DDS board or maybe you could use this info for a future DDS board? eg for a test signal generator that needs to be flat across 0-40MHz for example.
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Old 11th Oct 2017, 9:19 am   #14
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Default Re: SSB Signal Generator

Hi Jeremy.... The filter was "copied" from the Minikit DDS Module, and as far as I remember I borrowed the componentrs from the module I purchased.... The reason I made my own was that the board area was far too big with wasted space for tracks that were totally un needed....... I did buy some more inductors to enable me to make more modules, as and when... I did buy the AD9951 and I have the software to run it..however it seemed such a waste of performance when I am only running up to 10M + I.f.... as it is rather expensive... but if that is what is needed..
I had 5 pcb's based on the 9951 made in China,
But for the moment..... could I please accept your offer... and see how it goes.
I was planning on using the 9850 for HF and the 9951 for VHF.

Quote:
I think a suitable LPF + equaliser design could be fitted to your existing DDS board or maybe you could use this info for a future DDS board? eg for a test signal generator that needs to be flat across 0-40MHz for example.
I have no problems relaying the pcb......as you may know I have facilities here to make most pcb's,
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Old 11th Oct 2017, 10:13 pm   #15
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Default Re: SSB Signal Generator

I had a look at the pdf for the minikits VFO and there is a 70MHz filter for the 9851 version and a 30MHz filter for the 9850 version.

The 70MHz filter looks OK to me but there is something bad about the 30MHz version. I suspect that C4 and C5 (on their version of the circuit) should not be 180pF. I think they should be something like 82pF? Maybe you could email them to check. But either way, the filter cuts off at about 30MHz which is no good for your 39MHz requirement.

If you want an alternative filter here that is suitable for 39MHz then you first have to ask how much rejection do you want of the 47MHz spurious term caused by 125-39-39 = 47MHz. This term is typically 50-55dB down on these devices when the DDS is tuned to 39MHz.

If you want 60-65dB rejection then the equalised 39MHz LPF needs to reject 47MHz by at least 15dB. This would need a 7th order elliptic filter. i.e. one extra section compared to what you already have and it would need to be made with decent inductors, eg Micrometals T27-17 cores would be one option for the inductors. The equaliser circuit can be really easy, just a pi attenuator and an inductor and a cap. So don't worry about this.

I think it would be possible to get 0-40MHz with +/- 0.5dB ripple and 60dB rejection of the 47MHz term when tuned to 39MHz. It would be possible to make all this fit on your existing board with a few minor track mods.

Maybe it would be best if I made this filter/equaliser here and tried it out?
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Old 12th Oct 2017, 10:48 am   #16
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Default Re: SSB Signal Generator

Hi Jreremy...... I see the logic of what you are saying..... I guess I got the 30 Mhz version... thus that is why it dropped off so quickly.

Yes please....I can make the pcb do anything you suggest. I will be guided by you.
That would be fantastic.

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Old 12th Oct 2017, 9:44 pm   #17
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Default Re: SSB Signal Generator

Probably the first option would be to post up a 39MHz version of the minikits elliptic LPF. See below for such a 5th order LPF. Filters like this give a fairly sharp attenuation slope but the price for this is that the component quality and tolerance needs to be fairly decent. Ideally, you need to use 5% parts. C3 will probably be the most sensitive part in terms of tolerance. The inductors need to have a reasonably high Q. anything over 70 would be fine.

Because you are using this to drive a mixer, you probably don't need to maintain a very level output across 5-39MHz and the circuit below will probably deliver about 2 to 2.5dB droop when driven by the DDS as there will be 1.5dB due to sinx/x and maybe 0.6dB in the filter. But the filter may give up to 1dB droop depending on the components you use. So it could easily mean 2.5dB droop overall.

If you want to add some equalisation then maybe try the very basic equaliser circuit below. Simply place it after the LPF.

The design below gives very little attenuation at 47MHz but I don't think this matters in your case. A 47MHz LO spur probably won't give significant problems when mixing with 9MHz for the 10m band.

If you want to make something with better rejection at 47MHz I can show you a 7th order LPF design but this will need very good quality components. It will punish any attempt to use sloppy parts because the full performance will not be realised.
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Old 12th Oct 2017, 10:44 pm   #18
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Default Re: SSB Signal Generator

I managed to find my old AD9850 evaluation board and blow the dust off it and do some tests on it this evening. This is the official AD9850 evaluation board from Analog Devices but it is very old. It's an early revision A evaluation board and the AD9850 fitted to it has a 1996 date code. So it may not have the same performance (in terms of spurious) as a newer production device. At work we tend to get pre production AD parts earlier than the mainstream release date but this AD9850 could be a regular production part. I can't remember...

See below for a plot of the sinx/x response with no reconstruction filter inline. The evaluation board does not have any LPF on it and it just spits out the raw unfiltered DDS output to a 50R BNC test port. It shows the expected response and there is 1.5dB droop by 39MHz as expected. Note that you need an analyser with a very flat response across this range, otherwise the analyser response may mask the intended response. The Tek analyser here is running in direct mode where the signal is fed direct into the ADC in the analyser without any frequency conversion. So the response of the analyser is flat enough for this test. I've got another AD9850 eval board somewhere but I think it was the clockgen version that uses the onboard comparator. But if I find it I'll post up a plot from it.

I also measured the first image at 86MHz when the AD9850 is set to 39MHz and it showed the expected -6.8dB relative level as predicted by the excel spreadsheet.

Quote:
I was planning on using the 9850 for HF and the 9951 for VHF.
I also found a few other old DDS evaluation boards and I spotted that I still have the official Analog Devices AD9954 eval board here. I think it is similar to the AD9951 so I could post up some performance plots from it if you are interested? This is a much nicer DDS as it is much cleaner than the AD9850 in terms of spurious. You might decide to use the AD9951 on the HF band as well because the AD9850 has so many spurious terms sprinkled across the range at -70dBc.
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Old 13th Oct 2017, 12:46 pm   #19
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Default Re: SSB Signal Generator

Hyas Jeremy.....I must thank you for the work you have put in.... without your help and my friend Steve that writes the software.. my projects would be "dead ducks".. I will make a new PCB today for the DDS 9850... and another for the 9951... and let you know the outcome in a few days...I have no way to repay you.. except to give you and Steve glowing references in any article I write or lecture I give at various radio clubs.
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