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Old 15th Aug 2016, 10:57 pm   #41
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Default Re: SSB Exciter... HELP !!

Quote:
If you have a network analyser or a spectrum analyser with a tracker, sweep the filter with a resistive load, then pad out the resistance. Play around with the resistance until you get minimum ripple in the passband. Then try adding a bit of C to ground to see if you can get it a bit lower. If so go back and muck about with R a bit more. You should see a filter of the described bandwidth and with reasonable ripple now.
Yes, that's pretty much the way I did it with a spectrum analyser and Tgen. At filter frequencies below 10MHz I found that I could use a series Bourns trimmer resistor in the input and output and trim for lowest ripple on the analyser. Then double check with a regular fixed resistor(s) in place of the trimmer.

The filter impedance was then simply the sum of the resistor value + 50R and the resistor could then be replaced with a captap network that matched to this.

I had a lot of success with this method over the years.
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Old 16th Aug 2016, 1:44 am   #42
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Default Re: SSB Exciter... HELP !!

The other alternative to fitting a cap across R5 would be to pad the 47nF decoupling cap C1 with a big 47uF cap. This may work slightly better as long as you can maintain good decoupling here from AF through RF.
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Old 16th Aug 2016, 10:18 am   #43
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Default Re: SSB Exciter... HELP !!

Hi Jeremy. Thank you for the advice.... I will add the capacitor as suggested.... and also trim the gain.. at one point I did have a 1K across pins 2 & 3....... cannot remember why it was removed,, but its a SMD anyway..
Im getting there..... it looks like I will use the 9.0015 Filter.... and I have an old synthesiser that I can use to generate the L/O for upper and lower sidebands. I had a few samples from Plessey back in the day.. as this is a "one off" I am not bothered about using antique devices.
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Old 16th Aug 2016, 1:59 pm   #44
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Default Re: SSB Exciter... HELP !!

Hi Graham....Regarding the impedance matching of the Filter to the Mixer... I have done some of my own calculations... all be it.. probably wrong.. but here goes... The output Z of the mixer , I measured at 630R... the Input Z to the filter = 500R @ 32pf... Thus I calculated that the Top cap should be 133pf = 132 Z, the bottom cap should be 36pf = 491 Z ... If I want a resonant circuit... I ignore the 36pf in series and the inductor would be approx 2.4uH...... I hope this is nearly right..... To be honest... I have NO idea how to use a Smith chart, and without a commentary on your youtube posting, I didn't understand it...
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Old 16th Aug 2016, 5:20 pm   #45
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Default Re: SSB Exciter... HELP !!

Hi Peeps... After the help from Graham there have been a few modifications. I am hoping now I can progress.
The Audio pcb now has a switch to select Pre or Post VOGAD..this allows me to insert a test signal post Vogad, and use the Vogad with a microphone.
A matching circuit has been added to the Filter input, whose values may still be required to be changed.
A 27 ohm resistor added to the MC1496 between pins 2 & 3 to lower the gain slightly after adding a 47 uf decoupling cap to the junction of L1 and R3.
Any comments will be well received, even negative ones.
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Old 16th Aug 2016, 5:58 pm   #46
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Default Re: SSB Exciter... HELP !!

Oooooh!

Kristal Verarbeitung Neckerbishofsheim! KVG

Probably the poshest of all crystal filters. Certain amateurs in-the-know would have killed for KVG filters. Now an absorbed part of Vectron

David (C.E.P.E. division of Thomson-CSF were a bit more capable for difficult ones)
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Old 16th Aug 2016, 9:58 pm   #47
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Default Re: SSB Exciter... HELP !!

Maybe someone else can comment but the reason I suggested adding the 47uF cap (C12) across C1 was to prevent the possibility of having the AF drive waveform present on both of the differential pins 1 and 4. As drawn previously, there would have been a leakage path between these two pins at AF (via R4, R3 and L1) and with little phase shift between these inputs so it appeared very unconventional to have AF on both these pins. I think it is much more conventional to try and minimise the AF that can leak to pin 4. This extra cap helps a lot here?

Having seen the way you want to connect the filter 'directly' to the modulator in your latest circuit I think I would have experimented with the marked up circuit I've linked to below. You can see that it doesn't use a captap matching network on this side of the SSB filter.

This uses your existing transformer and also some load resistors to help control the range of load impedances seen by the MC1496 across the passband and stopband of the filter. It should also help with passband ripple once the resistor values and the transformer are optimised.

I've had a guess at the values in the circuit but they would probably need some tweaking. I don't know how lossy your transformer will be so I've assumed it a low loss type in the circuit below. If it is fairly lossy then the 3k3 resistors will need to go up in value or you could play with the turns ratio of the transformer?

My earlier captap matching network is better suited on the other side of the filter in case you wanted to match to 50R after the filter? Note also that I don't think I've used this chip beyond 1.4MHz in the past... Others may have more experience using it at 9MHz?
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Old 16th Aug 2016, 10:20 pm   #48
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Default Re: SSB Exciter... HELP !!

The reason I'm suggesting the load resistor method is because this circuit also works on receive as a mixer and I think it is risky to let the mixer look into a high impedance in the immediate stopbands of the filter. Often, an xtal filter can have a very high impedance on one side/stopband.

A big nearby/adjacent/unwanted station here could cause intermodulation in the receive mixer because the mixer voltage gain is a function of the impedance seen here. Big impedance = BIG voltage gain and the possibility of clipping at the MC1496 output pins leading to splatter in the mixer. So the resistors act as a ceiling for max impedance seen here as well as helping with filter matching once you get the turns ratio and the correct value for these resistors.

I guess the alternative is to have a lossy transformer and having just looked back at the picture of your real PCB then I'm going to guess that it might be quite lossy already! However, it might still be worth experimenting with adding large value load resistors here and use them to help with the matching. They might end up being over 10k ohm each but this depends on your transformer. I can also now see that it isn't practical to try and alter the turns ratio of that transformer.
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Old 16th Aug 2016, 10:39 pm   #49
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Default Re: SSB Exciter... HELP !!

Hi David and Jeremy.... David I bought the filters on Ebay for approx £25 for 3 on a pcb with high grade components including 3 x MC1496's
Jeremy.. This whole thing started after googling and finding a circuit by Julian Rosu VA3IUL.. 20m RX/TX .. I decided that it would scale down to 40M...
Ok about the filter re arrangement. I gather the "passband ripple" is the flatness or not at the top of the peak..I had not realised this should be more flat than it is,. so I will work on that..
At the moment the receive part is not really an issue but I take on board what you are saying and will look at the tank circuit tomorrow. I am using a Toko coil assy with ferrite core and outer adjusting core.. it may be the X10 style. There are 5 x 5 turns and 3T on the secondary...
My output from the filter was to a FET amp.. see attached..Its slightly changed now as there will only be one filter and no reed relay.. again this is part of the VA3IUL circuit.
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Old 17th Aug 2016, 12:49 am   #50
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Default Re: SSB Exciter... HELP !!

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Hi David and Jeremy.... David I bought the filters on Ebay for approx £25 for 3 on a pcb with high grade components including 3 x MC1496's
I can remember when you bought these. I was tempted to dive in and get a set for my own use...

At work I had the pleasure of designing various RF downconverters and receivers back in the 1990s and I often used (21.4MHz) KVG filters. I used to have their catalogue but it's long gone now. Looking back through an old company partlist and some notes I would have used filters like these:
KVG XF-214S14 (this was 30kHz BW)
KVG XF-214S57 (this was 50kHz BW)

I also spotted the KVG XF-214S82 with 150kHz BW and the KVG XF-214S84 with 300kHz BW in my notes. I'm not sure what was actually inside the wider BW filters and I'm not sure if I ended up using them. But the KVG filters were very pricey and had very impressive specs. We started using cheaper filters from Golledge and eventually stopped using crystal filters altogether as all our customers wanted wider bandwidths with digital IF technology. I started designing our own in house high order LC filters once the IF BW went up above about 100kHz.

It's definitely worth getting the best out of your KVG filters in terms of matching etc as they are capable of very good performance.
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Old 17th Aug 2016, 1:18 am   #51
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Default Re: SSB Exciter... HELP !!

To give you some idea of what your KVG filter will be capable of I had a rummage and found an old Yaesu 8.215MHz SSB filter in my box of crystal filters. This looks to be a very nice filter and I measured it on the VNA and exported a 2 port model of it to a computer. The VNA can do this very quickly.

I then used an RF simulation program to design a matching network for the filter and this produced a prediction for how the filter would look when matched with this network in terms of return loss, group delay and passband ripple. I then found/wound the required parts and fitted the matching network to the filter and compared the real filter and matching to the simulator prediction.

You can see the results below. This old Yaesu filter can give very low passband ripple when the matching network is optimised on a simulator. If I'd used a more complicated matching netowork and set up the auto optimiser I think I could have go it even better. But this was the result of maybe 20 minutes' work on the computer

The dark blue trace is the real filter with its matching network and the red trace is the simulation/prediction of how it could/should be. The best graph to look at is the one in the lower right corner as this shows ripple on a 1dB/div scale. It looks to have very low ripple

You can also see on this graph how well the simulator has predicted the return loss response of the filter. Given more time I think I could have made the two graphs agree even closer but this would have added little value.
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Old 17th Aug 2016, 10:17 am   #52
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Default Re: SSB Exciter... HELP !!

Hi Jeremy..First I noted the time you posted your last comments.. 1:24am.... I hope that was not due to my problem...but if it was.. Thanks.
Second.. I will add those two resistors in the tank circuit and let you know.. adding the matching caps and L certainly improved the signal level drive to the 2nd MC1496 7 Mhz converter.
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Old 17th Aug 2016, 12:22 pm   #53
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Default Re: SSB Exciter... HELP !!

Hi Jeremy. I tried damping resistors across the Tank inductor as suggested..however I was not getting any changes... then I realised..have we been talking at cross purposes..
I then connected the Rogol tracking generator output to the RF input of the modulator, wound up the Balance pot to get max rf out and then looked at the crystal filter performance... and Voila. Now I have the actual frequency response ... I then added various values and more importantly removed components.. and as you can see the responses are now clear.
The previous photo's were of the RF output of the filter with 2 X tone modulation.
Looking at the photo's I think the best response is the No Damping resistors but keep the matching network and maybe trim the component values.
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Old 17th Aug 2016, 9:06 pm   #54
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Default Re: SSB Exciter... HELP !!

The response does look good with the last plot. I suspect that what you have now is better than most hams will have homebrewed/produced in terms of ripple etc.

I suspect that your transformer is lossy enough to have the approx. equivalent of the 3300R resistors already in the form of core losses. If an inductor winding of about 3.5uH has a Qu of about 33 then its parallel resistance is 6700 ohms which is approx 3300 x2.
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Old 18th Aug 2016, 10:28 am   #55
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Default Re: SSB Exciter... HELP !!

Hi Jeremy. The Tank coil is a Toko Adjustable ... lots of ferrite... I cannot see any advantage of winding a normal coil.. sizewise anyway.
I have redesigned and built the IF Pcb... test it this AM... start on a better L/O. I cannot pull the crystals I have far enough. it may be a synthesizer chip.
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Old 23rd Aug 2016, 11:31 pm   #56
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Default Re: SSB Exciter... HELP !!

Hi peeps....yes I have been a bit quiet of late on the Exciter front... but I am progressing...I have now made a VXO for the 9 Mhz +/- 400 Hz. I didnt realise that I could pull a crystal up as well as down in frequency.. till I used the attached circuit.
It has been on test for a few hours with about 40 Hz drift, in OPEN to atmosphere changes, i.e in sunlight and a warm ish room. The drift is well within the crystal passband... and I wonder.. from you guys...if this will be ok, especially in a sealed box.
I also had difficulties reducing the RF out as it was way too much for the MC1496, but in the end a resistive divider did the trick.. it is now just above 100m/V pp.. from 8V pp.
I dont really want to make a synthesizer just for 2 frequencies.
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Old 29th Aug 2016, 3:32 pm   #57
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Default Re: SSB Exciter... HELP !!

Hi Peeps... I attach the Basic Block diagram... and the Audio output stage, which includes a Baxendall Filter to reduce upper frequency noise. There is a switch to allow more HF response if required. Anyone got any thoughts about a more simple filter... if not I will continue with this one... The resistor values were chosen after pots were set then removed and fixed resistors were fitted.
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Old 30th Aug 2016, 6:21 pm   #58
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Default Re: SSB Exciter... HELP !!

If all you need is a simple HF roll off, maybe a two stage simple ladder filter would do- if the source impedance is fairly low and the load fairly high, then a couple of RC stages of the form R-C followed by 10R-0.1C will give a good approximation to 40dB/decade roll off. Pick 1st stage R to be about twice source impedance.....
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Old 24th Sep 2016, 7:12 pm   #59
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Default Re: SSB Exciter... HELP !!

Hi Peeps....... 3 weeks on holiday is a long time in electronics...I started to build the 45W RF Linear FET amp from the bought from inscrutable chinese. "see Chinglish post"..I was happily adding the SMD components and realised there were more component /pad locations. I checked the PCB version which should have been V303. Instead they had sent a kit with a V306 pcb.
I contacted the vendor. just before I went away....and I received a reply fairly quickly, but as I was sunning myself I could not do anything about it.
In a nutshell, they had sent me a 100W Kit............ mostly the components are the same, but many are not. It has taken me the best part of 2 hours to replace some parts and check others.
The heatsink is a little difficult to source, but found one on ebay.
I attach the schematics of both versions for your interest.
Not got around to adding the fets yet or winding the coils... next job.
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File Type: pdf RF_AMP_2078_SCH_V306.pdf (23.1 KB, 261 views)
File Type: pdf _RF_AMP_2078_v303.pdf (54.7 KB, 136 views)
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