PCS Electronics TX - Sprogs problems

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NOYB
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PCS Electronics TX - Sprogs problems

Post by NOYB » Thu Jul 26, 2018 2:35 pm

Esteemed circuit designers and builders - do you have an ideas to help me improve the spurious on this TX? It is a MAX PRO 3000+ FM Exciter.

It has a nasty, noisy sproggy mess at around 50MHz (see attached photo). I've tried it with more than one power supply - current one is a new Mean Well SMPSU and I've tried adding big and small capacitors on its output to further smooth it.

Image

The manual for the unit is linked below. Unfortunately it does not include a schematic or circuit description.
http://www.pcs-electronics.com/software ... 00p_en.pdf

I'm just wondering if anyone knows anything about the design and the likely fix? Maybe something needs extra decoupling somewhere?
Failing that - any suggestions for a good BPF? - noddy instructions to make one, or good one to buy? Maybe a tweaky capacitor on the output filter might work?

All ideas and thoughts greatfully received. Thanks for your time.

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Re: PCS Electronics TX - Sprogs problems

Post by NOYB » Thu Jul 26, 2018 2:39 pm


MC Spanner
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Re: PCS Electronics TX - Sprogs problems

Post by MC Spanner » Fri Jul 27, 2018 1:04 am

That mess is typical of SMPS's and gets worse on load. Try a linear PSU. How's your spectrum analyser coupled to your TX output? The mess at around 50MHz is unlikely to be radiated by your TX. Move the analyser away from the PSU.

I am concerned however, about the elevated noise floor surrounding your carrier - you need to see if that's noise from the PSU actually being radiated by the TX (i.e. modulating the carrier in some way) by looking to see if it's still on the trace once you've got the SMPS away from the analyser and that nasty mess further down isn't showing. If you've still got stuff around the carrier, that's the part you'll have to sort out. Obviously filtering the PSU, although Meanwells are usually quite good, and other common treatments actually in the TX include decoupling or putting traps in the collector / drain circuits of the driver and/or PA stages. I'm not familiar with your TX though and without a circuit diagram that's about the limit of the help I can offer right now.

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Re: PCS Electronics TX - Sprogs problems

Post by nrgkits.nz » Fri Jul 27, 2018 2:46 pm

NOYB, are you using a scope or analyser? If it's a scope you're using, it will have a high impedance input more than likely and you're going to have reflections and all sorts of things going on there. Have you got the transmitter terminated into a 50ohm load while under test? All these things affect the reading you will get on the instrument. Some transmitters burst into self oscillation when presented with a bad or open load. Ideally you need a 50ohm attenuator to drop the output to the level required by your analyser, the input to the analyser must be 50ohm. Take a high res photo of the pcs board, I'd be interested to see what they're using as an oscillator, it could be using digital synthesis or one of those all in one fm chips that the Chinese use.

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Re: PCS Electronics TX - Sprogs problems

Post by radium98 » Fri Jul 27, 2018 3:04 pm

pcs very bad exciters i have problems once with the airport :( sad history where spirius fall on aviation frequency that time i think it was 118.xx mhz

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Re: PCS Electronics TX - Sprogs problems

Post by NOYB » Sun Jul 29, 2018 1:25 pm

Thanks for your thoughts. The TX is coupled directly into an analogue HP spectrum analyser directly via 50ohm pads and UR67 cable.

I had not thought of coupling from SMPSU to the analyser - so I just ran the PSU on its own with a wire "antenna" connected to the anlayser and laid across the PSU. It showed some crud up to about 10MHz, but nothing at 50MHz.

Here is a photo of the board in case it helps.

http://i67.tinypic.com/14y9u8w.jpg

Is it me - I can't see a raised noise floor around the carrier - or if it is, it's 70dB+ down so I'm not worried. the 50MHz crud is annoying though!

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Re: PCS Electronics TX - Sprogs problems

Post by radium98 » Sun Jul 29, 2018 3:09 pm

Can you share the control window software of it

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Re: PCS Electronics TX - Sprogs problems

Post by NOYB » Sun Jul 29, 2018 7:22 pm

I'm not using any software...

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Re: PCS Electronics TX - Sprogs problems

Post by radium98 » Sun Jul 29, 2018 7:34 pm

ok ,but i think there was a software that you can control the exciter from a window application.

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Re: PCS Electronics TX - Sprogs problems

Post by NOYB » Sun Jul 29, 2018 10:26 pm

I'm wondeering how the sw is related to the problem. Or do you just want a copy of the software?!

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Re: PCS Electronics TX - Sprogs problems

Post by MC Spanner » Mon Jul 30, 2018 12:31 am

Can't find a circuit for this thing but suspect it's an on-frequency VCO driving a chain ending up in something like an RD30HVF1. I'm sure someone will correct me.

Closer inspection of the trace makes me think the elevated noise floor up to about 140MHz is the "tail" of the crud centred around 45MHz. You're right - it's way down by the time you get to 100MHz. There's a repeating spike which looks like there's something going on at 18MHz somewhere. And yes, there's some real nastiness going on at just over 50MHz and it's mixed with lower frequency stuff which is creating the skirt around the peak. If it was related to the TX's RF chain then I'd expect to see something around your carrier but that looks clean and I can't see any close-in sprogs either.

So, I still think it's digital crap (and the small possibility it's getting into your PA chain, which is most likely quite wide) and it really does look like behaviour I've observed with many SMPSs including Meanwells, albeit the 400 or 500W models. You get a lot of crud appearing when the PSU is on load, and I don't know the spec of your PSU but 2 or 3 Amps required by your board shouldn't be asking too much of a reasonably specced unit. By the way, you implied your SMPS was off load when you did your test with the wire laid across the PSU so it might not have been representative - you need to be loading the PSU to see the crud.

You can rule out the SMPS by using a linear PSU. Or, a car battery should work with that board. There's a possibility other types of SMPS will produce different types of crud - so if you must use another SMPS, look closely at the analyser trace for variances from your observation with the Meanwell. If you can rule out the SMPS as the source of the noise then the next place I'd look is the MCU and other logic on that board. In this case, try running the board on low power and seeing if things change. You may also see changes in the makeup of that crud with a SMPS when using lower power too.

By the way what's that little loop of bell wire sticking up off the board near "LOW PWR" on the silk screen?

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Re: PCS Electronics TX - Sprogs problems

Post by NOYB » Mon Jul 30, 2018 9:27 am

Thanks MC Spanner

The bellwire is just my bodge for a jumper to set it between low and max power. I'm not using a PA or anything - running it at about 1w, but can run it up to 15w.

Yes the SMPSU was off-load when I did that test.

Yes I must try and find a 12v battery. I'm not convinced that is the problem though because I tried it with another PSU (Probably SM, I'm not sure) and the sprog pattern was exactly the same.

Shame I can't get a circuit diagram. I must say I've not found after-sales service to be a strength at PCS. Whenever you ask they just say "that model is obsolete, upgrade to a newer model"! I don't think I'll be buying anything more from them.

Thanks for the thoughts and ideas! I'm wondering, looking at the coils near the output, whether it might be possible to create some BPF response by adding some capacitiors in strategic places? I know a better fix would be to stop the crud generation in the first place though!

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Re: PCS Electronics TX - Sprogs problems

Post by MC Spanner » Mon Jul 30, 2018 1:24 pm

Hmm, OK then - sorry if I've led you down a blind alley there. Still looks like digital crap though, getting into the RF strip. There's quite a lot of logic on that board. Hard to know where/what to attack to improve matters without a layout / schematic, and converting the LPF to a bandpass response isn't trivial! At 50MHz you're 50dB down on your Watt though (effective 0.01mW), if that's any consolation, so even if you do radiate it, it won't go far. And like you say, in Band II it's at least 70dB down on your Watt.

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Re: PCS Electronics TX - Sprogs problems

Post by radium98 » Mon Jul 30, 2018 4:26 pm

i said we had a bad experience with this companie with 2 rds 5000 or 6000 i dodnt remmber and no luck to get help with comunication with pc while after a time we locally repaired the problem and they were brand new ,only one thing is good with their pallets 350 and in that time 600/800 watts

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Re: PCS Electronics TX - Sprogs problems

Post by OgreVorbis » Mon Aug 06, 2018 7:42 am

I've had this experience with their older boards. The maxpro 7015 and 8015 don't seem to have this issue. They still have a little bit of spurs, but at least -60-70dbc.

Also, don't touch their STMAX. It uses a single chip design that's even worse than the infamous BH1415F.

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Re: PCS Electronics TX - Sprogs problems

Post by OgreVorbis » Mon Aug 06, 2018 7:44 am

running it at about 1w, but can run it up to 15w.
That was another thing I remember about those. The spurs actually go down the more power you output. So at 15W, it might be less.

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Re: PCS Electronics TX - Sprogs problems

Post by radionortheast » Tue Aug 07, 2018 3:02 pm

the audio is better on the max pro 3000 + than pcimax 3000 + , it is still going i’ve had it since 2010, it was definately better than the pc cards that came after wards, it had a screen can round the oscilator was bang on frequency. It did alot on a small board, I don’t have a stereo encoder with the pro 3000 +, I quite tempted to get the 4w stereo transmitter, the st max the above poster has stopped me :tup

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Re: PCS Electronics TX - Sprogs problems

Post by NOYB » Sat Aug 11, 2018 6:56 am

OgreVorbis wrote:
Mon Aug 06, 2018 7:44 am
That was another thing I remember about those. The spurs actually go down the more power you output. So at 15W, it might be less.
Yes I'd noticed that too - but I want to run it low power for around the house. I guess I could burn power and then attenuate - but need to be sure it dissipates the heat well enough because it does get hot!

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Re: PCS Electronics TX - Sprogs problems

Post by Albert H » Mon Aug 13, 2018 12:29 am

I found that I've got an early 5 Watt PCS exciter here. It uses a dual gate FET oscillator, then a couple of little bipolar transistor stages and a 2SC1971 final. At 15V supply, it does just over 6 Watts, and is remarkably clean. It's an "at frequency" design, and the second harmonic is -71 dBc when correctly tuned, and I can't see any higher harmonics. The phase noise is low - it's a remarkably quiet carrier. This thing is so good that I'm thinking of boxing it up and using it as a test rig!

The output power is variable (the voltage to the driver stage can be varied), but when you get down to about 2 Watts, nasty things start happening - the output stage is then very under-driven and goes very dirty and no amount of trimmer tweaking can stabilise it. The whole thing is on a double-sided PCB about 10 X 12 cm with a separate small LCD PCB connected down a flat ribbon, and has output match trimmers which you peak using a bar-graph forward power indication on the LCD.

I was actually quite surprised by the good performance of this little board!
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Re: PCS Electronics TX - Sprogs problems

Post by NOYB » Tue Sep 04, 2018 3:52 pm

Yes I've come to the same conclusion. At full power it's OK. At lower power lots of nastyness going on! After sales service is awful though - it's just "that model is obsolete, buy a new one"!

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