Spectraman reviews of 2 new chinese TX's (look away if not interested)

[FMEnjoyer]

Blue one that EFR checked out as well, seems he measures a bit less harmonic and nothing below carrier.



VCO PLL - seems to be terrible according to him - 6 months old -45db or so in aircraft band



And this follow up of same one :



The reality of these is just cause they sent him a best case scenario review one what they might send to others remains completely unknowable so something you might get will have a strong second harmonic, they may just have doctored the one they sent for a review.

If they can get the second harmonic down to - 40 or so why cannot the fools who make this stuff add a low pass filter that would cost them $4 with minimal power losses 4 coil/4cap.

[radionortheast]

I don’t know if it was these but I liked it were he showed the difference between mono and stereo, it was a bh number chip, (a slinky number ) it was like a hedgehog when stereo was activated in mono it seemed alright, with a bh chip you could connect your own encoder, thats why they ended up with an mpx input.

i think they made an error trying to power it on 13.5v, i think they were maybe thinking, they would get extra watts, likely putting high voltage into the chip. (maybe it could end in a chip pan fire ) i was waiting for them to switch the scanner to nmb mode, they would hear the carrier drifting off frequency it was abit disappointing, they didn't show that.
testing something to see if it gets warm is a good test, specially of an rf output device, it takes a while for power supply to get warm, your best off looking at the voltage seeing if it drops, know if the power supply can handle it. if its been run to the max their might be different loading conditions so it might blow anyway, relax with the max lol

A bh puts out small spurs close to the aerial, likely hear it in ajoining rooms on other frequencies, likely with an aerial indoors, the qn puts out noise which covers the fm band which goes out further, with 5-7w with a loft or outdoor aerial it would be unlikely for your neightbours to hear anything on fm. The best option is if you wanted something to go to pcs buy something that isn’t a chip, stereo encoders will have a filtered output, which the bh chips don’t have, I suppose then you can hear shania without bothering anyway.

Theres not a tx that dosen’t have a filter, i think what people are meaning when their adding an extra, the amplifier isn’t good quailty so will spur without a match, its kind of stab in the dark that it works for them, without changing values of c. If you make a filter you connect to the back of any transmitter, changing the values of c get the perfect match, none of the signal will lost, get rid of spurs the amp maybe creating from having an imperfect match, it won’t take away the other stuff that the chip is creating, if it is one, which it doesn't have to be.

[radionortheast]

I think it was EFR who tried out one of these or a simliar one, i’m guessing it might of been a bh chip, with them been able to pick up other stations.

[Albert H]

Please stop wasting everyone's time with these worthless noise generators. NONE of the "transmitter" ICs are capable of truly clean operation (at ANY supply voltage).

These ICs were developed for just one purpose - to allow cordless connection of a portable CD or MP3 player to a car radio from inside the car's cabin. They were NOT designed to be amplified: they would just generate a couple of milliwatts of RF - possibly with a crude switching stereo coder as part of the deal - and that's it. Spectral purity wasn't an issue, because the signal strength was so low that it wouldn't get far outside the car. A PLL was included in later ICs, to keep the things on frequency, since most modern car radios also have PLLs for frequency stability.

The very first time I saw one of these things, it was in a little metal box, with a car aerial socket in it and a coaxial lead with a car aerial plug on the far end. There was also a +12V lead to power the thing. It was designed to be plugged directly into the car radio, and the car aerial was to be plugged into this little metal box, and there were a stereo pair of coaxial leads with stereo 3.5mm plug on the end coming out of the box, to plug into a CD or MP3 or cassette player. As I recall, it was a "Yamaha"-branded effort, and it could be tuned over part of Band II by means of a screwdriver adjustment. It didn't have a PLL, and it drifted!

If you want a cheap rig, learn how to build a proper one yourself. There are kits around for beginners, and they're not difficult to construct if you know how to solder neatly. If carefully constructed, you'll find that the output will be clean (many million times cleaner than one of those "audio sender" ICs), and sensible choice of operating frequency will avoid interference to other services. Forget these Chinese sprog-boxes!

[FMEnjoyer]

I have asked the mod to delete this, sorry to those who posted.

[Albert H]

Someone quite nearby to my Dutch demesne had one of these efforts, and when they fired it up, it wiped out band II (and part of the Airband) for a couple of kilometres. One of my neighbours - a radio amateur - tracked down the source during the first evening it was on. He told me that as he got closer to the site, the hash on his portable spectrum analyser got wider, until - when he was right outside the houseit was in - it was providing noise to over 1.4 GHz! It was marked as a "15W stereo transmitter", but most of the 15 W was on frequencies other than the one it was supposed to be generating.....

The owner of the device was aware that it was causing interference to other FM stations, but assumed that this was just a local phenomenon. He stopped using it as soon as the problem was explained to him.

My neighbour has offered to provide the lad with a proper 15W band II rig (despite being a radio amateur), and the kid is happy to pay for it. He'll be on the air (properly) in the next couple of days, and won't be causing gross interference! These Chinese sprog boxes are a disaster!

[yellowbeard]

In fairness, it'd be a struggle to make a proper 15 Watt stereo RDS box in a nice case and be making a profit for the hunnerd Euro aliexpress want for that yoke.
https://www.aliexpress.com/item/1005008511075549.html
Thats the detail the devil is in, it's cheap because it's cheap.

[SD-E1102]

I'll keep this thread up as it's quite informative to those who might be doing research on the Chinese txs.

[Albert H]

We put together a little prototype rig earlier today. It has a Kallitron ½f self-doubling oscillator with two tuning varicaps (and a voltage multiplier to allow it to cover the whole band), a TSA5511 PLL IC to tune it. There are separate varicaps for the modulation. There's a Soshin Band II filter after the oscillator (to make sure that no ½f gets through), and AT Tiny IC to program the PLL IC, a 4066 to turn off the data lines to the '5511 after the data is loaded (to stop the dreaded ticking) and to change the loop filter speed when locked (to allow low bass through). There's a stage of amplification feeding a RD15HVF, which uses the case as its heatsink. It consistently delivers just over 18 Watts very cleanly with a 15V supply whereever it's set to in the band, and fits neatly into a Hammond diecast box.

We added a simple current monitor to the output stage (if it goes into a bad match, the power to the PA stage is switched off), and it's also got an overheat alarm which also turns it off. There are no measurable in-band spurs, and the second is -74dBc at worst. The 3rd and higher harmonics are down in the "grass".

Frequency is fixed (though there's a programming socket), and it's mono. It has a 50Ω BNC output socket, a 3.5 mm jack socket for modulation, and a concentric power plug (the socket is on the power supply lead). There are a couple of status LEDs to show you what's going on. It's fully modulated by 3.086V RMS (PPM 6) and provides a 1kΩ input impedance. Current draw into a 50Ω dummy load is just under 2A. Modulation bandwidth is ample for stereo.

There are NO adjustments - it just does what it does very well.

We priced up the parts - it came to €61.91, which is just under £54. Neglecting construction time (just under 2 hours), it really didn't cost much!

This one was a prototype, and the board was etched here during the week. We're going to send the artwork to JLC and get a little pile of boards made. I'm also considering a frequency-agile version (possibly with an LCD display!). The client for these wants quite a few of them, so the development time hasn't been wasted.

You won't find ANY of the Chinese efforts that comes anywhere near these, at any price!

[RF-Head]

bla bla you always make the best transmitters.....
You make many boards but never show any picture (like now)
I think you make the transmitters in your head!
Sorry to say but maybe in the 80s you made a lot of transmitters but don't beleave you still make them

[Marcel]

Albert H,

It would indeed be nice to see some photos or diagrams, you often talk about the most fantasatic solutions but we never see practical solutions.