CZE-7C experimental mod: Helical filter

[BlackBeard]

What to do with a CZE-7C? I still have one of those chinese sprog boxes laying around that I bought a long time ago when I didn't know about proper VHF-equipment. As you know, it uses the BH1415F. I'm thinking of what to do with it now - maybe try something out. Would it somehow be possible to clean the output of the tx a little bit? I was thinking of a bandpass filter either directly after the ICs output (before the amping stage, probably hard to solder) or between the tx's output and the antenna. Of course this would only improve harmonics, spurs and overall band noise I guess, phase noise not so much - but I think a very narrow (bandpass) filter with a narrow bandwith would be hard to construct. I have a NanoVNA though, could also get a spectrum analyzer.

My idea was to construct a narrow helical bandpass filter and put it in between the tx and the antenna. I mean a filter like shown here: or here: I guess using self-constructed capacitors would be the better way. To calculate everything I wanted to use this calculator: https://www.changpuak.ch/electronics/He ... signer.php

[Krakatoa]

It will work and give you peace of mind getting rid of sprogs, BUT..., why not try to cleanup the source first?
I would build a separate oscillator with a mosfet or something that generates a clean signal and feed the RF amplifier strip from there.
Send a sample of RF to pin 9 of the existing BH1415 to use its internal PLL, and use the tuning voltage to control your external oscillator.
Keeping the BH chip and the oscillator power supplies separated and filtered from each other should yield a clean rig.
Then there is the stereo coder thing in the chip... well, that deserves another chapter...

[rigmo]

https://coil32.net/online-calculators/h ... ilter.html

[yellowbeard]

you can do all of that, but in the end you are just putting lipstick on a pig. The BH series chips are dirty, and dirty in close to the carrier frequency, my receiver auto tunes on to two of these before the fundamental - a helical filter would have little or no effect on these sideband spurii - and you're amplifying them in the next stage. It'd be better to use a proper exciter, but it'd probably cost £60 or so to make a decent stereo one. You bought a lemon...

[eiradioguy]

I heard a guy used one of these as an exciter for a 500w amplifier. Sounded just as you'd expect......

[Albert H]

The exciter I've been re-working lately costs around £28 in parts (cheaper if you shop around) and doesn't use anything exotic. The PLL uses discrete logic (though a PLL-chip version is entirely possible). The board size should end up at about the same as the old "Dawson" board, but give up to 12 Watts. Needless to say, it's designed for supreme harmonic and spurious cleanliness!

[BlackBeard]

The exciter I've been re-working lately costs around £28 in parts (cheaper if you shop around) and doesn't use anything exotic. The PLL uses discrete logic (though a PLL-chip version is entirely possible). The board size should end up at about the same as the old "Dawson" board, but give up to 12 Watts. Needless to say, it's designed for supreme harmonic and spurious cleanliness!

Thanks everyone

Rigmo, I found a similar calculator from Changpuak. Yours seems to be more sophisticated though.

Albert, is the design here on the forum?

[Albert H]

Not yet. When I'm happy with it, it'll be made available to all.

[Shedbuilt]

you can do all of that, but in the end you are just putting lipstick on a pig. The BH series chips are dirty, and dirty in close to the carrier frequency, my receiver auto tunes on to two of these before the fundamental - a helical filter would have little or no effect on these sideband spurii - and you're amplifying them in the next stage. It'd be better to use a proper exciter, but it'd probably cost £60 or so to make a decent stereo one. You bought a lemon...

Agreed entirely.

[Albert H]

Almost all of the "rig in a chip" devices are pretty useless in real transmitters. They're OK for their intended use - getting audio from a portable music player a couple of meters to your car antenna, so that you can hear your music player through the car radio receiver.

Unfortunately, the physical limitations of tiny silicon, and the tiny powers allowed for "licence-free" transmitters don't really allow the development of good quality transmitter ICs.

[radionortheast]

I have to agree, i’ve tried pretty much every chip out there, would be easy way to make a transmitter, they are available everywere, easy to tune with stereo. They are fine for in car transmitters, it's noise they put out across fm that is the problem when you amplifiy them, it will knock out every station, people in America use them live so far away from their nieghtbours, if you were on a farm it would be alright. You can see it on a tiny sa, it envelops the whole fm band in noise, a tall spike with a mountain at the bottom. A clear signal source you still see all fm stations, close proximity to the transmitter will reduce the signal strength of other fm stations.

The best you can get is if your transmitter on a low frequency to tune the rf amplifier, (different values of capactors in the output section using ic sockets) you will end up with abit of reduction in the top end of the band, it dosen’t stop the interference in the bottom end of the band.