HFU HF Underground
Technical Topics => The RF Workbench => Topic started by: Kage on November 25, 2015, 1730 UTC
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Just curious if one was to use a VFO for frequency generation how much it would take until it becomes obvious that the station is using a free running VFO?
Is 50hz drift over an hour varying +-25hz occasionally even notable on AM? (currently where I have my circuit zeroed in at, improving a tiny bit each time I try different capacitors)
I realize with SSB this could be an issue fine tuning on the listeners end but with AM I can't imagine this would be of issue correct?
I know there are a few people here who log stations down to 100hz, like 6925.5 for example, but as long as the signal is within a few 10s of hz would that many people even notice?
I worry because I damn near have this VFO built but worry that it will become blatantly obvious that I am using one to some people who are anal about logging frequency and I don't want to be that broadcaster that "drifts".
I know for a fact that pirates of the past used transmitters using VFOs, and it makes me wonder just how much they drifted over an hour or two of broadcasting after a basic warm up. Hell there are even pirates here still using tube based VFOs that broadcast. Those suckers have to drift a ton even when running to specification right?
Can anyone here that did that speak up and admit how much it drifted so?
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Is 50hz drift over an hour varying +-25hz occasionally even notable on AM?
Depends on the receiver bandwidth... but for the average AM detector it shouldn't be a problem. Years back I used to make VFO's with tubes and heating was a problem but I use to operate them on the 3885Khz AM window and no one said a thing.
The one thing you need to have is a good buffer between the VFO and the modulator stage to keep it from going into FM... now that can be a problem... :-)
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As a listener, I can say that 50 Hz of drift is no problem at all with AM. Even a few hundred Hz really isn't a big deal. It's a different story with SSB, of course, where even a few Hz can turn music into an annoying sound too painful to listen to.
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Hi Kage-
I've been watching your ongoing VFO build posts on your forum with great interest. Back in the 80s I occasionally ran a Johnson Valiant from the VFO when I needed to be where I had no rock. I never remember having had negative reports but then again, it was a long time ago :o What Vince said about buffering is true. FMing is a real issue, not only through the RF chain, but by perturbations through the power supply. With the good tight regulation you have with the LM431, that probably isn't an issue. Good luck with the project. BTW- nice pickup on that Amazon counter. Methinks I need one of those...
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Hi Kage-
I've been watching your ongoing VFO build posts on your forum with great interest. Back in the 80s I occasionally ran a Johnson Valiant from the VFO when I needed to be where I had no rock. I never remember having had negative reports but then again, it was a long time ago :o What Vince said about buffering is true. FMing is a real issue, not only through the RF chain, but by perturbations through the power supply. With the good tight regulation you have with the LM431, that probably isn't an issue. Good luck with the project. BTW- nice pickup on that Amazon counter. Methinks I need one of those...
Thanks for following. Yeah still lots of work to be done considering I am still at the VFO stage and haven't even moved on to the PA and modulator yet which will probably be a separate project, but at least that will be a lot simpler, or at least no where near as time consuming.
As I said on the other forum the stability is rock solid with almost any output load on the VFO. Even a direct short to the output has no real consequences to frequency stability. I am willing to bet running the VFO on its own power supply will help even that much more.
Yeah those amazon frequency counters are a steal. I can't believe they only cost $10 with free shipping! They are incredibly accurate and for the price you can pick up a few for various projects. Only issue I had with it is that it came in the mail in a small yellow envelope and was in bubble wrap, no protective box or anything, but it worked right out of the bubble wrap :P Just make sure to watch the youtube video on how to use the two buttons on the back to set 10/100hz measurement and brightness. The high brightness setting by default is blindingly bright.
They even sell one that can measure up to a few GHz for an extra few bucks.
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It sounds like your asking two questions; will people notice, or will it be a problem. The average Joe with an AM detector probably won't notice until the drift exceeds 500Hz or so under good conditions of propagation, he or she will simply notice a change in the perceived audio bandwidth as one edge of the audio sidebands become favored. This is why using a conventional continuous tuning receiver, AM signals sound flat when tuned correctly, and shrill as you tune to either side of the signal. People with AM sync or ECSS detectors won't really notice. Anyone with an SDR, as Chris has shown, will notice any small amount of drift over time, as the software allows you to look at long term trends. We can see when the heat in your place kicked on ;)
+-RH
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It sounds like your asking two questions; will people notice, or will it be a problem. The average Joe with an AM detector probably won't notice until the drift exceeds 500Hz or so under good conditions of propagation, he or she will simply notice a change in the perceived audio bandwidth as one edge of the audio sidebands become favored. This is why using a conventional continuous tuning receiver, AM signals sound flat when tuned correctly, and shrill as you tune to either side of the signal. People with AM sync or ECSS detectors won't really notice. Anyone with an SDR, as Chris has shown, will notice any small amount of drift over time, as the software allows you to look at long term trends. We can see when the heat in your place kicked on ;)
+-RH
Lol about seeing when the heat kicks in ;D
Yeah I guess I was asking how many people would really notice that little of drift or comment on it when logging a station.
I have my own project stabilized down to within 10-20hz drift +/- over an hour total now which is about as good as a VFO can get from everything I have read in ham articles on the subject. This is of course with the heat kicking in and cycling because of winter.
My other question was about how many pirates still use free running VFOs? Seems like everyone I recall logging were probably rock bound or similar but I fell into the category of one of those average people with a basic radio, not someone using a SDR to listen at that time.
Thanks to those cheap $12 TV tuner dongles now I am in that group of people playing with SDRs which is what triggered me to ask this in the first place 8)
Also sorry about that goofy post above, removed it since it wasn't relevant to the thread. Was a long day when I posted it heh.