In another post under 'Hombrew' I wrote about the MW Modulator that I built from a Practical Electronics magazine article a little while ago. I mentioned that a scope and frequency counter needed to be used to set up the VFO and the modulator. Whenever such test gear is mentioned, it instinctively tends to turn some people off, even if the actually possess such test gear, as they may assume that the project is complex and difficult to get going. I've attached some pics to show ow simple this really is.
The first pic shows the VFO frequency of my MW Modulator project displayed on my homebrew nixie tube frequency counter. I decided that a centre frequency of 650 kHz (460 Metres) would give a reasonable tuning spread either side of that frequency to find a vacant slot on a radio in the MW band, so need to adjust the core of the VFO till 650kHz was displayed. I then checked the tuning range of the variable cap by first trying it at the fully meshed position, then fully open, and read these frequencies on the counter. This happened to be 697 kHz and 612 kHz - 430M & 490 Metres, so that gave a nice spread across the MW band to find a vacant slot on which to tune in the modulator on the radio. Basically, you simply connect the device to the radio antenna socket, play whatever you want through the device from say an FM radio, i-pod or whatever, and tune across the band from 430 - 490 metres till the signal appears.
The second pic shows how the lead from the frequency counter (simply a loop of insulated wire to the input of the counter) is placed over or close to the VFO coil to get a read out on the counter of the tuned frequency.
The third pic shows a scope trace of an unmodulated sine wave of the centre frequency of 650 kHz (460M), and the final picture shows the sine wave from the VFO modulated by a 1kHz tone from a signal generator to show that the modulator is working.
If either the VFO or modulator didn't dsiplay these expected traces on the scope, or the desired frequencies on the counter, that would be a cause for troubleshooting the device, and using the scope and counter again to check if all was well. For example, if the coil that I'd wound was too low in frequency to give the desired band coverage, I'd have had to remove a few turns and test the coverage on the frequency counter again. If it was too high in fequency, I'd have had to add a few turns.
Just a basic explanation as to how useful scopes and frequency counters can be used very simply be in setting up homebrew projects or in fault finding on electronic equipment. I hope it's useful info, and helps inspire others to dabble about with homebrew, and get to know their test gear to diagnonse and rectify faults.
David.
The first pic shows the VFO frequency of my MW Modulator project displayed on my homebrew nixie tube frequency counter. I decided that a centre frequency of 650 kHz (460 Metres) would give a reasonable tuning spread either side of that frequency to find a vacant slot on a radio in the MW band, so need to adjust the core of the VFO till 650kHz was displayed. I then checked the tuning range of the variable cap by first trying it at the fully meshed position, then fully open, and read these frequencies on the counter. This happened to be 697 kHz and 612 kHz - 430M & 490 Metres, so that gave a nice spread across the MW band to find a vacant slot on which to tune in the modulator on the radio. Basically, you simply connect the device to the radio antenna socket, play whatever you want through the device from say an FM radio, i-pod or whatever, and tune across the band from 430 - 490 metres till the signal appears.
The second pic shows how the lead from the frequency counter (simply a loop of insulated wire to the input of the counter) is placed over or close to the VFO coil to get a read out on the counter of the tuned frequency.
The third pic shows a scope trace of an unmodulated sine wave of the centre frequency of 650 kHz (460M), and the final picture shows the sine wave from the VFO modulated by a 1kHz tone from a signal generator to show that the modulator is working.
If either the VFO or modulator didn't dsiplay these expected traces on the scope, or the desired frequencies on the counter, that would be a cause for troubleshooting the device, and using the scope and counter again to check if all was well. For example, if the coil that I'd wound was too low in frequency to give the desired band coverage, I'd have had to remove a few turns and test the coverage on the frequency counter again. If it was too high in fequency, I'd have had to add a few turns.
Just a basic explanation as to how useful scopes and frequency counters can be used very simply be in setting up homebrew projects or in fault finding on electronic equipment. I hope it's useful info, and helps inspire others to dabble about with homebrew, and get to know their test gear to diagnonse and rectify faults.
David.
