You are talking about frequency drift? Can you quantify the observed drift?
I asume it does.but why this does not happen in the KHz frequencies as well?
Hi,
I asume it does.
But with 1MHz a drift of 0.1% is 1kHz drift.
With 1kHz and a drift of 0.1% is just 1Hz of drift.
Klaus
Hi,
what causes the drift?
* a drifting power supply? --> then stabiize the power supply.
* temperature? --> you may find out which device is sensitive with an coolant spray or a soldering iron (no need to touch the device, just put the soldering iron tip close to the devices)
* what else?
Klaus
The tuning capacitor is 100pF. This is too small for LF to HF use but he uses this and he prefers switching more coils, than having a bigger capacitor and less coils. I am not sure why he does that, It may have to do with the constant output level of the generator, or for better fine tuning.Note also that the tuning control is the only component of a capacitive nature that determines the frequency. As L gets smaller, the effect of C gets greater. I would doubt the tuning capacitor is temperature stabilized (image is too small to read it's value). The classic fix is to add a capacitor in parallel with the tuning control (~47pF) and to select a type with an opposing temperature coefficient.
Brian.
there was much hum in higher frequencies above 10MHz, when listenning on AM. On SSB this hum was heard worse, like modulation. Why is that hapening?
It is possible the rectifiers are producing low level hum (assuming you are using a linear supply!). It is an effect caused by the recovery time of the diodes as the polarity across them reverses that produces short, sharp spikes at twice mains frequency. The reservoir capacitors are good for holding the overall charge but less effective at stopping very short pulses. The extra capacitors 'soften' the spikes at source.
Shielding in a metal enclosure is the only way to stop the proximity effect but if you need absolute stability, use a metal and thermally insulated box and add temperature regulation to it to keep it's internal temperature constant. It isn't difficult to do but you do generally have to leave it turned on for 10 minutes or so to allow the temperature to settle before use.
Brian.
It is possible the rectifiers are producing low level hum (assuming you are using a linear supply!). It is an effect caused by the recovery time of the diodes as the polarity across them reverses that produces short, sharp spikes at twice mains frequency. The reservoir capacitors are good for holding the overall charge but less effective at stopping very short pulses. The extra capacitors 'soften' the spikes at source.
Shielding in a metal enclosure is the only way to stop the proximity effect but if you need absolute stability, use a metal and thermally insulated box and add temperature regulation to it to keep it's internal temperature constant. It isn't difficult to do but you do generally have to leave it turned on for 10 minutes or so to allow the temperature to settle before use.
Brian.
Hum when connected to other equipment has been discussed elsewhere on this Forum. Basically, instead of being completely isolated, you are tethering it to a potential (the scope ground) which itself may not be noise free. Capacitive coupling from all parts of the circuit to things around it then introduce unwanted signals, predominantly hum from house wiring and appliances.
The other things that worries me now I can see the markings on the schematic is the use of a 12V secondary on the mains transformer which loses ~1.4V in the bridge rectifier then goes to a 12V regulator that needs about 15V minimum at it's input. The peak voltage of 12V RMS is about 17V so you only have a safety margin on voltage peaks of 17 - 15 - 1.4 = 0.6V, in other words it relies heavily on low ripple across the reservoir capacitor.
Good to see you covered the LED, that could also be a source of hum.
Brian.
Increasing the transformer voltage or using a low drop-out voltage regulator will give better results than both those circuits.
To minimize the effect of the scope, use two probes. Do not connect the ground clips on either one but connect the tips across the signal you want to measure. Use the scope's add and invert function to make the probes work as a differential input. If you have x10 probes, even better but make sure they are set the same.
Brian.
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