02-11-2012, 02:52 PM
(This post was last modified: 02-11-2012, 03:31 PM by Radio Fixer.)
Well John you dont give enough info on how you are doing this.
My method of seeing that the IF's resonate at around the right frequency, out of the chassis, is to feed one tuned circuit across the generator output via a resistor of say 47 to 100K (the higher the better depending how much O/P you can get from the gen and how sensitive the scope is). So the tuned cct has one side earthed and the other to the gen feed via the K resistor. Now I put a scope, with a X10 probe, across the tuned circuit and tune the gen like you did. At about the right IF freq, signal should appear on the scope and peak as you go thru resonance. Keep all leads short with good solid connections.
The X10 probe is essential as it is normally only 10pF in shunt with the tuned circuit, a X1 will be a lot more and put the circuit much more off tune.
Of course you want an accurate way of measuring freq and I assume you have a counter? This can be connected via a T to the gen or as I do using a coaxial changeover switch (between the feed to the IF and the counter). You certainly dont want to place the counter across the tuned circuit as that just adds more capacitance.
So you should get a fairly good idea of where the IF tuned circuit resonates. The IF cap is likely to be about 100pF so with the 10pF of the scope the freq will be slightly lower but as freq is inversly proportional to square root of C it wont be too much out.
Just for interest you can see if shorting the other winding out makes much difference... not done it lately but it shouldnt.
If you want, and have the freq counter, you can measure Q by measuring the freq at resonance and then tuning the gen frequency either side until the waveform amplitude drops by 3dB (to 70%). Note these two frequencies f low (fL) and f high (fH). Q is then f at resonance / (fH - fL). As an example say F resonance is 460 kHz and fH = 465 and fL = 455 then Q is 460 / 10 = 45.
Taking a quick look back at notes, I dont normally get as high a Q as that and 25 to 30 is about the norm. But this is not the same as a Q meter which I dont have and I take it as an indication only rather than an accurate measurement.
Another bit of info from these notes (1 chassis only but I have measured plenty) I got f resonances within 1% and 3% worse case and didnt try tuning them. Close enough to try in the chassis for me.
Hope this helps Gary
My method of seeing that the IF's resonate at around the right frequency, out of the chassis, is to feed one tuned circuit across the generator output via a resistor of say 47 to 100K (the higher the better depending how much O/P you can get from the gen and how sensitive the scope is). So the tuned cct has one side earthed and the other to the gen feed via the K resistor. Now I put a scope, with a X10 probe, across the tuned circuit and tune the gen like you did. At about the right IF freq, signal should appear on the scope and peak as you go thru resonance. Keep all leads short with good solid connections.
The X10 probe is essential as it is normally only 10pF in shunt with the tuned circuit, a X1 will be a lot more and put the circuit much more off tune.
Of course you want an accurate way of measuring freq and I assume you have a counter? This can be connected via a T to the gen or as I do using a coaxial changeover switch (between the feed to the IF and the counter). You certainly dont want to place the counter across the tuned circuit as that just adds more capacitance.
So you should get a fairly good idea of where the IF tuned circuit resonates. The IF cap is likely to be about 100pF so with the 10pF of the scope the freq will be slightly lower but as freq is inversly proportional to square root of C it wont be too much out.
Just for interest you can see if shorting the other winding out makes much difference... not done it lately but it shouldnt.
If you want, and have the freq counter, you can measure Q by measuring the freq at resonance and then tuning the gen frequency either side until the waveform amplitude drops by 3dB (to 70%). Note these two frequencies f low (fL) and f high (fH). Q is then f at resonance / (fH - fL). As an example say F resonance is 460 kHz and fH = 465 and fL = 455 then Q is 460 / 10 = 45.
Taking a quick look back at notes, I dont normally get as high a Q as that and 25 to 30 is about the norm. But this is not the same as a Q meter which I dont have and I take it as an indication only rather than an accurate measurement.
Another bit of info from these notes (1 chassis only but I have measured plenty) I got f resonances within 1% and 3% worse case and didnt try tuning them. Close enough to try in the chassis for me.
Hope this helps Gary
