If I have a conventional superhet with an IF of 465 kHz and I wish to check the alignment of the complete IF amplifier using the 'wobbulator' technique - a ramp-modulated FM signal with the ramp synchronised to the 'scope timebase - what is a sensible range of sweep times / frequencies to choose for this task? And if I am examining only one IF stage, what times / freqs. should I choose for that exercise?
Thanks for your thoughts.
Al. / Skywave.
Al
this a very good poser, unfortunately I do not have an answer, but I look forward to the comments of others here assembled.
I shall follow this with interest!
cheers Mark
a long time ago, but try a slow scan rate on scope 25/50 hz set wobulator centre sweep range to if frequencie ,nb this was for tv never used one for radio but should be the the same?
if this is wrong sorry
rob t
Thanks Rob.
50 Hz → 20 msec.; 25 Hz → 40 ms. Somehow I get the feeling that those times are far too short for a sweep time (a very short 'flyback' time is understood); somehow I get the feeling that something in the region of 0.25 sec. → 2 seconds is appropriate: I'm not sure. It is that certain "it feels right somehow" feeling that caused me to produce this very Thread in the first place: others have the relevant experience; I don't.
Al.
The sweep rate on mine is from 8-14mS and I typically use 10 as it is easy to sync it to avoid hum effects 'passing by' on the trace. Seems OK to me... the main thing is to choose the pick up point for the scope probe carefully. Some say the top of the volume control but if this is capacity coupled then it can slew the demodulated waveform. I get as close to the detector output as possible which often means a little RF on the waveform. However, a shunt cap of a couple of hundred pF or so cleans this up.
Gary
Thank you Gary for your reply. What is the equipment you use for that procedure, please?
Al.
(05-04-2011, 08:56 PM)Skywave Wrote: [ -> ]Thanks Rob.
50 Hz → 20 msec.; 25 Hz → 40 ms. Somehow I get the feeling that those times are far too short for a sweep time; somehow I get the feeling that something in the region of 0.25 sec. → 2 seconds is appropriate.
[edited - by Al.]
Al.
Must have been having an 'off-day' when I wrote that: what a load of nonsense!
Common sense tells us that on the one hand you need a sweep rate that is above a certain low value to avoid flicker on the screen. If it is
really too low. e.g. 1 to 2 seconds, the phosphor on the CRT screen will not have a long enough persistence time for the image to remain visible. (Assuming a 'scope tube with an 'ordinary' phosphor).
On the other hand, the sweep rate must be below a certain high value that will distort the resulting response, simply because the rate of change of the input frequency is faster than the rate that the item under test can respond to. (This is obviously related to the Q of the tuned circuits and the degree of the mutual coupling between windings).
Consequently, there is quite a wide range of suitable times, the exact timing somewhat dependant on the response time of the item under test: Gary's range of 8 → 14 msec. is quite sensible & suitable (most applications).
Right then - having subjected myself to a self-administered public reprimand
, further investigations have revealed that I was being led up the electronic garden path and consequently became confused. I had carried out the initial experiments using an IFT can from the 'junk box'. It proudly stated '465 kc.' on its case; it was lying! :@ Subsequent elementary measurements established that it was, in actual point of fact, a 1.6 MHz IFT !
Hence, when I set the sig. gen to sweep @ 465 kHz ± 50 kHz: no 'humps' !
And with that, the rest of this tale is obvious . . . but the experiments continue; perseverence
will prevail . . . watch this space . . .
Al.
As you say Al long sweep times are very tiring on a standard scope. If you have or can get hold of Radio Bygones Issue 124 then there is an article on the Knott Wob by Graham Gosling of East Coast Wireless. Its German and a wonderful piece of equipment. That has sweep times from 0.1 to 100 secs but the display does have a special long persistence phosphor.
The range on my home built piece of kit works OK on a standard phosphor Tek scope. There shouldn't be any slew rate affects until after the detector, as far as I can see. The coupling cap from it can cause a little slew so I put the scope probe on before that.
Gary