Radio Constructors' Circuit Archive

[Skywave]

Whilst browsing the 'Net - as you do - I came upon this:

http://vintageradio.me.uk/radconnav/misc.circuits/

There's some interesting-looking stuff in there. Worth a read, even if you don't plan on building any of it.

Al.

[tiv48]

thanks al some realy good stuff on there
rob t

[pwdrive]

I have this in my favourites for a while, all homebrew, mainly circuits.

http://www.qsl.net/va3iul/Homebrew_RF_Ci..._Ideas.htm

lawrence

[Skywave]

Re: Post #1. Amongst the listed circuits is one entitled "An automatic signal generator". There is no circuit - it is only the idea and a discussion of what it does, how it works and how you use it. The basic idea is a generator that can be used to check & adjust the RF alignment of the 'front end' of a superhet radio. The principle on which is works is as follows.
The unit contains a frequency changer. The two inputs to this are (1) a loosely-coupled capacitive probe that picks up the local oscillator signal from the set under test and (2) a free-running oscillator at the IF of the set under test. Consequently, the mixer output is at the RF of the receiver under test. As the receiver is manually tuned, the RF output of the automatic signal generator always presents the correct frequency to the receiver's 'front end'. And so you adjust the RF circuits accordingly.

O.K. fine; very clever. Or so I thought at first. Then I thought again: there's something not quite right with that idea : it assumes that the calibration of the receiver is correct to start with . I'll expand on that: suppose the calibration is incorrect. Then the RF output from the generator will be of the same error. Subsequently, the adjustment of the RF 'front end' circuits will also be out by the same error. Therefore, the prime function of this 'automatic generator' is defeated.

Now in 'real life', when re-alignment is performed, after the IF strip has been adjusted - with a calibrated signal generator - the next step is to adjust the local oscillator for correct calibration and tracking - again, using a calibrated signal generator. Now that the IF and L.O. are known to be 'on frequency', only then can the RF stages be adjusted - and adjusted correctly.

The whole article seems far too long and detailed for someone who has created an April Fool's joke - or even a hoax. Am I missing something in my analysis? :-/

Al.

[pwdrive]

Hello Al at the moment I agree, not to mention image frequency etc, what happens if the receiver under test has Osc below Fs as in some receivers on the highest band and Osc above Fs on the other bands as is often the case.
I had an idea once as regarding IF filtering, I looked at all these ideal IF response curves and they all seemed to want to achieve very steep sides with a flat top, a bit like a square wave pulse, so I started thinking as in a swept pulse arrangement, the pulse only being initated at the start of the lower end of the required IF bandwidth, it's duration would be equal to the total IF bandpass required or selected, then use this pulse to gate the IF amp, do I need locking up?

Lawrence.

[Skywave]

I see what you mean. If the IF response curve is, say, 6 dB down at F1 and F4, and is 'flat' between F2 and F3, then by gating the IF amp. so that it is only open for F2 to F3, you should, in theory, get a perfectly rectangular response. Now obviously in the real world there is the time of the amplifier to respond to that switching signal to consider and there is no such thing as a perfectly flat IF response curve. Like you, I get lots of innovative ideas, some of which I try. And I haven't had a visit by the men in white coats and a plain-coloured van with barred windows - yet :omg:. However, whenever I do get such ideas, there's always that creeping idea in the back of my mind that says "O.K., if this is such a smart idea, how come no-one else has done it yet - like the big boys who have millions to spend on R & D ?"

Al.

[pwdrive]

Got it in one I think Al, however if a colour burst of 4.3 Mhz can be gated every 64 micro seconds for the duration of that burst.....maybe not.... nyquist/sub nyquist and all that kind of stuff might get in the way, I think I am missing the third dimension.

Lawrence.