Precision Tuning

[JHGibson]

A simple receiver utilizing reaction can be quite sensitive, capable of receiving signals of a few microvolts. They have a drawback in that you only roughly know the frequency you are tuned to. To overcome this disadvantage, I added a cathode follower to the top of the tuned circuit and ran the output to a connector on the receivers rear panel and plugged the workbench frequency counter in. The cathode follower has a low capacity input together with a high input impedance and does not distract from the receivers performance.

Now if you want to know the exact frequency you are tuned to, just increase the reaction until you get a frequency reading. Conversely, if you know the frequency of the station you seek, put the detector in oscillation and tune until the required freqency is reached. Then back off on the reaction and there is your station, programming and ionispheric conditions permitting.
John.

[Brian R Pateman]

Well done John!

At last, a use for the frequency counter.

Regards,

[Al (astral highway)]

The cathode follower has a low capacity input together with a high input impedance and does not distract from the receivers performance.

Hello - nice work. This neatly solves a problem I've been thinking about - how to monitor the tuning frequency of an RF pre-amplifier to four decimal places' accuracy, without loading the tuned circuit.

Any chance you can post a sketch of the arrangement, please? I know what a cathode follower looks like, but nice to see it in relation to the tuned circuit.

[Skywave]

Astral - With the greatest of respect, I think you may have slightly misunderstood this

In the described application, it is because the detector is made to oscillate and oscillate at a frequency very close to that of the incoming signal that the freq. of this incoming signal is being determined by that oscillation and it is this oscillation that is being measured on the counter. In an RF pre-amplifier, the one thing we do not want is for it to become unstable - i.e. go into self-oscillation - which is what would be required in order to determine the freq. that this RF pre-amp. alone is tuned to, using the technique as described above.

Having said that, however, you could take the above-described reception technique and add a tunable RF pre-amp. in front of it - and manually adjust its tuning for a peak in the received signal, and therefore deduce (albeit approximately) the tuning of this RF pre-amp - but (unless I am mistaken - a distinct possibility ) - my reading of your post (above this) leads me to think that this is not what you have in mind.

The part that refers to the use of a cathode-follower as a method to reduce the loading on a tuned-cct. is valid.

The point I am trying to highlight is subtle - I hope I've made my thoughts clear - for what they're worth

However - if I have misunderstood you - then please accept my apologies - and we can move on.
Just trying to be helpful.

Al / Skywave

[Skywave]

A consequence of this technique is that it should enable one to produce a freq.-calibrated tuning scale . . . rendering further connection of the counter unecessary - no?
However, I assume here that the necessary-variable amount of applied reaction will only have a small de-tuning effect on the signal-determining L/C cct. - an assumption which may not be valid.

Just a thought.

Al / Skywave.

[Al (astral highway)]

Al, thanks for your feedback. I'm not quite sure if I do follow you, simply because I'm not sure what assumptions you thought I was making

Please let me clarify: I was thinking that I could add a cathode follower, as described, to the front end of a neutralised narrowband RF pre-amplifier.

I'll already know the ball-park tuning frequency (to within 1Mhz) but the appeal of knowing exactly what it's tuned to is high, especially during calibration.

I don't actually see why this can't be done as described.
Perhaps I'm being dense

[Station X]

Your RF pre- amp may be narrow band, but basically it'll amplify any signal on any frequency in its pass band.
The selectivity of a superhet receiver is determined by the IF circuits. The normal method of digitally indicating the frequency a radio is tuned to is to meaure the local oscillator frequency and subtract the IF from it. This is normally done automatically say by starting the count at 455 KHz rather than zero.

The tuning of the RF pre-amp should be accomplished by tuning it and the receiver for max signal.

[Al (astral highway)]

The normal method of digitally indicating the frequency a radio is tuned to is to meaure the local oscillator frequency and subtract the IF from it.

Thanks Graham. But in my circuit (it's not built yet, but let's assume that it is) the frequency at the output transformer of the mixer valve is not quite what I'm after. By design, that frequency is chosen to be output to a comms receiver's 20 metre band.

But I totally get your point. I could derive the frequency at the grid of the first stage manually by adding the frequency of the third harmonic of the crystal used in the oscillator (which is the precise figure to which the mixer's plate circuit is tuned) to the figure showing on my comms receiver's digital tuning readout.

So, good thinking. No need for an extra readout at all. Thanks

[Skywave]

Originally Posted by astral highway

Please let me clarify: I was thinking that I could add a cathode follower, as described, to the front end of a neutralised narrowband RF pre-amplifier.

That usage of a cathode follower - as a buffer amp. - is sound.

You're certainly not being dense :
I beginning to think that it is I who may have misunderstood you.

I read your post as implying that either:

(a) by making the RF amp. oscillate * - as like a regenerative detector - the resulting oscillation could then be used to determine the freq. that this RF amp. was tuned to - by coupling the counter to the RF amp. (via a cathode follower).
* This, of course, is a no-no.

or:

(b) that if the oscillation of the detector is read on the counter and the RF amp. is then peaked on the received signal, you then know the freq. that the RF amp is tuned to - approx. This, at best, will be very approx., since the Q of the L/C cct. in the RF amp. will not be sufficiently high to give an accurate reading.

I think we'd should go into PM mode if any more needs to be said on this - I don't won't to hog the Thread.

Sorry if I'm being a pain - just trying to help.

Al / Skywave.

[Station X]

Originally Posted by Station X

The normal method of digitally indicating the frequency a radio is tuned to is to meaure the local oscillator frequency and subtract the IF from it. This is normally done automatically say by starting the count at 455 KHz rather than zero.

I go that wrong The counter needs to be started at Max reading-455 KHz and allowed to roll over past zero.