Signal injecting

[Bigears]

Hi Guys

As I am trying to get my head around signal injecting would these to statements be correct.

Signal tracing is for AF

Signal injection is for RF

I am most grateful for any help and guidance you have given and thank you for the time taken to read this.

Regards Ken

[Station X]

Open to interpretation.

I'd say signal injection is putting a test signal in at some point in the circuit, generally working backwards from the speaker.

Signal tracing is seeing where the signal comes out, often at the speaker.

The signal would be AF for audio stages and RF (generally modulated) for IF and RF stages.

[Radio Wrangler]

For fault-finding a radio receiver there are two basic methods.

1) back to front start by checking the audio amplifier. Inject an audio signal, say at the volume control and listen to what (if anything) comes out the speaker. If this is OK, move on to injecting a modulated IF signal. Start with the end of the IF amplifier at the detector and work your way to the input of the IF amplifier. Reduce levels as you move earlier in the receiver chain to accommodate (and check!) its increasing sensitivity as there are more and more gain stages between the point of injection and the speaker. If all is OK so far (or you've fixed what you've found) Switch over to injecting a modulated RF signal at the frequency the set is tuned to. Inject into the mixer. If OK, work your way back through the RF amplifier(s) to the antenna input. Drop the injected level down low and check the set's overall sensitivity.

To do this properly you need a signal generator that can provide audio as well as modulated carrier at the IF and RF frequencies of your set. If it's well screened and has a good output attenuator, you can get a feel for the sensitivity at each stage and spot things which are down on gain. Adequate sig gens are not expensive. Plenty of people have them.

To do this crudely there are signal injectors which make pulses with harmonics all over the place. Cheap, handy, but no guide to whether the set is sensitive enough.

2) Front-to-back.... with this method, you stick a modulated signal in the input or possibly use one off-air. So you might need a signal generator to start with. Yjen you need a second receiver of some sort to probe down the receiver chain looking for modulated carrier at RF, Mixer and IF stages. It needs to be well screened else you don't know whether what you detect is from your probe or is just general pick-up. It also needs to be quite sensitive. You could use a variety of types of receiver but coverage of the IF means you need one with no IF gap. To be as versatile as a general sig gen in frequency coverage, you've just strayed into spectrum analyser territory. Not many people have these, chiefly due to price, even second-hand. But they can be very useful and do some very helpful tricks. You'll need an audio amp and speaker (computer powered speaker) to check audio.

Both methids have different advantages.

You can do one method frm the beginning and one method from the end to join up somewhere in the middle if you wish.

Experienced fault finders can cut the amount of testing to get to the fault. The basic 'successive approximation routine' says to take stab right in the middle of the structure and determine if the fault is in the first half or the second half. Then stab the middle of the faulty half and you've only got a quarter of the set to worry about. Continue narrowing in until you find the problem.

VERY experienced fault finders don't pich the centre by counting components or stages, they have an idea of likely faults annd pick a point with equal probability on the two sides of it.

I come from a background of designing new circuitry, building prototypes and having to get it going. I can have no confidence that it ever worked before, or even that another individual ever did. I'm completely on my own without any support. So I had to get organised at testing and fault-finding processes.

David

[Herald1360]

A simple piece of kit, sometimes called an "injectrace" (that was a Practical Wireless term for it) can be built which injects a broadband audio modulated signal or by plugging in an earpiece can be used as a tracer instead.

See p596 here:

https://worldradiohistory.com/UK/Pra...PW-1969-12.pdf

The B154 15V battery is rather expensive if you can find one but can be subbed by a suitably contained stack of button cells (look up B154 replacement on here) and the 2N2926s by the likes of BC547 etc. Note that the circuit rather crudely ignores the reverse base emitter breakdown for silicon devices of around 5V, though presumably the high value of the base bias resistor mitigates this.

[Bigears]

Thanks, Graham Dave and Chris

[DonaldStott]

For simple signal injecting and tracing I use this rather helpful device:-

http://www.velleman.co.uk/contents/en-uk/p261.html

Has proved invaluable on a number of occasions as it helps to quickly identify which stage of the set has the problem, especially if you follow the fault-finding advice from Radio Wrangler.

Lots of Posts on the construction and use of the Vellerman K7000 on this Forum.

[TrevorG3VLF]

I do not see why the PW design should not work on 5V.
If you wish to run on a higher voltage, then place a diode in each emitter.