FM multipath issues

[stevehertz]

Originally Posted by SteveCG

I was interested to read of the particular Trio tuner that stevehertz is using. I wonder whether there were other makes of tuner sold that had a MP distortion readout?

Yes, quite a few tuners had signal strength meters that could be switched for MP measurement. The Trio is somewhat unique in that it has an extra meter for that very purpose.

[G8HQP Dave]

If you have an oscilloscope you can see multipath. Put into X-Y mode. Connect X to the FM discriminator output. Connect Y to the signal strength voltage - if you can find it before filtering. The more wiggly the trace the more multipath you have.

[SteveCG]

G8HQP_Dave - an interesting tip. I'll have to see if it possible on the tuners I have.

[G8HQP Dave]

If you can't find a signal strength voltage try looking at the IF signal itself.

[stevehertz]

Some of the high end 70s tuners have MP output terminals on the back panel.

[TonyDuell]

My late father had a Pioneer tuner with sockets on the back labelled 'X' and 'Y' to link to a 'scope. I assume it was to detect multiipath problems.

[SteveCG]

I had a quick look at the circuit diagram of the Quad FM3 (serial no. in the 20,000s) I currently have. Not sure I have the connections to do what is suggested. The Quad does not have any agc line back to the front end and uses a CA3089E IF IC - so no discrete circuitry to connect to in order to get the IF amplitude envelope.

[G8HQP Dave]

Pin 13 of the 3089 gives signal strength, but it may have a capacitor added which will spoil its use for multipath detection.

[SteveCG]

G8HQP_Dave

I just checked the circuit diagram (and also the IF alignment instructions). Pin 13 on the 3089 appears to give an IF envelope (to be seen on a 'scope) - there is no capacitor present. So that looks hopeful, although what will be there when receiving a wanted broadcast (as distinct from a test wobbulator's output) is something to be checked.

[SteveCG]

I've been thinking about the type of tuner/receiver that is best suited for this experiment. And I think that an analogue tuned, with separate IF stages may be the best - as compared to the Ceramic filter followed by a (hard limiting) IF IC design that later tuners use. Indeed for more recent units I see that the complete tuner and decoder has disappeared into a screened box - now merely a bought-in part.

It seems, to me at any rate, that multi-path indication was a late 1970s fad. I wonder if somebody could give a list of tuners of that era that attempted to provide this information so that I could see if the service manuals - with circuit diagrams - were available on the web, and so see what the tuner manufacturers actually did?

[stevehertz]

Trio 600T, Pioneer TX-8100, Pioneer TX-9500, Kenwood KT-7001, Sansui TU-X1 etc

[SteveCG]

Thanks for the list stevehertz, I'll do some searching...

[stevehertz]

That would be great Steve, interesting.

[stevehertz]

Being able to ID multipath and where it's coming from is as important as being able to ID the exact direction of the main signal. It enables one to make slight adjustments to the antenna orientation so as to satisfy the needs or the balance of maximum direct signal strength vs MP corruption. So for example, it may well be that overall listening quality is improved with a reduced level of main signal, when MP is reduced as much as possible. A tuner is more able to cope with a reduced signal level and 'no' MP than it is a high signal level that is swamped with MP. I reckon..

[SteveCG]

A friend drew my attention to this web-page describing "Adding Tuner multi-path outputs":

http://ham-radio.com/k6sti/mpath.htm

which in turn has a link to this page describing a "Quadrature multi-path monitor":

http://ham-radio.com/k6sti/qmm.htm

Interesting reading.

[stevehertz]

I live very near to the soon (not soon enough) to be demolished Rugeley Power Station and it is notorious for causing reflected signals, particularly back in the days of analogue TV when ghosting was a major problem. But it also has a bad effect on VHF too.

I love DXing and receiving stations from all directions, so I don't want a static multi element yagi set up that limits me in that respect. And before anyone suggests it for the umpteenth time, I don't want a rotator either! That is until someone comes up with a wireless one ie no wires between rotator and control unit, just a mains (or whatever) supply to power the rotator. Otherwise you need to run a control cable from the roof mounted rotator down through walls into the ground floor room (in my case) where you have your control unit - not for me thank you. In fact, with wireless 'everything' being available these days, I'm amazed that no-one has done this yet.

[buggies]

A bit late to the discussion but I was flicking through some old mags today and noticed a two page article in Elektor with block diagrams suggesting making use of the 19kHz signal to make the measurement.

[stevehertz]

Very informative article George and a nice explanation of MP and its effects, thanks. In practise, if anyone has, or has access to a receiver that has MP indication, then once that has been used to align one's antenna for best MP rejection, then you need to do no more, it's set, a one-off adjustment.

[SteveCG]

I've been looking at the service manual (courtesy of HiFiengine) of the Rotel RT-824 AM/FM Tuner. This has a multipath meter readout.

From what I can see the meter is showing the AM detected envelope of the 10.7 Mc/s IF. In other words, the modulation of the IF signal due to multipath appears to the user just to be an AM modulated radio transmission; that is, what we are used to with LW/MW/SW radios. Or another way of putting it, the Rotel display is rather like what you would see on a VU meter if you were making a cassette/reel-to-reel recording.

To achieve this the Rotel's IF stage is split into a few stages with a tap off to the meter circuitry before any limiting process.

I wonder where this Rotel tuner sits in comparison to others?

[stevehertz]

Thanks Steve.

The Rotel RT-824 is not mentioned there, but for loads of information on almost any tuner that's ever been built take a look at Tuner Information Centre online. They do reviews, mods, comparisons (or shootouts as the Americans call them), photos, etc etc.

http://www.fmtunerinfo.com/index.html#updates

Slightly off topic, but any hifi tuner is only as good as it has been aligned at the factory, and/or drifted over the years. That's why it's misleading comparing the performance one tuner against another without both tuners having been accurately checked/realigned. Ok, you can easily find out which one is best 'as is', but not which one is potentially the best. Of course, a proper, accurate, realignment is not an easy job to perform, and can be a costly one for a trustworthy professional technician to perform for you. Just a thought.