Basic newbie FM tuner measurement questions

As an amateur to FM tuner testing, I have some questions about setting up some basic tests. Right now I just want to be able to apply a test signal and verify voltage levels in the tuner (per a service manual) are where they need to be.

I have a Panasonic VP-8193A FM signal generator that has been calibrated. I also have a Tektronix 475, 200MHz oscilloscope.

The first question I have is about the signal generator output level. It is defined as dBuV EMF. In looking that up I found this web site that shows formulas and has a conversion calculator for the various units such as dBuV EMF.

My VP-8193A test results (from the calibration check I had done) show that it is pretty much right on the money with its frequency and dBuV EMF output levels. But what if I want to check the levels with my own scope? So I decided to try and that is where I am out of my knowledge level and would appreciate some help.

The question then, is should I be able to connect a BNC to BNC cable from the VP-8193A signal generator to the oscilloscope and measure the voltage level correctly? The VP-8193A output is listed as 50 ohms. The scope input is listed as 1M ohm. When I try that, I see the sine wave on the scope has the expected frequency but I am not getting the peak voltage levels per the RF Unit Converter for the amplitude setting on my generator.
The FM tuner inputs for are for 75 ohm coax cable. Does that require a 50 to 75 ohm matching pad?

A second question is - I have seen tuner service manuals that say to input a signal at some frequency and a level of 1uV EMF. So I am trying to relate the VP-8193A dbuV EMF output value to a test input requirement of uV EMF units. The conversion chart I gave the link to appears to supply that but I am wondering how I can tell my test setup is actually correct?

The question then, is should I be able to connect a BNC to BNC cable from the VP-8193A signal generator to the oscilloscope and measure the voltage level correctly? The VP-8193A output is listed as 50 ohms. The scope input is listed as 1M ohm. When I try that, I see the sine wave on the scope has the expected frequency but I am not getting the peak voltage levels per the RF Unit Converter for the amplitude setting on my generator.
The FM tuner inputs for are for 75 ohm coax cable. Does that require a 50 to 75 ohm matching pad?

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The SG has a 50 o/p Z so to get a correct reading you need to terminate in 50 ohm Z, or it will usually show twice the V as it is open ckt, with no termination. The scope should have a 50ohm input termination switch, for each channel, if not use a 50 ohm feedthru terminator. Scope will show peak to peak so take p/p /2 x0.7071 to get Vrms.

A second question is - I have seen tuner service manuals that say to input a signal at some frequency and a level of 1uV EMF. So I am trying to relate the VP-8193A dbuV EMF output value to a test input requirement of uV EMF units. The conversion chart I gave the link to appears to supply that but I am wondering how I can tell my test setup is actually correct?

Click to expand...

dBuV is referenced to 1uV into 50 ohm Z. Since this a voltage measurement, it is dBuV=20 log base 10 Vout/Vref, Vref being 1uV. So 0dbuV is 1uV, 10uV is 20dBuV.
If you want to measure the tuner accurately, say quieting sensitivity, you need to match the 50 source Z with the 75 ohm termination Z (50-75 pad) & use appropriate coax cable as well. A matching pad is required, but note that the matching pad usually has some loss, so that has to be measured as well & factored into the gain/loss calculations.

Thanks for the reply.

I have ordered a feed through and terminators to try. I have also ordered a 50ohm to 75 ohm matching pad so I will retest once they arrive.

Got a T and 50 ohm termination plug today. That did the trick on the scope from the generator. I am still waiting on the 50 ohm to 75 ohm pad.

Update - I found that my FM boards I was checking were bad. Using a pair of good boards from a spare receiver, I am able to apply signals with the Panasonic signal generator and make the service manual measurements. However, I did discover in my ignorance on this subject, that it appears my Panasonic generator cannot function as a sweep/marker generator. There are a number of tests that call for a sweep generator so my test setup is missing a key item.

Sweeper for IF tuning

Yes, you need a IF sweeper to tune a stage that usually uses tuned coils, rather than say a Ceramic filter ckt. Probably could use a regular SG, but need to modify the procedure, since you have to do discrete measurements, a very time consuming process. I have never done this but I dio understand what you need to do.
I did find a ckt that acts as a IF sweeper(attached) , as far as having, a marker, I'd have to look further. ckt is basically a LC osc, that has a varactor in parallel with a fixed C, apply a sawtooth to the varactor to vary the freq markers help you to identify the span and center points. I recall using a Racal instrument many years ago.
Parts shown are obsolete, so you will have to find modern available subs, such as, BF998, and MPSH10.
Good Luck
Rick