Test equipment question

[Robb]

Do you have a passive pickup?
Didn't see one, and you definitely need that or else you will burn up your test gear!

 

[kaos513]

Never heard of the passive pick up what is it ? Thanks

 

[Robb]

Never heard of the passive pick up what is it ? Thanks

Let's say that you want to test the output freq of a radio with your freq counter. Well; how do you do that - since the radio's output is quite high? If you do hook it up directly to the radio's output, you WILL fry your freq counter. Or o-scope - for that matter.

What you then will have to use is a passive pickup to sample the signal at a much lower level to get a reading w/o wrecking anything expensive. A passive pickup is another name for a RF sampler.
Do you have one of those?
Like I said, I did not see one . . .

So you will have to read this thread from end to end, and DO NOT use your equipment until you have one and know how to use it:

https://www.worldwidedx.com/threads/rf-sampler.32110/

 

[kaos513]

yes i do haveone i made it my self will send the pic soon

 

[Shadetree Mechanic]

Well it's below 2V so I guess you're good to go. Enjoy your tuning endeavor...

The 2V figure if you look in a Service Manual is the level they want you to use into a SINAD meter...

Refer to Figure 2 in my posts above - referring to a simple ANALOG SINAD meter.

How it works, - it uses a Filter - simple PI filter and the switch connections set it up in two combinations.

One - a SERIES RESONATE circuit (a band pass) which GOES INTO THE METER and this is that you make sure to use 2 V or thereabouts - this is considered a loud enough signal and uses enough power to heard by normal hearing.

Once the tone can be picked up by the receiving radio...

To tune it, you simply send a tone or your "key of C" thru the radio transmitter for your receiver radio to pick up while listening on the same channel, or use a signal Generator and the TONE of "C" from your keyboard to check and tune to - into the radio, under test. And adjust the coil or the caps the ones used in the METER - for maximum indication on the SINAD meter. Peak out that baby! Adjust volume as you need to...

Now, flip the switch...

TWO - a SERIES RESONATE circuit now PARALLELS (Goes across) the meter leads - acts like a shunt - to remove signal arriving to the meter - but to do that, the tone has to meet several conditions.

One it is the same tuned frequency you adjusted for in Step 1 ... (Don't adjust the meters' tuning)
Two the tone CANNOT have artifacts - these can be square clipped saturated waves or sawtooth waveforms that can add an undesirable tone to the sound.
Three CANNOT have distortion or other noises present in the detected signal...added by the radio...

These conditions - when met, will reduce the needle of the SINAD meter - you're nulling it. To accomplish further annulments - you can adjust the RX strip tank tuning coils and RF gain and VR 2 accordingly to achieve the lowest needle deflection as possible.

The SINAD meter in the Analog sense is what was used way back in the day, but is still a viable tool today, for you now can see the distortion as lost, wasted power in both signal degradation and impacts to audio quality for listening. You can also hear how adding and removing these artifacts can affect your hearing those signals - as an ability to DISCERN the intelligence in the signals you want to hear.

That's good stuff, I am gong to put this in my note book. Thanks!

 

[TheRealPorkchop]

Never heard of the passive pick up what is it ? Thanks

I use that. But building your own is just as easy as buying one and possibly cheaper. I have connected my counter directly to a radio output without consequence (so far) but there’s a first time for everything.

Or Little Texas said there was anyway.

 

[kaos513]

Work in progress below is the circuit that soppose to go with the meter to power it up and get vu-readings on any test unit

 

[kaos513]

Is it ready for primetime?

 

[Handy Andy]

What are the details of the circuit PCB - does it also contain the tone generator needed for the SINAD to "sample"?

I see the L/R and 50K trimmers and can't see too many details of the op amp...

 

[kaos513]

just an amp to power the meter

 

[Handy Andy]

Gotcha' - didn't know you needed it but...

 

[kaos513]

Here is a closer look.

 

[kaos513]

When I purchased the meter a power circuit was required i had this board from my other vu meters. Works very well

 

[Handy Andy]

Ok, well it's pretty much the circuit above - but looks like its' passive like this threads' morphed to (which is fine - we need to see this) - I see the DB107S full wave bridge rectifier for the audio "sample" but don't you need two of them? One for each channel? Or does the op amp simply set the level and COMBINES both sides into one thru the rectifier then onto the metering?

This would be a cool low-level distortion metering for small signal amps...

Wish you well!

 

[kaos513]

Both channels are separate