diy IF transformers: first attempt.

[ppppenguin]

Also try unplugging the probe from the scope. I've no idea what that might do but it could reveal something.

[Amie]

OK having seen your last post I've repeated the measurements on the switchable probe....

Without being plugged into the scope, and the probe set on ×1  the Atlas measured it as 50pF.
Without being plugged into the scope, and the probe set on ×10 the Atlas measured it as 15pF.

Plugged in and set on ×1  the Atlas measured it as 1000pF
Plugged in and set on ×10  the Atlas measured it as 15pF

Rough and ready check for that anomalous reading. Put a 1n cap in parallel with the probe and see what happens. If the reading goes up to 2n then the original 1n is quite likely correct, however unlikely it seems. But I reckon you're more like to see something like 1.2n.

Plugged in and set on ×1  the Atlas measured it as 1200pF with a certified 1nf in parallel, (someone throw that penguin a mackerel, )

Is that calculable, or a pre existing observation?

Ta!
Amie.

[ppppenguin]

Sheer guesswork like that doesn't often come good. Smoked salmon for lunch! (Left over from V day yesterday)

200pF for the probe still seems a bit high, unless it has an unusually long cable. Maybe 2m or more. The source of the 1n reading still remains a mystery but it now looks like a measurement artefact of some unknown kind.

It's probably experience that makes engineers sniff out fishy(!) measurements. Amie, you've already gained more than about 99.9% (another bit of guesswork) of people your age.

[Amie]

Well, despite all, I did end up with some IF transformers that should work to some degree...

Smoked Salmon delicious! I like it with cream cheese and bagels, the food of engineers I was told. Sounds like someone had a good valentines day, (my birthday too!)

Thanks,
Amie.

[ppppenguin]

Happy Birthday! Ruth and I have a friend in Cambridge who has the same birthday.

*THE* place for smoked salmon and cream cheese bagels is here: https://spitalfieldslife.com/2009/12/11/...igel-shop/ We prefer it to the competitor just a few doors down. Both are open 24/7 (or were before the pandemic).

[Amie]

There can be a few disadvantages to having a birthday on the 14th of February I'm told, like having it on Christmas day...

I have been promised a trip to London after IT's all over, if my friend Grace can visit.... I couldn't pass up the chance of ambling a bit of Brick Lane, and always find an excuse to go into Duke of Uke, in Bethnal green.... now we have a recommendation for lunch....

I'll think of lox and bagels while I'm eating my baked beans....

[ppppenguin]

"Lox" is an American term. In one of his songs, Alan Sherman has the line: "...and the lox puts you in orbit A OK....". I wonder if he knew about the other meaning of lox, ie liquid oxygen. We have smoked salmon.

The curry places in Brick Lane aren't worth a visit. If you want Indian (or more strictly Pakistani/Bangladeshi) food round there walk down to Tayyabs in Whitechapel.

[Amie]

Lox is what one of dads old (US) colleagues used to call the salmon and cream cheese, a cultural term, though not strictly correct as it was salt cured salmon rather than smoked he used to have, so, yes smoked is better (and more accurate term)....

.... Now to get on with my local oscillator....

Thanks,
Amie.

[Kalee20]

I'm watching this carefully - I'm also aiming to be doing some trials this week, with IF amplifiers and transformers (as a matter of detail, I'm planning to use two RM5 ferrite core sets, which hopefully will have negligible external field, with coupling windings between them).

The 1nF that Amie reckoned on, for a x10 probe, alarmed me!

I'm in a different place from any capacitance measuring stuff right now. But I'm not away from my 'scope, a square-wave generator, and a box of resistors. So I had a play.

First thing: switch on 'scope (so any input circuitry is powered and behaving as normal) and use DMM to measure input resistance with my (non-switched) x10 probe fitted. Result: 10.07MΩ. In line with expectations.

Next, use two probes, two channels, to confirm that they show identical traces with my square-wave generator, and they are properly compensated. They do, and they are.

Next, add a 91kΩ resistor (came easily to hand!) in series with probe tip, and 'scope the square wave. This channel now shows exponentially slewing rise and fall, also a expected.

Such a rise, reaches 63% of final value, after one RC time-constant. Knowing R, and measuring time to 63%, I can calculate capacitance of the measuring setup, as seen at the probe tip.

The value of R in this, strictly is 91kΩ in parallel with 10MΩ, ie 83.4kΩ, and we also ought to allow that with 91kΩ added, the probe is no longer x10, but x10.1, however the error is only 1% and as can be seen, the square-wave rise isn't quite instantaneous, so there are other errors...

So, the time to reach 63%, using the amazing digitally-controlled measuring cursor* as shown is... 1.2μsec. Knowing R as 83.4kΩ, the value of C is thus 14.4pF.

It's a bit more than expected: I was reckoning on <10pF. But it is in line with figures discussed above, and even allowing for sources of error, I'd consider the result will be within 10% of the actual value.

* precision-controlled by digits of my right hand

[AdrianPH]

Amie;

Posting this in here as I note you do not want pm's.

It was only in the past day or so I have picked up on your age, and this is to say I think it is wonderful that you have an interest in electronics in general, but more so in this old technology. I sincerely wish you well in your experiments and learning. It is refreshing to see.

Good luck with the future and I hope your Mum feels your safe on this site and the feedback you get.

Cheers

Adrian