Testing crystals

[Crackle]

Thanks for the reading matter.
This is looking a bit technical, my brain doesn't like absorbing complex new technical things.

Mike

[Terry]

Don't panic, Mike!

I've been looking at your crystals - I hadn't realised before that the seller had provided a list of frequencies - so here are my findings.

The vast majority are, as I'm sure you will be well aware, completely outside the MW range but there are six that are not: 820/850/908/936/960/1024kHz

I'll start with 936kHz as that is the only one which is right on a 9kHz channel. It is used by 2 low power transmitters in the UK but far enough away that they shouldn't cause you any problems.  Outside the UK, the only transmitter I can find with any substantial power output is in Iran, so I don't think you need to worry about that! Should be an excellent choice for your micro Tx!

908kHz is a definite no-no! It is only 1kHz off Brookmans Park R5 (909kHz) - all 150kW of it!

820kHz looks good - virtually midway between the channels at 810 and 828kHz so any beats will be outside the IF passband. There is one high power Tx in Egypt on 819kHz, so this one looks good too.

850kHz. Authorised channels are 846kHz & 855kHz. Nothing of interest on 846, 1.5kW on 855 from Norfolk but the 5kHz beat should be outside the IF passband of most radios. High powered stations in Spain and Rumania. So, another one that looks good!

960kHz.  There is a 1.5MW(!) transmitter in Qatar on 954kHz but, again, the 6kHz beat should be outside the IF passband. There are 2 UK based low power transmitters on 963kHz, the only one of interest being 950W in Lea Bridge Road. Whether this would reach Basildon at night is difficult to say and will be influenced by both terrain and the design of the transmitter's aerial. A 3kHz beat could objectionable. You'd have to suck this one and see.

1024kHz. 200kW from Iran on 1024kHz plus a couple of low power distant UK locals here. Unlikely to cause any problems, I would have thought.

So, overall, you seem to have struck lucky with your haul! Getting one that is bang on a virtually empty channel is a particular bonus!

Browsing through the list I noted a 45233.3kHz crystal. This is only 0.5% off Channel B1 vision frequency so could be useful for a Band I modulator ...

Be very careful what you do with that 2182kHz crystal - it is/was an International Distress Frequency and, although the Coastguard no longer actively monitors it, I've now idea what the current situation is.

[Crackle]

Thanks Terry.
Yes 2182KHz, the old Trawler band calling and distress frequency.

Am I right in thinking the harmonic frequencies are multiples of the fundamental, and the 3rd harmonic is the best. Is this 3 times the frequency?
Is the same to be said for frequencies 3 x below the fundamental frequency.

Are the harmonics at a lower intensity (less efficient) if you see what I mean?

Puzzled Mike

[Murphyv310]

Hi Mike.
The problem with harmonics is that the higher the frequency you can have a better match to your aerial and get better range. At medium and long waves you'd need a rather long aerial for good matching.
If you use a Pi tank circuit there is less of a chance that harmonics will be a problem anyway. A flea power TX is hardly going to a problem TBH unless you add half a dozen 807's.

RE 2182khz, I have two Ship receivers here. One is a synthesised set and it sits on the distress frequency a lot of the time, in 10 years I've never heard a thing. Personally though I'd not consider using it.

[Crackle]

My tests seem to confer with your notes Terry.

820 sounds pretty clear here something on 829 and very faint background on 811 (Is my radio slightly off do you think)

850 I am picking up 2, maybe 3 stations on 855 all about the same strength at about 17:45 this evening. So sounds like the Norfolk local and couple of foreign ones.

908 R5 live booming out football rubbish.

936 loads of mush on this frequency fades out by 938, I should think a local signal will over come this with the AGC muting the gain.

960 not good, bleed through from Sunrise Radio on 963

1024 not too good as there is a station close at 1026

All my frequency readings seem to be 1kHz high.

Mike

[pwdrive]

Mike, so far as I understand it (limited knowledge) some xtals were specifically manufactured to operate at a particular overtone frequency.

Two modes of operation, series and parallel.

In series mode, shunt capacitance has little effect, at series resonance only the xtals motional inductance and motional capacitance are up for consideration and they are naturally more stable.

In parallel mode, shunt capacitance has an effect and by its nature won't be as stable, the shunt capacitance, including the xtals plates, leads and holder resonates with the Xtals motional inductance, that point lies between the series resonant frequency and the anti-resonant frequency.

The anti-resonant frequency is when the reactance of the xtals motional inductance equals the reactance of the xtals plates, leads and holder.

In frequency progression it's series resonant, parallel resonate and anti-resonant, then overtones etc.

Any errors then guilty as charged.

Lawrence.

[Crackle]

This morning I put together the Pierce oscillator circuit, it took ages to find an RF choke of about the right value in my spares box, I stuck a bit of ferrite up it to help take the value to 2.5mH.

It worked quite well for testing some crystals I had previously salvaged from old equipment..

Most crystals tested OK, coming up very close to their marked value. (probably the meter out by a bit)

6.144 MHz
   

6.250 MHz
   

4.000 MHz
   

But a couple were different.
This one marked as 40.000 was coming up as 13.33MHz
   

And this one marked as 3.2768 was randomly coming up as two different values 9.644 MHz and 3.277 MHz.
       

This one and another like it marked as 21.8MHz sold to me some time ago for a pantry TX did not seem to work at all.
The display on the meter was changing all the time.
   


Has anyone got explanations for the ones with odd readings?
Do you think the large 21.8MHz crystal would work in another circuit?
thanks


Mike

[pwdrive]

The 40.000 one might be 40MHz in overtone mode and it's oscillating at the fundamental in your circuit.

So far as I know most xtals designed to operate above 10/20 megs or so where operated at overtone frequency, the higher the fundamental the thinner the xtal so more difficult to manufacture.

Lawrence.

[Terry]

Yes, as Lawrence says, your 40MHz crystal is actually a 13.333MHz third overtone crystal. In normal operation you would have a resonant circuit tuned to 40MHz on the output.

The 3rd harmonic of 3.2768 is 9.8304MHz which doesn't agree with your reading so I wonder if the crystal is trying to operate in the wrong mode. Perhaps you should try it in a different circuit.

I'm fascinated by the idea of a 21.8MHz pantry transmitter - what, exactly, were you planning to use it for?

[pwdrive]

Can you post the schematic of the Pierce and how your FC is connected?

Lawrence.