Rafel,
See my comments below:
On 17-03-2017 22:35, Rafa? Lichwa?a wrote:
Hi Peter,
Thanks for this forwarded message!
So I noticed my first mistake: crystal filter has ~200 ohms Zin/Zout -
not 50, as I thought before.
Ok, so that means, attenuator has right Zin/Zout, but there is an
impedance mismatch between 1st AMP (just right after LPF) and mixer.
This AMP translates 50 ohms of the LPF to ~200 ohms, but mixer expects
50 ohms. Maybe that's why we can notice much improvement in the way
BITX works, when we remove that 1st AMP completely - then mixer has 50
ohms on all ports.
But I don't understand two things:
Second the terminal impedance of the crystal filter and the various
RF/IF amplifiers were optimized for 200 ohms the mixers are better at
50 ohms and there is no impedance matching to correct for that.
Yes, the lack of matching keeps the circuit simple but you pay the price of less than the best achievable performance from this circuit. In the real world you will need to measure and compare the results of both matched and unmatched circuits to see the performance difference and keep in mind that often what looks good on paper does not translate into a large practical (real world) difference, ie; the improvement may not be worth the added complexity.
Why mixers are better at 50 ohms?
I thought there is not much difference in mixing for different
impedances on mixer ports, but the point is to keep them EQUAL on each
port (nevermind if it is 50 ohm or 200 ohms - mixer will work fine as
long as those impedances are equal on each port). Is that correct?
When I saw this question it caused me to think about how I have been using DBM's for many years now. In truth I have never considered operating them at impedance's other than 50 ohms, all the books and circuits said 50 ohms so thats what I did. However, (I am not into the basic physics of these things) my understanding is that for the DBM to be effective the diodes need to be switched hard on/off as quickly as possible. It seems to me that it is probably easier to achieve this if the DBM is being driven from a low impedance source which ensures maximum current flow through the diodes.
And second thing: why RF/IF are optimized for 200ohms?
You should ask the designer, Farhan is active on this group ask him.
As it was explained by the author of that project, each AMP translates
impedance: 50ohms -> 220ohms and 220ohms -> 50ohms (for resistances
used in the circuit).
So I think there is no mismatch anywhere except this one between 1st
RF AMP and mixer. Is that correct?
That is not my understanding of how the bi-lateral amps work, again talk to Farhan.
Additional question at the end: is that a good idea to use ADE-1 (SMD
level 7 double balanced mixer) instead of those 1n4148 diodes and
"manual" transformers?
This link was posted by a member of our group in the past and compares the performance of various DBM's. I was very happy to see that the homemade 1N4148 DBM's performed so well against the commercial DBM's. Of course you will have to be careful in how you build the homemade DBM, match the diodes and be careful with the transformers but if you do a good job the results can be quite good. See link below:
Regards,
Rafal SP3GO
Best regards,
Peter VK1XP
18/03/2017
