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17th December 2011 17:41
Perry,
It isn't too critical, with the FTxx-43 the "xx" is the core diameter in 100ths of an inch. The 50 is half an inch, the 37 is smaller. At the power levels involved in the RX it doesn't matter at all, and either core would be fine. In fact just about any ferrite core would be OK as long as the final winding has enough inductive reactance.
For broadband transformers, like a DBM, the inductive reactance of the transformer should be at least 3-5 times as much as the load impedance. 200 Ohms or more in a 50 Ohm system is generally OK. At 3.5 MHz that means each winding should have 9-10 uH of inductance (the original uses 22 uH). For a FT50-43 core that is only 5 turns, more is fine (on a FT37 core 6 turns would be enough, the prototype uses 8 - plenty).
Regards,
Alan
8th December 2011 04:58
Your postings are a inspiration to the hobby and I congratulate you
You have “played” a lot with these receivers and I need your advice,
I have FT37-43 cores will that do for the DBmixer or will I have
problems. I cant seem to find ft38.should i look for FT50's or will FT37 do the job
Regards
6th August 2011 11:40
Bert,
No, not really, but they perform better if all ports see the same resistive load. If they are not terminated well the reflections can cause imbalances and allow breakthrough and intermodulation.
Regards,
Alan
6th August 2011 11:35
Hi Alan,
Do I always need a diplexer for diode mixers? I saw your "small 80m direct conversion rx" and find that you did not mention the importance of a good diplexer. Is it because the 200ohm pot and 100 ohm resistor which makes up a good 50ohm load for the VFO buffer?
15th August 2008 22:04
In mixers the diodes essentially act like switches, they are either high-Z or low-Z depending on if they are forward biased or not. Once forward biased the small signal model of a diode is basically a small resistor (nasty to calculate!) which can be largely neglected in this circuit.
The mixer transformers are broadband so the usual rules apply; you pick a winding reactance at least 3 times the load impedance the winding will see during use to get sufficient low-frequency performance. In a 50 ohm system you want at least +j150 Ohms of inductive reactance, at 3.5 MHz that works out to at least 6.8 uH. For an FT50-43 that is 4 turns minimum.
It is good practice to use more like 4 or 5 times the load impedance for the winding reactance. You can use too much however, as the winding inductance rises so does the leakage inductance, and it will resonate with the distributed capacitance determining the high-frequency cut-off for the transformer. While broadband transformers wound n-filar are more a transmission line loaded with ferrite they still suffer from leakage reactance like a conventional transformer.
This excellent page by LA7MI describes how to design broadband transformers for amplifiers. The general principles apply to trifilar 1:1 transformers like are used in diode ring mixers.
The RA3AAE mixer is a bit different, in fact I am not completely sure I understand how it works as well as it does.
In Michael's circuit the link-windings on the front-end filter tanks and the oscillator tank are determined by the load impedance to be reflected across the tank.
For the filter the bandwidth of the filter is a function of its load impedance, in designing double-tuned filters like this you pick the bandwidth desired, then calculate the Q to implement that bandwidth. The loaded Q for each resonator is then decided by the filter form-factor which you can lookup in tables, but root-2 times the Q for the width is a good starting point for a Butterworth shape, it takes into account the loading on each resonator. The coupling capacitor at the top of the resonators is also computed from the Qs. I suspect Michael hasn't been too cautious about the exact Q, with link coupling it is much harder to control the Q because you are limited to integer turns ratios - well you can do fractional taps and that is done at VHF in helical resonators, etc. Taps and link windings in general are mechanically troublesome, so a common practice is to design a tapped-capacitance match to reflect the load across the tank as a much higher impedance giving you the desired Q. Filter design is a huge topic, the EMRFD book has a good treatment of it. My calculator is a reasonable approximation, and PY2OHH has a good page on how to do it by hand for the Butterworth case.
The filter in my VXO receiver wasn't well designed, and was actually built empirically while sweeping it. You shouldn't do this, you should do the maths first! :)
The LO oscillator tap in Michael's circuit was probably selected to give the desired drive to the mixer. It also can't be too much or else it will load the oscillator excessively and degrade its performance or stop it all together. Too little and the voltage across the link won't be able to bias the diodes. The link winding drives the diodes and filter secondary which can't be a very high impedance load one the diode threshold is exceeded.
PA2OHH has a nice simple design using the Polyakov mixer. His novel PTO is interesting too, placing basically a shorted-turn around the oscillator tank coil to tune. He also takes some pains to balance the detector to reduce AM break-through.
15th August 2008 20:42
Hi Alan,
thanks for the extensive answer, this was really more then I was hoping for. It helped me getting started. I am now working on a DC receiver with Polyakov mixer from AA1TJ : http://mjrainey.googlepages.com/radio. Obne question that pops to mind, is how do you decide on the number of turns in DBM like yours in the 80M VXO or in Mikes Polyakov mixer. I mean diodes are highly non-linear.
Kind regards
cor
1st August 2008 13:43
Cor,
The core material to use depends on the application in the circuit. In the case of this receiver the mixer needs broadband transformers, so type-43 ferrite cores are used.
In general there are two core material types; powered iron and ferrite. The iron power cores are colour coded, the majority of the core is one colour and a small area on one face is a different one (or often "natural" or "clear" which shows through the core material colour). Sizes vary and cores are available from 1/5 of an inch to over 5 inches (sizes are imperial, but that isn't a huge problem). Larger cores are needed for higher powers and/or standing currents to prevent core saturation leading to non-linear distortions and core heating.
For chokes I generally use type-43 ferrite. I don't use much type-61 ferrite, but it is great for large value inductors and broadband transformers.
For general purpose inductors, say for tuned circuits, filters, etc then iron power cores are used. For top-band 160 metres blue/natural type-1 material is best. For up to 40 metres red/natural type-2 is used, and for the rest of HF and 6 metres yellow/natural type-6 is best. For VFO inductors where high stability is required white/natural type-7 material is better than yellow/natural type-6 but otherwise quite similar. There is a black/natural type-10 material for VHF work and tan/tan type-0 which is just phenolic and has almost a free-space permeability.
There are many other material types, especially for switchmode PSU use. Kits and Parts has a great page which summarises the properties of common cores available from Micrometals.
I keep a stock of types 2, 6, 7, and 43 in various sizes and a smaller quantity of assorted 1, 10, 61 and 0. Iron power toroids are coded T(size)-(material), where size is in 100ths of an inch and material is one of the iron power core material ID numbers above. Ferrite cores are coded similarly, FT(size)-(material). For example a T50-6 would be a yellow/natural (type-6) core half an inch in diameter and a FT150-43 would be a 1.5 inch type-43 ferrite core. The most common size I use is probably T50, but I like T37 for small applications and T100 or larger for antenna/atu work and applications where there is a lot of power involved. I also like FT23-43 cores for small VHF chokes and instrumentation transformers like directional couplers and RF ammeters. Bead and pig-nose forms are also available for ferrite.
I purchased mine from Kits and Parts and recommend them.
31st July 2008 20:20
Hi Alan,
how do you decide which toroid form to use, for example in the vxo 80 meter receiver? Do you keep a stock of some of the toroid forms?
Kind regards
cor
28th August 2013 12:13
Keith Ostertag wrote...