[Premium-Rx] New diode mixer

[Steve Stutman]

See also:


http://www.pan-tex.net/usr/r/receivers/smixerpic.htm



and the ever genteel:



http://www.fpqrp.com/bb0304.pdf


73,
Steve










Michael O'Beirne wrote:

> Hi everyone,
>  
>     This posting is aimed at non-RSGB members who do not have access to 
> RadCom. 
>     Pat Hawker's Technical Topics column in the current January 2005 
> issue describes a new diode mixer that is the subject of US Patent 
> 6,111,452 entitled "Wide Dynamic Range RF Mixers Using Bandgap 
> Semiconductors".  The patent is in the names of Fazi and Neudeck and is 
> assigned to the US Army.
>     The inventive step is to use bandgap material such as 
> monocrystalline silicon carbide (SiC) for the diodes in a double 
> balanced mixer rather than conventional narrow bandgap semiconductors.
>     Another wide bandgap material, GaN (gallium nitride), is currently 
> being used to make blue LEDs and the suggestion is made in Pat's piece 
> that it should prove equally effective for the same reasons.  [One could 
> I suppose discuss the pros and cons of LED mixers by reference to the 
> more traditional form of glowing mixer!!!]
>     Pat says that the patent provides an excellent discussion on the 
> operation of solid state diode switching mixers and the various methods 
> of increasing their dynamic range, and compares a conventional silicon 
> ring mixer with a mixer using SiC diodes in the same environment.  The 
> results are impressive.   But there is a cost - the LO power for a SiC 
> mixer needs to be 20dB more to provide the same 10dB loss because of the 
> higher turn-on voltage required.  As they say in Yorkshire : "You don't 
> get naught for naugt".
>     The source reference to this patent arises from an article by David 
> White (either WN5Y or KN5Y) in the March 2004 newsletter of the "Flying 
> Pigs QRP Club". 
>     The patent seems well worth reading, though whether one would be 
> able to deliver the required LO power is another matter.  We already 
> need about +17dBm to drive a high level diode mixer, so finding another 
> 20dB will not be easy.  I could just about do it with my ancient R&S 
> power signal generator, type SMLR.  It produced so much umph that it 
> burnt out one pi section of an expensive external Marconi UHF 
> attenuator, which did not please me one jot.
>     I cannot but feel that a better approach to mixer technology is to 
> use the H-Mode mixer  developed by Colin Horrabin, G3SBI, which uses a 
> high-speed bus switch.  The receive section of the CDG 2000 transceiver 
> (described in RadCom June - August 2002) incorporating such a mixer has 
> an IP3 of +40dBm with a noise figure of 10dB.  It also uses a very low 
> phase noise LO which achieved -140dBc / Hz at 9kHz offset from the 
> carrier, and -150dBc / Hz at just over 20kHz offset on the 20 metre 
> band.  The limitation in dynamic range was, in part, not the mixer but 
> the coils in the bandpass filters, being 13dB better for hand-wound 
> coils than for commercial Toko inductors.  This is rather like the old 
> RA1772 where the limiting factor in the prototype was the IP3 of the 
> roofing filter and not the, then, novel switching mixer using a ring of 
> four FETs.  All the original ferrite material in the filter had to be 
> removed in order to achieve the IP3 spec of +27dBm.  
>     In the CDG2000, with an IP3 of +40dBm and a noise floor of -130dB, 
> an IP dynamic range of 113dB was achieved.  That's a lot better than 
> many commercial premier receivers.
>     I hope this of some interest.
>     73s
>     Michael O'Beirne
>     G8MOB
>    
>  
>      
> 
> 
> ------------------------------------------------------------------------
> 
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