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The trouble with double balanced mixers

Hello folks !
I never knew that overdriving double balanced mixers can actually reduce their very purpose of carrier suppression . I found this out while watching a video on microwave mixers . This problem was plaguing my Bitx40 , one of the reasons why I didn't put a linear stage on it and come on air . Instead of using its own LC VFO , I use a DDS VFO external to the Bitx40 . It had stupendous amount of carrier in its output . Trying to figure out the cause , I changed the diodes , rewound the transformers , but nothing worked out . However reducing the drive level alleviated the problem . After doing a few simulations of this in TINA-TI and obtaining the results , I did the same with the actual Bitx40 PCB . The CRO output looks much better now compared to the previous cases . The tests on a receiver too is encouraging as I can hear my sound as an SSB signal in it . Needless to say , the BFO stage has to be tuned to match the crystal filter's range to ensure best operation . If everything goes well , I will attach a IRF510 linear and come on air soon !

73s 

Comments

  1. golden eagleAugust 24, 2015 at 10:36 PM

    The diode is used a s switch. To switch it on just 7dbm power, roughly 10 milli watt is required. also take note of the impedance. so many times at the output port a diplexer network is installed. And sometimes the 3db pad is put at the input to normalize the impedance. At the buffer stage many peoples have used 50 ohm.... so in nutshell,,, diodes needs very low power. Dillon has done very extensive experiments and published it. which might be available on net... I have few pages published in elector india.... de vu3inj.

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