VHF_Ensemble Antenna Isolation, "Subharmonic" mixing.


VHF Ensemble.
See below:- An explanation of "Subharmonic" mixing process.


Ground Isolation

Tony has not included antenna ground isolation in this model.
I found a BIG ground loop when I tried mine. To avoid this I removed the antenna socket (Not too easy!) and cut the ground plane on both sides with a small saw blade so it was isolated from the rest of the board. This is coupled at RF by 100pF capacitors soldered across the cut.

Tony suggests 0.01uF capacitors, my 100pF are smaller than optimum.
Tony also says the cut at the top could be made with a craft knife without removing the BNC socket.

To avoid cutting the ground plane an external transformer could be used similar to the HF Ensemble. I used this method because I thought it easier, worth a try and I was not certain of a suitable transformer design. It worked on my 2m version with no apparent problems and should be acceptable for the 50 and 70MHz versions.

Not too neat, you can do better.





Tony suggests using 0.01uF capacitors to keep the RF impedance really low, surface mount would be ideal. A look at reactance tables shows he is correct, using one 0.01 each side still leaves 80K Ohms reactance at 100Hz and should  eliminate ground loop effects.
 ValueReactance at 100Hz Reactance at 50MHz Reactance at 150MHz 
2 x 100pF3M Ohm16 Ohm1.4 Ohm 
2 x 1000pF  750K Ohm1.6 Ohm 0.5 Ohm 
2 x 0.01uF80K Ohm  0.16 Ohm 0.05 Ohm

I'll be increasing the value of my capacitors, I'd say 2 x 1000pF will be fine for 2 metres, my 2 x 100pF seem to make no noticeable difference to reception when shorted. 

"Subharmonic Mixing" used with some Lites> See here https://groups.yahoo.com/neo/groups/softrock40/conversations/messages/77430 Here Mike explains that the mixer is switched at 1/3 of the signal frequency. This is especially useful with VHF because it keeps frequencies within the range of the Si570 and the processing ICs. The LO in the centre of the band does not actually exist.



IMPORTANT:- Users of older Ensembles  before January 2014 see this page!  http://sites.google.com/site/g4zfqradio/softrock_ensemble_isolation_transformers

Index of my Softrock Related Pages https://sites.google.com/site/g4zfqradio/home

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