Hi TeamThanks to everyone that joined this discussion; my first conclusion after reading all the replies was that it seemed my pair of FETs were far from matched. Rather than trying to match two I simply swapped one of them for another one out of the FET draw.... the results are quite different.
I've updated the page here: http://g0mgx.blogspot.co.uk/2014/02/kiss-mixer-musings.htmlbut really I am finding little or no difference with 1 bias pot, two separate bias pots or just grounding the sources - the results are almost identical as far as I can see.
In my test setup I have a 20M signal into the LO port, 15MHz into the RF and therefore am wanting either the 20-15 = 5MHz signal or the 20+15 = 35Mhz signal. Under all combinations of bias setup I can alter the amplitude of the _unwanted_ signals, but make no difference to the _wanted _signals - they remain the same under all conditions.
Mark G0MGX On 01/03/2014 05:45, pakdeDar wrote:
Other questions to the experienced builder here according to the j310 used as KISS Mixer : 1. An article in the internet state that J310 was symmetric, symbolized by gate in the center between source and drain. On the contrary other article say that j310 was NOT symmetric. Q : what is the effect to interchange source and drain ? Could it be noticed clearly by ear ..our most sophisticated ham equipment Gift by our Creator ? I take simple solution for this case ..Just refer to the pin outline, wire the source to the common side and drain to the hot side. Am I correct ? 2. Is it possible to set jfet bias to 50 pct of and 50 pct on as Dan say using simple equipment such as DVM ( and ear setting of course ) but not using scope ? If yes, how ? I am interested to the spirit of KIS and N(ot) C(omplicated)...always remember Ham's spirit " Better to measure than not to measure " ( even using simple-homebrewed equipment ) Sudarmanta - YC1DCN Send from my TelakaspaBerrylayauw ® -----Original Message----- From: Joe Street <jstreet@xxxxxxxxxxxx> Sender: minima-bounce@xxxxxxxxxxxxx Date: Fri, 28 Feb 2014 23:20:37 To: <minima@xxxxxxxxxxxxx> Reply-To: minima@xxxxxxxxxxxxx Subject: [minima] Re: KISS Mixer Musings SST/U401 VGSth matched to within 5mv On 28/02/2014 11:13 PM, Joe Street wrote:Why not look for a monolithic pair? If they are fabricated on the same substrate the characteristics should be well matched. I haven't looked but surely there must be something off the shelf? On 28/02/2014 7:54 PM, Tayloe, Dan (NSN - US/Tempe) wrote:We need to bias the jfet channel for 50% on, 50% off. When a jfet is "on", it grounds its winding with its phasing for RF to IF transfer. Alternating jfets flips the signal to the opposite polarity. Rapidly flipping the signal polarity through this stage at the LO rate does the mixing. Thus there is no DC current, but we are trying to turn on/off AC paths. Since jfet pinch off thresholds vary so much from device to device, separate bias is best. As a matter of fact, if you had a bunch of these, it would be nice to match Idss and pinchoff voltage for these two jfets. - Dan Sent from my Windows Phone -----Original Message----- From: ext Joe Street Sent: 2/28/2014 5:10 PM To: minima@xxxxxxxxxxxxx Subject: [minima] Re: KISS Mixer Musings These FETs are not really 'biased' in the normal sense anyways because the drains are open circuit for DC so there is no bias current flowing. What you are doing is raising and lowering the potential of the whole channel. Variation in the device fabrication processes result in device to device variance in transconductance so perhaps in this strange circuit balance is more important?? On 28/02/2014 4:58 PM, Tayloe, Dan (NSN - US/Tempe) wrote:That might change the balance of the mixer, but does not change the bias on each gate. - Dan -----Original Message----- From: minima-bounce@xxxxxxxxxxxxx [mailto:minima-bounce@xxxxxxxxxxxxx] On Behalf Of ext Sandeep Lohia Sent: Friday, February 28, 2014 2:56 PM To: minima@xxxxxxxxxxxxx Subject: [minima] Re: KISS Mixer Musings http://4.bp.blogspot.com/-fIABMtTpyUE/Ut9yfPotc0I/AAAAAAAAAUQ/IG6TveVonrw/s1600/under+R&D.jpg NOTE : not yet tested live...IF you have two separate bias pots, You could bias the two JFETs with separate pots and adjust for similar drain Just thinking about this. I remember that the transconductance of JFETS varies quite a bit so the fact that the signals are not symetrical might be due to an imbalance in the device characteristics?Please take a look at the link and then any volunteers who can try and explain this to me most welcome!
