[minima] Re: KISS Mixer Musings
- From: Joshua Blanton <jtblanton@xxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Sat, 1 Mar 2014 16:41:54 -0500
Hello Mark and group,
I actually built this mixer a week-ish ago, and threw it up on the bench
for some testing - unfortunately, I did my testing on equipment at work,
and don't have my notes with me here, but I can speak to the
grounded-source behavior, I think. As a bit of background, I'm using
2SK192A-BL JFETs, because I have a large supply of them in my junkbox;
these will have lower max Idss, but I'm seeing 9dB conversion losses so I'm
not losing sleep over it - what's 3dB of loss from ideal among friends? I
also matched the two FETs I used for Vpinchoff, but did not match for Idss,
under the (possibly/probably mistaken) belief that pinchoff is more
critical to balance in this system. I must also explain that I only built
the mixer, and tested with two HP8640B generators for the LO and RF/IF
ports, looking at the other port with a spectrum analyzer. I built the
mixer to be a 6m-to-20ishMHz down-conversion, so my harmonics and their
mixer products were not nearby, and I didn't look at them in detail.
If you ground the sources, your input LO swing is going to cause the FET
gates to conduct as diodes, which will not be ideal from a noise figure
point of view (I believe - that's my understanding, anyway, and it makes
sense to me, I think). Ideally you swing your gate voltage from pinch-off
(FET dependent) to *just* below conduction (a diode-drop, so call it
0.6-0.7V), for your best variation in impedance; that's really the goal of
the FET. In an ideal world the pinch-off state would conduct nothing, and
the on-biased state would have 0 impedance, and thus switch the signal
hard-on and hard-off; also ideally the transition from one state to the
other would be instantaneous.
In my mixer, for some reason (that I did not explore enough to explain
here) my bias circuit was causing the mixer to massively increase
conversion losses, if I applied any bias at all, so I'm actually running my
mixer with no bias but include a 4.7k resistor to ground. I haven't ever
measured the voltage on the source, but I assume that it's floating up some
level based on LO drive and the Vgs conduction voltage... I believe that
my gates are conducting to generate this potential, which again is not
ideal from a mixer noise figure perspective. Most of my measurements were
looking at port-to-port leakage and conversion losses, but I can toss it
back up on the test bench and look at some of these other parameters next
time I have a minute at lunch. Next time I measure, I'm going to re-read
the section on mixer analysis in EMRFD, so that I take better measurements
:-)
All in all, this is a neat mixer - it appears to be robust (I did not see
compression on 0dBm input signals from RF->IF or IF->RF), and is certainly
simple. I look forward to playing with it more as I have time.
Josh, KB8NYP
On Sat, Mar 1, 2014 at 5:04 AM, Mark G0MGX <mark.g0mgx@xxxxxxxxx> wrote:
> Hi Team
>
> Thanks 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.html
>
> but 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> <jstreet@xxxxxxxxxxxx>
> Sender: minima-bounce@xxxxxxxxxxxxx
> Date: Fri, 28 Feb 2014 23:20:37
> To: <minima@xxxxxxxxxxxxx> <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 <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!
>
>
>
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