[minima] Re: KISS Mixer Musings
- From: Ashhar Farhan <farhanbox@xxxxxxxxx>
- To: minima@xxxxxxxxxxxxx
- Date: Mon, 3 Mar 2014 09:39:10 +0530
Mark and the gang,
What was the level of the 5 MHz signal into the mixer? It looks like it was
quite a strong one. I am a little confused.
1. The first picture at
http://2.bp.blogspot.com/-kIK-jtP4zAs/UxDN3oyTR-I/AAAAAAAAFps/DnpUkVjtnro/s1600/LO+Only.png
shows the local oscillator at the IF port with -10dbm output. What was
the
input level at the gate? it should be around 3 v peak-to-peap to drive the
J310s from pinch-off to full-on.
2. In the second picture with the RF signal at 5 Mhz, (
http://2.bp.blogspot.com/-NGDTM1XUkXU/UxDOB-QrjYI/AAAAAAAAFp0/sRNKTvuGntA/s1600/LO+15M+RF.png)
we see that a peak at 15 MHz that is 0 dbm! this is quite a high level
(more than 1 v peak). At this level, the IIP3 will substantially contribute
to the harmonic distortions.
Can you repeat these tests with the RF signal set to a much lower level?
Let's say around -20dbm?
- f
On Sun, Mar 2, 2014 at 3:11 AM, Joshua Blanton <jtblanton@xxxxxxxxx> wrote:
> 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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