Jfet mixers compared to diode mixers - Page 1
Forum section: electronics / rf-microwave

author: paul002
author_rank: Regular Contributor
timestamp: June 07, 2020, 11:33:13 am
content: I did an experiment to build some jfet mixers. My current receiver uses diode double balanced mixers which are easy to build. But literature suggest jfet mixers have more benifits like better ip3 and some gain. Made a small pcb and did some comparison. Unfortunaly my jfet mixers do not live up to expectation (right now) the suppression of LO and RF is much lower as the good old diode balance mixer. Also the insertion loss (was expecting some gain) is lower. In one measurement I get a IF of -48 dbm where as the diode mixer gives me -44 dbm. At 14 Mhz the jfet is a bit better in terms of gain -36 dbm where the diode mixer gives -38 dbm. But I can see some lift in the spectrum. If I lower the LO that lift drops but also the output of the mixer. I have a receiver without a pre-amp so a bit gain would be nice. Somebody have positive experience with these jfet mixers? Maybe they are a bit more complex than diode mixers and chances for mistakes are bigger.(IF output is 9Mhz, RF 14 Mhz and 3.5 Mhz, LO 5.5 and 23 Mhz)

author: David Hess
author_rank: Super Contributor
timestamp: June 08, 2020, 04:13:39 am
content: My recollection is that the main advantage of FET mixers is lower required local oscillator power for a given IP3.Matching of the FETs is still required for good rejection which presents more practical difficulties than with diodes.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: June 08, 2020, 11:48:03 am
content: Do you have to DC bias the mixer? Depending on what region the mixer is supposed to work at, it could be useful to bias the FETs so they are closer to turning on/off. That way you can even get gain out of active mixers.

author: paul002
author_rank: Regular Contributor
timestamp: June 08, 2020, 01:59:39 pm
content: Yes, they are biased. That is there is a source resistor and the gate is DC to GND. So you get some negative gate to source voltage. I could try to adjust this. But maybe first simulate the bias in Kicad / ngspice to see what the resistor should be. I don't use the described Jfets but j310. I also tried a alternative schematic. But that was even worse. So dropped that design.

author: M0HZH
author_rank: Regular Contributor
timestamp: June 08, 2020, 04:10:54 pm
content: J-FET mixers have some benefits over diode ring mixers, but matching is critical and harder to do than with diodes. Random FETs will give you random results.If you are looking to improve performance, you should really skip J-FET as they're old technology and look into quadrature mixers for the first mixer:http://www.norcalqrp.org/files/Tayloe_mixer_x3a.pdfhttps://martein.home.xs4all.nl/pa3ake/hmode/hmode_intro.htmlThis is the solution top-end heterodyne receivers (Hilbering, Elecraft, etc) are using. The disadvantage is you need multiple LO signals 90 degrees out of phase (or a LO at 4x the frequency and a divider with phased outputs).

author: paul002
author_rank: Regular Contributor
timestamp: June 08, 2020, 08:32:30 pm
content: Yes, you are right. JFET mixers are maybe not giving so much advantage and to critical to balance. This was also described by Ulrich Rohde in Ham Radio 1976 (from where I got the schematic) he preferred diode mixers because they are less critical to build. But I am a bit on a sentimental journey starting with EK90 now diode mixers wanted to try JFET mixers. Did not do homebrew building for 30 years and noticed that a lot has changed since the 1977 ... But the good old diode mixer is still performing well. H mode mixers are a bit complicated, but I was also thinking to try them but than as a mixer for a SDR radio. Creating 90 deg phase shift is possible using the si5351 or adding a johnson flipflop. Any way thanks for the articles.Quote from: M0HZH on June 08, 2020, 04:10:54 pm