On 21 Mar 2006 01:41:49 -0800, "Giancarlo Gian Moda, I7SWX"
Post by JJWhat is the IP3 of a typical dual gate MOSFET mixer?
Depening on the fet and circuit values around +3dbm.
Post by JJOne can also use HC4066 or 4053 as the switching element at lower freqs.
True. Doesn't work well at 6M at all.
Post by JJJJ
I really do not know what is the IP3 of a DG Mosfet as I never have
seen any value reported. I hope some readers may have data on this.
Certainly we have to look at IIP3 and OIP3 as the DGM mixer may have
high conversion gain 15-20dB and is somewhat limited in dynamic range.
Maybe it could have an OIP3 around +5dBm equivalent to an IIP3 of -10
to -20dBm .... more or less similar to the NE602.
Much better than NE602 on IP3, lower on conversion gain (6-10db).
The big differnce is the DGFET mixer is lack of balance so feed
through is an issue. However a pair used in a balanced arrangement
can be a very good low noise mixer. Many of the balanced FET designs
in past issues of Ham Radio and were from hams in Europe where
strong SW broadcast is the normal environment.
A thought on comparing mixers:
NE602, Low power needed around 2-3ma at 6V, IP3 -10dbm.
(for 18mW of power this gilbert cell offers good
perfomance). Low power needed is very small
to nonexistant due to internal OSC. Low NF of <5db at
45mhz. High conversion gain (typicall 15-17DB).
DGmosfet power needed around 4-6ma/12V, IP3 0 to 3dbm
Also low to no LO power needed. Conversion gains
of 5-8db typical. Properly matched, low noise.
Balanced DGmosfet(2) power needed ~8-10ma/12V, Ip3>3dbm
Low to minimal LO power needed. Low conversion gains
but low noise as well.
Ad831 high power needs but all the advantages of Gilbert cell
mixer with very high IP3s (variable with power
consumption setting).
DBM power needed zero (however 10mW of LO required!)
IP3 around 0DBM, requires post mixer IF amp or termination
and has 6-8db loss. System power required for mixer is
usually around 100mW or higher.
Fet switching mixers are all over the map on power required
some requiring little and some needed a great deal of LO power
to drive correctly, they may also require supporting pre and
post amps to insure noise figure and system gain.
Generally the more power, the better the IP3. In cases where power
is available this may not be an issue. Balanced designs regardless
of devices are more robust than single ended. For portable systems
where battery lifetime is a consideration that can be a determining
factor.
Post by JJThe DGM mixer was a common mixer in the first series of transistorized
RTX with valves PA (FT101Z, TS820 etc). The DGM has a good square law,
better than JFET.
JFETS in balanced active mixers do very well. It's also possible to
use two Jfets to replace a DG mosfet and in most designs that will
perform the same as the DGFET.
Post by JJ74HC4066 is OK at low frequency conversion but it has a haigh
conversion loss, around 8-10dB and so associated noise figure.
Old device and higher series on resistance.
Post by JJToday the switched mixer should use fast bus switches like the FST3125
family when looking at high performance mixers. The examples are the
CDG 2000 and STAR (Pic-A-Star) projects with IP3 around +40dBm and
+36dBm, where the H-Mode Mixer with FST3125 is used.
All of that is good but unless the post mixer amp, IF and filters
following it are up to the task and well matched the results can be
very disappointing. When going to that level of performance one
needs to look at the recieving system and examine carefully.
Allison