Allison
As I recall, the problem with the original Minima was that the 20 Mhz IF
leakage through the mixer (IF-RF) wasn't sufficient to prevent unwanted
transmit spurious at the IF frequency, and that the simple 2-band lowpass
filter at the input didn't have adequate performance to improve it enough. I
don't see any specification in the ADE-1 or SBL-1 datasheets for IF-RF
rejection, but my recollection is that numerous packaged and home-brew mixer
designs were evaluated with no definitive solution.
I think the proper solution for the original minima is a lower IF frequency
(maybe 9 MHz?) and switched bandpass filters at the front-end. One could use
2 CMOS bus switches and up to eight 2-pole bandpass filters to accomplish
this (I've done it and it works very nicely). The final product isn't much
more complicated than the 2-band minima lowpass approach and can be
implemented incrementally, adding one band filter at a time as desired. I
think one could also consider this as a souped-up BIT-X.
FWIW, my 2 cents and all that.
Joe
W3JDR
-----Original Message-----
From: allison
Sent: Wednesday, July 27, 2016 8:57 AM
To: minima@xxxxxxxxxxxxx
Subject: [minima] Re: for minima project
On 7/26/16 1:54 AM, Ashhar Farhan wrote:
There are many ways to continue with the Minima:MOs switches or a fully balanged mixer works.
First:
If one crosses over to the fulll H-mode mixer, it might solve the leakage problem. I have been trying to keep the design simple with the KISS mixer. Our (Joe and Mine) experiments wiht using MOS switches were encouraging.
Second:DBM is a easy way to get a balanced mixer. RF/IF gain is easy and cheap.
The other option is to just go with a diode mixer. I already have a transceiver like that in the works with Si5351. It is a low-cost design that works upto to 21 MHz and uses two oscillators for the BFO and the VFO. A a post-mix amp had to be added between the diode mixer and the crystal filter to provide decent termination to both sides as well as overcome the filter loss. This is a pretty simple design. The BFO and VFO are entirely software controlled. A single LPF that cuts off at 21 Mhz is all that it takes. I will soon post the details.
Third:capacitance from gate to drain is first order issue. Then the drain
The JFETs as mixers could be trouble as their gate-source and gate-drain can go into conduction easily. Instead, if we use BS170 or 2N7000 kind of switching MOSFETS it might just work.
The interesting thing is, though Minima itself was not a very replicable design, it did teach us quite a lot. It is a robust receiver and it has had a large number of spin-offs.