EMRFD Message Archive 9660
Message Date From Subject 9660 2014-01-17 20:22:45 Ashhar Farhan The Minima transceiver I have just finished a new transceiver called the Minima. Some on the list have already seen. The details are on http://www.phonestack.com/farhan/minima.htmlBriefly, the transceiver is a general coverage rig with minimum complexity.I've tried to keep the performance as best as one could given the constraints ofsimplicity and easy availablity of components. It sports a great receiver and clean transmitter. It is quite hackable.I hope you like it.In trying to keep the EMRFD list clear of Minima specific discussions, there's a separate mailing list to discuss it in detail for those interested in building or experimenting it with it. You can join that list from the web page.btw, we have found www.freelists.org a reasonable alternative to the yahoo groups, though it doesn't have a file sharing capability. but we are working on setting up a homebrew radio specific wiki for us all to contribute to.- farhan (VU2ESE) 9661 2014-01-17 21:17:42 Jerry Haigwood Re: The Minima transceiver Hi Fahan,I like the simplicity of your Minima transceiver. I have a correction for you. The 2N2222 is an NPN transistor and the 2N2907 is a PNP transistor but you know that. Notes on your schematic have them reversed. Here are a few questions:1. Is the extra transistor needed to drive the power amplifier shown on a different web page?2. How much xmit power is running through the crystal filter?3. What brand/part number of crystals did you use for the crystal filter and BFO/Carrier oscillator?Thanks for any help.Jerry W5JH
"building something without experimenting is just solder practice"----- Original Message -----
9662 2014-01-17 22:09:30 Ashhar Farhan Re: The Minima transceiver Jerry,I have updated the circuit with your corrections. Thank for sparing the eyeballs..I am sure that there are a number of other typos too. There was a problem with the relay T/R wiring, the t and r lines were swapped. I have fixed that too.1. the filter has about 200 mv on the input side (while transmitting). the output at the other end of the RF filter is about 50 mV.2. I am right now using a version of JBOT. http://www.phonestack.com/farhan/jbot.html with an extra stage of RF amplification and an added 6db attenuator at the input of this amp. I am not happy with any of these. I'd ideally want a low distortion class A chain feeding a push-pull amp.3. I picked up the crystals from the local components shop. They were marked 'MEC'. The other brand in the local shops is marked KDS. Both are chinese, both have mediocre Q. But they are quite cheap at Rs.10 per crystal (about 15 cents).- f 9663 2014-01-19 10:43:58 kb1gmx Re: The Minima transceiver
What method did you use for setting bias for the KISS mixer?
I am currently building it and using a packaged 9mhz filter pulled from a parts
dog FT200. In my case a 9:1 transformer applied to both ends will match the
filter in use. The plan is a 75/20/10m transceiver. The wide low pass filters
will be bandpass to meet my needs. Likely the power train for the transmitter
will be of the same pattern as JaBoT But the finals will be RD16s
and the driver a RD06 mosfets mostly because I have them and want to be in
the 10-20W range. I'll also have a switchable attenuator and RFpreamp.
I will use the K5BCQ SI570 based module as its already cooked.
9664 2014-01-19 11:03:30 Ashhar Farhan Re: The Minima transceiver Allison, I am glad to have attracted your attention to the little rig. I used bias to minimize the oscillator leakage. i guess there must be a way to bias it for best IIP3 without a two-tone test.i am quite unhappy with the oscillator leakage. it has resulted in having to use two low-pass filters. a single low pass filter with a notch for the IF would have been so much cooler. but engineering is not always about elegance.- farhan 9665 2014-01-19 15:11:41 kb1gmx Re: The Minima transceiver
Yes, engineering is the intersection of good, fast and cheap. Always compromise.
Its an interesting all discrete design (ignoring the 7805 which could be a zener).
However, I'd expect that RF is always on the port [t1] in this case facing the filters,
rather than the IF side. possibly swapping those two ports might make the traps
less needed. Its part of the problem with singly balanced mixers. The IF filter can
deal with unwanted leakage.
The solution for deriving the bias can be measure the Drain current at pinch off
and use that voltage. Also matching the fets for pinchoff may help. At least that's
how I'm approaching it. Another approach is to put a resistor (10K or so) at the
center tap of the driving transformer [t2] and bypass it for RF. Measure the voltage
across the resistor (result of gate to drain or source rectification) adjust bias until
that drops to zero or close to it. You will get feed through from LO to RF if the Gate
9678 2014-01-22 18:06:29 kb1gmx Re: The Minima transceiver
Looking at the schematic I had one of those moments that something was wrong.
I have to test this but the common point of T1 should be connected to the sources
of the fets via a dc path though a choke. This keeps the drains from becoming the
other end of the gate junction as a rectifying point (or diode feed through. Likely
the floating drain will work as is but it may complicate the LO feed through.
The gates should be reverse biased but if either source or drain of the fet is at DC
or can be the gate junction will be forward biased. The FET is symmetrical
(most are) so the source and drain are interchangeable. In a case where its
used as a switch the gate must not get more than about .5-.7V positive with
respect to either source or drain or the gate starts conducting and normal fet
This is why many of the misers using the FST3125 or other CMOS switchs
have bias usually about half Vcc to insure any of the spurious or parasitic
diodes in the structure should not conduct. That should ahppen at both sides
of the switch so that DC isn't being chopped and creating other noises.
That might help your leakage through the RF port of the mixer.
9679 2014-01-22 18:21:59 Ashhar Farhan Re: The Minima transceiver Allison,thanks for pointing that out. An explanation I found (though I don't remember where) was that the charge accumulates across the channel to help maintain bias on the other side too. I7SWX's note provides for a 10K resistor instead of the choke. I haven't tried the choke, but I have tried the 10K resistor, it didn't make much of a difference. Let me try putting in a choke to and report back. 9680 2014-01-23 14:35:48 bellettau Re: The Minima transceiver There is an extensive article on FST3125/6 biasing here, which is well worth reading. Conclusion is that 1/2 supply is not optimum.