Hi Tony, Hans et al,
As I had posted earlier, modelling the output transformer for QDX/QMX
is tricky business!
Proper modeling of the output transformer is complicated at the
outset. There are parasitic elements, frequency dependent
permeabilities and couplings , losses, etc.
.Ross's move to a RWTST design for the 12 Volt radios further
complicates simulation. The 12V RWTST transformer is a hybrid between
a transmission line transformer and a conventional transformer, as it
has elements of both. It is neither fish nor fowl.
Then there is the issue of the? peculiar binocular transformer core.
Ross posted a model which treated the windings through each aperture
of the binocular as separate, non-coupling inductors. Yes, when wound
in a way which avoids linking the central post of the binocular, that
is what happens! It is key to the operation of the Stockton SWR bridge
incorporated in QMX.
More Controversy!
JZ
On Tue, Sep 26, 2023 at 1:52?PM Tony Scaminaci <
tonyscam@...> wrote:
>
> Hi Hans,
>
> You make valid points, especially about modeling the output transformer. I’ll leave this question to JZ as he has more expertise in this particular area. I do think his model is fairly accurate though based on the spike and power output levels we measured
early on. The spike level was close to what you and others measured on the bench.
>
> John initially suggested replacing L14 with a small value resistor on the order of 1-2 ohms as a way of eliminating the key-up spike. We also experimented with keeping L14 and using the bypass cap. Both of these alternatives resulted in eliminating the spike
but I don’t recall what the effect was on power output. In this respect, you may be correct. These experiments were done with the BS170s and I took it one step further and tied the transformer directly to 12V which did result in a slight increase in power
output. All of these experiments were run at ~28 MHz in an attempt to improve power output on 10 meters.
>
> At this point, I began searching for a better transistor with similar specs to the BS170 but with two notable improvements: lower Vgs threshold and higher Vds breakdown voltage. Microchip’s TN0110 is an excellent drop-in replacement for QDX radios. Simply
flip them upside down and you’re in business. The 1V lower gate threshold is more suitable for a 5V drive level and the 100V minimum breakdown voltage is much more effective against the L14 spike voltage. As always, it’s best if the spike never exceeds 100V…
this could be achieved by a slight reduction in L14’s inductance.
>
> The TN0110 is back in stock at Mouser. I encourage you to build a QDX with this device and compare it directly against a QDX using the BS170. You’ll be pleasantly surprised at the increased power output and your customers will experience a much lower failure
rate of the PA. A win for everyone.?
>
> Regards,
>
> Tony
> AC9QY
>
>
> On Mon, Sep 25, 2023 at 12:25 AM Hans Summers <
hans.summers@...> wrote:
>>
>> Hi Tony
>>
>> Real world testing, it is easy to see the clear decrease in power output and efficiency if you omit L14 (the 10 turn FT37-43 choke). The difference in performance is notable and significant. The fact that your simulation produces a different result indicates
to me that something isn't quite right there. I suspect accurately simulating the output transformer may be critical, a simple coupled coil model may not suffice.
>>
>> The point about driver level is also quite interesting. Back when developing the original QCX I experimented with this. In my first prototype I had an entire ACT00 logic chip driving the three parallel BS170s with all four gates in parallel. Later I needed
to implement other features in the transceiver and for some of those, taking some of the NAND gates for other purposes would be useful or necessary. I wanted an inverter for the TX signal and also to be able to gate both the CLK2 signal to the PA and the signal
generator. The end result was that I only had a single gate left, to drive all three BS170 in parallel. I found that a little alarming, because there are three BS170s in parallel in QCX and they are Class E, and fast clean switching is very important. So of
course then I had to decide whether to add another ACT00 chip or perhaps add something some place else to free up gates again. To make that decision I made detailed power and efficiency measurements for 1, 2, 4 gate cases. I also tried one gate per BS170 gate
(3 gates used). I found that there was an increase in power output and efficiency when going from 1 gate driver to 2. But it was quite a slight small improvement. There wasn't any significant improvement in going from 2 to 4. Driving the gates one by one (one
gate per BS170 separately) actually made things slightly worse; for whatever reason, paralleled gates seem to work better. Accordingly with these observations in hand, I felt happy sticking with the single gate drive for QCX. Then when it came to QDX where:
>>
>> 1. There are 2 gates per driver not one;
>> 2. There are 2 parallel transistors not 3;
>> 3. It isn't Class E so it's a little less critical;
>> I felt entirely comfortable with 2 gates driving 2 parallel BS170s on each side.
>>
>> So I haven't tried doubling up a piggy back ACT08 but it would be an interesting experiment for someone; personally it would surprise me if doubling up made a significant or even noticeable improvement.
>>
>> 73 Hans G0UPL
>>
>>
>
>
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