One concern that I have is
that Farhan is publishing Q4 as a 2n2219A metal case npn but what shows
on the photos is a T39 case RF driver.. you take my point. If the
physical entity and the schematic do not agree it is pretty involved to
re-engineer a physical board. I suspect that the driver is a 2N3886.
Please inspect and advise before you contact me. TEF

----- Original Message -----

From:

[email protected]

To:

<[email protected]>

Cc:

Sent:

Sat, 04 Aug 2018 12:15:51 -0700

Subject:

Re: [BITX20] Harmonic performance - SSB vs CW

As a daughter board it would fully replace the back half and its a non-trivial
effort.

It could be done in? the same width but the depth would have to grow.

Ways to make it smaller would be careful choice of filters at the input
such as a mix of low pass and band pass with diode switching.? at the
output the existing filters with smaller relays (6 in all) would do and a
single TR relay.? DC switching can be fully done with transistors with
some of the pop reductions added (pulling the unpowered line to ground)

*The power amp you need a minimum of 4 stages but not 9 transistors.
*Mounting the finals to the underside so the can heatsink with insulators
to the case bottom or back cuts the needed space and would make for
better RF layout.? IRF510s are fine if employed correctly.?
*Dumping the paralleled devices saves space and cleans things up.
*Using balun cores can help with space.? The Q90 stage does not need
a FT37 size core when a BN43-2402 is more than large enough and likely
the thing device pre-driver a BN43-302 is more than enough.?

An example of small and packed.? That is a modified Kitsandparts 5W Class C amp
using IRF510s ( and heatsinks on the back) and a 2n3866 driver. It takes 10mw
and makes 10W flat from 3-10mhz and slow droop to 5.5W at 29mhz. Not a lot
of fancy hardware just a solid and dirt simple circuit.? The ruler is in inches.


Allison

No one is going to want to hear this but the design of the PA output filter is probably fatally flawed and it is likely due to board layout issues.

I have been puzzled how the high order harmonics of 80 meters, for example, could possibly survive a 7th order filter which should have well over 50 dB of attenuation at the 5th harmonic. So today I set out to test the results with a 9th order filter, figuring more is better. The harmonics remained. But in troubleshooting that installation the source of the issue was revealed.

In the attached screenshot the purple trace is the 80 meter CW output with the harmonics clearly far in excess of the green -43 dBc legal threshold. The blue trace is the output with L20 removed which disables the 80 meter filter and blocks RF output from the finals from reaching the output connector. (Don't worry..... the tests were carried out with the drive reduced so the output was only one half watt. And I have a bag of spare IRF510s on hand......). The harmonics remain! How can that be?

Pic #2 shows the path 80 meter RF follows through all three switching relays in order to reach the filter. It is a very novel and inventive concept to use three relays to switch both ends of 4 filters. Most manufacturers would require 8 separate relays to do this job. And for a very good reason.

In order for a filter to provide the required 50+ dB of attenuation, the filter itself must have 60 dB or more of input-to-output isolation. If it does not then the unwanted products simply bypass the filter and go directly to the output. That appears to be exactly what is happening in the filter complex of this unit.

There is no easy fix for this. No amount of on board filtering and tweaking is going to improve harmonic suppression until the path around the filters is blocked. I suspect that path is via the relays themselves but board layout often is critical as well in such cases. My guess is the only solution would be either extensive external filtering or a carefully laid out daughter card to replace the existing filters and relays.

WA8TOD

<841.jpg>

<Screen Shot 2018_08_04 at 8.21.51 AM.jpg>

From: [email protected] [mailto:[email protected]] On Behalf Of ajparent1/KB1GMX
Sent: Saturday, August 4, 2018 4:08 PM
To: [email protected]
Subject: Re: [BITX20] Harmonic performance - SSB vs CW

?

Rahul,

The LCL preselector is one I've tried.? The show stopper is the 500pf dual capacitor
and the 4 switches.? Works as shown but bulky.

That is also a three part and the third has an interesting filter as well.



Allison

Allison and Scott
Thinking old-school raises the question about using a broadband transformer to
raise the MOSFET output impedance up to around 5K and then use a Pi-network
as was done in ancient times to match almost anything.? Pre-aligning the Pi-net
on receive mode should allow sneaking up on desired power output without over-
stressing the MOSFETs.? Real problem might be in finding suitable variable
capacitors for the pi-network.? Younger hams probably have never experienced
the pain of an RF Burn.