When looking on the schematic of the ?BITX I am wondering why the LO inputs of the mixers must have a 6 dB attenautor.? Is there a reasonable explanation?

?

Thanks for the Spectrum Analyzer notes, I'll go over that thoroughly.

On Sat, Aug 11, 2018 at 12:56 PM, Jerry Gaffke wrote:

Yes, most of what I had in my last post was gleaned from your older posts.
I posted in response to Henning's warnng about using a better 45mhz transistor:
? ? "Using e.g. BFR106 instead could worsen the spur problem."

Yes, it improve the gain but we really don't need that as it just makes over drive worse.

Gordon, is the drawing in message #56429 basically what you are doing with your board ?

73 Kees K5BCQ

Hi Gordon:
Before you do all that rework, have you read my post of the ninth? My solution provides a 10dB improvement by design. You need to rewind the output transformer, change all the filter capacitors (uses the same inductors) and add a new 1:2 transformer. It can be done with NO artwork changes.

Howard

-------- Forwarded Message --------

Subject:	[BITX20] Proposed fix for Harmonics Issue
Date:	Thu, 09 Aug 2018 11:28:05 -0700
From:	Howard Fidel <sonic1@...>
Reply-To:	[email protected]
To:	[email protected]

After much thought, and reading all the threads, and speaking to Allison (who has not commented to me on this yet) I think I have a good solution to the harmonics issue, not the spurs. We know that the problem is capacitance between input and output of the filters. We estimated that it is 2 pF per a relay. For 80 meters, we have 4 relays, or 8 pF capacitance from input to output. You will note that the filter has a capacitor at its output. This acts as a voltage divider. The bigger we make this cap, the small the energy will be that comes through, since the larger the cap is, the more attenuation we get from the voltage divider. To get a filter with more capacitance we need to lower the impedance of the filter, so I used 25 ohms. This means that we need to rewind T11 to get a 25 ohm output impedance. The redesigned filter for 80 meters uses the same inductors as we now have, we just need to change the capacitors to the new values, and add a transformer to the output. If done on the board, this is the only artwork change. It can also be put in line with the output coax, and then no rework of the PCB artwork is needed. The simulation shows that there is a peak in the filter response caused by the 8 pF cap on the original design at 12.5 MHz and -53 dB. With the new filter this peak is at 10.5 MHz and is - 64 dB. I think a 10 dB improvement is all we need. The same can be done for each of the other filters, and the improvement will be the same, since the capacitance through the relays goes down as the frequency goes up (fewer relays). I have attached the simulation files and the filter schematic. I have to design a transformer.?

Howard

<Instrumentsfix.pdf>

<Instruments.pdf>

<80Mmodfilter.pdf>

Allison,

Yes, most of what I had in my last post was gleaned from your older posts.
I posted in response to Henning's warnng about using a better 45mhz transistor:
? ? "Using e.g. BFR106 instead could worsen the spur problem."

Here's Warren's plot for 15m:
? ??/g/BITX20/photo/65861/2
There's a fair bit of dirt showing that's lower in freq than the 15m fundamental.
Is that stuff real, or an artifact of the test setup?

Warren used a 1mhz RBW and 1mhz VBW, are the results accurate enough for our purposes?
I'm still chewing on this from your post? ?56613,
would be less ambiguous if I knew something about spectrum analyzers.


"FYI: if you run video bandwidth down far enough you can get a 10db?
error and also clean up the baseline, save for it will be wrong.??
Anything less than 30khz (for 3-30mhz span) on the Rigol and a less?
than 1:3 radio of RBW and VBW will yield low readings.? With the?
HP8568B less than 10khz and 1:3 give errors.? Also sweep rates less?
than 300mS give sampling errors.

The test in my case was radio, Bird model 43 wattmeter, NArda 30db attenuator,?
Telonics step attenuator, Spectrum analyzer.? ?With 10W indicated on the Bird
BOTH machines of known calibration will read low if set up wrong.? For the?
Rigol I could not get an accurate 10W (40DBM) indication with 3khz RBW
and 1Khz VBW (is was short by 11db) at 311mS sweep and 3-30mhz span.
If I lowered the span to 10mhz it was off by 3db.? At 1mhz the indication?
was under 1db of correct.? ?For the setup you used I add 10db.? If I make the?
VBW 30khz it gets better but its still off."

Jerry


On Sat, Aug 11, 2018 at 12:11 PM, ajparent1/KB1GMX wrote:

?I did this back in early June, your even had comments on it.??